chloro-2,3-dimethylpentane and minor: 4-chloro-2,2-dimethylpentane. I am not sure how the answer was arrived.
What are the major and minor products of 4,4-dimethyl-2-pentene with HCl? The answer given is major: 2-chloro-2,3-dimethylpentane and minor: 4-chloro-2,2-dimethylpentane. I am not sure how the answer was arrived.
The Correct Answer and Explanation is:
To determine the major and minor products of the reaction of 4,4-dimethyl-2-pentene with HCl, we need to understand how electrophilic addition of HX works, particularly Markovnikov’s rule and carbocation stability.
Step-by-step Mechanism:
1. Structure of 4,4-dimethyl-2-pentene:
This molecule is a six-carbon chain with a double bond between C-2 and C-3, and two methyl groups attached to C-4.
The structure looks like this:
CH₃–C(CH₃)₂–CH=CH–CH₃
2. Reaction with HCl:
HCl adds across the double bond. The π electrons from the double bond will attack the proton (H⁺) from HCl, forming a carbocation intermediate. The most stable carbocation determines the major product.
3. Carbocation formation:
Two possible carbocations can form depending on which carbon receives the proton:
- Proton adds to C-3, generating a carbocation at C-2:
This is a tertiary carbocation due to the adjacent carbon being bonded to three alkyl groups. This is the more stable intermediate. - Proton adds to C-2, generating a carbocation at C-3:
This is a secondary carbocation, less stable than the tertiary one.
Because the tertiary carbocation is more stable, it forms preferentially. According to Markovnikov’s rule, the H attaches to the carbon with more hydrogens (C-3), and Cl⁻ attaches to the more substituted carbon (C-2).
4. Nucleophilic attack:
- Cl⁻ attacks the tertiary carbocation at C-2 to form 2-chloro-2,3-dimethylpentane (major product).
- A small amount of the secondary carbocation also forms, leading to the minor product 4-chloro-2,2-dimethylpentane via Cl⁻ attacking the secondary carbocation.
Conclusion:
- Major product: 2-chloro-2,3-dimethylpentane
- Minor product: 4-chloro-2,2-dimethylpentane
The preference is driven by carbocation stability and follows Markovnikov’s rule.
