Markovnikov's rule explained

The rule

When H–X adds across a C=C, the hydrogen goes to the carbon that already has more hydrogens, and X goes to the more substituted carbon.

Why it works

The first step makes a carbocation. Adding H⁺ to the end carbon of propene gives a secondary carbocation; adding it to the middle carbon would give a primary one. Alkyl groups stabilise a positive charge (by induction and hyperconjugation), so the order of stability is tertiary > secondary > primary. The more stable carbocation forms faster, so its product is the major one.

The same idea with water

Acid-catalysed hydration follows the same rule: propene with steam and an acid catalyst gives propan-2-ol, not propan-1-ol.

Anti-Markovnikov additions

• HBr with peroxides adds by a radical mechanism. The Br radical adds first, to the end carbon, giving 1-bromopropane. (This only works with HBr.)
• Hydroboration–oxidation (1. BH₃·THF, 2. H₂O₂, NaOH) puts the OH on the less substituted carbon: propene → propan-1-ol.

Watch for rearrangements

Because there's a carbocation, a hydride or methyl group can shift to make a more stable cation before X adds. For example, 3-methylbut-1-ene with HBr gives mainly 2-bromo-2-methylbutane, not 2-bromo-3-methylbutane.

Practise choosing the right addition in this synthesis problem.