Question

Discuss the mechanism of the following reactions-
Dehydration of alcohols to form alkenes

Answer 1

We need to understand the general dehydration reaction of alcohols to generate alkenes. Different kinds of alcohol dehydrate via slightly different mechanisms. However, the fundamental concept behind each dehydration reaction is that the alcohol's –OH group contributes two electrons to H+ from the acid reagent, resulting in the formation of an alkyloxonium ion. This ion is an excellent leaving group, departing to create a carbocation. The nucleophile (the deprotonated acid) then attacks the hydrogen next to the carbocation, forming a double bond.

Complete answer:
Three stages are involved in the dehydration of alcohol to create alkene.
The first step is to create protonated alcohol. Because of the existence of two lone pairs of oxygen, alcohol behaves as a weak base. They react with strong acids such as HCl to create oxonium salt.
The second step is to produce a carbocation. To create ethyl carbocation, protonated alcohol removes a water molecule.
Proton elimination is the third stage in the formation of ethene. Because ethyl carbocation is reactive, it easily loses a proton to produce the ethene molecule.
Primary alcohols are eliminated molecularly (through the E2 mechanism), whereas secondary and tertiary alcohols are eliminated unimolecularly (E1 mechanism). The relative reactivity of alcohols in the dehydration process is as follows:
Methanol < primary < secondary < tertiary

Note:
We must note that because the alcohol dehydration process contains a carbocation intermediate, hydride or alkyl shifts can occur, relocating the carbocation to a more stable location. As a result, the dehydrated products are a combination of alkenes with and without carbocation rearrangement. Tertiary cations are more stable than secondary cations, which are more stable than primary cations due to a phenomenon known as hyperconjugation, which occurs when the interaction between the filled orbitals of neighbouring carbons and the singly occupied p orbital in the carbocation stabilises the positive charge in the carbocation.