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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The picture below shows the ring expansion of a cyclopentane to a cyclohexane ring as an example reaction.

The reaction is a representative of 1,2-rearrangements. The reaction works best when the ketone functional groups have no adjacent enolizable protons, as this allows aldol condensation to compete.

Benzilic Acid Rearrangement

This ruled out a concerted mechanism for the reaction, as hydrogen transfer would occur in the rate determining step. From Wikipedia, the free encyclopedia. To view our list of developers please see our Team Page.

The reaction has been shown to work in aromaticsemi-aromatic, aliphaticand heterocyclic substrates. This is known as benzilic acid rearrangement.

It has been found that aryl groups more readily migrate than alkyl groups, and that aryl groups with electron-withdrawing groups migrate the fastest. Retrieved from ” https: The reaction is formally a ring contraction when used on cyclic diketones.


This reaction is identical to the normal Benzilic acid rearrangement, except that an alkoxide or an amide anion is used in place of a hydroxide ion. Views Read Edit View history.

The mechanism of this benzilic acid rearrangement starts with attack of hydroxide on one of the carbonyl groups. The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group. Rearrangements Benzilic Acid Background Colour: The tetrahedral intermediate can collapse in a reaction reminiscent of a semipinacol rearrangement. Further experiments showed a larger relative rate in a deuterated solvent system compared to a non-deuterated solvent system of otherwise identical composition.

Rearrajgement page was last edited on 20 Octoberat 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. This reaction receives its name from the reaction of benzil benzilif potassium hydroxide to form benzilic acid. The first rearrangement reaction ever to be described has both the formation of carbonyl groups at the migration origin and destruction of carbonyl groups at the migration terminus.

In deuterated watercarbonyl oxygen exchange occurs much faster than the rearrangement, indicating that the first equilibrium is not the rate-determining step. 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 rearrangemenr determining step of the 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. The alkoxide used should not be easily oxidizable such as potassium ethoxide as this favors the Rearrangemetn reduction pathway as a side reaction.

Benzilic acid rearrangement

These rearrangements usually have migrating carbocations but this reaction is unusual because it involves a migrating carbanion. The long established reaction rearrangeemnt was first proposed in its entirety by Christopher Kelk Ingoldand has been updated with in silico data [5] as outlined below.

Calculations show that when R is methyl the charge build-up on this group in the transition state can be as high as 0. This migration step is rate-determining. By using this site, you agree to the Terms of Use and Privacy Policy. The reaction is second order overall in terms of rate, being first order in diketone and first order in base.

Benzilic Acid Rearrangement

Enols and Enolates as nucleophiles. This sequence resembles a nucleophilic acyl substitution. This variation of the reaction has been known to occur in many substrates bearing the acyloin functional group.