Ortho-nitrophenol, C6H4.OH N02(1-2), crystallizes in yellow needles which melt at 45° C. and boil at 214°C. Para-nitrophenol, C6H4.
OH N02(1.4), crystallizes in long colourless needles which melt at 114°C. Meta-nitrophenol, C6H4.OH N02 (1.3), is prepared from meta-nitraniline by diazotizing the base and boiling the resulting diazonium salt with water.
Torray's observations on nitromalonic aldehyde, N02 CH(CHO)2,formed by acting on mucobromic acid, probably CHO CBr:CBr:000H, with alkaline nitrites; this substance condenses with acetone to give p-nitrophenol, and forms [I.3.5]-trinitrobenzene when its sodium salt is decomposed with an acid.
He has also shown that the nitrophenols yield, in addition to the colourless true nitrophenol ethers, an isomeric series of coloured unstable quinonoid aci-ethers, which have practically the same colour and yield the same absorption spectra as the coloured metallic salts.
Thus para-nitrophenol has colourless molecules, but an intensely yellow negative ion.
If an alkali is added, however, a highly dissociated salt of para-nitrophenol is formed, and the yellow colour is at once evident.
Instances of its application are found in the separation of orthoand para-nitrophenol, the o-compound distilling and the p- remaining behind; in the separation of aniline from the mixture obtained by reducing nitrobenzene; of the naphthols from the melts produced by fusing the naphthalene monosulphonic acids with potash; and of quinoline from the reaction between aniline, nitrobenzene, glycerin, and sulphuric acid (the product being first steam distilled to remove any aniline, nitrobenzene, or glycerin, then treated with alkali, and again steam distilled when quinoline comes over).
Soc., 1900, 77, pp. 99 et seq.) nitrated para-oxyazobenzene with dilute nitric acid and found that it gave a benzene-azo-ortho-nitrophenol, whereas quinone are not attacked by dilute nitric acid.