Term
| Why are melting points carried out? |
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Definition
1) To characterize a compound
2) To record a physical characteristic for a new compound
3) To establish purity of a known compound |
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Term
| What is the definition of the melting point of a pure compound? |
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Definition
| 1) A pure solid should melt in between a narrow range of temperature, usually less than one degree Celsius (A solid is considered pure if the melting point does not rise after recrystallization). |
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Term
| What is the criteria for impurity? |
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Definition
| 1) The impurity must be soluble in the compound, so an insoluble impurity will not depress the melting point |
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Term
| What forces affect melting point? |
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Definition
1) IMF such as London forces (hydrocarbons), dipole-dipole forces (EN atoms), hydrogen bonding (OH and NH groups), and ionic forces.
Larger / symmetrical molecules have higher melting points, R and S isomers have the same melting point, but the racemate or diastereomers will usually possess different melting points |
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Term
| What is the definition of eutectic point? |
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Definition
1) The lowest point on the melting point composition diagram where a mixture has a very narrow melting point range
2) The point at which two solids and their liquid solutions are in equilibrium |
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Term
| What is the definition of eutectic temperature? |
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Definition
| 1) The lowest temperature at which a mixture at a certain concentration will start melting |
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Term
| What is a mixed melting point experiment? |
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Definition
| 1) It is used to confirm the identity of an unknown compound. If A and B are different compounds than a mixture of the two would have a depressed melting point. To carry out the experiment, mix unknown with a small quantity of unknown compound and observe the melting point |
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Term
| What is the definition of a melting range? |
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Definition
| 1) The melting range is defined as when the first drop of liquid is observed to the temperature when the sample is completely liquefied |
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Term
| What happens if you have too much sample? |
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Definition
| 1) If too much is in the capillary, there will be a differential in temperature throughout the sample |
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Term
| What happens if the sample is coarse? (Not finely powdered). |
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Definition
| 1) If the sample is in chunks, the air pockets in the sample will not allow the sample to be packed properly |
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Term
| How should the sample be heated in the melting point experiment and why? |
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Definition
| 1) Slowly, to give enough time for heat to be transferred equally from the heating block top the sample to the thermometer |
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Term
| What may happen if you heat the sample too quickly? |
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Definition
| 1) Because the heat reaches the sample before before the thermometer, it is probable that the thermometer reading will be lower than the actual temperature at the melting point |
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Term
| Why does the sample shrink before melting? |
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Definition
| 1) Because of crystalline structure changes |
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Term
| What are the criteria for the sample before packing and how should it be packed? |
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Definition
| 1) The sample should be dry (because moisture from solvents like water will act like impurities and depress the melting point), finely powdered, packed tightly (by rapping on a hard surface or by dropping into a two foot length of glass tubing on a hard surface) and it should not be more than two-three mm high. |
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Term
| What is the purpose of recrystallization? |
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Definition
| 1) To purify a mixed compound |
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Term
| What should you consider when picking a solvent? |
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Definition
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Term
| What are the requirements for solvents with respect to solute and impurities? |
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Definition
1) Solute: Dissolves the solute completely when the solvent is hot; precipitates solute completely when the solvent is cold; does not react with the solute; nonflammable; nontoxic; inexpensive
2) Impurities: Does not dissolve at all or dissolves impurities very well even if the solvent is cold. |
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Term
| What are the requirements for solvents for solvent pairs? |
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Definition
| 1) Both solvents have to be miscible in each other; one solvent has to be a poorer solvent for the solute. |
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Term
| Why are solvent pairs used? |
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Definition
| 1) They are used when no one solvent has ideal properties with respect to a solute |
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Term
| What are the requirements when cooling? Why do small powdery crystals form and how do you prevent them? |
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Definition
| 1) Must be cooled slowly, otherwise the solid will come "crashing out" of solution as powder, precipitating impurities along with it; muse use paper towels to insulate the test tube; if disturbed / moved too many nuclei for crystallization to form, leading to small crystals that come out of the solution along with impurities |
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Term
| What is the definition of superheating? |
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Definition
| 1) Superheating occurs when a very clean liquid in a very clean vessel is heated to a temperature above its boiling point without ever actually boiling |
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Term
| How do boiling sticks and stones prevent superheating? |
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Definition
| 1) In boiling sticks, air trapped in the wood comes out of the stick and forms nuclei on which even boiling can occur. Boiling chips are composed of porous unglazed porcelain material that is filled with air in numerous fine capillaries. With heating the air expands to form the fine bubbles on which even boiling can take place. Once the liquid cools, it will fill these capillaries and the boiling chip becomes ineffective |
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Term
| Why does product get lost in crystallization, collecting, and washing steps? |
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Definition
| 1) There will always be some solute soluble in the cold solvent. Some crystals adhere to the filter paper or dissolve during the washing step |
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Term
| Why are large crystals preferred? |
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Definition
| 1) They are easier to filtrate and easier to wash free of adhering impure solvent |
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Term
| Why is it important that the boiling stick is removed during the cooling process? |
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Definition
| 1) So the crystals don't form on the boiling stick; the solution must be saturated |
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Term
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Definition
Crystals come out of the solution as an oil when:
1) The saturated solution temperature or the boiling point of the solvent is higher than the melting point of the solute
2) If the melting point of the solute is depressed to a point such that a low melting eutectic mixture of the solute and solvent comes out of the solution (the remedy is to lower the temperature at which the solution becomes saturated by simply adding more solvent). |
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Term
| What is the purpose of activated charcoal? How does it work? |
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Definition
