Question

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?

A) (Z)-3-methylpent-2-ene
B) (E)-3-methylpent- 2-ene
C) 2-methylpent-2-ene
D) 2,3-dimethylbut-2-ene
E) none of the above

Answer 1

The unknown alkene yielding a racemic mixture of (2S, 3S) and (2R, 3R)-3-methylpentan-2,3-diol after reaction with MCPBA followed by acidic work-up is (E)-3-methylpent-2-ene.

Step-by-step explanation:

The reaction of an unknown alkene with meta-chloroperoxybenzoic acid (MCPBA) in dichloromethane followed by acidic work-up (H2O/H+) has led to the formation of a racemic mixture of (2S, 3S) and (2R, 3R)-3-methylpentan-2,3-diol. In order to generate such diol products, the starting alkene must have a double bond in a position that allows for the formation of two new chiral centers upon the addition reaction.

The resulting racemic mixture indicates that the reaction has taken place at a symmetrical or prochiral double bond, which enables both faces of the double bond to be attacked with equal probability, thus yielding both enantiomers. Given these characteristics, the correct alkene must have the double bond at the 2-position and cannot be hindered by steric effects that would favor the formation of one enantiomer over the other.

Therefore, the specific structure of the alkene used in the reaction is (E)-3-methylpent-2-ene (Option B). The presence of the alkyl group (methyl) on the third carbon ensures the formation of diols with methyl groups at the 3-position, while the E-configuration allows for a symmetrical approach of MCPBA during the epoxidation step and subsequently equal probability of attack from both sides during the hydrolysis step that forms the diol, leading to a racemic mixture.