Stereochemistry Of Electrophilic Addition

Electrophilic addition of $B{r}_2$ to an alkene proceeds through a cyclic bromonium ion intermediate rather than an open carbocation, and this intermediate controls the stereochemistry. When $B{r}_2$ approaches cyclohexene, one bromine forms a three-membered bromonium ring bridging both alkene carbons, blocking one face of the molecule. The liberated bromide ion then attacks a carbon from the opposite face, a backside $S_{N}2$-like attack, so the second bromine adds anti to the first. The result is trans-1,2-dibromocyclohexane, with the two bromines on opposite faces. The cis option is wrong because syn addition would require both bromines to approach the same face, which the bromonium mechanism forbids. The option placing both bromines on one carbon is incorrect because addition occurs across the double bond, one bromine per carbon. The claim of non-stereospecific random addition is false; the bromonium ion makes the reaction strictly anti and therefore stereospecific. The bromonium intermediate is favoured over an open carbocation because bromine's lone pairs can bridge the two carbons, distributing the positive charge and avoiding a high-energy localised cation. This bridged geometry is precisely what forces the incoming bromide to approach from the side opposite the bromonium, sealing the anti outcome. This is the NCERT-supported mechanism of halogen addition to alkenes. A consistency check: opening a bridged bromonium ion by backside attack must give anti products, confirming the trans dibromide.