Question

Question: a) What is the major alkene formed when A is dehydrated with ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$ ? (b) What is the major alkene formed when A is treated with ${\mathbf{POCl}}_{\mathbf{3}}$and pyridine? Explain why the major product is different in these reactions.

Short answer

Answer

a.

b.

The major product is different in both reactions as in the first reaction, product formation depends on carbocation stability, whereas in reaction B, the product formation occurs via elimination reaction.

Step-by-step solution

Step-by-Step SolutionStep 1: Dehydration of alcohols

Dehydration reactions can also be termed dehydrogenation reactions. The combination of protic acids with alcohol leads to the generation of an alkene.

Reaction of alcohols with POCl3 and pyridine

Conversion of alcohols to alkenes can be done with the help of reagents ${\mathbf{POCl}}_{\mathbf{3}}$and pyridine. ${\mathbf{POCl}}_{\mathbf{3}}$functions similar to a strong acid in dehydration reactions.

Predicting the alkenes formed by the reaction of compound A with various reagents

a. In the reaction of compound A with sulphuric acid the protonation of alcohol occurs first, leading to the formation of a secondary carbocation.

This carbocation undergoes hydrogen atom migration leading to the formation of a tertiary carbocation. This is followed by the elimination of proton, leading to the formation of alkene. The reaction is given as:

Alkene formed when compound A is dehydrated with ${\mathrm{H}}_2{\mathrm{SO}}_4$

b. The reaction of compound A occurs by E2 elimination reaction leading to the formation of alkene. The reaction can be given :

In the dehydration of compound A using sulphuric acid, the product formation depends on carbocation stability. In the treatment of compound A with${\mathrm{POCl}}_3$ and pyridine, the reaction takes place by the E2 mechanism.

Hence, different products are obtained in the reaction.