Benzilic acid rearrangement

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Benzilic acid rearrangement

The benzilic acid rearrangement is the rearrangement reaction

Rearrangement reaction

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule ....
of benzil

Benzil

Benzil is the organic compound with the formula 2, generally abbreviated 2. This diketone is used as an intermediate in organic synthesis....
with potassium hydroxide

Potassium hydroxide

Potassium hydroxide is the inorganic compound with the formula potassiumhydroxide. Along with sodium hydroxide, this colourless solid is a prototypical "strong base"....
to benzilic acid. First performed by Justus Liebig in 1838 this reaction type is displayed by 1,2-diketones

Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
in general. The reaction product is an a-hydroxy

Hydroxy

'Hydroxy' can refer to:* In chemical nomenclature, the prefix "hydroxy-" shows the presence of a hydroxyl functional group * An abbreviation for the medication hydroxyzine, which is commonly sold under the brand names Atarax, Ucerax, Serecid, and Vistaril....
-carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
.

Certain acyloin

Acyloin

Acyloins are a class of organic compounds in organic chemistry sharing a common functional group consisting of a hydroxyl group placed on the a-position of a carbonyl group....
s also rearrange in this fashion.

This diketone reaction is related to other rearrangements: the corresponding keto-aldehyde (one alkyl group replaced by hydrogen) rearranges in a Cannizzaro reaction

Cannizzaro reaction

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base -induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position....
, the corresponding 1,2-diol reacts in a pinacol rearrangement

Pinacol rearrangement

The pinacol rearrangement or pinacol-pinacolone rearrangement is a method for converting a diol to a carbonyl compound in organic chemistry....
.

reaction is a representative of 1,2-rearrangement

1,2-rearrangement

A 1,2-rearrangement or 1,2-migration or 1,2-shift or Frank C. Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound....
s.

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Encyclopedia

The benzilic acid rearrangement is the rearrangement reaction

Rearrangement reaction

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule ....
of benzil

Benzil

Benzil is the organic compound with the formula 2, generally abbreviated 2. This diketone is used as an intermediate in organic synthesis....
with potassium hydroxide

Potassium hydroxide

Ketone

Hydroxy

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
.

Certain acyloin

Acyloin

Cannizzaro reaction

Pinacol rearrangement

The pinacol rearrangement or pinacol-pinacolone rearrangement is a method for converting a diol to a carbonyl compound in organic chemistry....
.

Reaction mechanism

The reaction is a representative of 1,2-rearrangement

1,2-rearrangement

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer Electron shell#Valence shell instead of the eight valence electrons that ensures maximum stability ....
s but this reaction is unusual because it involves a migrating carbanion

Carbanion

A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons ....
. The long established reaction mechanism

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs .Although only the net chemical change is directly observation for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism....
updated with in silico

In silico

In silico is an expression used to mean "performed on computer or via computer simulation." The phrase is coined in analogy to the Latin language phrases in vivo and in vitro which are commonly used in biology and refer to experiments done in living organisms and outside of living organisms, respectively....
data is outlined in scheme 2.

A hydroxide

Hydroxide

In chemistry, hydroxide is the name for the Diatomic molecule anion OH-, consisting of oxygen and hydrogen atoms, usually derived from the Dissociation of a base ....
anion attacks one of the ketone

Ketone

Nucleophilic addition

In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a p bond is removed by the creation of two new covalent bonds by the addition of a nucleophile ....
to the hydroxyl

Hydroxyl

Hydroxyl in chemistry stands for a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent bond. The neutral form is a hydroxyl Radical and the hydroxyl anion is called a hydroxide....
anion 2. 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 in a concerted

Concerted reaction

In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved....
step with reversion of the hydroxyl group back to the carbonyl group. This sequence resembles a nucleophilic acyl substitution

Nucleophilic acyl substitution

Nucleophilic acyl substitution describes the substitution reaction involving nucleophiles and acyl compounds. Acyl compounds are carboxylic acid derivatives including esters, amides and acid halides....
. Calculations show that when R is methyl the charge build-up on this group in the transition state

Transition state

The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate....
can be as high as 0.22 and that the methyl group is positioned between the central carbon carbon at a separation of 209 pm.

The carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
in intermediate 4 is less basic than the hydroxyl anion and therefore proton transfer takes place to intermediate 5 which can be protonated in acidic workup to the final a-hydroxy

Hydroxy

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
6. 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. They also provide a shuttle for the efficient transfer of one proton in the formation of intermediate 5.

From a molecular orbital

Molecular orbital

In chemistry, a molecular orbital is a mathematical function that describes the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region....
point of view this rearrangement may at a first glance not obvious. Contrary to a carbocationic rearrangement as in the Wagner-Meerwein rearrangement

Wagner-Meerwein rearrangement

A Wagner-Meerwein rearrangement is a class of carbocation 1,2-rearrangement rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon....
in which the empty carbocationic orbital interacts positively and symmetry allowed with the filled pi orbital HOMO

Homo

Homo may refer to:In science:* Homo , the genus including modern humans and closely related species such as Neanderthals* Highest Occupied Molecular Orbital, in chemistry: see HOMO/LUMO...
of the central C-C bond (situation A in scheme 3), a filled carbanionic orbital should not be able to escape a symmetry forbidden MO overlap with the LUMO

Lumo

Lumo is a 2007 documentary film about twenty-year-old Lumo Sinai, a woman who fell victim to "Africa's First World War." While returning home one day, Lumo and another woman were gang-raped by a group of soldiers fighting for control of the Congo during the 1994 Rwandan genocide....
which is the empty antibonding

Antibonding

Antibonding is a type of chemical bond. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct Atomic nucleus....
pi orbital having one node (situation B).

In reality a 1,2-diketone LUMO is a 4 electron system without any nodes in the central C-C bond and a symmetry allowed transition is possible (Situation C). In other words the transition states of both a carbocationic rearrangement and the benzilic rearrangement obey the Woodward-Hoffmann rules

Woodward-Hoffmann rules

The Woodward?Hoffmann rules devised by Robert Burns Woodward and Roald Hoffmann are a set of rules in organic chemistry predicting the stereochemistry of pericyclic reactions based on orbital symmetry....
because the involves respectively 2 electrons and 6 electrons (n=0 and 1 in the 4n+2 Hückel's rule

Hückel's rule

In organic chemistry, H?ckel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemistry Erich H?ckel in 1931....
).

Variations

A variation of this reaction occurs in certain steroid

Steroid

A steroid is a terpenoid lipid characterized by a carbon skeleton with four fused rings, generally arranged in a 6-6-6-5 fashion.Steroids vary by the functional groups attached to these rings and the oxidation state of the rings....
s. In the so-called D-Homo Rearrangement of Steroids a cyclopentane ring expands to a cyclohexane ring with added base .

Benzilic acid rearrangement

Reaction mechanism

Variations

Further reading

External links