Question

What will be the major product obtained on dehydration of propan-1,2-diol ?

I am not being able to understand which one among the 2 lone pairs (of 2 oxygen atoms) is going to attack the proton.

Answer 1

I predict the major product to be propanal.

Explanation:

The sulfuric acid protonates both oxygen atoms.

Protonation at `"C1"`

However, loss of water from `"C1"` produces a primary carbocation, which is quite unstable.

Protonation at `"C2"`

Loss of water from `"C2"` produces a secondary carbocation, which is much more stable.

Loss of an α-hydrogen

The cation loses a proton preferentially from `"C1"` to form an enol, which rapidly tautomerizes to the more stable aldehyde.