Question

During dehydration of alcohols to alkenes by heating with concentrated sulphuric acid, the initiation step is

• A. Formation of carbocation
• B. Loss of $\left(H_{2}O\right)$ molecule
• C. Deprotonation to form alkene
• D. Protonation of alcohol

Answer 1

The correct option is D Protonation of alcohol

Dehydration of alcohol is an E1 elimination reaction. The rate of the reaction depends on the stability of the carbocation formed.

Mechanism of dehydration of alcohol:
Step 1: Protonation of alcohol molecule.
The $H_{2}S{O}_4$ will protonate the alcohol group to form ${}^{+}O{H}_2$, this step occur very fast.

Step 2: Loss of leaving group (Slowest step)
The formed ${}^{+}O{H}_2$ leaves as $H_{2}O$ leaving behind a carbocation.
Step 3: Loss of proton to give alkene


Hence, initial step is protonation of alcohol.