halonium ion
/ ha-LOH-nee-um /
A halonium ion is the curious bridged intermediate that forms when a halogen molecule like Br2 adds to a double bond. Instead of one carbon grabbing the bromine and leaving a normal carbocation on the other, the bromine straddles both carbons at once, forming a three-membered ring with a positive charge on the bromine. Picture the halogen as a tiny clip clamped over the top of the former double bond, bridging the two carbons.
Why does it bridge rather than form an open cation? Bromine and chlorine have lone pairs, and one of those lone pairs reaches over to the carbon that would otherwise be electron-poor, sharing electrons and closing the ring. This bridge does two important jobs. First, it locks the geometry: the bottom face of the former double bond is now blocked, so any nucleophile must come in from the opposite (anti) face, which is why halogenation and halohydrin formation add anti. Second, it suppresses rearrangement, because there is no fully open carbocation to shift into.
The halonium ion is the key that unlocks the stereochemistry of a whole set of reactions. When Br2 adds to a cyclic alkene, the bromonium ion forces the two bromines onto opposite faces, giving the trans (anti) dibromide; when water or another nucleophile opens the ring, it attacks the more-substituted carbon (which bears more positive character) from the back, setting both the regiochemistry and the anti stereochemistry of halohydrins. It is a small bridge with large consequences.
Adding Br2 to cyclohexene proceeds through a bromonium ion; bromide then attacks from the back face, giving trans-1,2-dibromocyclohexane rather than the cis isomer — direct evidence that a bridged ion, not an open cation, is involved.
The bridging halogen blocks one face, forcing the nucleophile to add anti.
The bridge is most symmetric for bromine and iodine; for chlorine, and when one carbon is much better at bearing positive charge, the ion can be lopsided and behave partly like an open carbocation, eroding the perfectly anti outcome.