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.

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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

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Catalyzed Hydrolysis

Acid-catalyzed hydrolysis involves the reaction of an ester with water in the presence of an acid, leading to the formation of an alcohol and a carboxylic acid. The acid donates a proton to the ester, enhancing its electrophilicity, which facilitates the nucleophilic attack by water. This mechanism typically includes protonation steps and the formation of a tetrahedral intermediate.

Curved Arrows in Mechanisms

Curved arrows are used in organic chemistry to illustrate the movement of electrons during chemical reactions. A curved arrow starts from a nucleophile (electron donor) and points to an electrophile (electron acceptor), indicating the formation of a bond. Understanding how to correctly use these arrows is crucial for depicting reaction mechanisms accurately.

Formation of Methyl Acetate

The reaction between acetic acid and methanol to form methyl acetate is an example of an esterification reaction, where an alcohol reacts with a carboxylic acid. In this process, the hydroxyl group of acetic acid is replaced by a methoxy group from methanol, resulting in the formation of the ester methyl acetate and water. This reaction is reversible and can be driven to completion by removing water.