Lecture Notes on Ester Hydrolysis
Introduction
- Ester Formation: Previously discussed in Fischer esterification.
- Reactants: Carboxylic acid and alcohol.
- Catalyst: Acid.
- Products: Ester and water.
- Focus of Current Lecture: Ester hydrolysis (reverse reaction of esterification).
- Goal: Convert ester back to alcohol and carboxylic acid.
Mechanism of Ester Hydrolysis
- Equilibrium Shift: Increase water concentration to shift equilibrium left.
- Key Step: Breaking the bond to form alcohol (R') and carboxylic acid.
Detailed Mechanism
- Formation of Hydronium Ion:
- Protonation of Ester:
- Lone pair on carbonyl oxygen picks up a proton.
- Forms a protonated carbonyl, making it more electrophilic.
- Nucleophilic Attack:
- Water acts as a nucleophile, attacking the carbonyl carbon.
- Pushes electrons onto oxygen, forming a new bond.
- Deprotonation:
- Water removes a proton, forming two OH groups.
- Protonation of Leaving Group:
- Protonate one of the oxygens (turns into a better leaving group).
- Reformation of Carbonyl:
- Electrons move to reform carbonyl, releasing alcohol as a leaving group.
- Final Deprotonation:
- Water removes proton, yielding carboxylic acid.
Products: Carboxylic acid and alcohol.
Example Reactions
- Example 1: Hydrolysis of methyl salicylate.
- Ester bond breaks to form salicylic acid and methanol.
- Reaction conditions can shift equilibrium (referring to Fischer esterification).
- Example 2: Different ester starting materials.
- Highlights formation of carboxylic acid from ester bond cleavage.
Transesterification
- Definition: Conversion of one ester into another using alcohol other than water.
- Example: Using butanol instead of water.
- Results in new ester and methanol.
- Methanol's low boiling point used to shift equilibrium.
Conclusion
- Equilibrium Considerations: Reaction conditions determine direction.
- Industrial Use: Transesterification is a valuable reaction in chemical industries due to its versatility in producing different esters.