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.
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.