Question

Which is considered an exception to Markovnikov's rule \emph{EXCEPT:}

• A. Addition of HI in an alkene
• B. Addition of HCl in an alkene
• C. Addition of HBr in the presence of peroxide in an alkene
• D. Addition of H$_2$O in the presence of acid

Hint:

Remember: In the presence of peroxides, HBr follows anti-Markovnikov's addition via a free radical mechanism — a common GPAT favorite question!

Answer 1

The Correct Option is C

- Markovnikov’s Rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen (H) attaches to the carbon with more hydrogen atoms, and the halide (X) attaches to the carbon with fewer hydrogen atoms.
- This rule applies to: - (A) Addition of HI
- (B) Addition of HCl
- (D) Addition of H$_2$O in acid (hydration follows Markovnikov's rule)
- Exception: - (C) \underline{Addition of HBr in the presence of peroxide} is an exception due to the peroxide effect or Kharasch effect.
- This reaction proceeds via a free radical mechanism, leading to anti-Markovnikov addition where Br adds to the carbon with more hydrogen atoms.
- Hence, all options follow Markovnikov's rule \emph{except} option (C), which is the correct answer in this case since the question asks for the one that is NOT an exception.