Question

Why cyclohexa-3,5-dien-1,2-dione not show keto enol toutomerism?

Answer 1

Tautomerism essentially requires the presence of α-hydrogen (its the hydrogen atom present generally adjacent to the functional group, or the hydrogen atom present on the carbon which is adjacent to the carbon attached to the functional group).

cyclohexa-3,5-dien-1,2-dione has an α-hydrogen, however, its a vinylic hydrogen(hydrogen attached to a carbon atom which is sp2 hybridized), its very difficult to abstact such a hydrogen, and hence it becomes very less acidic. Therefore, no abstraction of α-hydrogen from it, by a base, hence, it does not show tautomerism!