Halogenation in general

Antimony(lll) fluoride, " antiinony(lll) fluoride activated with pentavalent antimony salts [anlimony(V) fluoride, antimony(V) chloride, antimony(V) bromide, or by the addition of bromine or chlorine to form pentavalent antimony salts in situ], SbFjX, and antimony(V) fluoride substitute active halogens. In general, antimony fluorides can substitute halogens in polyhalogenated compounds when more than one possible site of fluori-natioii is present, the following order of reactivity is observed (where X = Br or C ) ... 

Using A-chlorosuccinimide (NCS), 4,5-disubstituted imidazoles afford 2-chloro substituted imidazoles. However, if other positions are available, chlorination goes preferentially at C-5 first, then C-4, and lastly, C-2. Selective chlorination (or halogenation in general) at the 2-position can be achieved via deprotonation using LDA, at the 2-position, followed by quenching with NCS or NBS (Scheme 65) <2003JME3463>. 

In unsaturated aliphatic systems the most important reactions are those of czirbonyl compounds of the type —COX, in which X is a good leaving group such as halogen. In general, most displacement reactions of anionic nucleophiles on the carbonyl carbon atom of acyl halides involve an addition-elimination mechanism " (e.g. equation 16). In such reactions bond-formation is in advance of bond-rupture... 

Ingram (1962) has discussed many of the methods used for the determination of halogens. In general, the decomposition methods are similar to those used for sulfur, and in many cases sulfur and halogens may be determined on aliquots of a single sample decomposition. The general reactions are as follows (X represents either Cl, Br, or I)... 

Table 19.1 summarizes the six categories of activators and deactivators. Notice the unique position of the halogens. In general, activators are ortho-para directors, while deactivators are meta directors, but halogens are the exception. 

Thus, as can be seen, the halogenation of maleic acid derivatives has contributed a great deal toward understanding the stereochemical and electronic mechanism of halogenation in general. Synthetically, high yields of the products are obtained making these products attractive intermediates for a variety of uses. 

---