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.
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.
Frequently Asked Questions
What are the main methods for hydrating alkenes to form alcohols?▾
The three primary methods are acid-catalyzed hydration (using H₂O/H⁺), hydroboration-oxidation (HBO), and oxymercuration-demercuration (OMD). Each method yields alcohols but differs in regioselectivity, stereoselectivity, and mechanism.
How does Markovnikov's rule apply to alkene hydration?▾
Markovnikov's rule states that in the addition of an unsymmetrical reagent (like H-OH) to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that already has more hydrogen atoms. The hydroxyl group (OH) then adds to the more substituted carbon atom. This rule is followed in acid-catalyzed hydration and oxymercuration-demercuration.
What is the key difference between Hydroboration-oxidation and Oxymercuration-demercuration?▾
Hydroboration-oxidation (HBO) results in anti-Markovnikov addition of water, meaning the OH group adds to the less substituted carbon of the alkene, and the addition is syn. Oxymercuration-demercuration (OMD), on the other hand, follows Markovnikov's rule, adding the OH group to the more substituted carbon, and the net addition is anti.
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