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Magazine Chapter 12: PROBLEM 12.42 (page 489)
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 SN2 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
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