| 1) When a chemical reaction produces high molecular weight byproducts that are highly colored, the impurities can be adsorbed onto the surface of activated charcoal by boiling the solution with charcoal |
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Term
| What are the remedies for unsaturated and supersaturated solutions? |
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Definition
| 1) On a microscale, there is a tendency to use too much solvent so that on cooling the hot solution little or no material recrystallizes. The remedy is to evaporate some of the solvent by careful boiling and repeat the cooling process. If the solution is supersaturated, obtain a seed crystal and / or scratch the bottom of the test tube with a glass rod. |
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Term
| What are the reasons for carrying out recrystallization? |
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Definition
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Term
| What are the requirements for choosing a solvent? |
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Definition
1) Remember like dissolves like:
-Polar solvents dissolve polar solutes
-Non-polar solvents dissolve non-polar solutes |
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Term
| What are the requirements for solvents with respect to solute and impurities? |
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Definition
For solutes, solvents must:
1) Dissolve solute completely when solvent is hot
2) Precipitate solute completely when solvent is cold
3) Not react with solute
For impurities, solvents must:
1) Not dissolve impurities at all, so that the impurities can be filtered out from the hot solvent
2) Dissolve impurities very well when the solvent is cold, so impurities along with the cold solvent can be removed from crystals via filtration |
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Term
| What are the requirements for solvents and solvent pairs? |
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Definition
1) Both solvents have to be miscible in each other (mix in a homogenous way)
2) One solvent has to be a poorer solvent for solute than the other
3) Used when no one solvent has ideal properties with respect to a solute |
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Term
| Why are solvent pairs used? |
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Definition
| 1) Solvent pairs are used when no single satisfactory solvent can be found |
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Term
| What are the desirable and undesirable qualities of water, ethanol, ethyl acetate, and acetone (common crystallization solvents) |
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Definition
Water:
1) Cheap, nonflammable, nontoxic, and dissolves large variety of polar organic molecules
2) High boiling point and high heat of vaporization make it difficult to remove from crystals
Ethanol:
1) High boiling point makes it a better solvent for less polar molecules; evaporates readily from crystals
2) Esters may undergo an interchange of alcohol groups on recrystallization
Ethyl Acetate:
1) Has right combo of moderately high boiling point and volatility needed to remove it from crystals
Acetone:
1) Low boiling point means there is not much difference in the solubility of a compound at its boiling point compared to about 22 degrees Celsius |
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Term
| How do you minimize loss of product in crystallization, including collecting and washing steps? (Why does the product get lost in these steps). |
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Definition
1) Product gets lost in these steps because the crystals can adhere to filter paper or dissolve during the washing process
2) To minimize loss, collect in Hirsch funnel through vacuum filtration. It is important to use filter paper, so that the frit of the funnel does not get clogged with insoluble impurities |
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Term
| What are the requirements when cooling? |
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Definition
1) The cooling process must be slow. If not, precipitation (as powder) will occur with the desired solute and no exclusion will occur (the impurities will precipitate along with it)
2) To ensure slow cooling, wrap paper towel to insulate test tube |
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Term
| What are the remedies for unsaturated and supersaturated solutions? |
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Definition
1) For unsaturated solutions (occur when solution is removed of impurities by charcoal) the solution should be boiled off the solvent under an aspirator tube or gently blown off using a gentle stream of air or nitrogen in the hood.
2) For supersaturated solutions (saturated solution cools and no crystals form), a seed crystal should be use or the bottom of the test tube should be scratched |
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Term
| What is the definition of superheating? How can it be prevented? |
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Definition
1) Superheating is when a solution is heated above its boiling point without actually boiling the solution
2) To prevent it, you can use a boiling stick or boiling chip. They work by air being trapped in it, forming nuclei on which boiling can occur. |
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Term
| Why do crystals oil out? What are remedies for this problem? |
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Definition
1) Oiling out occurs when the temperature of the solution or boiling point of the solvent is higher than the melting point of the crystals.
2) The remedy is to lower the temperature at which the solution becomes saturated with the solute by adding more solvent. It if persists, another solvent should be used. |
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Term
| How do small powdery crystals form and how can this be prevented? Why are larger crystals preferred? |
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Definition
1) If the flask is moved during the experiment many nuclei will form and small crystals will be created with a large surface area. They will not be easy to filter out and wash clean.
2) You want large crystals because they are easily separated by filtration and clearly washed free of adhering impure solvents |
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Term
| What is the purpose of activated charcoal? How does it work? |
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Definition
1) The purpose of activated charcoal is to absorb impurities that give off color to solution
2) The activated charcoal has an extremely large surface area per gram and can bind a large number of molecules to this surface (functions like a boiling chip) |
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Term
| Why is it important to remove the boiling stick during the cooling process? |
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Definition
| 1) Boiling sticks should be removed because crystals can grow on them |
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Term
| Describe specifically what is done when employing the concept of solubility in the process of recrystallization. |
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Definition
| 1) Solute is dissolve in appropriate hot solvent to yield a solution |
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Term
| Describe specifically what occurs when employing the concept of saturation level in the process of recrystallization. |
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Definition
| 1) The cooling solvent causes the crystal solution to become saturated with respect to the solute |
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Term
| Describe the concept of exclusion as it relates to the process of recrystallization. |
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Definition
| 1) Impurities are excluded (dissolved in solution) and get a single pure substance |
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Term
| A seed crystal is an example of a point of nucleation. Provide two other examples of points of nucleation in the process of recrystallization. |
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Definition
1) Speck of dust
2) Scratch wall on test tube |
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Term
| How do you use a solvent pair to recrystallize a solid? (Explain procedure briefly). |
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Definition
| 1) Dissolve the solute in a better (more solubilizing) solvent and add poorer (less solubilizing) solvent to the hot solution until it becomes cloudy and the solution is saturated |
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Term
| In the recrystallization experiment, you found that you did not get 100% recovery. Explain how solute may have been lost (specifically) in the washing step. |
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Definition
| 1) Some crystals will dissolve during washing |
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Term
