FATTY ACIDS, a highly important class of compounds of carbon, hydrogen and oxygen, in which one group, the car boxylic) group, CO.OH, has a hydrogen atom replaceable by metals and basic radicles. One class of fatty acids can be regarded as derived from the hydrocarbons (see HYDROCARBON) of the paraffin series, through the substitution of a hydrogen atom by the acidic carboxyl group. Thus, counting hydrogen as the first hydrocarbon, we get the two series: Hydrocarbon Fatty or Aliphatic Acid Hydrogen, H-H Formic Acid, H-COOH Methane, CH3-H Acetic Acid, CH3-COOH Ethane, C2H5-H Propionic Acid, C2Hs-COOH Proprane, C3H7-H Butyric Acid, C3H7-COOH Butane, C4H9-H etc.
The earlier acids in the series are treated in separate articles (q.v.; see also CHEMISTRY: Organic: Aliphatic Compounds). Other fatty acids are similarly related to the ethylene and acety lene groups of "unsaturated" hydrocarbons; further complications are introduced in hydroxy-acids, where one or more hydroxyl (–OH) groups may replace hydrogen atoms besides those re placed by the carboxyl group. Here will be discussed only those higher members of the series which play an important part in the chemistry of the naturally occurring glycerides (q.v.; see also OILS AND FATS) .
Some of these fatty acids may contain as few as four carbon atoms in the molecule, others as many as 24; the most common natural fatty acids contain 12, 16 or 18 carbon atoms in each molecule. There are two main types of these fatty acids, "sat urated" and "unsaturated," and it is essential to know something of the difference between them.
In saturated fatty acids each carbon atom is united with as much hydrogen as it can hold and palmitic acid, C16H32C2, the most widespread saturated acid, is formulated, for instance, as The most common members of this group are lauric (C11H23•COOH), myristic (C13H27•COOH), palmitic (C15H31•COOH), and stearic acids; from (capric acid) upwards they (and the corresponding glycerides) are solids at the ordinary temperature and the melting point increases with increasing number of carbon atoms, stearic acid, for example, melting at 71 ° C. The melting-point of a fat is determined by the particular acids present in greatest quantity.
In unsaturated acids there is a deficiency of hydrogen in each molecule, so that one or more pairs of carbon atoms, instead of being completely saturated with hydrogen have some of their chemical combining power left unsatisfied (–CH : CH–) . In consequence, such bodies, especially if several of these unsaturated centres are present in the molecule, can become attached to other elements, for example, atmospheric oxygen, and this is the basis of the utility of linseed and other oils in paints ; fixation of oxygen at the unsaturated centres causes the oil to become a solid tenacious film. Obviously this capacity for combining with oxygen, useful in the case of var nishes, may be a detriment in other cases, where it causes the development of rancidity in edible fats or even soaps. Of the acids with one unsaturated centre, oleic acid is by far the most common and is present in practically all known fats ; other similar acids are palmitoleic, C15H29.COOH, and erucic, whilst linoleic acid, and linolenic acid, COOH, contain respectively two and three unsaturated centres.
The position, in the molecule of a fatty acid, of the pair of carbon atoms which form an unsaturated centre, is known in several cases, for example in: Oleic acid CH3• (CH2) 7•CH : CH. (CH2) Linoleic acid CH3• (CH2)4-CH : CH•CH2•CH: CH- (CH2) 7•COOH Palmitoleic acid CH3• (CH2) 5•CH : CH• (CH2) 7• COOH Erucic acid CH3• (CH2) 7•CH: CH. (CH2) ii.COOH Obviously it is important to be able to assess the average mag nitude of the fatty acids present in naturally occurring glycerides, and also their state of "unsaturation." The former value is arrived at by measurement of the amount of caustic alkali neces sary to combine exactly with the fatty acid present, and is known as the saponification number; it is inversely proportional to the molecular weight of the fat. The unsaturation is measured by the power of the fat under examination to combine with iodine under specified conditions, the percentage of iodine absorbed being known as the iodine value of the fat.
Finally, it may be mentioned that compounds of the fatty acids with alcohols other than glycerol are widely distributed in nature, the alcohols in question being usually similar in chemical struc ture to palmitic and stearic acids (for example, cetyl alcohol C16H33.OH, or ceryl alcohol, Such compounds are classified as waxes, and include beeswax, spermaceti, carnauba and other waxes; these substances are useful in polishes and were formerly used in candles, but the waxes as a class have much less utility than the fats. (See GLYCERINE; OILS, FATS AND WAXES, etc.)