Term
| Give a brief overview of elimination and the general things that go on in all elimination reactions |
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Definition
Elimiation requires that 2 bonds be broken and 2 bonds be made. Both of the broken bonds are to C. One new pi bond to C is made during this process |
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Term
| How do elimination and substitution compare in terms of entropy? What does this result mean? |
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Definition
Elimination is entropically quite favorable to substitution and therefore becomes preferential over substitution as the temperature is raised |
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Term
| What would happen if you were to break in the CX bond first when undergoing elimination? |
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Definition
| X- would be slow to break away but once the positive carbon is rendered a negatively charged base ion would react very quickly with the residual hydrogen. Therefore the first step of this reaction is the rate determining step |
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Term
| What would happen if you were to break the CH bond/make the BH bond first in an elimination reaction and how would this affect the reaction? |
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Definition
The base would be slow to react with the hydrogen but once the carbon has the lone pair of electrons the X would be very quick to break away. Therefore, the first step is the rate determining step for this reaction |
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Term
| What would happen if you were to make and break the bonds simultaneously in an elimination reaction and what effect would this have on the overall reaction? |
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Definition
The transition state in this case could have different variants and therefore neither a specific rate determining step nor true mechanism can be derived from this information
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Term
| What are some general facts about the concerted mechanism of elimination? |
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Definition
It's a one step mechanism with one transition state. The rate law is k[alkyl halide][base], so the reaction is second order. In the end, that means that the reaction is an elimination, second order reaction (or E2) |
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Term
| What are some factors which would affect the value of k in the E2 elimination? |
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Definition
The leaving group (I>Br>Cl>>F), the substrate (tertiary>secondary>primary), the nature ofthe base and the solvent |
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Term
| What does the selectivity factor more accurately quantify? |
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Definition
It quantifies the preference for forming the more substituted (and therefore more stable) alkene |
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Term
| Why might a less regioselective compound still be more chemically selective? |
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Definition
If the based used is very small by comparison then that is the likely case |
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Term
| Which product goes through a reaction process with more free energy release (and therefore less stability): a more substituted alkene or a less substituted alkene? |
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Definition
The less substituted alkene will result in the higher free energy |
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Term
| In regards to the effects of steric hindrance in the regiochemistry of the E2 reaction, what effect would a larger halide have on the final products from various reagents? |
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Definition
A larger halide would give results that favor the more stable substituted double bond |
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Term
| In regards to the steric hindrance in the regiochemistry of the E2 elimination, what effect would a larger organic base have on the products: would the more stable 2-butene or the less stable 1-butene form? |
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Definition
For a larger base more of the less stable 1-butene will form. For smaller organic bases more of the more stable 2-butene product will form |
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Term
| What is so strange in regards to alkyl flourides and their regioselectivity in the E2 reaction? |
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Definition
They will end up forming more of the less substituted product (about 7:3) |
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Term
| Is it better for the base to attack a hydrogen at the more or the less substituted carbon? |
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Definition
It's better for it to attack at the less-substituted carbon; that way the resulting organic compound will be more stable in terms of regioselectivity |
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Term
| Let's say that an organic compound with a 1-ene and 4-halide were to react with OH-. Would the major product result in a 1,3-diene setup or a 1,4-diene setup? |
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Definition
The 1,3-diene setup would be the major product. In this case there would be a pattern of sp2 carbons which is a good thing for the compound's stability |
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Term
| What are the primary differences between anti-periplanar/trans-coplanar elimination and syn-periplanar/cis-coperiplanar elimination? |
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Definition
With anti-peri/trans-co the halide is distinctly negative while the base and the hydrogen are bonded; this is due to the fact that the hydrogen and the halide are opposite each other in the orientation. In syn-peri/cis-co the hydrogen and halide are on the same side as each other in the orientation and therefore get pulled off together |
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Term
| In regards to the syn/anti versions of the E2 reaction, what do each refer to? |
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Definition
Anti elimination means that one component of each carbon which branches off and is lost is from opposite ends (and they don't eclipse each other or any atoms in the compound). Syn elimination means that one of the components eclipses the other and each branch off right from there and leave the compound alone in regards to R group positioning |
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Term
| Why does the stereochemistry of the E2 elimination prefer the E product over the Z product? |
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Definition
Not only is the free energy of the anti conformation lower than that of the gauche but the free energy of the E state is lower than that of the Z state |
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Term
| What is important to note about the stereochemistry of the E2 reaction in cyclohexanes? |
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Definition
