What are the products expected from the reaction of 1-cyclohexylethene with HBr, and supply the plausible mechanism?
The correct answer and explanation is:
Correct Answer:
The reaction of 1-cyclohexylethene with HBr gives 1-bromo-1-cyclohexylethane as the major product, following Markovnikov’s rule.
Explanation (Plausible Mechanism and Details):
When 1-cyclohexylethene reacts with hydrogen bromide (HBr), the addition reaction proceeds through an electrophilic addition mechanism. This type of reaction is common for alkenes reacting with hydrogen halides.
1-cyclohexylethene is an alkene with the double bond between the carbon connected to the cyclohexyl ring and a terminal methyl group. The first step in the reaction involves protonation of the double bond.
Step 1: Protonation of the alkene
The π electrons from the double bond act as a nucleophile and attack the proton (H⁺) from HBr. According to Markovnikov’s rule, the proton will attach to the carbon that already has more hydrogen atoms. This results in the formation of the more stable carbocation on the carbon adjacent to the cyclohexyl group.
In this case, the carbocation forms on the carbon bonded to the cyclohexyl ring because this site can stabilize the positive charge better due to hyperconjugation and slight inductive effects from the ring.
Step 2: Nucleophilic attack by Br⁻
The bromide ion (Br⁻), generated after the proton transfer, quickly attacks the carbocation to form the final product.
The final product is 1-bromo-1-cyclohexylethane, where the Br atom is added to the more substituted carbon.
This reaction proceeds without rearrangement because the initially formed carbocation is already stable. There are no neighboring hydrides or alkyl groups to shift for further stabilization.
Summary:
- The reaction follows Markovnikov addition.
- The intermediate carbocation forms on the more stable carbon.
- Br⁻ attacks the carbocation to give the major product: 1-bromo-1-cyclohexylethane.