Elemental halogens

Tertiary phosphines and primary and secondary phosphines can be oxidized by elemental halogen to halophosphine halides. 

Halogenation. Normally, 2-halopropane derivatives are prepared from isopropyl alcohol most economically by reaction with the corresponding acid haUde. However, under appropriate conditions, other reagents, eg, phosphoms haUdes and elemental halogen, also react by replacement of the hydroxyl group to give the haUde (46). 

However, as HOX is consumed, hydrolysis is retarded because of build-up of free amine. Consumption of hypohalous acid through reaction with HX can result in formation of elemental halogen HOX + + X X2 + H2 O (1—3). The tendency for halogen formation is much greater for HOBr and... 

Alkyl side chains in both pyrazines and quinoxalines are susceptible to halogenation by elemental halogens (28JCS1960, 68TL5931) and under radical conditions with NBS (72JOC511). Thus, bromination of 2-methylquinoxaline with bromine in the presence of sodium acetate... 

Additions of elemental halogens to unsaturated compounds are among the most common preparations of halogenated fluoroorganics. The transformations are usually fairly clean and proceed in good yields. Besides the numerous examples of halogen addition tofluoroalkenes and fluoroalkyl-substituted alkenes, additions to perfliioropropyl vinyl ether [2] and fluormated styrenes [7, 4] have been reported. Both ionic and free-radical processes occur (equations 1 and 2)... 

Conjugated, cross-conjugated, and homoconjugated fluoroalkenes react with halogens to yield predominantly 1,4-adducts Results from the reactions of a series of conjugated fluoroalkenes with elemental halogens are summarized in Table 4 In nearly all cases, the fraMs-l,4-addition products are formed exclusively [Il ... 

Like simple elemental halogens, iodine monochloride reacts with conjugated fluorodienes to yield mostly 1,4-addition products. These bidirectional reactions lead to mixtures of regioisomers, as shown in Table 5 [//]. 

Note Such halogenation can be done by addition of elemental halogen to alkenyl or alkynyl groups or by replacement of one or more of the hydrogen atoms attached to alkyl or aryl substituents, using a variety of reagents. 

Cyclic ylide complexes are readily oxidized not only by aggressive reagents like elemental halogens but also by milder oxidants like disulfides, or even mercaptanes in the presence of molecular oxygen, as shown for a reaction with pyridine-2-thiol in air (Equation (34)).182 A similar reaction is known to take place with thiuramdi-dulfides.2,180... 

Halogenation of the B12Hi22- ion with N-halosuccinamide leads to mono- and di-halosubstituted dodecaborates, while reaction with elemental halogens finally led... 

The elemental halogens are not the only source of electrophilic halogen atoms, and, for some synthetic purposes, other positive halogen compound may be preferable... 

Element/Halogen Exchange in Organic Synthesis 3.1 Search for Halogen Equivalents... 

Favorable elements for the element/halogen exchange at alkyl groups are according to our investigations localized in the marked small area of Scheme 14. 

We now turn to reagents which are important in context with the element/halogen exchange Compounds of the type 8 would be interesting reagents for nucleophilic halomethylation but are quite unstable due to carbene formation. Compound 9 would be an attractive synthetic equivalent for the synthons 8. As will be shown... 

Pyrimidines undergo halogenation at position 5 (Eq. 5-17), while guanine reacts at position 8. Adenine is quite unreactive. Elemental halogens or a variety of other halogenating reagents may be used. Of special... 

Let us take as an example the formation of alkali metal halides (solid), MX, from the solid metal, M, and one-half mole of gaseous elemental halogen, X2. Indicating the energy change associated with this reaction by Q one may write... 

Just as atoms shrink when converted to cations by loss of an electron, they expand when converted to anions by gain of an electron. As shown in Figure 6.2 for the group 7A elements (halogens), the expansion is dramatic. Chlorine, for example, nearly doubles in size, from 99 pm for the neutral atom to 184 pm for the chloride anion. 

Reaction with Halogens The alkali metals react rapidly with the group 7 A elements (halogens) to yield colorless, crystalline ionic salts called halides ... 

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