When magnesium is heated in fluorine or chlorine or in the vapour of bromine or iodine there is a violent reaction, and the corresponding halide compounds are formed.
The most important of the halide salts is the chloride which, in the hydrated form, has the formula MgC1 2.6H 2 O.
It combines with the halide derivatives of boron and silicon to form, e.g.
Meyer (Ber., 18 94, 2 7, p. 510 et seq.) showed that in benzenoid compounds ortho-substituents exert a great hindering effect on the esterification of alcohols by acids in the presence of hydrochloric acid, this hindering being particularly marked when two substituents are present in the ortho positions to the carboxyl group. In such a case the ester is best prepared by the action of an alkyl halide on the silver salt of the acid, and when once prepared, can only be hydrolysed with great difficulty.
A Klages (Ber., 1902, 35, pp. 2633 et seq.) has shown that if one uses an excess of magnesium and of an alkyl halide with a ketone, an ethylene derivative is formed.
The products formed by the action of the Grignard reagent with the various types of organic compounds are usually thrown out of solution in the form of crystalline precipitates or as thick oils, and are then decomposed by ice-cold dilute sulphuric or acetic acids, the magnesium being removed as a basic halide salt.
The majority of these halide compounds are decomposed by water, with the formation of basic salts.
The atomic weight of silicon has been determined usually by analysis of the halide compounds or by conversion of the halides into silica.