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Benzoin condensation

Benzoin condensation

Overview
The benzoin condensation is a reaction (often called a condensation reaction
Condensation reaction
A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...

, for historical reasons) between two aromatic aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s, particularly benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...

. The reaction is catalyzed
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 by a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

 such as the cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

 anion or an N-heterocyclic carbene
Persistent carbene
A persistent carbene is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula 2C:, where the 'R's are various functional groups...

. The reaction product is an aromatic 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 α-position of a carbonyl group.- Nomenclature :Common types of ketols include:...

 with benzoin
Benzoin
Benzoin is an organic compound with the formula PhCHCPh. It is a hydroxy ketone attached to two phenyl groups. It appears as off-white crystals, with a light camphor-like odor. Benzoin is synthesized from benzaldehyde in the benzoin condensation...

 as the parent compound. An early version of the reaction was developed in 1832 by Justus von Liebig
Justus von Liebig
Justus von Liebig was a German chemist who made major contributions to agricultural and biological chemistry, and worked on the organization of organic chemistry. As a professor, he devised the modern laboratory-oriented teaching method, and for such innovations, he is regarded as one of the...

 and Friederich Woehler during their research on bitter almond oil
Almond
The almond , is a species of tree native to the Middle East and South Asia. Almond is also the name of the edible and widely cultivated seed of this tree...

. The catalytic version of the reaction was developed by Nikolay Zinin
Nikolay Zinin
Nikolay Nikolaevich Zinin was a Russian organic chemist.-Life:He studied at the University of Kazan where he graduated in mathematics but he started teaching chemistry in 1835. To improve his skills he was asked to study in Europe for some time, which he did between 1838 and 1841...

 in the late 1830s, and the 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 observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

 for this organic reaction
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

 was proposed in 1903 by A. J. Lapworth
Arthur Lapworth
Arthur Lapworth was a Scottish chemist.He was born in Galashiels, Scotland, the son of geologist Charles Lapworth and educated at St Andrew's and King Edward's School, Birmingham. He graduated in Chemistry from Mason College...

.

In the first step in this reaction, the cyanide anion (as sodium cyanide) reacts with the aldehyde in a nucleophilic addition
Nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....

.
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Encyclopedia
The benzoin condensation is a reaction (often called a condensation reaction
Condensation reaction
A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...

, for historical reasons) between two aromatic aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s, particularly benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...

. The reaction is catalyzed
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 by a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

 such as the cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

 anion or an N-heterocyclic carbene
Persistent carbene
A persistent carbene is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula 2C:, where the 'R's are various functional groups...

. The reaction product is an aromatic 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 α-position of a carbonyl group.- Nomenclature :Common types of ketols include:...

 with benzoin
Benzoin
Benzoin is an organic compound with the formula PhCHCPh. It is a hydroxy ketone attached to two phenyl groups. It appears as off-white crystals, with a light camphor-like odor. Benzoin is synthesized from benzaldehyde in the benzoin condensation...

 as the parent compound. An early version of the reaction was developed in 1832 by Justus von Liebig
Justus von Liebig
Justus von Liebig was a German chemist who made major contributions to agricultural and biological chemistry, and worked on the organization of organic chemistry. As a professor, he devised the modern laboratory-oriented teaching method, and for such innovations, he is regarded as one of the...

 and Friederich Woehler during their research on bitter almond oil
Almond
The almond , is a species of tree native to the Middle East and South Asia. Almond is also the name of the edible and widely cultivated seed of this tree...

. The catalytic version of the reaction was developed by Nikolay Zinin
Nikolay Zinin
Nikolay Nikolaevich Zinin was a Russian organic chemist.-Life:He studied at the University of Kazan where he graduated in mathematics but he started teaching chemistry in 1835. To improve his skills he was asked to study in Europe for some time, which he did between 1838 and 1841...

 in the late 1830s, and the 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 observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...

 for this organic reaction
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

 was proposed in 1903 by A. J. Lapworth
Arthur Lapworth
Arthur Lapworth was a Scottish chemist.He was born in Galashiels, Scotland, the son of geologist Charles Lapworth and educated at St Andrew's and King Edward's School, Birmingham. He graduated in Chemistry from Mason College...

