What NEET Asks
- NEET frequently tests the regioselectivity of HX addition, especially Markovnikov's rule and its exceptions.
- Questions often involve identifying major products, considering carbocation stability and rearrangements.
- Expect 1-2 questions from Haloalkanes & Haloarenes, carrying 4 marks each, where this concept can be crucial.
Key Points
- Electrophilic Addition: Alkenes react with hydrogen halides (HX = HCl, HBr, HI) via electrophilic addition.
- Markovnikov's Rule: In unsymmetrical alkenes, the negative part of the reagent (X-) adds to the sp2 carbon atom with fewer hydrogen atoms (or more alkyl substituents).
- Carbocation Intermediate: The reaction proceeds through a carbocation intermediate (rate-determining step), whose stability order is 3° > 2° > 1°.
- Carbocation Rearrangement: If a more stable carbocation (e.g., from 2° to 3°) can be formed through a 1,2-hydride or 1,2-alkyl shift, rearrangement will occur, leading to rearranged products.
- Anti-Markovnikov's Rule (Peroxide Effect): Only with HBr, in the presence of peroxides (e.g., ROOR), the addition occurs via a free radical mechanism and follows anti-Markovnikov's rule.
- Reactivity Order: The reactivity of hydrogen halides follows HI > HBr > HCl, due to decreasing bond strength.
Must-Know Formula / Reaction
- Markovnikov's Addition:
R-CH=CH2 + HX → R-CH(X)-CH3(X adds to carbon with fewer H) - Anti-Markovnikov's Addition:
R-CH=CH2 + HBr --(peroxide)--> R-CH2-CH2-Br(Br adds to carbon with more H)
Common Mistakes
- Students often forget to check for carbocation rearrangements, leading to incorrect major products.
- Don't confuse Markovnikov's rule with anti-Markovnikov's rule; the peroxide effect is specific to HBr.
- Ignoring the relative reactivity order of HX (HI > HBr > HCl) can lead to errors in predicting reaction rates.
Rapid Revision
Master HX addition: Markovnikov's rule is the default, X to fewer H, via carbocation. Always look for carbocation rearrangements (1,2-shifts) for major products. Remember, only HBr with peroxides follows anti-Markovnikov's rule (free radical mechanism).