Electrophilic Addition of Hydrogen Halides

Hydrogen halides add to alkenes by the same electrophilic addition mechanism as bromine, with one difference in how the electrophile arises.The H-Br bond is already polar: bromine is more electronegative than hydrogen, so the molecule has a permanent dipole, H^δ+-Br^δ−. The δ+ hydrogen atom is the electrophile. (With Br₂, the dipole has to be induced by the alkene.)Mechanism for ethene + HBrStep 1: a curly arrow goes from the C=C bond to the δ+ hydrogen atom. At the same time a curly arrow goes from the H-Br bond to the bromine atom. The H-Br bond breaks heterolytically, forming a bromide ion, Br⁻. The H atom is now bonded to one carbon, and the other carbon becomes positively charged: the carbocation CH₃CH₂⁺.Step 2: the bromide ion donates a lone pair to the positive carbon (curly arrow from the lone pair to C⁺), forming a C-Br bond. The product is bromoethane, CH₃CH₂Br.The same mechanism applies to HCl and HI. For example, cyclohexene and HCl form a carbocation, which is then attacked by a chloride ion to give chlorocyclohexane.Exam points: show the dipole on H-Br with δ+ and δ−; start the first arrow at the double bond, not at a carbon atom; draw the lone pair on the halide ion and start the last arrow from it; and put the + charge on the correct carbon of the carbocation.