.

Reaction mechanism


In the first step in this reaction, the cyanide anion (as sodium cyanide) reacts with the aldehyde in a nucleophilic addition
Nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....

. Rearrangement of the intermediate results in polarity reversal
Umpolung
Umpolung or polarity inversion in organic chemistry is the chemical modification of a functional group with the aim of the reversal of polarity of that group. This modification allows secondary reactions of this functional group that would otherwise not be possible. The concept was introduced by...

 of the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group, which then adds to the second carbonyl group in a second nucleophilic addition. Proton
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....

 transfer and elimination of the cyanide ion affords benzoin as the product. This is a reversible reaction
Reversible reaction
A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as...

.

The cyanide ion serves three different purposes in the course of this reaction. It acts as a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

, facilitates proton abstraction, and is also the leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...

 in the final step. The benzoin condensation is in effect a dimerization and not a condensation because a small molecule like water is not released in this reaction. For this reason the reaction is also called a benzoin addition. In this reaction, the two aldehydes serve different purposes; one aldehyde donates a proton and one aldehyde accepts a proton. 4-Dimethylaminobenzaldehyde is an efficient proton donor while benzaldehyde is both a proton acceptor and donor. In this way it is possible to synthesise mixed benzoins, i.e. products with different groups on each half of the product.

Scope


The reaction can be extended to aliphatic
Aliphatic compound
In organic chemistry, aliphatic compounds are acyclic or cyclic, non-aromatic carbon compounds.Thus, aliphatic compounds are opposite to aromatic compounds.- Structure :...

 aldehydes with base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 catalysis in the presence of thiazolium salts; the reaction mechanism is essentially the same. These compounds are important in the synthesis of heterocyclic compounds. The addition is also possible with enone
Enone
An enone is an unsaturated chemical compound or functional group consisting of a conjugated system of an alkene and a ketone. The simplest enone is methyl vinyl ketone or CH2=CHCOCH3....

s; for instance methyl vinyl ketone
Methyl vinyl ketone
Methyl vinyl ketone is a reactive organic compound classified as an enone. It is a colorless, flammable, highly toxic liquid with a pungent odor...

 is a reagent in the Stetter reaction
Stetter reaction
The Stetter reaction is an organic reaction involving the nucleophile catalyzed conjugate addition of an aldehyde to a Michael acceptor such as an enone. The reaction product is a 1,4-dicarbonyl compound...

.

In biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, the coenzyme thiamine
Thiamine
Thiamine or thiamin or vitamin B1 , named as the "thio-vitamine" is a water-soluble vitamin of the B complex. First named aneurin for the detrimental neurological effects if not present in the diet, it was eventually assigned the generic descriptor name vitamin B1. Its phosphate derivatives are...

 is responsible for biosynthesis of acyloin-like compounds. This coenzyme also contains a thiazolium moiety, which on deprotonation
Deprotonation
Deprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...

 becomes a nucleophilic carbene.

In one study, a custom-designed N-heterocyclic carbene
Persistent carbene
A persistent carbene is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula 2C:, where the 'R's are various functional groups...

 (NHC, the framework is related to thiazolium salts) was found to facilitate an enantioselective intramolecular
Intramolecular
Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...

 benzoin condensation (Scheme 2).
This finding was confirmed in another study with a slightly modified NHC using DBU
DBU (chemistry)
1,8-Diazabicyclo[5.4.0]undec-7-ene, or more commonly DBU, is a chemical compound and belongs to the class of amidine compounds. It is used in organic synthesis as a catalyst and complexing ligand and a strong non-nucleophilic base.It is used as a curing agent for epoxy; it is used as a protecting...

 as the base instead of potassium tert-butoxide
Potassium tert-butoxide
Potassium tert-butoxide is the chemical compound with the formula 3COK. This colourless solid is a strong base useful in organic synthesis. It exists as a tetrameric cubane-like cluster...

(Scheme 3).