Q51E - Organic Chemistry (Mcmurry)

Question

Epoxides are reduced by treatment with lithium aluminum hydride to yield alcohols. Propose a mechanism for this reaction.

Step-by-Step Solution

Answer

The mechanism is below:

Step 1: Reduction using lithium aluminium hydride

Lithium aluminium hydride is a strong reducing agent used in chemical reactions.

 These reagents are source of hydride nucleophile due to the presence of metal-hydrogen bond. Addition of hydride anion to the ketone, aldehyde and epoxide yield corresponding alcohol.

Step 2: Reduction of epoxide using lithium aluminum hydride

The mechanism is shown below:

Reduction of epoxide

• In the first step epoxide get attacked with hydride anion of nucleophile, results an alkoxide with aluminum hydride as byproduct. 
• In the next step alkoxide get fused with aluminum hydride.
• In last step protonation from water break the O-Al bond result the final product.

In metal hydride reduction the resultant alkoxide salt in second step is insoluble and needs to be hydrolyzed with water to get the alcohol product. Thus, protonation of alkoxide is done using hydrolysis reaction to get the final product cyclohexanol.