the compound that exhibits tautomerism.???

the compound that exhibits tautomerism.???


1 Answers

askIITians Faculty 164 Points
8 years ago


It is the type of isomerism in which two functional isomers exist together in equilibrium. The two forms existing in equilibrium are called as tautomers. For example, the compound acetoacetic ester has two tautomers – one has a keto group and other has an enol group:

Out of the two tautomeric forms, one is more stable and exists in larger proportion. In above, normally 93% of the keto form (more stable) and only 7% of the enol form (less stable i.e. labile) exist.

The equilibrium between the two forms is dynamic, i.e., if one form is somehow removed by making a reaction, some of the amount of the other form changes into the first form so that similar equilibrium exists again. Thus, whole of the acetoacetic ester shows the properties of

both ketonic group as well as the enolic group. Thus, it adds on HCN, NaHSO3etc. due to the presence of >C=O group and it decolourises bromine water and gives dark colouration with FeCl3due the presence of >C—OH group. Due to the presence of keto and enol form

this type of tautomerism is known as keto-enol tautomerism. It is the most commonly observed type of tautomerism.

Keto-enol tautomerism is generally observed in those compounds in which either a methyl (CH3—), methylene (—CH2—), or a methyne (>CH-) group is present adjacent to a carbonyl (—CO—) group as in acetoacetic ester above. In other words, it can be said that keto-enol tautomerism is possible in only those carbonyl compounds in which atleast one a-hydrogen atom is present so that it may convert the carbonyl group to enol group.

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