Electrophilic Addition Mechanism

Alkenes react with electrophiles by electrophilic addition. The mechanism is shown with curly arrows: each curly arrow shows the movement of a pair of electrons. It starts at a bond or a lone pair and points to where the electrons go.The mechanism for ethene and bromine happens in two steps.Step 1: as Br₂ approaches the C=C bond, the π electrons repel the electrons in the Br-Br bond, inducing a dipole. The bromine nearer the double bond becomes slightly positive (δ+). A curly arrow goes from the C=C bond to the δ+ bromine, and a second curly arrow goes from the Br-Br bond onto the δ− bromine. The Br-Br bond breaks by heterolytic fission: both electrons go to one bromine atom, forming a bromide ion, Br⁻. The other bromine is now bonded to one carbon, and the other carbon has lost a share of electrons and carries a positive charge. This ion is a carbocation intermediate.Step 2: the bromide ion uses a lone pair to attack the positive carbon (a curly arrow from the lone pair on Br⁻ to C⁺). A new C-Br bond forms and the product is 1,2-dibromoethane.Evidence for the carbocation: if ethene is bubbled into bromine water containing sodium chloride, 1-bromo-2-chloroethane forms as well as 1,2-dibromoethane. The chloride ions must have attacked a positive intermediate, which shows that the two bromine atoms do not add at the same time.