Cannizzaro reaction

The Cannizzaro reaction is a reaction of names of organic chemistry. It was named after its discoverer, the Italian chemist Stanislao Cannizzaro ( 1826-1910 ). Aldehydes which have a hydrogen atom bonded to the carbon atom which is in α -position to the carbonyl group, to react in said reaction in a strongly basic medium, to the salt of a carboxylic acid and an alcohol. A strongly basic medium, for example, concentrated sodium hydroxide solution, the result is the sodium salt of a carboxylic acid and the alcohol.

Is an α - hydrogen atom in the aldehyde present, the aldol reaction proceeds preferentially.

Overview reaction

Disproportionation reaction in the aldehydes which have a hydrogen atom in α -position to the aldehydic carbon atom in the presence of strong bases.

As an oxidation product is formed, the appropriate salt of a carboxylic acid, as a reduction product of the alcohol. The residues of the aldehydes can be alkyl or aryl groups. The alkyl group may have no α - hydrogen. Additionally, the radicals can be identical.

Reaction mechanism

In this mechanism, the Cannizzaro reaction using a general Aldehys, explains without α - hydrogen atom. The reaction takes place in an aqueous solution of concentrated sodium hydroxide solution as an example.

A hydroxide ion attacks the aldehyde 1 to nuclephil, so that the simple deprotonated diol 2 forms. This is again attacked by nucleophilic hydroxide ion. It forms the dianion 3 under formation of a carbonyl group is split off a hydride ion, which in turn has been attacking an aldehyde group on the carbonyl. The intermolecular hydride shift is determined by the electron-donating effect of the O - allows substituents. The hydride transfer must take place within a complex molecule, as the hydride ion would otherwise react with the existing always in the water protons to hydrogen. The intermediate aldehyde corresponding alcoholate 4 and 5 is produced carboxylate The intermediate 4 is protonated by the solvent, in an acid -base reaction to alcohol 6. The carboxylate 5 can now turn out with the sodium hydroxide solution of the sodium salt of the corresponding carboxylic acid (not always).

Crossed Cannizzaro reaction

Since carboxylic acids are easier to synthesize by other means, the Cannizzaro reaction generally used for recovery of the alcohol. As more of a mole of an aldehyde to half of the alcohol and the other half of the carboxylic acid is created, the atom economy of the conventional Cannizzaro reaction, is low ( only up to 50% of the aldehyde can be reduced to the desired alcohol, the other 50% be oxidized ). Therefore, the more frequently is "crossed Cannizzaro reaction " is used, in which formaldehyde is used as an additional component.

The formaldehyde is attacked by a hydride base and then to the second aldehyde from, which is then reduced to the alcohol as a formate formed oxidatively ( salt of formic acid ) from the formaldehyde:

This measure increases the yield of alcohol. For example, the tetravalent arises, bearing four hydroxy alcohol pentaerythritol by triple aldol reaction of formaldehyde and acetaldehyde followed by a crossed Cannizzaro reaction.