Halogens Group Hydrochloric acid Hydrogen

Manufacture of many important amino intermediates used for dyes and other purposes is usually by conversion or replacement of a substituent. For example, as already mentioned, in substituted nitro compounds, the nitro groups may be reduced with iron/hydrochloric acid, hydrogen and catalyst, or zinc in aqueous alkali. Partial reductions can be brought about with sodium sulfide. Amino groups may be introduced by replacing halogens in the aromatic ring. Another approach to amination is by attack on a substituted aromatic compound with ammonia or amines. Thus, for example, direct amination of p-chloronitrobenzene (15a) in the presence of a copper catalyst affords p-nitroaniline (15b) in almost quantitative yield l,4-dichloro-2-nitrobenzene (16) is converted in a similar way to 4-chloro-2-nitroaniline (17). Reactions of ammonia with carboxylic acids or anhydrides are carried out on an industrial scale. 

As with other groups, halogens can substitute hydrogen in organic compounds containing additional functional moieties such as carboxylic acids to form acid chlorides, e.g. acetyl chloride CH3COCI. These are reactive acidic compounds liberating hydrochloric acid on contact with water. 

The Group VIIA elements, or halogens, are reactive. Chlorine (CI2), a pale greenish-yeUow gas, is prepared commercially hy the electrolysis of aqueous sodium chloride. Its principal uses are in the preparation of chlorinated hydrocarbons and as a bleaching agent and disinfectant. Hydrogen chloride, HCl, is one of the most important compounds of chlorine aqueous solutions of HCl are known as hydrochloric acid. 

Hydriodic acid and phosphorus (22), like hydrochloric or hydrobromic acid replaces the hydroxyl of tropine by a halogen atom. The reductive elimination of this iodine atom from iodotropane by nascent hydrogen (Zn + HCl) provided the first preparation of the parent substance, tropane (dihydrotropidine) (I, R = CH3). (A more direct preparation of tropane is by the catalytic reduction of tropidine over platinum black (159a)). Tropane reacts readily with hydrogen chloride at an elevated temperature eliminating methyl chloride with the formation of a new saturated base, nortropane (94). The methyl group eliminated from tropane must have been attached to the nitrogen, for nortropane, in contrast to tropane, forms a well defined A-nitroso derivative. The isolation of methyl chloride and the secondary amine, nortropane, confirms the earlier assumption of the... 

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