Question

The reaction of an \(\alpha\)-diketone with concentrated sodium or potassium hydroxide to give the salt of an \(\alpha\)-hydroxyacid is given the general name benzil-benzilic acid rearrangement. It is illustrated by the conversion of benzil to sodium benzilate and then to benzilic acid. Propose a mechanism for this rearrangement. O=C(C(=O)c1ccccc1)c1ccccc1 O=C(O)C(O)(c1ccccc1)C(O)(c1ccccc1)c1ccccc1 Benzil Sodium benzilate Benzilic acid

Short answer

Question: Describe the benzil-benzilic acid rearrangement mechanism. Answer: The benzil-benzilic acid rearrangement mechanism involves the following steps: 1) Nucleophilic attack by OH⁻ on the carbonyl carbon of benzil, forming a tetrahedral intermediate. 2) Proton transfer from the α-carbon to the negatively charged oxygen atom, forming an enolate intermediate. 3) Phenyl group migration to the carbonyl carbon, resulting in the formation of a new C-C bond and breaking the C-C bond of the enolate. 4) Protonation of enolate by a water molecule, forming sodium benzilate. 5) Keto-enol tautomerization of sodium benzilate, eventually forming benzilic acid.

Step-by-step solution

Draw structures of benzil, sodium benzilate, and benzilic acid

Draw the structures of the reactant benzil, the intermediate sodium benzilate, and the product benzilic acid, which are given by their respective SMILES notation for clarity: Benzil: O=C(C(=O)c1ccccc1)c1ccccc1 Sodium benzilate: O=C(O)C(O)(c1ccccc1)C(O)(c1ccccc1)c1ccccc1 Benzilic acid: O=C(O)C(O)(c1ccccc1)C(O)(c1ccccc1)c1ccccc1

Analyze the structural changes in the reaction

Observe the structural changes through the reaction. In this rearrangement, the nucleophile OH⁻ (from sodium or potassium hydroxide) attacks the carbonyl carbon of benzil, followed by the migration of the phenyl group and tautomerization to form the product benzilic acid.

Propose the mechanism for benzil-benzilic acid rearrangement

The proposed mechanism for benzil-benzilic acid rearrangement can be summarized in the following steps: 1. Nucleophilic attack by OH⁻: The nucleophile OH⁻ attacks the carbonyl carbon of benzil, forming a tetrahedral intermediate. The negative charge is now on the oxygen atom of the carbonyl group. 2. Proton transfer: A proton from the α-carbon (next to the carbonyl carbon) is transferred to the negatively charged oxygen atom, forming an enolate intermediate. 3. Phenyl group migration: The phenyl group on the α-carbon migrates to the carbonyl carbon (now bearing a negative charge), resulting in the formation of a new C-C bond between the phenyl group and the carbonyl carbon, while breaking the C-C bond of the enolate. 4. Protonation of enolate: The enolate intermediate is protonated by a water molecule, forming sodium benzilate. 5. Tautomerization: Sodium benzilate undergoes keto-enol tautomerization, which involves a proton transfer between the α-hydroxy group and the carbonyl oxygen, eventually forming benzilic acid. By following this proposed mechanism, benzil is successfully transformed into benzilic acid through the benzil-benzilic acid rearrangement.