Fusion with caustic alkali

The most important reaction of the sulphonic acids is their conversion into phenols by fusion with caustic alkalis. When they are fused with potassium cyanide, nitriles are obtained, e.g. benzonitriie from ben-zenesulphonic acid. 

Once the presence of a sulphonate group has been estabhshed (and, if possible, the phenol isolated), the compound may be characterised by the preparation of a derivative. It must be remembered that both sulphoxides RSOR and sulpJiones RSOjR yield sulphur dioxide on fusion with caustic alkali and acidification. 

Silver apparatus. The chief uses of silver crucibles and dishes in the laboratory are in the evaporation of alkaline solutions and for fusions with caustic alkalis in the latter case, the silver is slightly attacked. Gold vessels (m.p. 1050 °C) are more resistant than silver to fused alkalis. Silver melts at 960 °C, and care should therefore be taken when it is heated over a bare flame. 

Upon fusion with caustic alkali (for experimental details, see Section IV,33,1) and acidification of the aqueous extract, hydrogen sulphide is evolved (detected by lead acetate paper). This test is given by all organic compounds of divalent sulphur (RSH, R SR and R SSR ). 

This is a white powder which fuses at a red heat giving a black glass. It is not affected by aqueous acids and alkalies, nor even by hot nitric acid, but may be hydrolysed by fusion with caustic alkalies. On account of these properties it is represented as a polymer (PNO) of high molecular weight and probably cyclic structure (see below).5... 

Another important technical reaction of the sulfonic acids is hydrolysis by fusion with caustic alkalis. In qualitative work, this is of minor importance. 

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