Why is acetic anhydride added slowly to a dichloromethane solution of the other reactants?

Acetic anhydride is the only monocarboxylic acid anhydride which is an acylating agent capable of modifying reactions. The dichloromethane is the reaction solvent.

The acetic anhydride should be added slowly through a separatory funnel to other reactants so as to avoid a runaway reaction, also known as thermal runaway. By doing this the reaction mixture boils as gently as possible while stirring. This phenomenon occurs in situations where an increase in temperature changes the conditions in a way that leads to destructive result brought about by a further increase in temperature.

Acetic Acid is considered one of the excellent source of Acyl (CH₃CO-) group. It is also one of the the most reactive derivative of Carbonyl groups.

As we know that the bond formation is an Exothermic reaction. Greater the rate of formation of new bonds greater will be the amount of heat released.

Hence, Acetic Anhydride being more reactive will produce more heat on reaction with other reactants and generates more heat. In order to prevent the reaction mixture from bursting the reaction is often carried out at lower temperatures and also the rate of addition of Acetic Anhydride is slowed down.