Ch15 : Synthesis of Esters

Chapter 15: Alcohols, Diols and Thiols

Synthesis of Esters

Reaction type: Nucleophilic Acyl Substitution

Summary

This reaction is also known as the Fischer esterification.
Esters are obtained by refluxing the parent carboxylic acid with the appropraite alcohol with an acid catalyst.
The equilibrium can be driven to completion by using an excess of either the alcohol or the carboxylic acid, or by removing the water as it forms.
Alcohol reactivity order : CH₃OH > 1^o > 2^o > 3^o(steric effects)
Esters can also be made from other carboxylic acid derivatives, especially acyl halides and anhydrides, by reacting them with the appropriate alcohol in the presence of a weak base (see chapter 20)

Study Tip:
The carboxylic acid and alcohol combination used to prepare an ester are reflected by the name of the ester,
e.g. ethyl acetate (or ethyl ethanoate), CH₃CO₂CH₂CH₃ can be made from CH₃CO₂H, acetic acid (or ethanoic acid) and HOCH₂CH₃ (ethanol).

This general "disconnection" is shown below:

MECHANISM FOR REACTION FOR ACID CATALYSED ESTERIFICATION
Step 1: An acid/base reaction. Protonation of the carbonyl makes it more electrophilic.
Step 2: The alcohol O functions as the nucleophile attacking the electrophilic C in the C=O, with the electrons moving towards the oxonium ion.
Step 3: An acid/base reaction. Deprotonate the alcoholic oxygen.
Step 4: An acid/base reaction. Need to make the -OH leave, so convert it into a good leaving group by protonation.
Step 5: Use the electrons of an adjacent oxygen to help "push out" the leaving group, a water molecule.
Step 6: An acid/base reaction. Deprotonation of the oxonium ion reveals the carbonyl in the ester product.