Write mechanism of acid dehydration of ethanol to yield ethene.

Mechanism: The formation of ethylene and diethyl ether from ethanol and conc. H₂SO₄ may be explained as follows:
(a) Formation of ethene or ethylene:
Step 1. Due to the presence of two lone pairs of electrons on oxygen, alcohols act as weak bases. Therefore, they react with strong mineral acids (HCl, H₂SO₄ etc.) to form oxonium salts.


Step 2. The presence of a positive charge on the highly electronegative oxygen atom weakens the C—O bond. Thus, the protonated ethanol readily eliminates a molecule of H₂O to form ethyl carbocation.

This step is slow and hence is the rate-determining step of the reaction.
Step 3. The ethyl carbocation formed in step 2 being a reactive chemical species, readily loses a proton to form the ethene molecule.