Halogenation polyhalogenation

Bromochloro-5,5-dimethyl hydantoin halogenation, polyhalogenated derivs. 

Ha.logena.tlon, One review provides detailed discussion of direct and indirect methods for both mono- and polyhalogenation (20). As with nitration, halogenation under acidic conditions favors reaction in the benzenoid ring, whereas reaction at the 3-position takes place in the neutral molecule. Radical reactions in the pyridine ring can be important under more vigorous conditions. 

Heterocychc compounds range from those, such as furan which is readily halogenated and tends to give polyhalogenated products, to pyridine which forms a complex with aluminum chloride that can only be brominated to 50% reaction (23). 

There is also the haloform reaction (effect of a haloform on a ketone in a basic medium), already described with halogen derivatives on p.272 (the danger is more related to the interaction of the polyhalogen derivative with the base, according to the author). A large number of accidents involved the ketone as much as butanone. The accident below illustrates the danger of this reaction ... 

The reductive removal of halogen can be accomplished with lithium or sodium. Tetrahydrofuran containing /-butanol is a useful reaction medium. Good results have also been achieved with polyhalogenated compounds by using sodium in ethanol. 

The applicability of these methods is restricted to polyhalogenated compounds, since the inductive effect of the halogen atoms is necessary for facilitating formation of the carbanion. 

Skalsky HL, Farris MW, Blanke RV, et al. 1979. The role of plasma proteins in the transport and distribution of chlordecone (Kepone ) and other polyhalogenated hydrocarbons. In Nicholson WJ, Moore JA, eds. Annals of the New York Academy of Sciences, Health Effects of Halogenated Aromatic Hydrocarbons International Symposium, New York, NY, USA, June 24-27, 1978. New York, NY New York Academy of Sciences, 320 231-237. 

These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms m their structures. For example. 

Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds. Many of these compounds are useful in industry and agriculture. Some polyhalogen compounds are described in this seetlon. 

With polyhalogen derivatives the amount of the potassium hydroxide must be equivalent to the number of halogens to be replaced by hydrogen. 

The influence of electron density distiibution in the radical-anioa on the rate of carbon-halogen bond cleavage was discussed on page 94. This effect makes possible the selective dehalogenation of polyhalogen aromatic compounds. Examples... 

In contrast, in basic solution halogenation tends to proceed to polyhalogenated products. This is because the inductive effect of a halogen accelerates base-catalyzed... 

Fluorine has been used for the generation of extremely strong electrophilic halogenating agents in electrophilic iodination and bromination of deactivated aromatic substrates in highly acidic reacton media. Polyhalogenation of more activated aromatic substrates is also possible (Fig. 90) [231-233]. 

Treatment of perhalogenated and highly halogenated alkenes with sulfur tetrafluoride in the presence of lead(IV) oxide, which is most likely converted in situ into lead(IV) fluoride, results in addition of fluorine to the C = C bond of the alkenes. The addition proceeds at 100 C to afford per- and polyhalogenated alkanes 5 in yields ranging from a few percent to almost quantitative.190 Representative examples are given. 

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