Question

As a method for the preparation of alkenes, a weakness in the acid-catalyzed dehydration of alcohols is that the initially formed alkenes (or mixture of alkenes) sometimes isomerizes under the condition of its formation. Write a stepwise mechanism showing how 2-methyl-1-butene might isomerize to 2-methyl-2-butene in the presence of sulphuric acid.

Answer 1

In presence of 2-methyl-1-butene isomerizes to 2-methyl-2-butene.
In the first step, a hydrogen ion is added to the terminal carbon atom joined by double bond. The positive charge is on tertiary carbon atom. The tertiary carbocation formed is more stable than secondary carbon atom.
Then a hydrogen ion is lost from neighbouring carbon atom to form a double bond.
If the hydrogen ion is lost from the same carbon atom (to which it was added,) 2-methyl-1-butene is obtained. But when hydrogen ion is lost from another carbon atom, then 2-methyl-2-butene is obtained.