Read Which Mechanism Is Running Before Predicting Regiochemistry
Four addition methods put -OH or -X on opposite carbons of the same alkene depending on mechanism — knowing which method goes via a carbocation (Markovnikov) versus a radical or concerted pathway (anti-Markovnikov) resolves almost every regiochemistry question here.
Markovnikov and Anti-Markovnikov Addition to Alkenes is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Simple HX/H2O addition follows Markovnikov's rule
Plain HX or H3O+/H2O addition to an unsymmetrical alkene puts H on the carbon with MORE hydrogens already (and X/OH on the more substituted carbon), because the reaction goes through the more stable carbocation — propene + HI gives 2-iodopropane (secondary cation), not 1-iodopropane.
Peroxide-mediated HBr addition reverses the regiochemistry — but only for HBr
HBr specifically, in the presence of peroxides, adds via a free-radical mechanism instead of the ionic one, giving the ANTI-Markovnikov product (n-propyl bromide from propene, not isopropyl bromide) — this peroxide effect is unique to HBr; HCl and HI do NOT show a peroxide effect because their radical propagation steps are thermodynamically unfavourable for those halogens.
Hydroboration-oxidation gives the anti-Markovnikov alcohol with syn stereochemistry
BH3 adds boron to the LESS substituted carbon (anti-Markovnikov placement) via a concerted, non-carbocation mechanism, and H2O2/OH- then replaces B with OH with retention of configuration — this combination is the standard route to the anti-Markovnikov alcohol, and for a cyclic alkene it also delivers syn addition (both new groups add to the same face).
Oxymercuration-demercuration gives the Markovnikov alcohol without rearrangement
Hg(OAc)2/H2O followed by NaBH4 places OH on the MORE substituted carbon (Markovnikov, like simple acid-catalysed hydration) but goes through a bridged mercurinium ion rather than a free carbocation — this avoids the carbocation rearrangements that can complicate simple acid-catalysed hydration, making it the preferred lab method for a clean Markovnikov alcohol.
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- ✓Simple HX and acid-catalysed H2O addition follow Markovnikov's rule (via the more stable carbocation).
- ✓HBr + peroxide is the ONE case that reverses to anti-Markovnikov (free-radical mechanism) — HCl and HI don't show this effect.
- ✓Hydroboration-oxidation gives the anti-Markovnikov alcohol with syn stereochemistry, via a concerted (non-carbocation) mechanism.
- ✓Oxymercuration-demercuration gives the Markovnikov alcohol without carbocation rearrangement, via a bridged mercurinium ion.
Don't apply the peroxide (anti-Markovnikov) effect to HCl or HI — it is specific to HBr, because only HBr's radical chain propagation steps are both thermodynamically favourable enough to proceed.
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