Cannizzaro's reaction

Explanation

Aldehydes which do not have hydrogens on `alpha-` carbon atom do not undergo aldol condensation. They undergo Cannizzaro.s reaction when heated in the presence of an alkali. Such aldehydes are `C_(6)H_(5)CHO,HCHO,C Cl_(3)CHO`, etc.
`HCH=O+HCH=Ounderset(H_(2)O)overset(overset(" "-)(OH))(to)CH_(3)OH+HCOOH`
One molecule of an aldehyde undergoes oxidation and the other undergoes reduction
`C_(6)H_(5)CHO+C_(6)H_(5)CHOunderset(H_(2)O)overset(overset(" "-)(OH))(to)C_(6)H_(5)CH_(2)OH+C_(6)H_(5)COOH`
If two aldehyde groups are present in the same molecule, intramolecular Cannizzaro reaction may take place. for example,

Cross Cannizzaro reaction
When two different aldehydes which do not have any `alpha`- hydrogen, are treated with concentrated alkali, the reaction is called crossed cannizzaro reaction. This reaction is of little synthetic importance because it will give a mixture of all the four possible products. If, however, one of the aldehydes is formaldehyde, cross-Cannizzaro reaction is of great synthetic importance since it is always the formaldehyde which is oxidised to give sodium formate while the other aldehyde is reduced to the correspending alcohol. For example,