The acids which, when combined with glycerol, produce fats are of two general types. The first of these are the so-called "fatty acids" having the general formula CnH2n+{1}·COOH. These are the "saturated" acids, i.e., they contain only single-bond linkages in the radical which is united to the ·COOH group; hence, they cannot take up hydrogen, oxygen, etc., by direct addition. The second type are the "unsaturated" acids belonging to several different groups, as discussed below, but all having one or more double-linkages between the carbon atoms of the alkyl radical which they contain. Because of these double linkages, they are all able to take on oxygen, hydrogen, or the halogen elements, by direct addition. When exposed to the air, for example, these "unsaturated" acids, or the oils derived from them, take up oxygen, increasing in weight, and becoming solid or hard and stiff. Hence, natural oils which contain considerable proportions of glycerides of these "unsaturated" acids are known as "drying oils" and are largely used in the manufacture of paints, varnishes, linoleums, etc.; while oils which contain little of these glycerides are known as "non-drying," and are used for food, for lubrication, or for other technical purposes in which it is essential that they remain in unchanged fluid condition when exposed to the air.
The following are some of the more important of the acids which occur as glycerides in natural fats: Saturated Acids:
- (a) Acetic, or stearic, acid series—general formula, CnH2n+1·COOH.
- (1) Formic acid, H·COOH, occurs free in nettles, ants, etc.
- (2) Acetic acid, CH3·COOH, occurs free in vinegar.
- (3) Butyric acid, C3H7·COOH, in butter fat.
- (4) Capric acid, C9H19·COOH, in butter fat and cocoanut oil.
- (5) Myristic acid, C13H27·COOH, in cocoanut oil and spermaceti.
- (6) Palmitic acid, C15H31·COOH, in palm oil and many fats.
- (7) Stearic acid, C17H35·COOH, in most fats and oils.
Intervening members of this series, such as caprylic acid, C7H15·COOH, and lauric acid, C11H23·COOH, are also found in smaller quantities in cocoanut and palm nut oils, in butter fat, and in spermaceti; while higher members of the series, as arachidic acid, C19H39·COOH, and lignoceric acid, C23H47·COOH, are found in peanut oil; and cerotic acid, C25H51·COOH, and melissic acid, C29H59·COOH, in beeswax and carnauba wax. Unsaturated Acids:
- (b) Oleic acid series—general formula, CnH2n-1·COOH.
- (1) Crotonic acid, C3H5·COOH, occurs in croton oil.
- (2) Oleic acid, C17H33·COOH, occurs in many fats and oils.
- (3) Brassic acid, C21H41·COOH, occurs in rape-seed oil.
- (4) Ricinoleic acid, C17H32OH·COOH, occurs in castor oil.
- (c) Linoleic acid series—general formula, CnH2n-3·COOH.
- (1) Linoleic acid, C17H31·COOH, occurs in linseed and other drying oils.
- (d) Linolenic acid series—general formula, CnH2n-5·COOH.
- (1) Linolenic acid, C17H29·COOH, occurs in many drying oils.
It will be observed that all of these acids contain a multiple of two total carbon atoms. No acid containing an uneven number of carbon atoms has been found in a natural fat. Furthermore, the acids which occur most commonly in natural fats are those which contain eighteen carbon atoms; in fact, more than 80 per cent of the glycerides which compose all animal and vegetable fats are those of the C18 acids. This fact, in addition to the one that the sugars and starches all contain multiples of six carbon atoms in their molecules, indicates a very great biological significance of the chain of six carbon atoms. This has been alluded to in connection with the discussion of the biological significance of molecular configuration (see [page 57]) and will be mentioned again in other connections.
THE ALCOHOLS WHICH OCCUR IN NATURAL FATS
Glycerol, as has been pointed out, is by far the most common alcoholic constituent of natural fats and oils. This substance, which is familiar to everyone under its common name "glycerine," is a colorless, viscid liquid having a sweetish taste. It is a very heavy liquid (specific gravity 1.27) which mixes with water in all proportions and when in concentrated form is very hygroscopic.
Glycerine is made from fats and oils by commercial processes which clearly prove that the constitution of fats is as described above. The fat is boiled with a solution of caustic soda and is decomposed, the sodium of the alkali taking the place of the glyceryl (C3H5) group, the latter combining with three (OH) groups from the three molecules of alkali necessary to decompose the fat. A sodium salt of the organic acid, or soap, and glycerol are thus produced, and are separated by saturating the hot solution with common salt, which causes the soap to separate out as a layer on the surface of the liquid, which, on cooling, solidifies into a solid cake, which is then cut and pressed into the familiar bars of commercial soap. From the remaining solution, the glycerine is recovered by evaporation and distillation under reduced pressure. Taking stearin, a common fat, as the example, the reaction which takes place in the above process may be expressed by the following equation:
| C3H5(C17H35·COO)3 | + | 3NaOH | = | 3C17H35COONa | + | C3H5(OH)3 |
| Stearin | Sodium stearate—a soap | Glycerol |