| In the recrystallization experiment, you found that you did not get 100% recovery. Explain how solute may have been lost (specifically) in the collecting step. |
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Definition
| 1) Some crystals will adhere to the filter paper after having been washed |
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Term
| In the recrystallization experiment, you found that you did not get 100% recovery. Explain how solute may have been lost (specifically) in the crystallizing step. |
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Definition
| 1) There is always some solute left in cold solvent, so 100% recovery is not possible |
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Term
| You find your percent recovery in the recrystallization experiment is larger than 100%. What step was not done efficiently? |
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Definition
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Term
| What other effect would you observe on a physical characteristic of the crystal if the drying step is not carried out efficiently? |
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Definition
| 1) The crystal will still be wet |
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Term
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Definition
| 1) Crystals come out of the solution as oil; occurs when the temperature of the solution is above the melting point of the crystals |
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Term
| One of the remedies for oiling out in a recrystallization process is to add more solvent to the solution. What does this serve to do? |
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Definition
| 1) This lowers the temperature at which the solution becomes saturated |
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Term
| What would occur if you forget to insulate the test tube containing the saturated solution with paper towels in the recrystallization experiment? |
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Definition
| 1) The solution will cool too quickly and solid will come crashing out of solution as powder along with impurities |
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Term
| What would occur if the boiling stick is left in the crystallization solution while it is cooling? |
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Definition
| 1) Crystals may form on the boiling stick |
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Term
| How do you complete the drying of crystals after keeping the crystals for a few minutes on the Hirsch funnel? |
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Definition
| 1) Squeeze crystals in between filter paper and let crystals air dry on a watch glass |
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Term
| Define supersaturation with regards to recrystallization. |
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Definition
| 1) When a saturated solution cools and no crystals form |
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Term
| What should be done in the distillation apparatus to ensure that the temperature of the vapor distilling through the sidearm is accurately recorded? |
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Definition
| 1) Thermometer bulb must be below sidearm |
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Term
| How do you avoid the rupturing of the apparatus while carrying out a distillation? |
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Definition
| 1) Never distill in an airtight system because this is a closed system |
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Term
| Why can a boiling chip only be used once? |
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Definition
| 1) Cool liquid will fill the capillaries in the boiling chip once it is used |
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Term
| What two observations can be made about the distillation of a pure liquid? |
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Definition
1) Boiling point of the liquid is the temperature of the vapor
2) Temperature remains constant throughout distillation |
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Term
| When you set up the apparatus for distillation, how do you minimize the release of vapors into the hood? Assume that all connectors are snug-fitting. |
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Definition
| 1) End of sidearm must be inside receiver vial |
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Term
| Why is it important not to distill liquids to dryness? |
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Definition
| 1) Peroxides can form; dry residue can be explosive |
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Term
| How do you ensure that vapor condenses in the sidearm of the distillation apparatus? |
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Definition
| 1) Use a wet paper towel to cover the condenser |
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Term
| What is the purpose of distillation? |
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Definition
| It is a way to purify liquids; liquids are separated based on boiling points |
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Term
| Why is a wet paper towel used to cover the condenser (sidearm) in the distillation experiment? |
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Definition
| 1) To ensure that the vapor condenses in the sidearm |
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Term
| What must the position of the thermometer be in the distillation experiment? |
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Definition
| 1) It must be below the sidearm so it accurately records the temperature of the vapor distilling through the sidearm |
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Term
| What must the position of the sidearm be in the distillation experiment? |
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Definition
| 1) It must be inside the receiver vial to minimize the release of vapors into the hood (loss of liquid) |
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Term
| What are the uses of simple and fractional distillation and what kinds of mixtures are suitable for each kind of distillation? |
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Definition
1) Simple distillation does not yield complete separation; it is used when the boiling point difference between the two liquids is large and when one liquid composition is less than 10%
2) Fractional distillation is a series of continuous distillations; it is used when the boiling point difference between the two liquids is small and the two liquid compositions are substantial |
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Term
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Definition
| 1) They are mixtures with a fixed composition that cannot be altered by simple or fractional distillation. They can be minimum, below the pure boiling points of both liquids, or maximum, above the pure boiling points of both liquids |
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Term
| What is the purpose of vacuum distillation? |
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Definition
| 1) It is a distillation of miscible mixtures carried out under reduced pressure; thus requiring a lower boiling point. It is used when neither ordinary distillation, nor steam distillation is practical for compounds that decompose at their boiling points or are sensitive to oxidation |
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Term
| What is the purpose of steam distillation? |
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Definition
| 1) It is usually used with immiscible liquids. One of the liquids is usually water. The boiling point of the mixture is below the boiling point of each pure component. It only works if the compound is volatile. It is carried out by passing steam into a boiling flask. |
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Term
| What requirement is necessary so that the boiling point remains constant in steam distillation? |
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Definition
| 1) Adequate amount of water and organic component must be present to saturate the vapor space |
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Term
| What is the definition of "hold up"? |
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Definition
| 1) It is unrecoverable distillate that wets the column packing |
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Term
| How do you insulate the apparatus in the distillation experiment? |
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Definition
| 1) Wrap the apparatus with foul to minimize heat loss |
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Term
| What are the criteria for the packing material in the distillation lab? |
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Definition
1) Easy to insert
2) Doesn't come out
3) Large surface area
4) Good heat transfer
4) Low hold up
5) If the packing material is too dense, pressure changes cause non-equilibrium conditions.
