Question

The chemistry of deuterium is nearly identical to that of hydrogen. One area of difference is that C-D bonds are about 1.2 kcal/mol stronger than C-H bonds. As a result, reaction rates tend to be slower if a C-D (as opposed to C-H) bond is broken in a rate-limiting step. This important fact can lead to great mechanistic insight and is called the kinetic isotope effect.

a) Propose a mechanism to explain each product in the following reaction.

Br KOH OH

b) When the following compound reacts under the same conditions, the rate of the substitution reaction is unchanged, but the rate of the elimination reaction is 7 times slower. Explain.

c) When the following compound is treated with sodium methoxide in methanol, two elimination products are possible. Explain why the deuterated product predominates by about 7:1.

Answer 1

c) when aldol reaction is carried out in D2O, deuterium may be incorporated in acetone but not in aldehyde because in case of -CHO second step which include attack of carbanion on carbonyl is fast. So carbanion does not have time to interact with D2O which makes deuterium to predominate the reaction.

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