Asymmetrical Alkenes

An asymmetrical (unsymmetrical) alkene has different groups on the two carbons of the C=C, such as propene, CH₃CH=CH₂. When it reacts with HBr, the H can add to either carbon, so two products are possible: a major product and a minor product.The product depends on which carbocation forms in step 1:• H adds to carbon 1: the positive charge is on carbon 2, which is bonded to two other carbons. This is a secondary carbocation, CH₃CH⁺CH₃. Br^– then gives 2-bromopropane.
• H adds to carbon 2: the positive charge is on carbon 1, bonded to only one other carbon. This is a primary carbocation, CH₃CH₂CH₂⁺, which gives 1-bromopropane.Carbocation stability: tertiary > secondary > primary. Alkyl groups are electron releasing (they have a positive inductive effect). They push electron density towards the positive carbon, which spreads out the charge and stabilises the ion. The more alkyl groups on the positive carbon, the more stable it is.The more stable carbocation forms more easily and faster, so it leads to the major product. For propene and HBr the major product is 2-bromopropane and the minor product is 1-bromopropane.Worked example: 2-methylpropene, (CH₃)₂C=CH₂, with HBr. H adding to the CH₂ carbon gives (CH₃)₃C⁺, a tertiary carbocation, so the major product is (CH₃)₃CBr, 2-bromo-2-methylpropane.