6) If the packing material has too large a surface area, it will absorb or "hold up" the distilled material |
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Term
| What are the precautions about distilling to dryness? Distilling in a close / airtight system? |
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Definition
1) Dry residue of liquids can be explosive
2) The apparatus may rupture |
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Term
| What are the sources of error in distillation? |
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Definition
1) Loose connectors cause vapor to escape
2) Temperature variations due to airflow in hoods
3) Large surface area of microscale apparatus make it easy to lose heat to the environment |
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Term
| What is the behavior temperature during a simple or fractional distillation? |
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Definition
| 1) The temperature does not remain constant. It increases throughout distillation because the composition of vapor that is distilling varies throughout. Some lower boiling point component comes out first and then the vapor is enriched with higher boiling point components |
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Term
| What precautions should be taken for spotting in the TLC experiment? |
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Definition
1) Must be careful not to flake off adsorbent because the solvent will not ride up the plate effectively
2) Make spots small and compact by touching lightly on the TLC plate; making large spots will cause smearing
3) Sample should not be too concentrated
4) Label spots with a pencil |
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Term
| What precautions should be taken for developing in the TLC experiment? |
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Definition
1) Make sure the solvent level is below the spotting line so that the solvent does not dissolve samples on the spotting line
2) Make sure the cap is placed on the developing chamber so that the solvent does not evaporate during the developing stage; this will cause the TLC plates to be messy and ineffective
3) Make sure solvent does not rise up past the solvent front, if it does spots in visualization stage will be difficult to evaluate and the Rf will be too high
4) Make sure filter paper is in the developing chamber to saturate the developing chamber with solvent vapors |
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Term
| What precautions should be taken for visualizing in the TLC experiment? |
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Definition
1) Mark spots with a pencil as the carbon in the tip is inert
2) Trace spots right away
3) Use iodine chamber for alkanes, alcohols, and ethers which do not absorb UV light |
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Term
| Why should bores of capillaries be small? |
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Definition
| 1) The bores should be small so that the sample which is provided on the TLC plate is not too concentrated and so that smearing is prevented |
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Term
| What are typical characteristics for solvents used as eluents? |
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Definition
| 1) Ethyl acetate (liquid) as eluent |
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Term
| Characteristics of solvents that allow them to migrate rapidly? |
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Definition
1) The solvent should have low viscosity(migrate quickly)
2) The solvent should have a low boiling point (easily evaporated) |
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Term
| Adsorbent materials and specific uses (which adsorbent would be better for other substances) |
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Definition
Alumina:
1) More active
2) Separates nonpolar substances, such as hydrocarbons, alkyl halides, ethers, aldehydes, and ketones
Silica Gel:
1) Less active
2) Separates more polar substances such as alcohols, carboxylic acids, and amines |
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Term
| What is the order of solute migration? |
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Definition
Highest / fastest (elute with nonpolar mobile phase)
1) Alkane hydrocarbons
2) Alkyl halides (halocarbons)
3) Alkenes (olefins)
4) Dienes
5) Aromatic hydrocarbons
6) Aromatic halides
7) Ethers
8) Esters
9) Ketones
10) Aldehydes
11) Amines
12) Alcohols
13) Phenols
14) Carboxylic acid
15) Sulfonic aicds
Lowest / slowest (need polar mobile phase to elute) |
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Term
| What are the six uses of TLC? |
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Definition
1) Determine number of components in a mixture
2) Determine identity of two substances
3) Monitor the progress of a reaction
4) Determine the effectiveness of purification
5) Determine the appropriate conditions for a column chromatographic separation
6) To monitor column chromatography |
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Term
| What are the effects of using too polar / nonpolar of a solvent? |
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Definition
1) If too polar, the spots will rise to the solvent front; too nonpolar and they will remain on the bottom of the TLC plate
2) Using eluent with too high a polarity with respect to samples (polar) use silica
-All spots will remain towards the bottom of the TLC plate
-High Rf values
3) Using an eluent with too low a polarity with respect to samples (nonpolar)use alumina
-All spots will stay towards the bottom of the TLC plate
-Low Rf values |
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Term
| Visualization techniques - Why were certain kind of visualization techniques used for certain types of compounds? |
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Definition
1) UV light is used for conjugate compounds
-TLC plates normally contain a flourescent indicator that makes them glow green under UV wavelength, yielding blue or purple spots
2) Iodine vapor - for most compounds
like alkanes, alcohols, and ethers that do not absorb UV light
-Will adsorb iodine vapors to form brown spots |
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Term
| Why is pencil never used to mark a plate? |
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Definition
| 1) A pencil is always used because the graphite (carbon) is inert |
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Term
| Why is ink never used to mark then plate? |
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Definition
| 1) If ink is used to mark the plate, it will chromatograph just as any other organic compound and give flawed results |
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Term
| What are the precautions to take when spotting, developing, and visualizing? |
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Definition
When spotting:
1) Be sure not to flake off adsorbant
2) Make spots small and compact
3) The sample should not be too concentrated
When developing:
1) Make sure the solvent line is below the spotting line so that the solvent doesn't dissolve the samples
2) Make sure the top is placed back on the jar so that the solvent does not evaporate (evaporation would cause the development of the TLC plate to be messy and ineffective)
3) Make sure the solvent does not rise past the solvent front
4) Make sure the filter paper is in the developing chamber with solvent vapors
When visualizing:
1) Trace the spots right away |
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Term
| Why are certain visualization techniques used for certain types of compounds? |
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Definition
1) UV light is used for conjugated compounds
2) Iodine vapor is used for most compounds |
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Term
