Rearrangement of benzilic acid. 1. Rearrangement of benzilic acid Prepared by: Aras jabar & shaxawan rahim university of silemany school of. This is known as benzilic acid rearrangement. The mechanism of this benzilic acid rearrangement starts with attack of hydroxide on one of the carbonyl groups. The Benzilic Acid Rearrangement Leads to Ring Contraction. Learn about Benzilic Acid Rearrangement Mechanism with the Help of our Free Online Tutors.
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This reaction receives its name from the reaction of benzil with potassium hydroxide to form benzilic acid.
First performed by Justus von Liebig in it is a classic reaction in organic synthesis and has been reviewed many times before.
The reaction has been shown to work in aromaticsemi-aromatic, aliphaticand heterocyclic substrates. The reaction works best when the ketone functional groups have no adjacent enolizable protons, as this allows aldol condensation to compete. The reaction is rexrrangement a ring contraction when used on cyclic diketones.
It has been found that aryl groups more readily migrate than alkyl groups, and that aryl groups with electron-withdrawing groups migrate the fastest. The reaction is a representative of 1,2-rearrangements. These rearrangements usually have migrating carbocations but this reaction is unusual because it involves a migrating carbanion.
The long established reaction mechanism was first proposed in its entirety by Christopher Kelk Ingoldand has been updated with in silico data  as outlined below.
The reaction is second order overall in terms of rate, being first order in benzikic and first order in base.
Benzilic Acid Rearrangement
A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to form the alkoxide 2. The next step requires a bond rotation to conformer 3 which places the benzioic group R in position for attack on the second carbonyl group.
This migration step is rate-determining. This sequence resembles a nucleophilic acyl substitution. Calculations show that when R is methyl the charge build-up on this group in the transition state can be as high as 0. Calculations show that an accurate description of the reaction sequence is possible with the participation of 4 water molecules taking responsibility for the stabilization of charge buildup.
Rearrangements – Benzilic Acid
They also provide a shuttle for the efficient transfer of one proton in the formation of intermediate 5. The above mechanism is consistent with all available experimental evidence. In deuterated watercarbonyl oxygen exchange occurs much faster than the rearrangement, indicating that the first equilibrium is not the rate-determining step. Further experiments showed a larger relative rate in a deuterated solvent system compared to a non-deuterated solvent system of otherwise identical composition.
This was explained as being due to the greater relative basicity of the deuterated hydroxide anion compared to the normal hydroxide anion, and was used to indicate that hydrogen migration did not occur in the rate determining step of the reaction. This ruled out a concerted mechanism for the reaction, as hydrogen transfer benzi,ic occur in the rate determining step.
This reaction is identical to the normal Benzilic acid rearrangement, except that an alkoxide or an amide anion is used in place of benzolic hydroxide ion. The alkoxide used should not be easily oxidizable such as potassium ethoxide as this favors the Meerwein—Ponndorf—Verley reduction pathway as a side reaction.
The reaction is second order overall in terms of rate, being first order in terms of alkoxide and first order in terms of diketone.
Benzilic acid rearrangement – Wikipedia
This variation of the reaction has been known to occur in many substrates bearing the acyloin functional group. The picture below shows the ring expansion of a cyclopentane to a cyclohexane ring as an example reaction. From Wikipedia, the free encyclopedia. The base-catalysed reactions of 1,2-dicarbonyl compounds”.