Ortho-nitrophenol, C6H4.OH N02(1-2), crystallizes in yellow needles which melt at 45° C. and boil at 214°C. Para-nitrophenol, C6H4.
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.