| What are specific uses of the adsorbant materials? |
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Definition
1) Alumina is more active and is used for nonpolar substances
2) Silica is less active and is used for more polar substances |
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Term
| What are the effects of using too polar or too nonpolar a solvent? |
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Definition
1) If you use too polar a solvent, all the spots will rise to the solvent front
2) If the solvent is too nonpolar all the spots will remain on the bottom of the TLC plate |
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Term
| What are the characteristics of solvents that allow them to migrate rapidly? |
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Definition
1) The solvent should have low viscosities
2) The solvents should have low boiling points so that they can be easily evaporated |
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Term
| Why should the bores of capillaries be small? |
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Definition
| 1) So that once a liquid is drawn into them, it will not flow out to form a drop and lead to large, concentrated, drops |
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Term
| What are the uses of TLC? |
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Definition
1) Determine number of components in a mixture
2) Determine identity of two substances
3) Monitor the progress of a reaction
4) Determine the effectiveness of purification
5) Determine the appropriate conditions for a column chromatographic separation
6) To monitor column chromatography |
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Term
| What is the purpose of extraction? |
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Definition
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Term
| Why are extractions carried out in the sequence they are (weak base, strong base)? |
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Definition
| 1) If a strong base is used first, two compounds may be separated from the others at the same time |
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Term
| What are the reactions of acid-base extractions? |
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Definition
| 1) The sodium bicarbonate, a base, is reacted with benzoic acid, to form the water soluble sodium benzoate ion. The addition of the strong base, sodium hydroxide, triggers a reaction between sodium hydroxide and tert-butyl phenol, to form the water soluble sodium tert-butyl phenoxide io. To transfer water form the tert-butyl methyl ether to the aqueous layer, sodium chloride was added to the first tube. To remove the microscopic droplets of water suspended in the organic layer, calcium chloride pellets, the drying agent, where added to the solution. To regenerate the benzoic acid and tert-butyl phenol, hydrochloric acid was added to the benzoate and tert-butyl phenoxide ions |
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Term
| What are the requirements for extraction solvents? |
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Definition
1) Should readily dissolve the substance to be extracted at room temperature
2) Should not be miscible with water
3) Should not react with the solute
4) Should have low boiling point for easy removal
5) Should not be flammable or toxic
6) Should be inexpensive |
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Term
| What are emulsions and how are they caused? What are the remedies? |
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Definition
1) Emulsions are a suspension of one liquid as droplets in another
2) To prevent them, shake the mixture gently and make the aqueous layer highly ionic
3) They're caused by shaking the mixture |
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Term
| How do you decrease the solubility of an organic substance in an aqueous layer? |
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Definition
| 1) Make the aqueous layer highly ionic by adding NaCl |
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Term
| Why was a drying agent used in the extraction experiment? What observations indicate than an organic layer is dry? What substance was used in the experiment as a drying agent? What are the criteria for choosing a drying agent? |
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Definition
1) To remove microscopic droplets of water suspended in the organic layer
2) Upon completion of drying the solution becomes clear
3) Calcium chloride pellets were used as the drying agent
4) Drying agents should:
-Not react with solute
-Remove water quickly
-Be efficient
-Easy to recover solute |
|
|
Term
| What are the advantages of using calcium chloride as a drying agent when compared to other drying agents? |
|
Definition
Advantages of calcium chloride pellets:
1) Calcium chloride is fast and effective
2) Clumps in the presence of excess water so you know how much to add by observing behavior
3) Does not disintegrate into a fine powder
4) Pellets are suitable for microscale experiments where the solvent is removed from the drying agent with a Pasteur pipette
5) Can also remove a contaminating alcohol
Disadvantages of other drying agents:
1) Drierite (calcium sulfate) hard to tell if enough has been used
2) Magnesium sulfate is too finely powdered so it is difficult to remove during filtration
3) Molecular sieves are impossible to ascertain whether enough has been used
4) Sodium sulfate is a very poor drying agent because it is slow and not very efficient |
|
|
Term
| How do you ensure that all product is transferred from the drying agent? |
|
Definition
| 1) You add small amounts of ether to the drying agent two more times after letting them dry for five minutes and use a pipette to transfer the ether to the holding vial |
|
|
Term
| What are the advantages of t-butylmethyl ether and disadvantages of ether? |
|
Definition
Advantages of t-butylmethyl ether:
1) Does not form peroxides
2) Cheaper
3) Less volatile
4) Explosion limits are narrower
5) Low toxicity
6) Ignition temperature is higher
Disadvantages of ether:
1) Highly flammable
2) Easily forms peroxides |
|
|
Term
| Why don't you get 100% recovery in the extraction experiment? |
|
Definition
1) The solute is never completely transferred to the organic layer
2) In the acid / base reactions, all of the ion formed is never completely transferred to the aqueous layer |
|
|
Term
| Provide four methods for remdying the problem of emulsion. |
|
Definition
1) Shake mixture
2) Make aqueous layer ionic with NaCl
3) Vacuum filtration
4) Centrifugation |
|
|
Term
| Why are ethanol and methanol not good extraction solvents? |
|
Definition
| 1) Soluble (miscible) in water |
|
|
Term
| Besides the fact that water is not miscible with the organic solvents used in extraction, why is water used with organic solvents in extraction? |
|
Definition
1) Not highly flammable or toxic
2) Relatively inexpensive |
|
|
Term
| Besides the evolution of CO2 in acid / base extractions, what is another source of build up of pressure? |
|
Definition
|
|
Term
| You dissolve a mixture of phenol (weak acid), aniline (a base) and napthalene (neutral compound) in ether. You extract the mixture with NaOH. What is contained in the aqueous layer? |
|
Definition
|
|
Term
| You dissolve a mixture of aniline (a base) and napthalene (a neutral compound) in ether. You extract the mixture with HCl. What is contained in the aqueous layer? |
|
Definition
|
|
Term
| Why was there bubbling when HCl was added to the aqueous extract containing sodium benzoate? |
|
Definition
| 1) HCl was reacting with excess NaHCO3 to form CO2 gas |
|
|
Term
| You added HCl to the aqueous layers to generate benzoic acid and 4-tertbutyl phenol in the extraction experiment. What else did the HCl react with in the aqueous layers in this process? |
|
Definition
| 1) Unreacted weak base and strong base remains in the aqueous layer that would react with HCl |
|
|
Term
