Propose a second mechanism for the reaction of benzoic acid with acetyl chloride to give acetic benzoic anhydride. This time, let the other oxygen of benzoic acid serve as the nucleophile to attack the carbonyl group of acetyl chloride. Because proton transfers are fast between these oxygen atoms, it is difficult to differentiate between these two mechanisms experimentally.

Verified step by step guidance

1. Step 1: Begin by identifying the reactants and their roles. Benzoic acid (C₆H₅COOH) has two oxygen atoms: one in the hydroxyl group (-OH) and one in the carbonyl group (C=O). Acetyl chloride (CH₃COCl) contains a highly electrophilic carbonyl carbon due to the electron-withdrawing effect of the chlorine atom.
2. Step 2: In this proposed mechanism, the carbonyl oxygen of benzoic acid will act as the nucleophile. The lone pair of electrons on the carbonyl oxygen attacks the electrophilic carbonyl carbon of acetyl chloride, forming a tetrahedral intermediate. Represent this step as: $C_6{H}_{5}C=O+\mathrm{CH}{3}_{-}\mathrm{COCl}\to \mathrm{tetrahedral}\mathrm{intermediate}$.
3. Step 3: The tetrahedral intermediate is unstable and undergoes a rearrangement. The chloride ion (Cl⁻) is expelled as a leaving group, and a proton transfer occurs between the hydroxyl group and the newly formed oxygen atom. This step stabilizes the intermediate and forms the acetic benzoic anhydride product.
4. Step 4: Highlight the role of proton transfer. Proton transfers between the hydroxyl oxygen and the carbonyl oxygen of benzoic acid are fast and reversible. This makes it challenging to experimentally distinguish between this mechanism and the one where the hydroxyl oxygen acts as the nucleophile.
5. Step 5: Conclude by noting that both mechanisms lead to the same product, acetic benzoic anhydride. The difference lies in which oxygen atom of benzoic acid initiates the nucleophilic attack on acetyl chloride.

Key Concepts

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a fundamental reaction in organic chemistry where a nucleophile attacks the electrophilic carbon of a carbonyl group, leading to the substitution of a leaving group. In the context of benzoic acid and acetyl chloride, the nucleophile (the oxygen atom of benzoic acid) attacks the carbonyl carbon of acetyl chloride, forming a tetrahedral intermediate that eventually leads to the formation of an anhydride.

Proton Transfer Mechanism

Proton transfer mechanisms involve the movement of protons (H⁺ ions) between atoms, which can significantly influence reaction pathways. In the proposed reaction, rapid proton transfers between the oxygen atoms of benzoic acid can complicate the identification of distinct mechanisms, as these transfers can occur before the reaction products are formed, making it challenging to track the specific roles of each oxygen atom.

Carbonyl Reactivity

The reactivity of carbonyl compounds is a key concept in organic chemistry, as the carbonyl carbon is electrophilic and susceptible to nucleophilic attack. In the reaction between benzoic acid and acetyl chloride, the carbonyl group of acetyl chloride is particularly reactive due to the presence of the electronegative chlorine atom, which enhances the electrophilicity of the carbonyl carbon, facilitating the nucleophilic attack by the benzoic acid.