8. Elimination Reactions

Given a secondary alkyl bromide reacting with a strong base such as $O_{2}H-$ (ethoxide ion) in ethanol, which mechanism is most likely to occur: $SN1$, $SN2$, $E1$, or $E2$?

Silver-assisted solvolysis of bromomethylcyclopentane in methanol gives a complex product mixture of the following five compounds. Propose mechanisms to account for these products.

(b)

Write a balanced equation for each reaction, showing the major product you expect.

(b)

Predict the major product(s) of the following elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(d)

After a proton is removed from the OH group, which compound in each pair forms a cyclic ether more rapidly?

c.

Pure (S)-2-bromo-2-fluorobutane reacts with methoxide ion in methanol to give a mixture of (S)-2-fluoro-2-methoxybutane and three fluoroalkenes.

b. Propose a mechanism to show how (S)-2-bromo-2-fluorobutane reacts to give (S)-2-fluoro-2-methoxybutane. Has this reaction gone with retention or inversion of configuration?

Pure (S)-2-bromo-2-fluorobutane reacts with methoxide ion in methanol to give a mixture of (S)-2-fluoro-2-methoxybutane and three fluoroalkenes.

a. Use mechanisms to show which three fluoroalkenes are formed.

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(a)

Write a balanced equation for each reaction, showing the major product you expect.

(d)

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

h. 1-bromo-1-methylcyclopentane heated in methanol

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

g. 1−bromo−1−methylcyclopentane + NaOEt in ethanol

Predict the major and minor elimination products of the following proposed reactions (ignoring any possible substitutions for now). In each case, explain whether you expect the mechanism of the elimination to be E1 or E2.

(a)

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

a. 1−bromo−1−methylcyclohexane + NaOH in acetone

Under second-order conditions (strong base/nucleophile), S_N2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide.