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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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.

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Acid Catalyzed

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.

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General Mechanism

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.

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