8. Elimination Reactions

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).

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

(b) <IMAGE> 

Identify the bonds that break and form in the following elimination reactions.

(b) <IMAGE>

If a quaternary ammonium ion can undergo an elimination reaction with a strong base, why can’t a protonated tertiary amine undergo the same reaction?

b. Indicate how each of the same factors affects an E2 reaction.

1. the strength of the base

2. the concentration of the base

3. the solvent

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-

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.

Draw the structure of DDE.