The reaction of acetic anhydride with water is a nucleophilic acyl substitution reaction, producing acetic acid. Here's a step-by-step breakdown of the mechanism:
Step 1: Nucleophilic Attack
* The oxygen atom of water acts as a nucleophile and attacks the electrophilic carbonyl carbon of one of the acetyl groups in acetic anhydride. This is due to the partial positive charge on the carbonyl carbon and the electron-rich nature of the oxygen in water.
Step 2: Formation of Tetrahedral Intermediate
* The attack of water breaks the π bond of the carbonyl group and forms a tetrahedral intermediate. The oxygen atom of water becomes negatively charged, while the carbon atom of the carbonyl group becomes tetrahedral and carries a positive charge.
Step 3: Leaving Group Departure
* The negatively charged oxygen atom of the tetrahedral intermediate attacks the carbon atom of the adjacent carbonyl group. This leads to the departure of a carboxylate ion as a leaving group, forming a protonated form of acetic acid.
Step 4: Deprotonation
* The protonated acetic acid is deprotonated by a water molecule, releasing a proton and forming acetic acid.
Overall Reaction:
(CH3CO)2O + H2O → 2 CH3COOH
Note: This reaction is reversible, but the equilibrium lies heavily towards the formation of acetic acid.
Mechanism Diagram:
```
O
||
CH3-C-O-C-CH3 + H2O -----> CH3-C-OH + CH3-C-OH
|| |
O H
```
This reaction is a classic example of nucleophilic acyl substitution and illustrates the reactivity of anhydride functional groups. It also demonstrates the importance of water as a nucleophile in various chemical reactions.
