Question

Using the mechanism for the acid-catalyzed hydrolysis of an ester as your guide, write the mechanism—showing all the curved arrows—for the acid-catalyzed reaction of acetic acid and methanol to form methyl acetate. Use HB+ and :B to represent the proton-donating and proton-removing species, respectively.

Accepted Answer

Protonation of the carbonyl oxygen: The reaction begins with the protonation of the carbonyl oxygen in acetic acid by the acid catalyst (HB+). This increases the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack. Represent this step with a curved arrow from the lone pair on the carbonyl oxygen to the proton (H) of HB+. Nucleophilic attack by methanol: Methanol acts as a nucleophile and attacks the carbonyl carbon of the protonated acetic acid. Draw a curved arrow from the lone pair on the oxygen of methanol to the carbonyl carbon. This forms a tetrahedral intermediate with a positively charged oxygen on the methanol group. Proton transfer: A proton transfer occurs to stabilize the intermediate. The positively charged oxygen on the methanol group donates a proton to a base (:B), forming a neutral intermediate. Represent this step with a curved arrow from the oxygen-hydrogen bond to the base (:B). Formation of the ester: The intermediate undergoes elimination of water. The lone pair on the oxygen of the hydroxyl group reforms the carbonyl bond, and a curved arrow is drawn to show the departure of water as a leaving group. This step forms methyl acetate. Regeneration of the catalyst: The acid catalyst (HB+) is regenerated by the protonation of the base (:B) with a proton. Draw a curved arrow from the lone pair on the base (:B) to the proton (H) to complete the catalytic cycle.

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Key Concepts

Acid-Catalyzed Hydrolysis

Curved Arrows in Mechanisms

Formation of Methyl Acetate