Exams › JEE Advanced › Chemistry

On protonation by H+ (Markovnikov addition), which of the following alkenes gives a secondary (2 degree) carbocation as the more stable intermediate?

1. A methyl-substituted cyclohexene (1-methylcyclohexene), on protonation with H+
2. An exocyclic methylene attached to a cyclohexane ring (methylenecyclohexane), on protonation with H+
3. 2-methylpropene (isobutylene), on protonation with H+
4. An allylic amine-containing alkene, on protonation with H+

Correct answer: An exocyclic methylene attached to a cyclohexane ring (methylenecyclohexane), on protonation with H+

Solution

Protonation follows Markovnikov's rule, generating the most stable carbocation. 1-Methylcyclohexene and 2-methylpropene both give tertiary carbocations. The allylic amine case gives a resonance/heteroatom-stabilized cation. Methylenecyclohexane (CH2=ring): protonating the terminal =CH2 carbon places the positive charge on the ring carbon, which is bonded to two other ring carbons -> a secondary carbocation. Hence methylenecyclohexane gives a 2 degree carbocation.

Related JEE Advanced Chemistry questions

• Even though the chlorination of methane releases energy, why is a high temperature necessary for the reaction to occur?
• Acetylene (HC≡CH) undergoes the following two-step transformation: Step 1: Passed over a red-hot iron tube at 873 K (cyclic trimerisation). Step 2: The product of Step 1 is treated with CO, HCl, and AlCl3 (Gattermann-Koch reaction). How many carbon atoms in the final product are sp2 hybridised?
• Which of the following species is non-aromatic (i.e., neither aromatic nor antiaromatic)?
• Among the following compounds, which one produces ONLY oxalic acid (HOOC-COOH) as the major product when treated with O3 followed by H2O2?
• Which of the following reactions proceeds in the forward direction as written?
• What reagent(s) can convert 1,3-dibromopropane (Br-CH2-CH2-CH2-Br) into acetylene (H-C≡C-H)?

⚔️ Practice JEE Advanced Chemistry free + battle 1v1 →