Acid chlorides are a particularly reactive derivative of carboxylic acids in which the OH group is replaced with a Cl.
Acid chlorides are made by reacting a carboxylic acid with thionyl chloride (SOCl2). The reaction occurs via nucleophilic attack of the carboxylic acid on the sulfur of the SOCl2. Chlorine is kicked out and the resulting chloride ion deprotonates the molecule, creating a carbonyl connected to a good leaving group, SO2Cl. Then a chloride anion attacks the carbonyl, which then loses the SO2Cl leaving group, making an acid chloride.
What drives the reaction of carboxylic acid and SOCl2 to completion?
After the SO2Cl leaving group is expelled, SO2Cl loses the chloride ion and forms SO2 gas which then bubbles out of solution, pushing the equilibrium to completion via Le Chatelier’s principle.
Why is acid chloride used?
Acid chloride is much more reactive than other derivatives of carboxylic acids. This is for 3 reasons: resonance, inductive effects, and ability to function as a leaving group. As a result of resonance and inductive effects (chlorine is electronegative), the chlorine tends to withdraw electron density from the carbonyl, making it more electrophilic and able to react under nucleophilic acyl substitution.
See the following charge density map of a carboxylic acid (left) vs an acid chloride (right), and notice how the acid chloride has a much more electron poor carbonyl, making it electrophilic.
Furthermore, chloride ion can function as a leaving group, allowing it to more easily undergo nucleophilic acyl substitution than for example a carboxylic acid, which contains OH, a poor leaving group.
The general order of reactivity of carboxylic acid derivatives is as follows:
Acid chloride > acid anhydride > ester > amide