Question

Why presence of more than 1 hydroxyl groups on the same carbon atom makes the molecule(of which the carbon atom is a part)unstable?

Answer 1

I don't quite know the reason, but probably the reason is the presence of two hydroxyl group on the same carbon atom makes it very highly electropositive and vulnerable to attack by very weak nucleophiles. So a water molecule escapes leaving behind C=O.

HOPE THIS HELPS!!