Question

Question: Addition of HBr to which of the following alkenes will lead to a rearrangement?

Short answer

Answer

The addition of HBr leads to rearrangement in alkene c.

Step-by-step solution

Rearrangement of carbocations

The carbocation undergoes 1,2-H shift to become more stable, thus forming the major product of the reaction.

Carbocation stability

The more substituted carbocation is more stable. Due to the electron-donating tendency of the alkyl group, the carbocation centers achieve stability to some extent.

Products of the given reactions

The addition of HX to alkenes enables carbocation formation by the addition of the hydrogen atom of HX to the less substituted carbon atom in the C=C bond.

Carbocations undergo a 1,2-H shift to achieve stability.

a.

Addition of $H^{+}$leads to the formation of a $3°$carbocation, which is a very stable intermediate. Hence, it does not undergo rearrangement.

b.

Formation of $2°$carbocations takes place and both of which are not able to form a more stable $3°$carbocation upon rearrangement. Hence, no rearrangement takes place.

Carbocation in b

c.

One of the $2°$carbocation undergoes rearrangement to form a more stable $3°$carbocation.

Carbocation in c