Hydrolysis of Esters

Hydrolysis of Esters
Reaction type:  Nucleophilic Acyl Substitution
Summary

• Carboxylic esters hydrolyze to the parent carboxylic acid and an alcohol.
• Reagents : aqueous acid (e.g. H₂SO₄) / heat, or aqueous NaOH / heat (known as "saponification").
• These mechanisms are among some of the most studied in organic chemistry.
• Both are based on the formation of a tetrahedral intermediate which then dissociates.
• In both cases it is the C-O bond between the acyl group and the oxygen that is cleaved.

Related Reactions

• Fischer esterification
• Hydrolysis of Amides

Reaction under BASIC conditions:

• The mechanism shown below leads to acyl-oxygen cleavage (see step 2).
• The mechanism is supported by experiments using ¹⁸O labeled compounds and esters of chiral alcohols.
• This reaction is known as "saponification" because it is the basis of making soap from glycerol triesters in fats.
• The mechanism is an example of the reactive system type.

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.
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:

• Note that the acid catalyzed mechanism is the reverse of the Fischer esterification.
• The mechanism shown below also leads to acyl-oxygen cleavage (see step 5).
• The mechanism is an example of the less reactive system type.

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.
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.

CHALLENGING problems

alcohol preparation how

simple conversion problems

di bromination a problem

solve this problem

solution

reimer tiemann reaction a variation with ring expansion

Reimer–Tiemann reaction _ an electrophillic substitution or carbene attack.

Reimer Tiemann's reaction

On treating phenol with chloroform in presence of sodium hydroxide at 340 K, a - CHO group is introduced at ortho position of benzene ring. 

The reaction is an example of carbene based reactions where

it is also considerd to be an electrophillic attack on benzene  .

This reaction is known as Reimer - Tiemenn reaction. 

This results in the formation of O - hydroxybenzaldehyde (salicylaldehyde) and para-hydroxybenzaldehyde, the ortho isomer being the major product.

The intermediate substituted benzal chloride is hydrolysed in presence of alkali to produce salicylaldehyde.

Reimer Tiemenn reaction is an electrophilic substitution reaction. The first step is generation of electrophile.

                               

Dichlorocarbene contains a sextet of electrons and thus is a strong electrophile.