Cannizzaro reaction

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Cannizzaro reaction

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro

Stanislao Cannizzaro

Stanislao Cannizzaro, Fellow of the Royal Society was an Italy chemist. He is remembered today largely for the Cannizzaro reaction and for his influential role in the atomic-weight deliberations of the Karlsruhe Congress in 1860....
, is a chemical reaction

Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
that involves the base

Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
-induced disproportionation

Disproportionation

Disproportionation or dismutation is used to describe two particular types of chemical reaction:* A chemical reaction of the type: 2A ? A' + A" where A, A' and A" are different chemical species....
of an aldehyde

Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
lacking a hydrogen atom in the alpha position. Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and benzoic acid from the treatment of benzaldehyde with potash

Potash

Potash is the common name given to potassium carbonate and various mined and manufactured salts that contain the element potassium in water-soluble form....
(potassium carbonate).

The oxidation product is a 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....
and the reduction

Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
product is an alcohol

Alcohol

In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
.

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Reaction mechanism

The first reaction step is nucleophilic addition

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 ....
of the base (for instance the 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....
anion) to the carbonyl carbon of the aldehyde

Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
. The resulting alkoxide is deprotonated to give a di-anion, known as the Cannizzaro intermediate. Formation of this intermediate requires a strongly basic environment.

Both intermediates can react further with aldehyde to transfer a hydride

Hydride

Hydride is the name given to the Electric charge ion of hydrogen, H-. Although this ion does not exist except in extraordinary conditions, the term hydride is widely applied to describe Chemical compound of hydrogen with other chemical element, particularly those of Periodic table group 1–16....
, "H"^-. The hydridic character of the C-H is enhanced by the electron-donating character of the alpha oxygen anion. This hydride transfer simultaneously generates a hydroxyl anion and a carboxylate. Further evidence for the hydridic character of the Cannizzaro intermediate is provided by the formation of H₂ by its reaction with water.

Only aldehydes that cannot form an enol

Enol

Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Enols and carbonyl compounds are in fact isomers; this is called keto-enol tautomerism:...
ate ion undergo the Cannizaro reaction. The aldehyde cannot have an enolizable proton. Under the basic conditions that facilitate the reaction, aldehydes that can form an enolate instead undergo aldol condensation

Aldol condensation

An Aldol condensation is an organic reaction in which an enolate ion reacts with a carbonyl compound to form a ?-hydroxyaldehyde or ?-hydroxyketone, followed by dehydration to give a conjugated enone....
. Examples of aldehydes that can undergo a Cannizaro reaction include formaldehyde

Formaldehyde

Formaldehyde is a chemical compound with the chemical formula H2CO. It is the simplest aldehyde. Formaldehyde exists in several forms aside from H2CO: the cyclic trimer trioxane and the polymer Polyoxymethylene....
and aromatic aldehydes such as benzaldehyde

Benzaldehyde

Benzaldehyde is a chemical compound consisting of a benzene ring with an aldehyde substituent. It is the simplest representative of the aromatic aldehydes and one of the most industrially used members of this family of compounds....
.

Variations

A special condition is the crossed Cannizzaro reaction. This variation is more common these days because the original Cannizzaro reaction yields a mixture of alcohol and carboxylic acid. For example any aldehyde with no alpha hydrogens can be reduced when in the presence of formaldehyde

Formaldehyde

Formic acid

Formic acid is the simplest carboxylic acid. Its formula is hydrogencarbonoxygenOH or CH2O2. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stingers....
and the corresponding alcohol is obtained in a high yield although the atom economy

Atom economy

Atom economy describes the conversion efficiency of a chemical process in terms of all atoms involved. In an ideal chemical process the amount of starting materials or reactants equals the amount of all products generated and no atom is wasted....
is still low.

Cannizzaro reaction

Reaction mechanism

Variations

See also