Bondline / Reactions

Fischer esterification

Acid makes the carboxylic acid carbonyl more electrophilic; the alcohol adds, a proton moves, water leaves and a proton is lost. Every step is reversible.

Acyl substitution · 5 steps

Conditions

Carboxylic acid + alcohol (often in excess) with conc. H₂SO₄, heat; remove water to drive it forward.

Mechanism in 5 steps

Red curly arrows show where each pair of electrons moves.

OOHHOH2+OHOH+HO++
Step 1The carbonyl oxygen is protonated.
OHOH+HOO+HHOOHMeOH+
Step 2Methanol attacks the carbonyl carbon.
O+HHOOHOH2O+OH
Step 3A proton moves to one of the OH groups (in practice via the solvent), making a good leaving group.
OH2O+OHHO+OH2O+
Step 4Water leaves as the C=O reforms.
HO+OH2OOOH3O+++
Step 5Loss of a proton gives the ester.

About acyl substitution reactions

Carboxylic acid derivatives react by nucleophilic addition followed by elimination: the nucleophile adds to C=O, then the leaving group is expelled and the C=O reforms.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

Open in Bondline