Substituting one hydroxyl group into each of these residues, we obtain radicals of the type - CH 2.
On the chromophoreauxochrome theory (the nitro group being the chromophore, and the hydroxyl the auxochrome) it is necessary in order to explain the high colour of the metallic salts and the colourless alkyl and aryl derivatives to assume that the auxochromic action of the hydroxyl group is only brought strongly into evidence by salt formation.
By the action of phosphorus pentachloride, the hydroxyl group is replaced by chlorine.
By the entrance of amino or hydroxyl groups into the molecule dyestuffs are formed.
When heated with concentrated hydrochloric acid the amino group is replaced by the hydroxyl group and the phenolic eurhodols are produced.
The hydrogen of the hydroxyl group in phenol can be replaced by metals, by alkyl groups and by acid radicals.
The meta-nitrocompound, which is precipitated last, is then reduced, and the amino group so formed is replaced by the hydroxyl group by means of the Sandmeyer reaction.
It is convenient first to consider the effect of introducing one, two, or three hydroxyl (OH) groups into the - CH 3, > CH 2, and >CH groups, which we have seen to characterize the different types of hydrocarbons.
A second hydroxyl group may be introduced into the residues - CH 2.
A third hydroxyl group may be introduced into the - CH: 0 residue with the formation of the radical - C(OH) :0; this is known as the carboxyl group, and characterizes the organic acids.
An important class of compounds, termed amines (q.v.), results from the condensation of alcohols with ammonia, water being eliminated between the alcoholic hydroxyl group and a hydrogen atom of the ammonia.
This group may be considered as resulting from the fusion of a carbonyl (:CO) and a hydroxyl (HO-) group; and we may expect to meet with compounds bearing structural resemblances to the derivatives of alcohols and aldehydes (or ketones).
Considering derivatives primarily concerned with transformations of the hydroxyl group, we may regard our typical acid as a fusion of a radical R CO - (named acetyl, propionyl, butyl, &c., generally according to the name of the hydrocarbon containing the same number of carbon atoms) and a hydroxyl group. By replacing the hydroxyl group by a halogen, acid-haloids result; by the elimination of the elements of water between two molecules, acid-anhydrides, which may be oxidized to acid-peroxides; by replacing the hydroxyl group by the group. SH, thio-acids; by replacing it by the amino group, acid-amides (q.v.); by replacing it by the group - NH NH2, acid-hydrazides.
By transformations of the carbonyl group, and at the same time of the hydroxyl group, many interesting types of nitrogen compounds may be correlated.
The introduction of hydroxyl groups into the benzene nucleus gives rise to compounds generically named phenols, which, although resembling the aliphatic alcohols in their origin, differ from these substances in their increased chemical activity and acid nature.
The phenols more closely resemble the tertiary alcohols, since the hydroxyl group is linked to.
OH, in which we will assume the hydroxyl group to occupy position I, is converted into brombenzene, which is then converted into benzoic acid, C 6 1-1 5 -COOH.
These three acids yield on heating phenol, identical with the substance started with, and since in the three oxybenzoic acids the hydroxyl groups must occupy positions other than I, it follows that four hydrogen atoms are equal in value.
Soc. 61, p. 367): If the hydrogen compound of the substituent already in the benzene nucleus can be directly oxidized to the' corresponding hydroxyl compound, then meta-derivatives predominate on further substitution, if not, then orthoand paraderivatives.
Thus a double bond of oxygen, as in the carbonyl group CO, requires a larger volume than a single bond, as in the hydroxyl group - OH, being about 12.2 in the first case and 7.8 in the second.