8. Elimination Reactions

Practice your electron-pushing skills by drawing a mechanism for the following E2 reactions.

(b)

cis-1-Bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane both react with sodium ethoxide in ethanol to form 4-tert-butylcyclohexene. Explain why the cis isomer reacts much more rapidly than the trans isomer.

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion?

e.

Justify on a reaction coordinate diagram the fact that a strong base like sodium amide (NaNH₂) results in a faster E2 elimination than does sodium hydroxide (NaOH).

Suggest a mechanism for the following elimination reactions.

(c)

Practice your electron-pushing skills by drawing a mechanism for the following E2 reactions.

(a)

Draw the elimination products for each of the following E2 reactions; if the products can exist as stereoisomers, indicate which stereoisomers are obtained.

e. 3-chloro-3-ethyl-2,2-dimethylpentane + high concentration of CH₃CH₂O^-

When 2-bromo-3-phenylbutane is treated with sodium methoxide, two alkenes result (by E2 elimination). The Zaitsev product predominates.

b. When one pure stereoisomer of 2-bromo-3-phenylbutane reacts, one pure stereoisomer of the major product results. For example, when (2R,3R)-2-bromo-3-phenylbutane reacts, the product is the stereoisomer with the methyl groups cis. Use your models to draw a Newman projection of the transition state to show why this stereospecificity is observed.

c. Use a Newman projection of the transition state to predict the major product of elimination of (2S,3R)-2-bromo-3-phenylbutane.

d. Predict the major product from elimination of (2S,3S)-2-bromo-3-phenylbutane. This prediction can be made without drawing any structures, by considering the results in part (b).