| What is the disadvantage of using molecular sieves over calcium chloride pellets as a drying agent? |
|
Definition
| 1) Difficult to determine by appearance whether enough has been used |
|
|
Term
| What is the advantage of using calcium chloride pellets over using the older granular form? |
|
Definition
| 1) Pellets don't disintegrate to powder |
|
|
Term
| List two reasons that make sodium sulfate a poor drying agent compared to the calcium chloride pellets. |
|
Definition
1) Slow
2) Not very efficient in removing water |
|
|
Term
| What are the disadvantages of using magnesium sulfate as the drying agent instead of calcium chloride pellets? |
|
Definition
| 1) So finely powdered that it always requires careful filtration |
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|
Term
| When transferring a layer from one test tube to another using a pipette with a rubber bulb attached, it is advised to grasp an empty tube in the same hand as the full tube to avoid losing any solution. What is the cause of the possible loss of solution from the pipette? |
|
Definition
| 1) High vapor pressure of solvent causes it to dribble out of pipette |
|
|
Term
| What two observations indicate than an organic layer has gone from wet to dry when you add drying agent (assume it is a powder) to the organic layer? |
|
Definition
1) Drying agent stops clumping
2) Organic layer goes from cloudy to clear |
|
|
Term
| In the separation of benzoic acid, 4-tertbutyl phenol, and biphenyl, what would occur if sodium hydroxide had been used in the first extraction? |
|
Definition
| 1) Would extract both benzoic acid and 4-tertbutyl phenol into aqueous phase |
|
|
Term
| When carrying out the acid-base extractions in lab, you notice you were asked to extract with sodium bicarbonate only once while the procedure directs you to carry out the extraction with a second amount of sodium bicarbonate. Why might the omission of the second extraction cause you to obtain a less than 100% recovery of the substance you are extracting? |
|
Definition
| 1) Some solute is still dissolved in ether (or organic or other) layer |
|
|
Term
| How do acid / base reactions work? |
|
Definition
1) Use base to react with an organic acid to form an ionic compound (water soluble) which is pulled into the aqueous layer
2) Use acid to react with an organic base to form an ionic compound (water soluble) which is pulled into the aqueous layer |
|
|
Term
| How do you decrease the solubility of organic substances in the aqueous layer? |
|
Definition
| 1) Make the layer ionic by adding NaCl |
|
|
Term
| Why are drying agent used? What observations indicate that an organic layer is dry? |
|
Definition
1) Drying agents are used to remove all traces of water from the organic layer
2) The solution will become clear |
|
|
Term
| What is the criteria for choosing a drying agent? |
|
Definition
1) Must not react with solute
2) Must remove water quickly
3) Must be efficient
4) Must be easy to recover solute |
|
|
Term
| What are the advantages of using calcium chloride as a drying agent when compared to other drying agents? |
|
Definition
1) Clumps together in the presence of excess water
2) Pellets do not disintegrate into a fine powder
3) Faster and more effective |
|
|
Term
| How do you ensure all product is transferred from the drying agent? |
|
Definition
| 1) Wash the drying agent left in the tube with several small quantities of pure solvent |
|
|
Term
| What are the advantages of t-butylmethyl ether and the disadvantages of diethyl ether? |
|
Definition
1) t-butylmethyl ether does not easily form peroxides and can be stored for a loner time
2) Diethyl ether is highly flammable and easily forms peroxides |
|
|
Term
| Why don't you get 100% recovery in extraction? |
|
Definition
1) Incomplete acid / base reactions
2) Loss of product during transfer steps |
|
|
Term
| What are the factors that affect SN1 / SN1 reactions? |
|
Definition
SN1 Reactions:
1) The stability of the carbocation affects the rate of the reaction
2) The order of reactivity is 3 > 2
3) Primary, methyl, and vinyl alkyl halides do not undergo SN1 reactions
4) Nulceophile can have an indirect effect as a solvent by decreasing the concentration of the alkyl halide
5) Rearrangements (methyl and hydride shifts) are likely
6) Polar protic solvents (ethanol or water) are preferred as they stabilize the transition state, lowering the energy of activation
SN2 Reactions:
1) Second order, so dependent upon the concentration of the nucleophile and alkyl halides
2) Reactivity is determined by hinderance
3) Rate of reactivity is CH3X > 1 > 2
4) Polar aprotic solvents (acetone or DMSO) are preferred as they do not solvate the nucleophile and therefore do not impede the rate of the reaction
Both:
1) Temperature increase reaction rate
2) Good nucleophiles are required: I > Br > Cl |
|
|
Term
| What is the importance of using the exact amount of alkyl halide and nucleophile in each test tube? |
|
Definition
| 1) For SN2 reactions, the amount of nucleophile affects the rate and would add another variable. For all reactions, alkyl halide concentration affects the rate |
|
|
Term
| What is the cause of a possible false positive in the nucleophilic substitution experiment? |
|
Definition
| 1) The alkyl halide can act as a solvent in which the nucleophile is insoluble |
|
|
Term
| Why don't vinyl and aryl halides undergo SN1 or SN2 reactions? |
|
Definition
1) They don't undergo SN1 reactions because the high energy, carbocation intermediates they would form are relatively difficult to form
2) They don't undergo SN2 reactions because the electrons in the double bonds repel the nucleophile which is either an ion or polarized neutral species |
|
|
Term
| What are the products of SN1 and SN2 reactions when the starting alkyl halide is chiral and why? |
|
Definition
1) In SN2, The products are opposite the starting material due to inversion
2) In SN1, the product is 50:50 (racemic) mixture of enantiomers because the planar intermediate can form a bond with the nucleophile on either face |
|
|
Term
| Why do Br- and Cl- not attack the alkyl iodides that form in the SN2 reaction carried out? |
|
Definition
| 1) NaCl and NaBr precipitate so there are no free Br- and Cl- ions in solution to attack the alkyl iodides |
|
|
Term
| What factors promote SN2 in the NaI in acetone reactions? |
|
Definition
1) Minimum steric hinderance
2) Polar aportic solvent
3) Good nulceophile
4) Good leaving group |
|
|
Term
| What factors promote SN1 in the AgNO3 in ethanol reactions? |
|
Definition
1) Stable carbocations
2) Polar aprotic solvent
3) Good nucleophile
4) Good leaving group |
|
|
Term
| What side products may form in SN1 and SN2 reactions? |
|
Definition
| 1) In SN1, hydride or methyl shifts are likely to take place. Alkenes are also likely as a result of E1 |
|
|
Term
| Why wouldn't E2 reactions occur in the NaI in acetone reaction? |
|
Definition
| 1) There is no possibility of an E2 side product since I is a weak base while still being a good nucleophile |
|
|
Term
| Why would E1 reactions occur in the AgNO3 in ethanol reaction? |
|
Definition
| 1) Ethanol is a weak nucleophile so it undergoes SN1 reactions. The intermediate in E1 reactions is the same for SN1 (they compete) |
|
|
Term
| What is the order of reactivity for SN1 reactions? |
|
Definition
| 1) Tertiary > Allylic alkyl halide > Secondary |
|
|
Term
| Why can vinyl or aryl halides not react through an SN1 mechanism? |
|
Definition
| 1) The carbocation is difficult to form due to double bond |
|
|
Term
| Why does chirality not matter in SN1 reactions? |
|
Definition
| 1) SN1 yields racemic products (50:50 enantiomer) because the nucleophile can bond to either side of the planar SP2 hybridized carbocation intermediate |
|
|
Term
| Are rearrangements possible in SN1? Why do they occur? |
|
Definition
| 1) Hydride or methyl rearrangement may occur in order to relocate the carbocation to a more stable (substituted) site. This occurs when a carbon in the immediate vicinity is more substituted than the carbon with the initial carbocation. |