The hydrogen involved and the halide should be in an axial position in order for the reaction to proceed. Axial anything is typically more unstable than its equitorial counterparts |
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Term
| What happens in regards to the stereochemistry of the E2 elimination in cyclohexanes when there are 2 axial hydrogens and one axial bromide? What about no axial hydrogens? |
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Definition
In the case of 2 axial hydrogens you'd get 2 products and if there is anything such as an R group then the double bond attached to the carbon linked to the R group will form the major product. In the case of no axial hydrogens in cyclohexanes there would be no E2 product at all |
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Term
| What are some general properties regarding the dissociative mechanism for elimination? |
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Definition
It is a 2 step mechanism with 2 transition states (first step is rate determining), the rate law = k[alkyl halide] (which is first order) and therefore the reaction is first order elimination, or E1 |
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Term
| What are the similarities and differences between the E1 and SN1 reactions? |
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Definition
They're both kinetically the same reaction subject to the same rate determining factors. The only difference is the final result after the RD step and thus E1 conditions are the same as SN1. This just adds one more reaction a carbocation can undergo |
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Term
| How do larger nucleophiles and larger tertiary halides affect whether a E1 or SN1 reaction will take place (and therefore result in a ene or more complex ane product)? |
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Definition
The larger the nucleophile and the tertiary alkyl halide the more the ene (elimination product) will be favored |
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Term
| What is important to note when looking at a graph of the free energy of an E1 reaction with a 1-ene and 2-ene product? |
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Definition
Although the end results have differing free energy amounts (the 2-ene product in this case) the first step (the RDS) is equal for both; once this product is formed and stabilizes a little bit the two tracks split off from each other as soon as they start to rise in free energy again |
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Term
| What is important to note about the stereoselectivity of the E1 reaction in regards to a cyclohexane with a halide? |
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Definition
It can undergo an E1 reaction at higher temps if the halide is equitorial (but not axial). A hydrogen will replace the halide which will then be paired up with another hydrogen through a 1,2-H shift; this will then undergo an E1 reaction and will result in a racemic mixture of a cyclohexene with a double bond (this racemization factor is given that the central carbon and surrounding carbons have at least similar groups or no groups attached) |
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Term
| What conditions will favor an E1 reaction over an E2 reaction? |
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Definition
E1 is favored when a weak base is present (often the solvent) and a protic polar solvent |
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Term
| Under what conditions is an E2 reaction favored over an E1 reaction? |
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Definition
An E2 reaction is favored by a high concentration of a strong base and an aprotic polar solvent |
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Term
| What is the difference between E1 and E2 in terms of reacitivty in various degrees of alkyl halides? |
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Definition
Tertiary then secondary alkyl halides undergo E1 reactions but primary aklyl halides can also undergo E2 reactions |
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Term
| For the SN1 reaction what are the stereochemical products compared to the reactants? |
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Definition
Both isomers (R and S) are formed (racemization) |
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Term
| What is the stereochemistry of the products of an E1 reaction? |
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Definition
Both E and Z isomers are formed (but more of the E is formed; this is determined by the complexity around the double bond) |
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Term
| What is the stereochemistry of the products formed from the SN2 reaction? |
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Definition
Only the inverted product is formed |
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Term
| What is the stereochemistry of the products formed from the E2 reaction? |
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Definition
Both E and Z isomers are formed from the anti elimination. If the beta carbon has only one hydrogen then the orientation of the ONE stereoisomer depends on the configuration of the reactant |
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Term
| What are the expected products of primary alkyl halides when comparing SN2 and E2 reactions? |
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Definition
Substitution is primarily expected but if steric hindrance is present in the alkyl halide or nucleophile elimination will be favored |
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Term
| What products are expected from primary alkyl halides in regards to SN1 and E1 reactions? |
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Definition
They can't undergo SN1/E1 reactions |
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Term
| Would a secondary alkyl halide be more likely to undergo SN2 or E2 reactions? |
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Definition
They could undergo both. However, the stronger and bulkier the base and the higher the temperature, the greater the percentage of elimination |
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Term
| Would a secondary alkyl halide be more likely to undergo SN1 or E1 reactions? |
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Definition
It could undergo both and there are no outstanding conditions or restrictions on this option |
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Term
| Would a tertiary alkyl halide be more likely to undergo SN2 or E2 reactions? |
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Definition
It could only undergo elimination |
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Term
| Would a tertiary alkyl halide be more likely to undergo SN1 or E1 reactions? |
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Definition
Both would occur and there are no outstanding conditions or restrictions on either |
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Term
| What reaction should be used if you want to get an alkene from an alkyl halide? |
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Definition
Always use the E2 (the E1 is rarely appropriate for synthesis) |
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