Halonium ions, structure

The C2H4Br ions produced in this way are relatively stable and have been shown by nmr to have the cyclic halonium ion structure. 

Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. The following reaction illustrates the preference for ortho and para substitution in the nitration of chlorobenzene. 

The regioselectivity of addition is established when water attacks one of the car bons of the halonium ion In the following example the structure of the product tells us that water attacks the more highly substituted carbon... 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

The transition-state structures for fluorination, chlorination and bromination were obtained by ab initio MO calculation82. Chlorination and bromination were found to proceed via three-centred geometries (cyclic halonium ions) leading to awfi-addition. In contrast, fluorination involves a four-centred transition state which is consistent with the observed yyw-stereoselectivity82. 

Heteroaromatic Halophenium Ions. Halophenium ions are a class of halonium ions analogous to thiophene, furan, and pyrrole. To date, no parent halophenium ions 189 are known, but many stabilized tetraphenyl iodophenium, benzoiodophenium, and dibenzoiodophenium ions have been prepared by Beringer and co-workers.419,420 Some of them have been analyzed by X-ray crystal structure investigations. Representative examples are ions 190-192. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

The Markovnikov orientation observed in halohydrin formation is explained by the structure of the halonium ion intermediate. The two carbon atoms bonded to the halogen have partial positive charges, with a larger charge (and a weaker bond to the halogen) on the more substituted carbon atom (Figure 8-5). The nucleophile (water) attacks this more substituted, more electrophilic carbon atom. The result is both anti stereochemistry and Markovnikov orientation. 

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