Unsymmetrical Molecules

When a hydrogen halide adds to an unsymmetrical alkene, such as propene, the H can join either carbon of the C=C, so two different products are possible:CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major) or CH₃CH₂CH₂Br (minor)2-bromopropane is the major product and 1-bromopropane is the minor product. The reason lies in the carbocation formed in step 1.If the H adds to the end carbon (CH₂), the positive charge ends up on the middle carbon: a secondary carbocation, CH₃C⁺HCH₃. If the H adds to the middle carbon, the charge is on the end carbon: a primary carbocation, CH₃CH₂CH₂⁺.Alkyl groups are electron releasing (they have a positive inductive effect). They push electron density towards the positive carbon, spreading out (stabilising) its charge. The more alkyl groups attached to the C⁺, the more stable the carbocation:tertiary > secondary > primaryThe more stable carbocation forms more readily, so the product made from it is the major product. This is summarised by Markovnikov's rule: the H atom adds to the carbon that already has more H atoms.Example: 2-methylpropene, (CH₃)₂C=CH₂, with HBr can form a tertiary carbocation, so the major product is 2-bromo-2-methylpropane.The same idea applies to hydration: propene and steam give mainly propan-2-ol, with some propan-1-ol.