organic anhydride and the organic peroxide may be thus exhi

(1272) The table on the preceding page will assist in explaining the relations of the principal classes of the derivatives of the monobasic acids.

§ III. THE FATTY ACIDS.

(1273) THE fats and fixed oils when saponified yield a number of acids, which belong to two different homologous series, both of which are regarded as monobasic. Stearic and palmitic acids

2n

may be taken as the types of one series, in which the general formula of the normal acids is (EH). The general formula of the other series is (EH); this series is represented by oleic acid, but only a few terms of it are known. By the oxidation of the members of these two groups a third series of acids may be obtained, to which suberic and succinic acids belong. The acids of this group are dibasic: their normal hydrates have the general formula (EH2-2). In the following table the

Ꮎ

corresponding acids in each of these three groups are enumerated:

(a) Acids belonging to the Acetic or Stearic Series (HE,H2n-102). (1274) THE acids which are comprised in this series are monobasic; they constitute one of the most numerous and most

374

ACIDS BELONGING TO THE STEARIC ACIDS.

carefully examined groups of homologous compounds; all the terms of which, commencing with the formic and proceeding as far as the rutic, are well known. It has been remarked, that in the terms which contain more carbon than the rutic, the number of atoms of carbon in all the acids which have been satisfactorily examined, is divisible by 2; although many chemists admit the existence of acids corresponding in composition to all the hydrocarbons from EH,, increasing step by step by successive increment of EH,, up to (20H40). The fusing point of Brodie's cerotic acid (H) is, however, so much lower than that which might have been expected from an acid so high in the series, that Heintz has suggested that this exceptional case may possibly arise from the acid being a mixture of two others; since he finds it to be a general rule, that a mixture of any two of the solid acids of this series has a fusing point considerably below that of either of its components (1293). As the number for n increases, the boiling point rises, and at the same time the acid property decreases excepting in particular cases; the lower terms generally displace those above them in the series from their combinations with bases; for example, formic or acetic acid will displace the valeric.

The acids belonging to this group may all be distilled, either partially or completely, without undergoing decomposition. Many of them exhibit a remarkable peculiarity in the volume of vapour which they furnish at different temperatures; for instance, the formic, the acetic, the butyric, and the valeric acids, at low temperatures, furnish a much smaller volume of vapour than they do at more elevated temperatures

At the higher temperatures, and at all points above them (until the acid undergoes decomposition), each atom of the compound yields 2 volumes of vapour; but at low temperatures the vapour volume does not correspond exactly to any specific multiple of the volume of hydrogen.

ACIDS BELONGING TO THE STEARIC ACIDS.

375

All of the acids of this group when melted, with the exception of the formic and the acetic, exhibit the properties of an oil which is imperfectly soluble in water; though some of the lower members of the series are soluble in water to a considerable extent they are all abundantly soluble both in alcohol and ether.

These acids are regarded as monobasic, though several of them form both neutral and acid salts with the alkalies; an acid formiate, an acid acetate, and an acid stearate of potassium, for example, may be readily obtained. Many of them yield with oxide of lead, not only the usual normal salt, but also basic salts, which contain 3 equivalents of oxide of lead for each equivalent of acid. Many of these basic salts are freely soluble in

water.

The remarkable relation which these acids bear to the alcohols has already been pointed out; the acid containing an atom of oxygen more than the alcohol, and two atoms of hydrogen less, whilst the number of atoms of carbon in the two compounds is the same (p. 38);—for every alcohol there is a corresponding acid, which may be formed from the alcohol by a regulated process of oxidation; this oxidation may sometimes be effected directly-as in the conversion of wine alcohol into vinegar-by the operation of finely divided platinum; but more usually it is necessary to resort to indirect means, such as heating, the alcohol with caustic potash; in which case hydrogen is eliminated, whilst oxygen enters into the compound, the general form of the equation being:

The action of gaseous chlorine upon the acids of this group gives rise to the formation of chlorinated acids, in which a certain number of atoms of the hydrogen in the radicle is displaced by an equal number of atoms of chlorine; but the basicity or saturating power of the acid is not altered. In some acids this substitution takes place at ordinary temperatures; in others, heat is necessary, and in some the direct rays of the sun are required to bring about the reaction. In many cases more than one chlorinated acid may be formed from the original acid; for example:

In many cases corresponding compounds with bromine may be obtained, such as bromacetic (H,,H,Br2) and dibromacetic (H,Є,HBг,,) acids. The iodized substitution-products cannot be obtained unless the brominated bodies are first formed; on heating these with potassic iodide, double decomposition occurs, and an iodized acid is obtained.

When a brominated or a chlorinated acid is boiled with oxide of silver and water, the acid sometimes breaks up completely, as occurs with trichloracetic acid, but usually an exchange of each atom of chlorine or bromine for one of hydroxyl (HO) takes place. Bromacetic (E,H,Br.) may thus be converted into glycolic acid (H), and dibromobutyric (EH.Br.) into dioxybutyric acid (H ̧ ̧); as for example :

Dibromobutyric acid.

3

Dioxybutyric acid.

2

If the mono-brominated acids be treated with an alcoholic solution of ammonia, an amido-acid is the result (Perkin), whilst the ammonium bromide is separated; for instance:

Є2H2Br2+ 2 H2N = €2H ̧(H ̧N)→ ̧ + H ̧NBr.

2

3

2

Several of these acids also furnish nitro-acids, when treated with fuming nitric acid; such, for example, as the following: