Electronegative halogens

Mixed halide additions are most satisfactorily interpreted as proceeding via a halonium complex (1), in which the less electronegative halogen approaches from the less hindered a-side. The energetic preference for a pm-planar transition state leading to the diaxial product (2) is sufliciently great that the... 

Br—Cl, I—Cl, etc., attack occurring through the less electronegative halogen as this will constitute the electrophilic end of the molecule. The two species above are thus found to effect bromination and iodination, respectively. t- s+... 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Cornforth et al.54 introduced a variant of the Cram model to be used when one of the substituents is highly electronegative (halogens, etc.). The electronegative group X is oriented anti to the oxygen, to minimize dipolar interactions. In other words, X plays the role of the bulky substituent. 

A carbanion can act as a leaving group if it is stabilized somehow. We have already seen several examples of this. For example, CX3 , a methyl carbanion stabilized by the inductive effect of three electronegative halogen atoms, leaves in the haloform reaction (see Section 20.2). 

Mercury bis[pentafluorotelluro(trifluoromethyl)amide] reacted with elemental halogens and interhalogens to yield ha]o(trifluoromethyl)amino tellurium pentafluoride2,3. The less electronegative halogens of the interhalogens join the amide-nitrogen atom. 

Unlike a normal carbocation, all the atoms in a halonium ion have filled octets. The three-membered ring has considerable ring strain, however, which, combined with a positive charge on an electronegative halogen atom, makes the halonium ion strongly electrophilic. Attack by a nucleophile, such as a halide ion, opens the halonium ion to give a stable product. 

The overall reaction is a reduction because it replaces the electronegative halogen atom with a hydrogen atom. In particular, this reaction provides a way to label a compound with deuterium (D or 2H, a heavy isotope of hydrogen) at any position where a halogen is present. 

In this reaction, the less electronegative halogen is found in the corresponding hypohalous acid where it has a +1 oxidation state. 

The tendency of the hydrogen atom to form the negative ion is much lower than for the more electronegative halogen elements. This may be seen by comparing the energetics of the formation-reactions ... 

In earlier literature there are reports of the formation of perhalides on treatment of phenothiazine with bromine and iodine these are now considered to be charge-transfer complexes. The phenothiazine component normally acts as a donor and the electronegative halogens as acceptors, but here total transfer of electrons occurs and the phenothiazine is oxidized. Phenothiazine and 3,7-dimethylpheno-thiazine behave similarly in the presence of strong organic acceptors, like dicyanobenzoquinone and dichlorodicyanobenzoquinone. ... 

A solid polyhalide salt with the structure M+ tends to dissociate into a simple monohalide derivative of the cation M+ with the most electronegative halogen, and a neutral halogen or interhalogen. 

---