Keto – enol tautomerism: When one of the two tautomer forms is keto and the other is the enol form, this type of tautomerism is called keto-enol tautomerism.
example: The two tautomer forms of acetoacetate ester have one keto form and the other enol form and this compound exhibits keto-enol tautomerism. In order to demonstrate keto-enol tautomerism, it is necessary that the compound contains a carbonyl group (keto or aldehyde group) and at least one alpha hydrogen atom present. At least one carbon atom in the carbon atoms attached to the –CO – group. But a hydrogen atom must be present.
On this basis, acetone(CH3COCH3), Cyclohexanone(C6H10O), acetophenone (C6H5COCH3), propaldehyde(CH3CH2CHO) and Phenole(C6H5OH) can exhibit keto-enol tautomerism but benzophenone(C6H5COC6H5), benzaldehyde(C6H5CHO) does not exhibit keto enol tautomerism.