Question

explain how phenol is unstable in ketoform?​

Answer 1

In the case of phenol, the keto form results in the loss of the delocalized aromatic system, and this form is higher in energy (less stable) than the enol form which is lower in energy due to the delocalization of the electrons in the aromatic system.  

Answer 2

The keto-enol equilibrium for phenol lies entirely on the enol side due to thermodynamic stabilization provided by aromaticity. so phenol is more stable than its keto form.