Reactions of Esters

Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution

Interconversion Reactions of Esters

conversion of esters to other carboxylic acid derivatives
esters
acids
   amides 

Reaction type:  Nucleophilic Acyl Substitution

Summary

Hydrolysis of Esters

hydrolysis of esters
Reaction type:  Nucleophilic Acyl Substitution

Summary

Related Reactions Reaction under BASIC conditions:
MECHANISM OF THE BASE HYDROLYSIS OF ESTERS

Step 1:
The hydroxide nucleophiles attacks at the electrophilic C of the ester C=O, breaking the p bond and creating the tetrahedral intermediate.
hydrolysis of an ester using hydroxide
Step 2:
The intermediate collapses, reforming the C=O 
results in the loss of the leaving group the alkoxide, leading to the carboxylic acid.
Step 3:
An acid / base reaction. A very rapid equilibrium where the alkoxide functions as a base deprotonating the carboxylic acid (an acidic work up would allow the carboxylic acid to be obtained from the reaction).

Reaction under ACIDIC conditions:

MECHANISM OF THE ACID catalyzed  HYDROLYSIS OF ESTERS
Step 1:
An acid/base reaction. Since we only have a weak nucleophile and a poor electrophile we need to activate the ester. Protonation of the ester carbonyl makes it more electrophilic.
hydrolysis of an ester with acid catalysis
Step 2:
The water 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 oxygen that came from the water molecule.
Step 4:
An acid/base reaction. Need to make the -OCH3 leave, but need to convert it into a good leaving group first by protonation.
Step 5:
Use the electrons of an adjacent oxygen to help "push out" the leaving group, a neutral methanol molecule.
Step 6:
An acid/base reaction. Deprotonation of the oxonium ion reveals the carbonyl in the carboxylic acid product and regenerates the acid catalyst.

 

Reduction of Esters
(review of Chapter 15)

reduction of carboxylic acids using LiAlH4
Reactions usually in Et2O or THF followed by H3O+ work-ups
Reaction type:  Nucleophilic Acyl Substitution then Nucleophilic Addition
Summary
REACTION OF LiAlH4 WITH AN ESTER

Step 1:
The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the ester. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex.
reduction of an ester using hydride
Step 2:
The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, this produces a ketone as an intermediate.
Step 3: 
Now we are reducing an aldehyde. 
The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the aldehyde. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex. 
Step 4:
This is the  work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the primary alcohol product from the intermediate complex. 

 

  Reactions of RLi and RMgX with Esters
(review of Chapter 14)

reaction of RLi or RMgX with esters
Reaction usually in Et2O followed by H3O+ work-up
Reaction type:  Nucleophilic Acyl Substitution then Nucleophilic Addition

Summary:

REACTION OF RMgX WITH AN ESTER

Step 1:
The nucleophilic C in the organometallic reagent adds to the electrophilic C in the polar carbonyl group of the ester. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex.
addition of Grignard reagent to an ester
Step 2:
The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, this produces a ketone as an intermediate.
Step 3:
The nucleophilic C in the organometallic reagent adds to the electrophilic C in the polar carbonyl group of the ketone. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex.
Step 4:
This is the  work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the alcohol product from the intermediate complex.