reduction of esters and carboxylic acids

Question

reduction of esters and carboxylic acids

Conditions of reaction?

products formed?

special notes ?

mechanisms/Example of reaction?

Answer 1

Esters are actually carboxylic acid derivatives, and react similarly to particular reducing agents.

Both require a strong reducing agent: LiAlH₄ (a.k.a. LAH). This is followed up by a protonation step, typically H₃O⁺. Diborane (B₂H₆) in ether can also be used to reduce carboxylic acids to alcohols, which is also followed by H₃O⁺.

Carboxylic acids (RCOOH) and ethers are relatively unreactive to other reduction methods, such as H₂/Pt.

1. LAH

2. H₃O⁺

Ester / RCOOH ––––––––––> Primary Alcohol

The LAH mechanism is simple:

(1) Hydride (^–:H) attacks the carbonyl (C=O) carbon

– this opens the carbonyl π bond

(2) The carbonyl reforms and either ^–OR or ^–OH leaves, for ester or carboxylic acid.

– an aldehyde intermediate forms

(3) Another hydride attacks the carbonyl carbon again

– this opens the carbonyl π bond again

– this is the end of step 1, the C=O had been fully reduced

(4) H₃O⁺ is used to quench the resulting alkoxide.

– this works because hydronium has a pKa of –1.7 and alcohol has a pKa ~16-18.