A reaction of an unknown alkene with MCPBA in dichloromethane followed by work-up with H₂O/H+ yielded, as the major product, a racemic mixture of (2S,3S) and (2R,3R)-3-methylpentan-2,3-diol. What is the specific structure of the alkene used in the reaction?

Question

Quizplus · Accepted Answer

The reaction of MCPBA with an alkene leads to epoxidation, yielding an oxirane ring. The subsequent hydrolysis of the epoxide produces a diol. The stereochemistry of the diol is determined by the regioselectivity of the epoxidation and the stereochemistry of the starting alkene. The fact that the product is a racemic mixture of (2S,3S) and (2R,3R)-3-methylpentan-2,3-diol indicates that the starting alkene was achiral and its epoxidation occurred with no preference for a particular face. Therefore, the starting alkene must be a symmetric, disubstituted alkene. Of the choices given, only (Z)-3-methylpent-2-ene fulfills this criterion. Its epoxidation with MCPBA followed by hydrolysis would give a racemic mixture of (2S,3S) and (2R,3R)-3-methylpentan-2,3-diol as the major product.

A Reaction of an Unknown Alkene with MCPBA in Dichloromethane