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.

Answer 2

Markovnikov's rule states that when an unsymmetrical alkene reacts with a reagent like HX, the hydrogen atom attaches to the carbon that already carries more hydrogen atoms, while the other part attaches to the more substituted carbon. Most additions to alkenes follow this pattern, but one of the four reactions listed here goes against it. Checking the mechanism of each reaction identifies which one breaks the rule.

1. Addition of HI in an alkene: This addition proceeds through a carbocation intermediate, an ionic or electrophilic mechanism, and the proton adds to the carbon with more hydrogens while iodide adds to the more substituted carbon, exactly as Markovnikov's rule predicts.
2. Addition of HCl in an alkene: This also proceeds through the same ionic mechanism as HI addition, forming the more stable carbocation first, so chlorine ends up on the more substituted carbon and the reaction again follows Markovnikov's rule.
3. Addition of HBr in the presence of peroxide in an alkene: Peroxides change the mechanism completely by generating a bromine free radical instead of a carbocation. This radical adds first to the carbon that gives the more stable radical intermediate, which reverses the usual positioning, and bromine ends up on the carbon with more hydrogens instead of the more substituted one. This flips the normal outcome and is known as the peroxide effect or Kharasch effect.
4. Addition of H2O in the presence of acid: Acid catalyzed hydration proceeds through a carbocation intermediate just like HI and HCl addition, so the hydroxyl group ends up on the more substituted carbon, again following Markovnikov's rule.

Three of the four reactions proceed through a carbocation pathway and follow Markovnikov's rule, while the peroxide catalyzed addition of HBr switches to a radical pathway that gives the opposite regiochemistry.

So the correct answer is Addition of HBr in the presence of peroxide in an alkene.