Predict which of the following alkenes reacts the fastest with HCl -[Free] B106
Predict which of the following alkenes reacts the fastest with HCl . ACH3CH2CH2CH2CH=CH2 B-CH3CH2CH=CHCH2CH3 C-CH3CH2CH=CHCH2CH3 D(CH3)2C=CHCH2CH3
![Predict which of the following alkenes reacts the fastest with HCl -[Free] B106](data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20640%20208'%3E%3C/svg%3E)
Question:
Predict which of the following alkenes reacts the fastest with HCl:
- A. CH₃CH₂CH₂CH₂CH=CH₂
- B. CH₃CH₂CH=CHCH₂CH₃
- C. CH₃CH₂CH=CHCH₂CH₃
- D. (CH₃)₂C=CHCH₂CH₃
Answer:
🔍 Understanding the Mechanism
The addition of HCl to alkenes follows Markovnikov’s Rule, where the proton (H⁺) adds to the carbon of the double bond with the greater number of hydrogen atoms. The rate-determining step is the formation of a carbocation. The more stable the carbocation, the faster the reaction proceeds.
🧪 Analyzing the Options
Option A: CH₃CH₂CH₂CH₂CH=CH₂ (1-hexene)
- Terminal alkene.
- Forms a primary carbocation upon H⁺ addition.
- Primary carbocations are relatively unstable → slow reaction.
Option B and C: CH₃CH₂CH=CHCH₂CH₃ (2-hexene)
- Internal alkenes.
- Protonation leads to formation of a secondary carbocation.
- More stable than primary → moderate reactivity.
Option D: (CH₃)₂C=CHCH₂CH₃
- Double bond is adjacent to a carbon bonded to two methyl groups.
- Protonation leads to a tertiary carbocation.
- Tertiary carbocations are highly stabilized by hyperconjugation and inductive effects → fastest reaction.
✅ Final Conclusion
The fastest reacting alkene with HCl is:
D. (CH₃)₂C=CHCH₂CH₃
This is because it forms a tertiary carbocation intermediate, which is the most stable among the options, leading to the fastest reaction rate.