|
|
Term
| What solvents should be used for SN1 reactions? What is their role in the mechanism? |
|
Definition
1) Polar protic
2) They stabilize the transition state more than polar aprotic solvents and lower the activation energy of the reaction |
|
|
Term
| What type of leaving group should be used for SN1? |
|
Definition
| 1) A good leaving group is necessary (the weaker the base the better) |
|
|
Term
| The rate of an SN1 reaction increases with the concentration of __________ __________. |
|
Definition
|
|
Term
| Why does the amount of solvent matter in SN1 reactions? |
|
Definition
| 1) Affects the concentration of alkyl halide |
|
|
Term
| What is the primary factor in determining the rate of an SN1 reaction? |
|
Definition
| 1) The rate of carbocation formation |
|
|
Term
| How does temperature affect the rate of SN1 reactions? |
|
Definition
| 1) Increases the rate by increasing the amount of kinetic energy / collision of molecules |
|
|
Term
| What is the structure of alkyl halides for SN2 reactions? |
|
Definition
| 1) Methyl > Primary > Secondary |
|
|
Term
| Why do vinyl or aryl halides not undergo SN2 reactions? |
|
Definition
| 1) Electron density (double bond) repel the nucleophile |
|
|
Term
| The __________ concentration affects the rate of an SN2 reaction. |
|
Definition
|
|
Term
| Why do Cl and Br not attack alkyl iodides in SN2 reactions? |
|
Definition
| 1) Cl and Br do not attack alkyl iodides in acetone because they precipitate out and are not free in solution to attack the alkyl halides |
|
|
Term
| What solvents should be used for SN2 reactions? How do they affect the mechanism? |
|
Definition
1) Polar aportic
2) Salvation of nucleophile cannot occur, therefore there can be no interference from other molecules |
|
|
Term
| Both the concentration of the __________ and the __________ __________ affect the rate of SN2 reactions. |
|
Definition
1) Nucleophile
2) Alkyl halide |
|
|
Term
| What is the configuration of SN2 reactions? |
|
Definition
| 1) Will always be inverted, from R to S and vice versa |
|
|
Term
| What factors promote SN2 in the NaI / acetone reaction? |
|
Definition
1) Acetone does not favor ionization of the alkyl halide
2) NaI readily dissolves in nonpolar solvent due to dielectric constant of 21 |
|
|
Term
| What factors inhibit the SN2 reaction in AgNO3 with ethanol? |
|
Definition
1) Solvation of nucleophile
2) Ethanol and water favors ionization of alkyl halide
3) Nitrate ion is a poor nucleophile
4) Silver ion coordinates with electron pair of halogen to weaken carbon-halogen bond |
|
|
Term
| Why does E2 not occur in the NaI / acetone reaction? |
|
Definition
| 1) Iodide ion is a weak base (strong base needed for E1) |
|
|
Term
| Why may E1 occur as a side reaction in the ethanol / water reactions? |
|
Definition
| 1) E1 has the same intermediates as the SN1 reactions. |
|
|
Term
| What are the false positives that can occur during the nucleophilic substitution experiment? |
|
Definition
| 1) The false positive that may occur results from the alkyl halide acting as a solvent in which the nucleophile is insoluble |
|
|
Term
Indicate whether the following situation would increase, decrease, or not affect the reaction rate of an SN1 reaction.
1) Decrease temperature
2) Change the leaving group from Br to F
3) Decrease the concentration of the nucleophile
4) Change solvent from ethanol to water
5) Increase concentration of alkyl halide |
|
Definition
1) Decrease
2) Decrease
3) No effect on rate
4) Increase
5) Increase |
|
|
Term
Indicate whether the following situations would increase, decrease, or have no effect on the reaction rate of an SN2 reaction:
1) Increase temperature
2) Change the leaving group from Br to I
3) Decrease the concentration of the nucleophile
4) Change solvent from ethanol to acetone
5) Decrease the concentration of alkyl halide
6) Decrease temperature
7) Change the leaving group from Br to Cl
8) Increase the concentration of the nucleophile
9) Change the solvent from acetone to water
10) Increase the concentration of alkyl halide
11) Change the solvent from water to acetone |
|
Definition
1) Increase
2) Increase
3) Decrease
4) Increase
5) Decrease
6) Decrease
7) Decrease
8) Increase
9) Decrease
10) Increase
11) Increase |
|
|
Term
| How can Le Chatlier's principle be used in the esterification reaction? |
|
Definition
| 1) In the experiment, Le Chatlier is applied by removing water via azeotropic distillation. Water can also be removed by drying with moleclar sieves. Another option is to use an excess of the cheaper reagent to drive the reaction forward |
|
|
Term
| Why was one starting material used over another in the esterification experiment? |
|
Definition
| 1) The boiling points of acetic acid, 1-butanol, and butyl acetate are extremely close; it would be difficult to separate the product from the starting materials. Excess of the starting material would be a significant impurity to the product |
|
|
Term
| What are two methods for removing water? |
|
Definition
1) Drying with molecular sieves
2) Azeotropic distillation |
|
|
Term
| Why can you only use molecular sieves instead of other drying agents as an alternate method of removing water? |
|
Definition
| 1) Most other drying agents will not remove water at the temperatures required to make esters |
|
|
Term
| Why was the apparatus set up the way it is (on an angle, wrapping the condenser in a wet paper towel, etc)? |
|
Definition
1) A cork is used instead of a septum so that layer separation can be observed clearly
2) The condenser is wrapped with a wet paper towel to ensure that vapor condenses in the sidearm
3) The apparatus is tilted on an angle so that the vapor condenses and runs down the sidearm; when the layers separate, the denser water remains in the sidearm and the lighter ester plus the alcohol layer runs down into the reaction flask. The angle allows for the isolation and collection of the product |
|
|
Term
| What was the role of each step of the esterification reaction, including the simple distillation that was asked for in the procedure but not carried out? |
|
Definition
1) The resin (acid catalyst), acetic acid, and 1-butanol are heated in a flask and allowed to reflux (heated to the solvent's boiling point without evaporating) so that the vapors condense and the aqueous layer can be separated from the ester
2) The water is drained out from the sidearm and the ester is separated from the resin with a Pasteur pipette
3) CaCl1 pellets are added to remove microscopic droplets of water suspended in the organic layer |
|
|
Term
| What is the purpose of the IR in the esterification experiment and the interpretation of the IR spectrum? |
|
Definition
1) The type of functional groups in organic molecules can be identified by IR spectroscopy
2) IR spectra are inverted, with the strongest absorptions at the bottom |
|
|
Term
| How could TLC have been used during the esterification experiment? |
|
Definition
1) TLC could have been used to assess the purity of the product
2) If taken during the reaction, it could have been used to monitor the reaction's progress; it would be possible to see the reactants disappear and the products appear. |
|
|
Term
| What was the role of resin and its advantages? What is the composition of resin? |
|
Definition
1) Esterification using a carboxylic acid and alcohol requires an acid catalyst so resin fulfills this role
2) Resin is a polystyrene that has sulfonic acid groups on some of the phenol groups and is easily removed during filtration |
|
|
Term
| What are the disadvantages of using traditional acid catalysts and the purpose of each of the steps needed to be carried out if traditional acid catalysts are used? |
|
Definition
1) If H2SO4 was used, it would be necessary to dilute the reaction mixture with ether, wash the ether layer with water, sodium carbonate solution, and saturated sodium chloride solution then dry the ether layer using calcium chloride pellets before evaporating the ether.
