Q43E - Organic Chemistry (Mcmurry)

Question

21-43 The following reaction, called the benzilic acid rearrangement, takes place by typical carbonyl group reactions. Propose a mechanism (Ph=phenyl).

Step-by-Step Solution

Answer

The Benzilic acid rearrangement has transformed a $\mathbf{\alpha }\mathbf{-}$ diketones to a $\mathbf{\alpha }\mathbf{-}$ hydroxy acid in the provided reaction.

Step 1: Introduction to the Concept

In the presence of a strong base, these ketones undergo a rearrangement, yielding salts of L-hydroxy carboxylic acids.

The name of the reaction is given benzilic acid rearrangement both di-phenyl or diaryl ketones because the early investigations of the reaction were performed on Benzil and the product produced was benzil acid.

Ketones yield well, with $\mathbf{Ar}$ leading the way in yield, however  $\mathbf{\alpha }\mathbf{-}$diketones containing enolisable a-protons are difficult to get due to competing aldol condensation processes.

Step 2: Solution Explanation

The benzilic acid rearrangement is an irreversible reaction in which alkyl, aryl, or phenyl group on an electron-deficient carbon atom undergoes a 1,2-shift. The mechanism's first step is the addition of a nucleophile across a $\mathbf{\text{C=O}}$  bond to produce a tetrahedral intermediate, which is similar to the first nucleophilic addition step of carboxylic acid derivatives' nucleophilic acyl substitution.

1. Nucleophilic addition: 

The phenyl/or aryl group on the carbon atom of the neighboring $\mathbf{\text{C=O}}$ group migrates to create a d-hydroxyl carboxylic acid salt by proton transfer in the next step.

The initial phase of nucleophilic addition has been discovered to be reversible and quick, while the second step, migration, as same as migration aptitude.

Further reflection led to the idea of examining the migration group, as well as the rate of movement and migration aptitudes.

The starting benzil is symmetrical, which means that both the aryl (as in the provided reaction) and phenyl groups are comparable and have the same migration ability.

The ability of an aryl group to migrate rises when it contains a strong electron withdrawing group, according to isotopic labeling research. Thus,

In consequence, the substituent group's election withdrawing –R effect on  inductively withdraws electron density from the alkoxide data-custom-editor="chemistry" ${\mathrm{O}}^{-}$ ion's electronic "push" on the tetrahedral carbon, thereby dropping this substituted  to the neighboring electron deficient carbonyl carbon. The corresponding stable carboxylate anion is formed by a fast proton transfer provided by the medium containing (${\mathbf{\text{H}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}^{\mathbf{+}}$ ) [acidic].

The conversion of a ten stable anion into a more stable anion is the drawing force for the rearrangement.