Exercise 13: - Elimination reactions

1. When 3-bromo-2,2-dimethylbutane is heated with a dilute solution of sodium ethoxide in ethanol, reaction follows first-order kinetics. Along with substitution, elimination occurs to yield 2,3-dimethyl-2-butene and 2,3-dimethyl-1-butene. Propose a mechanism for their formation.

2. Propose a mechanism for the formation of (Z)-2-bromo-2-butene from both (R,R) and (S,S)-2,3-dibromobutane via a dehydrobromination reaction.

3. Partial dehydrohalogenation of (1R,2R)-1,2-dibromo-1,2-diphenylethane gives (Z)-1-bromo-1,2-diphenylethene.

a. Draw the structure of (1R,2R)-1,2-dibromo-1,2-diphenylethane.

b. Draw the structure of (Z)-1-bromo-1,2-diphenylethene.

c. Propose a mechanism for this reaction.

4. The rate of dehydrohalogenation of cis-1-bromo-4-tert-butylcyclohexane is proportional to the concentration of both the bromide and the base whereas the rate of dehydrohalogenation of the trans isomer is only proportional to the concentration of the bromide.

a. Draw the structure of cis-1-bromo-4-tert-butylcyclohexane.

b. Propose a mechanism for the dehydrohalogenation of the cis-1-bromo-4-tert-butylcyclohexane.

c. Draw the structure of trans-1-bromo-4-tert-butylcyclohexane.

d. Propose a mechanism for the dehydrohalogenation of the trans-1-bromo-4-tert-butylcyclohexane.

e. Explain why the trans-1-bromo-4-tert-butylcyclohexane cannot react by a second order mechanism.

The bulky tert-butyl group has to be equatorial forcing the bromine into an equatorial position. The anti arrangement necessary for an E2 elimination is not possible.

5. Base treatment of (2R,3S)-2-bromo-3-deuteriobutane gave, in addition to the deuterated 1-butene, only (Z)-2-deuterio-2-butene and (E)-2-butene. Explain.