2) Ether would be used as a solvent for extraction
3) Sodium bicarbonate is used to separate the organic layer from the aqueous layer
4) NaCl is used to transfer any traces of water from the ether layer to the aqueous water
5) CaCl2 pellets are added to remove microscopic droplets of water suspended in the organic layer |
|
|
Term
| What are some characteristics of the catalytic acid in the esterification experiment? |
|
Definition
1) The catalyst is never used up
2) Always regenerated at the end of the reaction
3) Speeds up the reaction rate |
|
|
Term
| What are the sources of error for esterification? |
|
Definition
1) Loose connectors allow vapors to escape
2) Temperature variations due to airflow in hoods
3) Large surface area if microscale apparatus makes it easy to lose heat to the environment
4) Inaccurate measuring of reagent, inaccurate weighing
5) Some aqueous layer in the sidearm may have slid back down into the reaction flask
6) Loss of product during transfer to vial |
|
|
Term
| What are some reasons why the reaction may not go to completion in esterification? |
|
Definition
1) Ineffective removal of water so there is no shift in equilibrium
2) Running down of aqueous layer from the sidearm into the reaction flask |
|
|
Term
| What would you be separating the ester from if you carried out simple distillation? Why would it be okay to carry out a simple distillation on this mixture? |
|
Definition
1) Simple distillation would further purify the ester. It would be separated from water
2) It would be okay to carry out the distillation because the starting material is very small (despite close boiling points) and this the product is relatively already pure |
|
|
Term
| How is reduction characterized in organic chemistry? |
|
Definition
| 1) By an increase in hydrogen bonds or decrease in bonds to EN atoms (such as oxygen) |
|
|
Term
| How is oxidation characterized in organic chemistry? |
|
Definition
| 1) Decrease in hydrogen bonds and or an increase in bonds to EN atoms (such as oxygen) |
|
|
Term
| Why is NaBH4 used instead of LiAlH4 during reductions? |
|
Definition
1) Milder
2) More selective reduces of ketones and aldehydes
3) Does not react violently with water or alcohols |
|
|
Term
| What was ethanol used as a solvent instead of ether in the reduction reaction? |
|
Definition
| 1) Alcohol was used instead because it is key in the 1st step of the mechanism since it protonates the oxygen |
|
|
Term
| Why must ether be used as a solvent if LiAlH4 is used? |
|
Definition
| 1) LiAlH4 would react violently if otherwise |
|
|
Term
| Why was excess NaBH4 used in the reduction reaction? |
|
Definition
| 1) To ensure that the reaction goes to completion, accounting for reactions with solvent and decomposition of the reducer due to reaction heat |
|
|
Term
| What are the side products of reduction and how do they form? |
|
Definition
1) RR hydrobenzoin
2) SS hydrobenzoin
3) R benzoin, S benzoin
4) They form because the carbonyl is SP2 making it trigonal planar and open to attack from either the top or bottom. The others form because of an incomplete reduction of the molecule |
|
|
Term
| What is the role of each addition of water in the reduction experiment? |
|
Definition
1) 1st serves to hydrolyze the borate ester intermediate
2) 2nd is added to saturate the solution and start the crystallization process, thus its just involved in the purification, not the reaction |
|
|
Term
| What is the role of the ice bath in reduction experiment? |
|
Definition
| 1) Since the reaction is exothermic it controls the reaction rate |
|
|
Term
| Why is recrystallization carried out in reduction experiment? |
|
Definition
| 1) Helps to separate the benzoin from the hydrobenzoin since they are completely different compounds |
|
|
Term
| Why is melting point carried out in the reduction experiment? |
|
Definition
|
|
Term
| What are the sources of error in the reduction experiment? |
|
Definition
|
|
Term
| What are side products of aldol condensation and why do they form? |
|
Definition
1) Benzalacetone due to an incomplete reaction
2) 4-hydroxy-4-methyl-2-pentonone due to enolate of acetone reacting with another acetone
3) Water or alcohol can also be given off |
|
|
Term
| What are the drawbacks of mixed aldols and how do you minimize the mixture of products? |
|
Definition
1) Mixed aldol reactions have a tendency to give a mixture of products
2) To minimize mixtures the following should occur:
-Use a reactant with no alpha hydrogen (and excess of this reactant)
-Use a more reactive aldehyde in the presence of a less reactive ketone
-Deprotonate a carbonyl with a strong base like LDA to completely convert it to the enolate |
|
|
Term
| What is the role of sodium hydroxide in the aldol condensation experiment? |
|
Definition
| 1) Allows for an addition product to form |
|
|
Term
| What is the role of water and its source in the aldol condensation experiment? |
|
Definition
1) Water is used to wash the crystals and remove hydroxide
2) Allows for condensation product to form |
|
|
Term
| In the aldol condensation experiment, why are the crystals washed with water and what happens if they are not? |
|
Definition
1) Crystals should be rinsed with water to remove hydroxide
2) OH- is a catalyst that must be removed or the experiment will continue to carry out as it is regenerated |
|
|
Term
| Why does water leave easily in the aldol condensation experiment? |
|
Definition
| 1) Water leaves easily due to the resulting stabilizing conjugation |
|
|
Term
| Requirements for starting materials for aldol condensation? |
|
Definition
1) Aldehydes, ketones, and esters can react to give addition or condensation products
2) In order for a reaction to form an aldol condensation product, the carbonyl that forms that enolate must have at least two alpha hydrogens to form the condensation product |
|
|
Term
| Why are enolates formed in the aldol experiment? |
|
Definition
1) In order form an enolate to form, the carbonyl must have at least one alpha hydrogen
2) Enolates are more reactive than enols or enol ethers and can attack electrophiles directly to make the reaction run better |
|
|
Term
| What is the catalytic base of the aldol experiment? |
|
Definition
| 1) Hydroxide which is not consumed and regenerated at the end |
|
|
