State why it is initially unexpected for alkenes to undergo electrophilic addition with bromine. Explain why this reaction does indeed occur.
Bromine exists as Br2, and since both bromine atoms are identical - their electronegativities identical, - the electron density is distributed evenly across the molecule (it is non-polar, the atoms have equal share of the bonding electrons). There is no apparent electron deficiency to form an attraction between the bromine molecule and the alkene. Alkenes are unsaturated hydrocarbons, which means they contain a double bond. This double bond is a region of high electron density. As the bromine molecule approaches the double bond, the identically charged electron regions repel each other, inducing a (temporary) dipole in the bromine molecule (now electron density is unevenly distributed). Heterolytic fission occurs within the bromine molecule, the electron deficient bromine atom joins the hydrocarbon and also, a bromide ion is released, to be used in a later stage of the mechanism.
