Question

Treating 1 -methyl-1-vinylcyclopentane with \(\mathrm{HCl}\) gives mainly 1 -chloro-1,2-dimethylcyclohexane. CC1CCCCC1(C)Cl 1-Methyl-1-vinyl- cyclopentane 1-Chloro-1, 2-dimethyl- cyclohexane (a racemic mixture) Propose a mechanism for the formation of this product.

Short answer

Question: Propose a mechanism for the formation of 1-chloro-1,2-dimethylcyclohexane from 1-methyl-1-vinylcyclopentane and HCl. Answer: The mechanism involves an electrophilic addition of HCl to 1-methyl-1-vinylcyclopentane, forming a carbocation intermediate, followed by ring expansion to a more stable cyclohexane ring, and finally nucleophilic attack by the chloride ion to form 1-chloro-1,2-dimethylcyclohexane.

Step-by-step solution

Identifying the Reactive Species

In this reaction, the reactants are 1-methyl-1-vinylcyclopentane and hydrochloric acid (HCl). They will react to form the 1-chloro-1,2-dimethylcyclohexane. The HCl will act as the electrophile, interacting with the double bond in the vinyl group of the 1-methyl-1-vinylcyclopentane molecule.

Formation of a Carbocation

The electron-rich double bond (vinyl group) in the 1-methyl-1-vinylcyclopentane molecule attacks the electrophilic H atom in the HCl molecule, resulting in the formation of a new bond between the carbon and the hydrogen atom, and leaving a chloride ion (Cl-). This step generates a carbocation intermediate, specifically at the 1-methyl position due to the stabilization provided by the adjacent methyl group.

Ring Expansion

To form the 1-chloro-1,2-dimethylcyclohexane product, the carbocation intermediate must undergo a ring expansion. The breaking of a C-C bond adjacent to the carbocation generates a new carbocation intermediates which is more stabilized and forms a six-membered cyclohexane ring due to hyperconjugation and lesser ring strain (cyclohexane rings are more stable than cyclopentane rings).

Nucleophilic Attack by Chloride Ion

Now that we have the more stable carbocation intermediate with its six-membered ring, the nucleophilic chloride ion (Cl-) that was generated in Step 2 will attack the carbocation. This forms a new C-Cl bond, resulting in the formation of the racemic product mixture, 1-chloro-1,2-dimethylcyclohexane. In summary, the proposed mechanism involves the initial formation of a carbocation intermediate due to the electrophilic addition of HCl, followed by ring expansion and finally the nucleophilic attack of chloride ion to form the final product, 1-chloro-1,2-dimethylcyclohexane.