Question

Ethanol on reaction with acetic anhydride gives:
A. acetic ester
B. formic ester
C. ethanoic acid
D. ethyl acetate and ethanoic acid

Answer 1

Hint: It contains two molecules of an acid, with loss of a molecule of water. Addition of water to the anhydride regenerates two molecules of the carboxylic acid. The reaction between acetic anhydride and ethanol requires heating in the absence of catalysts.

Complete step by step answer:
The word anhydride means without water. Preparation of acetic anhydride is given in the reaction below:
\[{\text{R}} - {\text{COOH + R}} - {\text{COOH}} \rightleftarrows {\text{R}}{\left( {{\text{CO}}} \right)_2}{\text{O + }}{{\text{H}}_2}{\text{O}}\]
    Carboxylic acid anhydride
Anhydrides composed of two different acids are called mixed anhydrides.

Acid anhydrides also known as acetic anhydrides, react with alcohol to produce esters and carboxylic acids. It does not require a catalyst, but still requires heating. The reaction is given below:

$ + {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} \to $

$ + {\text{C}}{{\text{H}}_3}{\text{COOH}}$
The mechanism involves four steps-nucleophilic attack by alcohol, deprotonation by pyridine, leaving group removal and protonation of carboxylate.
When acetic anhydride reacts with alcohol, it does not produce two molecules of ester, but an ester and ethanoic acid. While ethanoic acid reacts with alcohol, it produces ester. This does not occur with the acid which is formed by reacting with alcohol in the mixture. This is because this reaction occurs in the absence of catalysts. For the acids and alcohols to react, we need to heat and catalysts like sulfuric acid should be used.
So, the correct answer is option C.

Additional information Acetic anhydrides are not as reactive as acyl chlorides. When acyl chlorides react with alcohol, hydrochloric acid is produced instead of ethanoic acid.

Note:
In this reaction, pyridine acts as solvent. This reaction is similar when anhydrides react with water and phenol since all of them have hydroxyl groups. In water, the hydroxyl group is attached to hydrogen. In alcohols, hydroxyl groups are attached to alkyl groups. While in phenols, the hydroxyl group is attached to the benzene group.