14.2.5—Inductive effects of alkyl groups

Alkyl groups have a positive inductive effect: they push electron density toward a positively charged carbon. More substituted carbocations are therefore generally more stable: tertiary > secondary > primary.

When HX adds to an unsymmetrical alkene, the pathway that forms the more stable carbocation is favoured. The hydrogen adds so that the halide ends up on the more substituted carbon: the Markovnikov product.

Propene + HBr gives mainly 2-bromopropane, because proton addition can form a secondary rather than primary carbocation.

This is a product-prediction rule, not a claim that the final product contains a free carbocation or that every addition gives one product only.