25 августа, 2015 
Pokraskin The hydroxyl group can be introduced into a molecule in various ways.
(1) Fusion of a sulfonic acid with sodium hydroxide. This method is often referred to as “potash fusion,” a term applied when potassium hydroxide was used almost exclusively. The cheaper sodium hydroxide is now almost always used. Dilute or concentrated hydroxide is employed, depending on the conditions. In using dilute sodium hydroxide (30 to 60 per cent), it is necessary to work in an autoclave if the reaction temperature is higher than the boiling point. The pressure makes no direct contribution to the reaction, but is necessary, unfortunately, to maintain the reaction temperature. Some variations of the procedure follow, (a) Alkali fusion in the presence of an oxidizing agent, whereby, in addition to the replacement of the sulfonic acid group by hydroxyl, a second hydroxyl group is introduced (see alizarin, page 314). (b) Alkali fusion in the presence of an alkaline-earth hydroxide, which precipitates the sulfite formed in the reaction as the insoluble alkaline-earth sulfite, and thus prevents reduction of. the final product (anthraquinone series, see 2- aminoanthraquinone, page 229).
(2) Replacement of a labile halogen by the hydroxyl group, e. g., dinitrophenol from dinitrochlorobenzene, and many other similar preparations (see sulfur black T, page 337).
(3) Heating a diazonium salt, sometimes in the presence of copper salts (hydro — quinone from p-aminophenol, or guaiacol from o-anisidine).
(4) Treatment of an amine with bisulfite, and hydrolysis of the intermediate compound (Bucherer’s method, page 182).
(5) Heating as amine with acid or alkali under pressure.
(6) Frequently, by treatment with weak alkali, an ortho sulfo or nitro group, or a chlorine atom, in a diazonium salt is smoothly replaced by the hydroxyl group.
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