Propose a mechanism for the reaction of benzoic acid with acetyl chloride to give acetic benzoic anhydride.

Verified step by step guidance

1. Step 1: Identify the reactants and the product. The reactants are benzoic acid (C₆H₅COOH) and acetyl chloride (CH₃COCl), and the product is acetic benzoic anhydride (C₆H₅COOCOCH₃). This is an acylation reaction where an acid chloride reacts with a carboxylic acid to form an anhydride.
2. Step 2: Recognize the role of acetyl chloride as an electrophile. The carbonyl carbon in acetyl chloride is highly electrophilic due to the electron-withdrawing effect of the chlorine atom. Benzoic acid acts as a nucleophile, with the oxygen atom of the hydroxyl group attacking the electrophilic carbonyl carbon of acetyl chloride.
3. Step 3: Describe the nucleophilic attack. The lone pair of electrons on the oxygen atom of the hydroxyl group in benzoic acid attacks the carbonyl carbon of acetyl chloride, forming a tetrahedral intermediate. This intermediate is unstable and will collapse to eliminate a leaving group.
4. Step 4: Explain the elimination of the leaving group. The tetrahedral intermediate collapses, expelling the chloride ion (Cl⁻) as the leaving group. This results in the formation of a mixed anhydride intermediate, where the benzoic acid and acetyl groups are connected.
5. Step 5: Finalize the mechanism. The mixed anhydride intermediate stabilizes to form acetic benzoic anhydride, completing the reaction. The product contains two carbonyl groups connected by an oxygen atom, characteristic of an anhydride.

Key Concepts

Acid-Base Reactions

In organic chemistry, acid-base reactions involve the transfer of protons (H+) between molecules. In the context of the reaction between benzoic acid and acetyl chloride, benzoic acid acts as a Brønsted acid, donating a proton to the acetyl chloride, which is a reactive acyl chloride. This step is crucial for initiating the formation of the anhydride product.

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a fundamental reaction mechanism in organic chemistry where a nucleophile attacks an acyl compound, leading to the substitution of a leaving group. In this case, the hydroxyl group from benzoic acid acts as a nucleophile, attacking the carbonyl carbon of acetyl chloride, facilitating the formation of the acetic benzoic anhydride.

Formation of Anhydrides

Anhydrides are formed through the condensation of two carboxylic acids or a carboxylic acid and an acyl chloride, resulting in the loss of water. The reaction between benzoic acid and acetyl chloride exemplifies this process, where the hydroxyl group from benzoic acid and the chloride from acetyl chloride are eliminated, yielding acetic benzoic anhydride as the product.