Experience has shown that such mono-derivatives as nitro compounds, sulphonic acids, carboxylic acids, aldehydes, and ketones yield as a general rule chiefly the meta-compounds, and this is independent of the nature of the second group introduced; on the other hand, benzene haloids, amino-, homologous-, and hydroxy-benzenes yield principally a mixture of the orthoand para-compounds.
Aminoand oxy-benzenes, aminophenols, quinones, and oxycarboxylic acids.
It is to be noted that only traces of the aromatic amines are produced by heating the halogen substituted benzenes with ammonia, unless the amino group be situated in the side chain, as in the case of benzylamine.
The ortho-compounds condense to azimido benzenes, the metacompounds yield azo-dyestuffs, and the para-compounds yield bis-diazo compounds of the type XN2ï¿½C6H4ï¿½N2X.
They are readily reduced to the corresponding hydroquinones or para-dihydroxy-benzenes, and also combine with hydroxylamine hydrochloride to form nitrosophenols, ON C 6 H 4.
Alkylated amino-azo-benzenes are also known, and are formed by the coupling of diazonium salts with alkylated amines, provided they contain a free para.
It is convenient to restrict the term to compounds in which the hydroxyl group is attached to an aliphatic residue; this excludes such compounds as the hydroxy-benzenes, naphthalenes, &c., which exhibit many differences from the compounds derived from the aliphatic alkyls.