Acetic anhydride is hydrolyzed in water much more rapidly when a small amount of pyridine is added. Pyridine acts as a nucleophilic catalyst.
The overall stoichiometry is
(CH₃CO)₂O + H₂O → 2 CH₃COOH.
(a) Propose a sequence of elementary steps that shows how pyridine participates in the mechanism. Clearly indicate the intermediate in which pyridine is covalently bound to a reacting species.
(b) Explain why the mechanism in part (a) is classified as covalent catalysis rather than binding/orientation or surface catalysis.
(c) Discuss why the formal concentration of pyridine remains essentially unchanged from the beginning to the end of the reaction, even though pyridine is consumed in an early elementary step.
(d) Predict qualitatively how protonating the pyridine (to form pyridinium ion) before the reaction would affect the observed rate. Justify your answer in terms of the mechanistic role of pyridine.