Synthesis of Esters

Summary

• This reaction is also known as the Fischer esterification.
• Esters are obtained by refluxing the parent carboxylic acid with the appropriate 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)

MECHANISM FOR REACTION FOR ACID catalyzed 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.

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