The attempt was made by Remsen and Palmer (A.G.) to accomplish this by (a) reducing the nitro compound to the amide, (b) making the diazo compound and (c) boiling this with absolute alcohol. According to generally accepted views this should effect the removal of the diazo group and its replacement by hydrogen.
Experiments however showed that the replacement was made not by hydrogen but by the ethoxy group -OC2H5. This method was therefore impracticable.
A modification of this method was suggested by an observation of Baeyer and Liebermann that if phenyl hydrazine be boiled with a dilute solution of copper sulphate the hydrazine group is replaced by hydrogen and benzene thus formed. Hence it was believed that if the hydrazine compound should be made from diazo compound mentioned above, the corresponding hydrocarbon, i.e. toluene o-sulphonic acid could be obtained. The results of experiments showed that this afforded a practicable method of preparing toluene ortho-sulphonic acid.
After experimenting with various modifications of the method the following was found to be the best adapted to the purpose.
The potassium salt of toluene p-nitro-o-sulphonic acid is easily obtained, as already stated, by heating p-nitro toluene on the water bath with three times its weight of fuming H2SO4, neutralizing with chalk and to the solution of calcium salt thus obtained adding a slight excess of K2CO3. On filtering from the precipitated CaCO3 and evaporating slightly, the salt is obtained in long needle shaped crystals of a pale straw yellow color. This is
| ⎧ | CH3 | ||
| C6H3 | ⎨ | SO2OH | (o) |
| ⎩ | NO2 | (p). |
The reduction of the nitro group is best effected by means of tin and HCl, in the proportion, salt 5 parts, tin 6 parts and concentrated HCl 30 parts.
The amido acid forms a compound with tin which crystallizes from the HCl together with stannous chloride. This compound may be broken up and the tin removed by continued boiling with water.
A better method of removing the tin is by dissolving the compound in Na2CO3. This forms a salt with the amido acid and throws down the tin as Sn(OH)2, a white flocculent precipitate. On filtering and adding to the solution conc. HCl, the free amido acid is deposited in characteristic colorless, rhombic crystals, having the formula
| ⎧ | CH3 | |
| C6H3 | ⎨ | SO2OH |
| ⎩ | NH2 |