Question

Show the mechanism for the generation of the acylium ion if an acid anhydride is used instead of an acyl chloride for the source of the acylium ion.

Short answer

The answer is,

Mechanism

Step-by-step solution

Acid anhydride

The acid anhydride reacts with lewis acid-like Aluminium chloride to generate the electrophile acylium ion.

Aluminum chloride is a lewis acid that can accept the lone pair of electrons from the acid anhydride.

Explanation

The mechanism for generating the acylium ion from an acid anhydride is shown below.

Mechanism

The lone pair on the oxygen atom attacks the Aluminium chloride and donate the lone pair electrons to it. The intermediate generated then undergoes electron rearrangement to give the acylium ion.