Reactive halogen atoms-hydrogen

A very reactive halogen atom, such as that of mi acyl or sulfonyl halide, is replaced by fluorine by the action of almost any inorganic fluoride. The most convenient method consists in heating gently a mixture of an acyl or sulfonyl chloride with zinc or antimony fluoride in an apparatus which permits the acyl fluoride to distil as it is formed. The acyl fluoride usually boils about 40° lower than the chloride, and its removal from the reaction mixture results in quantitative yields. Com- plete interchange also can be effected with hydrogen fluoride, but more elaborate equipment is required. Good results have been reported for the synthesis of formyl and acetyl fluorides from mixtures of fonnic or... 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

An alkyl halide (also known as a haloalkane) is an alkane in which one or more hydrogen atoms have been replaced with halogen atoms, such as F, Cl, Br, or I. The functional group of alkyl halides is R—X, where X represents a halogen atom. Alkyl halides are similar in structure, polarity, and reactivity to alcohols. To name an alkyl halide, first name the parent hydrocarbon. Then use the prefix fluoro-, chloro-, bromo-, or iodo-, with a position number, to indicate the presence of a fluorine atom, chlorine atom, bromine atom, or iodine atom. The following Sample Problem shows how to name an alkyl halide. 

A halogen directly attached to the 1,2,4-triazine ring is very reactive and can be replaced by most other nucleophiles as outlined in Scheme 9. Tlie reactivity toward neutral nucleophiles decreases from the 5-position to the 3- and 6-positions, while that towards anionic nucleophiles decreases from the 5-position to the 6- and 3-positions. Among nucleophiles which displace halogen atoms are water, alcohols, amines, hydrazine, hydroxyl-amine, thiols and hydrogen sulfide. It is also reported that the bromine in 6-bromo-1,2,4-triazine-3,5-dione can be replaced by fluorine (323) or by a cyano group (324). 

A considerable wealth of dynamical information has been obtained from studies of halogen atom reactions. The majority of these studies concentrate either on the reactions of fluorine atoms which tend to be faster and more exoergic than the other halogen atom reactions or those reactions which involve the production of a hydrogen halide molecule which may then be studied using infrared chemiluminescence methods. From a chemical point of view, this makes a review of this nature incomplete, but it does reflect the scope of the experimental studies conducted so far. Because of experimental difficulties in reagent preparation, the reduced reactivity and the inability of infrared methods... 

Grignard reagents are not enough basic to abstract a vinylic hydrogen, nor do they participate in halogen metal exchange. As shown, they instead substitute the reactive chlorine atom in a-chloroenamines. More recent work has shown that 87 a, R = Me can be obtained from co-chloroacetophenone and tris-dimethylamino) arsine in 86 % yield I61). 

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