Return to Contents Chapter 19: Carboxylic Acids Ch 19 contents
Preparation of Esters
esterification of carboxylic acids
Reaction type:  Nucleophilic Acyl Substiution

Summary

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:
disconnecting esters

 
MECHANISM FOR REACTION FOR ACID CATALYSED ESTERIFICATION
Step 1:
An acid/base reaction. Protonation of the carbonyl makes it more electrophilic.
acid cat. esterification
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, creating the tetrahedral intermediate.
Step 3:
An acid/base reaction. Deprotonate the alcoholic oxygen.
Step 4:
An acid/base reaction. Need to make an -OH leave, it doesn't matter which one, 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 neutral water molecule.
Step 6:
An acid/base reaction. Deprotonation of the oxonium ion reveals the carbonyl in the ester product.
 
What is the B: used here ? If we look at the reaction, there are several bases present already. It could be the conjugate base A- of the acid catalyst, or a C=O group (as in step 1), or an R-OH group as in step 4, or a water molecule as in step 6.

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© Dr. Ian Hunt, Department of Chemistry, University of Calgary