Question

Why is geminal diols structure unstable?

Answer 1

Not all geminal diols are unstable. Geminal diols are formed by the hydration of carbonyl compounds. They may also lose a molecule of water to give back the parent carbonyl compound. Addition of water to carbonyl compound is a reversible reaction and the equilibrium may be shifted towards either compound (diol or carbonyl compound) depending on the stability.
The diols of the last 3 carbonyl compounds are stable, while that of acetone is unstable. In case of acetaldehyde, both are in almost equal proportion.
Vicinal diols are stable as they cannot lose water molecule so easily.