Question

Question: Draw the structure of two different epoxides that would yield 2-methylpentan-2-ol [ (CH₃)₂CH(OH)CH₂CH₂CH₃] when reduced with LiAlH₄ .

Short answer

Answer

Epoxide 1- 2-methyl-2-propyl oxirane

Epoxide 2- 3-ethyl-2,2-dimethyloxirane

Step-by-step solution

Epoxides

Alkenes react with peroxy acids to form epoxides. In these reactions, a single oxygen atom is added to the alkene.

Peroxy acids cleave the weak pi bond in alkenes and form two new C-O bonds during the reaction.

It has a concerted mechanism to form a three-membered ring by syn addition.

Reaction of epoxides with lithium aluminum hydride

Lithium aluminum hydride is a reducing agent that can provide hydride ions. The compound is abbreviated as LAH.

When epoxides are treated with LAH, it reduces the epoxides into alcohols by a nucleophilic substitution reaction.

The reaction follows S_N2 mechanism and therefore has a concerted mechanism.

LAH attacks the epoxide from the least hindered site making alcohol the major product.

Epoxides that can give methypentan2-ol on reduction

In LAH reduction, the LAH attacks the epoxide from the least hindered site to form an alkoxide which is hydrolyzed to form alcohol.

To obtain methylpentan-2-ol as the reduction product, the hydride attack should not be favored at the second carbon. Therefore, epoxides that do not favor hydride attack on the second position can give methylpentan-2-ol.

The two epoxides that can give methylpentan-2-ol on reduction are:

Epoxide 1- 2-methyl-2-propyl oxirane

Epoxide 2- 3-ethyl-2,2-dimethyloxirane