Halogenation and Dehalogenation

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

STEVEN E. ROKITA, PhD, is Professor in the Department of Chemistry and Biochemistry at the University of Maryland. His research interests lie in sequence and conformation specihc reachons of nucleic acids, enzyme-mediated activation of substrates and coenzymes, halogenation and dehalogenation reactions in biology, and aromatic substitution and quinone methide generation in bioorganic chemistry. 

If 5-benzyl-3,7-diaryl-4,6-dihydro-l,2,5-triazepines (2Q2) are treated with bromine or iV-bromosuccinimide, a process involving consecutive halogenation and dehalogenation has been proposed. The reaction sequence (Scheme 116) has been postulated to pass through bicyclic intermediates (80AHC(27)241>. 

Halogenation and Dehalogenation Reactions. Usually, reactions of this type take place readily without utilizing catalysts. However, when selectivity of the desired product is low or it is necessary to run the reaetion at a lower temperature, the use of a catalyst is desirable. Supported copper and silver halides can be used for the halogenation of hydrocarbons. Hydrochlorination reactions can be carried out with mercury copper or zinc halides. 

Halogenation and dehalogenation [36], Friedel-Crafts-type alkylation [37], 0-and iV-dealkylation [38], carboxylation [39], and decarboxylation [40], isomerization [41], acyloin [42], and aldol reactions [43]. Even Michael... 

Although halogenation and dehalogenation reactions can be catalyzed by enzymes, it is doubtful whether these reactions will be used widely, mainly because the corresponding conventional chemical methods are highly competitive. 

Halogenation and dehalogenation of aromatic compounds are summarized in Scheme 12. 

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