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

Step 1: Understanding the Concept:
Markovnikov's rule predicts that in the addition of an acid like HX to an unsymmetrical alkene, hydrogen goes to the carbon already holding more hydrogens, and the rest of the reagent goes to the more substituted carbon. Reactions that follow a carbocation pathway generally obey this rule.

Step 2: Key Formula or Approach:
The reaction that breaks this rule is the one whose mechanism does not go through a carbocation. The addition of HBr in the presence of peroxide switches the mechanism to a free radical chain reaction instead, which is why it gives the opposite orientation, and this is called the peroxide effect.

Step 3: Detailed Explanation:
Addition of HI and addition of HCl both proceed through a carbocation forming on the more substituted carbon, since that carbocation is more stable, and the halide then attaches there, matching Markovnikov's rule.
Addition of H2O in the presence of acid also proceeds through a carbocation, so the hydroxyl group ends up on the more substituted carbon, again matching the rule.
Addition of HBr with peroxide present generates a bromine radical first, and this radical adds to the carbon that forms the more stable radical, which turns out to be the carbon with fewer substituents. The hydrogen atom then adds to the more substituted carbon, giving the reverse of the usual Markovnikov placement.

Step 4: Final Answer:
Addition of HBr in the presence of peroxide is the reaction that goes against Markovnikov's rule, due to its radical mechanism.