Question

Propose a mechanism for each of the following reactions:

a.

b.

Short answer

The mechanism of the given reactions are:

a.

b.

Step-by-step solution

Inter and Intramolecular reactions

A reaction is said to be intramolecular if it describes a phenomenon or process limited within the single molecule or structure.

A reaction is said to be intermolecular reaction if it describes a process between two or more reactant species or molecules.

Proposed mechanism for Reaction (a)

In the 1^st step of the reaction mechanism (a), HCl added to the alkene results in the formation of the secondary carbocation that undergoes a 1, 2-hydride shift to form a tertiary carbocation. This tertiary carbocation is an electrophile that can either add to double bond in a second molecule (intermolecular) or add to the benzene ring within the same molecule of the reactant (intramolecular).

Here, the intramolecular reaction is favoredbecause it forms a stable five-membered ring. After the electrophile addition to the benzene ring, the base B perform deprotonation in the reaction mixture removes a proton and make the molecule stable by restored its aromaticity.

Mechanism of (a)

Proposed mechanism for Reaction (b)

In the 1^st step of the reaction mechanism (b),HCl added to the alkene results in the formation of the secondary carbocation that undergoes a 1, 2-hydride shift to form a tertiary carbocation. This tertiary carbocation is an electrophile that can either add to double bond in a second molecule (intermolecular) or add to the benzene ring within the same molecule of the reactant (intramolecular). The given product of the reaction has two benzene rings and there are twice as many carbon in the product as in the reactant indicates that two reactant molecule react in an intermolecular reaction.

In this case intermolecular reaction is favored,because intramolecular reaction would lead to a highly strained three membered ring. Thus, the electrophile that is formed in the intermolecular reaction can add to benzene ring in an intramolecular reaction to form a stable five-membered ring.

After the electrophile addition to the benzene ring, the base B perform deprotonation in the reaction mixture removes a proton and make the molecule stable by restored its aromaticity.

Mechanism of (b)