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.

1 Answer
Jan 24, 2018

I predict the major product to be propanal.

Explanation:

The sulfuric acid protonates both oxygen atoms.

Protonation at "C1"

C1 ProtonationC1 Protonation

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

Protonation at "C2"

C2 protonationC2 protonation

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

Loss of an α-hydrogen

DeprotonationDeprotonation

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