Term
| What characterizes a pure organic compound? |
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Definition
| A pure organic compound has no detectable impurities |
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Term
| For what type of compounds is recrystallization the major mode of purification? |
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Definition
| Recrystallization is the major mode of purification for solids |
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Term
| In its simplest form what does the process of recrystallization involve? |
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Definition
| Recrystallization involves the preparation of a hot saturated solution, which is allowed to cool |
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Term
| What occurs as a hot saturated solution gradually cools? |
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Definition
| The solute precipitates as the hot saturated solution gradually cools |
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Term
| In order to obtain a purer sample what must be true about the impurities? |
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Definition
| In the best case scenario the impuritites dissolve in the hot solvent and remain in solution even as hot saturatede solution is cooled. Alternatively the impuritites do not dissolve in the hot solvent. This would require a hot, gravity filtration step after which the hot saturated solution would be allowed to cool. |
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Term
| How does the rate of cooling affect crystal growth |
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Definition
| Slow cooling promotes slow crystal growth. Slow crystal growth promotes the formation of larger pure crystals. Rapid cooling promotes rapid crystal growth. Rapid crystal growth promotes smaller, less pure crystals. |
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Term
| What useful adage can guide one during the trial-and-error process of solvent selection? |
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Definition
| The adage like-dissolves-like is a useful guide during solvent selection. |
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Term
| Generalizations may be quite useful in predicting solvent suitablility.. |
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Definition
1) A substance is most soluble in that solvent to which it is closely related structurally
2) Low melting solids are generally more soluble in a given solvent than high-melting compounds of comparable structure
3) In any homologous series, the higher members become more and more "hydrocarbon-like" in their physical properties.
The less hydrocarbons the more soluble
The more functional groups presence the more soluble |
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Term
| What high polarity solvents are commonly used for recrystallization? |
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Definition
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Term
| What two low polarity solvent are commonly used for recrystallization? |
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Definition
| Diethylether and ethyl acetate |
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Term
| Why should one use a steam bath or hot-water bath, rather than a hot plate or open flame, to heat solvents such as ether, hexane, and petroleum ether? |
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Definition
| These solvents are very flammable |
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Term
| What is the most crucial aspect of recrystallization? |
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Definition
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Term
| What characterizes an effective recrystallizaion solvent? |
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Definition
A) chemically inert
B) High capacity for the solute of interest at the boiling point of the solvent and have a low capacity for the solute of interest at a temperature just about the solvents freezing point
C) Dissolve impurities at all temperature or not dissolve impuritites at any temperature |
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Term
| What rule of thumb should be followed in order to select an Erlenmeyer flask of appropriate size? |
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Definition
| 2-3 times larger than the volume of solvent one expects to use. |
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Term
| In the absence of specific solubility data what rule of thumb shoule be followed in order to select an appropriate solvent quantity? |
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Definition
| Solvent volume should be 20-40 times the mass of the sample to be recrystallized |
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Term
| What characterizes the manner of solvent addition? |
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Definition
| Solvent is added incrementally |
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Term
| How are insoluble impurities removed from the hot, saturated solution? |
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Definition
| Insoluble impurities are removed from the hot, saturated solution by hot gravity filtration |
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Term
| Under what circumstances are seed crystals used? |
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Definition
| Seed crystals are used when solute does not crystallize from the cooling saturated solution. |
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Term
| Aside from mechanical losses during recrystallization what other factors might lead to low recovery of material? |
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Definition
A) Too much solvent used
B) Crystallization during hot, gravity filtration
C) Vacuum filtration performed prior to complete crystallization |
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Term
| For how long should one dry a product that has been recrystallized from a high boiling-point solvent? |
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Definition
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Term
| What steps are undertaken in order to judge whether or not a particular solvent will be suitable recrystallization solvent? |
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Definition
Step 1
Combine a small quantity of the test solvent and a small quantity of sample in a test tube at room temp. Mix by shaking. If the sample dissolves the solvent will not be an effective recystallization solvent. If the sample does not dissolve proceed to step 2
Step 2
Heat the contents of the test tube to the boiling point of the solvent. If the sample does not dissolve the solvent will not be an effective recrystallization solvent. If the sample dissolves proceed to Step 3.
Step 3
allow the contents of the test tube to gradually coo. If crystals do not form with 10-20 minutes the solvent will not be an effective recrystallization solvent. If crystals do form within 10-20 minutes the solvent is likely to be an effective recrystallization solvent. |
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Term
| What are the three basic steps in recrystallization? |
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Definition
A) Preparation of a hot, saturated solution at teh solvent's boiling point followed by removal of any insoluble impuritites
B) Gradual cooling of the hot, saturated solution
C) Collection of crystalline material by vacuum filtration |
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Term
| Identify two methods for transferring a soling to the recrystallization flask? |
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Definition
A) The use of creased weighting paper
B) The use of creased weighing paper in conjunction with a powder funnel |
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Term
| Why is it advisable to set aside a small amount of crude material before performing a recrystallization? |
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Definition
| For use as seed crystals should they be required |
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Term
| After the flask has cooled to room temperature is there any advantage to cooling it in a ice-water bath? |
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Definition
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Term
| What is the standard system for collecting products purified by recrystallization? |
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Definition
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Term
| Why is it advisable to employ a trap flask during Vacuum filtration? |
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Definition
| To prevent the flow of water from the water aspirator to the filter flask. |
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Term
| Why should one pull some of the recrystallizing solvent through the filter paper, by application of the vacuum source, prior to filtration? |
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Definition
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Term
1)What quantity of water should be used to recrystallize a 1.2 g quantity of salicylic acid (solubility of salicyclic acid in water: 1g/460 mL at 25 degrees, 1g/15 mL at 100 degrees)
2) How much salicylic acid will crystallize when the solution prepared above is allowed to cool to 25 degrees?
3) What is the maximum percent recovery of salicyclic acid in this scenario? |
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Definition
1)At 100 degrees minimum quanity of water needed to dissolve all the salicylic acid. 1.2g x 15mL/1g = 18.0 mL
2) Mass of saliclic acid that remains in solution after cooling 25 degrees is 18.0 mL x (1g/460mL) = .04g
Mass of salicyclic acid that crystallizes afer cooling to 25 degrees is 1.2g-.04g= 1.16g
3) Max percent recovery is 1.16g/1.2g x 100 = 96.7% |
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Term
| See question 30. Unit II. |
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Definition
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