Reactions of the Free Halogens

The known oxo acids of the halogens are listed in Table 13-4. The chemistry of these acids and their salts is very complicated. Solutions of all the acids and of several of the anions can be obtained by reaction of the free halogens with water or aqueous bases. We discuss these reactions first the term halogen refers to chlorine, bromine, and iodine, only. 

Reactions of the Free Halogens 24-6 The Hydrogen Halides and Hydrohalic Acids... 

Write equations describing general reactions of the free halogens, X2, with (a) Group lA (alkali) metals, (b) Group IIA (alkaline earth) metals, and (c) Group IIIA metals. Represent the metals as M. 

SECONDARY REACTIONS. The reactions of the free radicals include (1) abstractions (of H atoms, with preference for tertiary H, and of halogen atoms), (2) addition to double bonds, which are very efficient scavengers for radicals, (3) decompositions to give both small molecule products, such as CO2, and (4) chain scission and crosslinking of molecules. 

Consideration of the halogenation of quinoline and isoquinoline is complicated by uncertainties relating to the mechanisms of reaction of the free base this applies particularly to quinoline. 

All the compounds of the type XX are reactive materials that undergo some reactions that are similar to those of the free halogens. For example, they are all oxidizing agents and usually add to double bonds in organic molecules. They also react with water ... 

Nitrate in the presence of bromide and iodide The brown ring test for nitrates cannot be applied in the presence of bromides and iodides since the liberation of the free halogen with concentrated sulphuric acid will obscure the brown ring due to the nitrate. The solution is therefore boiled with sodium hydroxide solution until ammonium salts, if present, are completely decomposed powdered Devarda s alloy or aluminium powder (or wire) is then added and the solution gently warmed. The evolution of ammonia, detected by its smell and its action upon mercury(I) nitrate paper (see Section III.38, reaction 1) and upon red litmus paper, indicates the presence of a nitrate. 

Iodine can be introduced into aromatic compounds by the action of the free halogen, if the reaction is carried out under conditions which bring about the removal of the hydriodic acid formed as the result of the substitution. For example, iodo-benzene is formed when benzene is heated with iodine and nitric acid, mercuric oxide, iodic acid, or other substances which react with hydriodic acid. Reactions of this kind are used in a limited number of cases only as a means of preparing iodo derivatives. Free iodine also converts aniline, C6H5.NH2, into substitution-products. In this case the hydriodic acid formed is removed, as the result of the formation of a salt of aniline, C6H5NH2.HI. 

As iodine substitution-products are not formed readily by the action of the free halogen on hydrocarbons, iodo-derivatives are usually prepared from amines by means of the diazo reaction. 

R = ethyl, R = cyclohexyl) decompose at 80°C to either SnS (nitrogen atmosphere) or SnO (air atmosphere). SnS is obtained from the thermal decomposition of bis(N,N-diethyl-dithiocarbamato)tin(II) [133] and bis(N,N-diethyl-dithiocarbamato) dihalotin (IV) complexes in inert atmosphere [134]. The solid reaction intermediate obtained during the thermal decomposition of the second coordination compound is dihalotin (IV) sulfide which subsequently decomposes to SnS with liberation of the free halogen [134]. 

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

A second mole of halogen adds with greater difficulty oxidative side reactions can be minimised by halogenating an ester instead of the free alcohol (26). 

In aqueous solution, OF2 oxidizes HCl, HBr, and HI (and thek salts), Hberating the free halogens. Oxygen difluoride reacts slowly with water and a dilute aqueous base to form oxygen and fluorine. The rate of this hydrolysis reaction has been determined (23). 

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