8. Elimination Reactions

A chemist allows some pure (2S,3R)-3-bromo-2,3-diphenylpentane to react with a solution of sodium ethoxide (NaOCH₂CH₃) in ethanol. The products are two alkenes: A (cis-trans mixture) and B, a single pure isomer. Under the same conditions, the reaction of (2S,3S)-3-bromo-2,3-diphenylpentane gives two alkenes, A (cis-trans mixture) and C. Upon catalytic hydrogenation, all three of these alkenes (A, B, and C) give 2,3-diphenylpentane. Determine the structures of A, B, and C; give equations for their formation; and explain the stereospecificity of these reactions.

Predict the elimination products of the following reactions, and label the major products.

b. trans-1-bromo-2-methylcyclohexane + NaOCH₃ in CH₃OH

Predict the elimination products of the following reactions, and label the major products.

a. cis-1-bromo-2-methylcyclohexane + NaOCH₃ in CH₃OH

When the following stereoisomer of 2-bromo-1,3-dimethylcyclohexane is treated with sodium methoxide, no E2 reaction is observed. Explain why this compound cannot undergo the E2 reaction in the chair conformation.

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

c.

d.

a. Design an alkyl halide that will give only 2,4-diphenylpent-2-ene upon treatment with potassium tert-butoxide (a bulky base that promotes E2 elimination).

b. What stereochemistry is required in your alkyl halide so that only the following stereoisomer of the product is formed?

One of the following dichloronorbornanes undergoes elimination much faster than the other. Determine which one reacts faster, and explain the large difference in rates.

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)

(a)

The cis-diastereomer undergoes E2 elimination 500 times faster than the trans form. Explain.

Which reactant in each of the following pairs undergoes an elimination reaction more rapidly? Explain your choice.

a.

b.

Explain why only a substitution product and no elimination product is obtained when the following ­compound reacts with sodium methoxide:

For each of the following compounds, draw the product that forms in an E2 reaction and indicate its configuration:

b. (1S,2R)-1-bromo-1,2-diphenylpropane

Why do cis-1-bromo-2-ethylcyclohexane and trans-1-bromo-2-ethylcyclohexane form different major products when they undergo an E2 reaction?

Rank the following from most reactive to least reactive in an E2 reaction:

Which isomer reacts more rapidly in an E2 reaction: cis-1-bromo-4-tert-butylcyclohexane or trans-1-bromo-4-tert-butylcyclohexane? Explain your answer.