What NEET Asks
- NEET frequently tests mechanisms of alkene hydration methods (acid-catalyzed, HBO, OMD).
- Questions often involve predicting major products based on regioselectivity (Markovnikov/anti-Markovnikov) and stereoselectivity (syn/anti).
- Expect rearrangement questions for acid-catalyzed hydration, which carry moderate weightage.
Key Points
- Acid-catalyzed hydration (H₂O/H⁺): Follows Markovnikov's rule, involves carbocation intermediate, prone to rearrangements (hydride/alkyl shifts). Forms alcohols.
- Hydroboration-oxidation (HBO) ((BH₃)₂/THF, then H₂O₂/OH⁻): Anti-Markovnikov addition, syn addition of H and OH. No carbocation formation, thus no rearrangements.
- Oxymercuration-demercuration (OMD) (Hg(OAc)₂/H₂O, then NaBH₄/OH⁻): Markovnikov's addition, anti addition of H and OH (net effect). Proceeds via a mercurinium ion, no carbocation rearrangements.
- Regioselectivity: Markovnikov's rule adds H to the carbon with more hydrogens and OH to the carbon with fewer hydrogens.
- Anti-Markovnikov's rule adds H to the carbon with fewer hydrogens and OH to the carbon with more hydrogens.
Must-Know Formula / Reaction
1. Acid-Catalyzed Hydration: R-CH=CH₂ + H₂O ( \xrightarrow{\text{H}^+} ) R-CH(OH)-CH₃ (Markovnikov's Product) Mechanism: Protonation of alkene → carbocation formation (rearrangements possible) → water attack → deprotonation.
2. Hydroboration-Oxidation (HBO): R-CH=CH₂ ( \xrightarrow{\text{1. (BH₃)₂/THF}}{\text{2. H₂O₂/OH⁻}} ) R-CH₂-CH₂OH (Anti-Markovnikov's Product) Mechanism: Concerted syn addition of B-H, followed by oxidation of C-B bond with retention of configuration.
3. Oxymercuration-Demercuration (OMD): R-CH=CH₂ ( \xrightarrow{\text{1. Hg(OAc)₂/H₂O}}{\text{2. NaBH₄/OH⁻}} ) R-CH(OH)-CH₃ (Markovnikov's Product) Mechanism: Formation of a cyclic mercurinium ion, nucleophilic attack by water, followed by reductive demercuration.
Common Mistakes
- Students often forget to check for carbocation rearrangements in acid-catalyzed hydration, leading to incorrect products.
- Don't confuse the regioselectivity: HBO is anti-Markovnikov, while H₂O/H⁺ and OMD are Markovnikov.
- Ignoring stereochemistry: HBO is syn addition, OMD is anti addition (though often not explicitly drawn in NEET options).
Rapid Revision
Master alkene hydration by remembering: H₂O/H⁺ (Markovnikov, rearrangement, carbocation), HBO (Anti-Markovnikov, syn, no rearrangement), OMD (Markovnikov, anti, no rearrangement). Quick recall of these three methods' characteristics is crucial.