Although the fatty and aromatic groups of chemical compounds are differentiated so strongly that it is not easy to convert a rep resentative of either group into a representa tive of the other one, such a transformation is nevertheless possible. Thus Berthelot showed that when acetylene gas is passed through a red-hot tube it is converted into benzene, naph thalene and other members of the aromatic group; and an example of the opposite kind of transformation, in which an aromatic compound is converted into a fatty compound, is afforded by the formation of methyl chloride, CH2C1, when hydrochloric acid gas is passed through boiling metaxylene, GH, (CH,),. It has been pointed out that there is probably much signifi cance in the fact that the carbohydrates, which are the chief constituents of .plants, are fatty compounds containing six (or a multiple of six) atoms of carbon; while the products ob tained by the dry distillation of coal (that is, the fossilized remains of such plants), are chiefly aromatic compounds, in which six car bon atoms are united in the form of a "benzene nucleus? It is likely, in view of this fact, that most of the carbohydrates might be trans formed into members of the aromatic group, by the prolonged action of great pressure and moderately high temperature.
Fundamental among the fatty compounds are three series of hydrocarbons, which contain only carbon and hydrogen and are known re spectively as the paraffins, the olefines and the acetylene series. The simplest member of the paraffin series is methane ("marsh gasp), CH4, from which the higher members are derived by successive additions of the group CH* as has already been illustrated in connection with the derivation of propane from ethane. The paraffins are very numerous, and many of the higher members of the series have been but little studied. As will be seen from their mode of derivation from methane, they all come under the general formula G,H2n+2, where n may have any value from unity up to a limit which probably exists, but is at present undeter mined. Isomeric forms of the paraffins are possible when n is greater than 3, and the num ber of such possible isomeric forms increases rapidly as the value of n increases. Thus in the case of butane, C211,2, there can be but one isomer, while the compound Culirl, known as tridecane, is theoretically capable of existing in no less than 802 distinct forms. See ISO MERT S If.
The olefine series of hydrocarbons begins with ethylene (aolefiant gas))) C2114, from which the higher members may be regarded as derived, just as before, by successiye additions of CH2.
The general formula for the define series therefore is CDR.. and many isomers are known here also. The acetylene series, which has not been so thoroughly studied as the lower paraf fins and the olefines, begins with acetylene gas, Cali, and has the general formula Gap-a. See HYDROCARBONS.
Among the derivatives of the fatty hydro carbons, the fatty alcohols and fatty acids re quire special mention. Alcohols, as a class, are
considered under Atconot. In general, they may be regarded as derived from the hydro carbons by replacing one or more of the hydro gen atoms by an equal number of molecules of hydroxyl (OH). They are known as °mono utrihydric, etc., according as one, two, three, etc., atoms of hydrogen are replaced in the original hydrocarbon. A vast number of monohydric alcohols are known, but the number of recognized dihydric alcohols (also known as "glycols?') is far smaller. Five trihydric fatty alcohols are known, of which glycerin, Gli.(OH)/, is the only familiar ex ample; and only one fatty tetrahydric alcohol, erythrite, C211.2(OH)4, is known, though a few that are still more highly hydrated have been prepared. The most important series of the monohydric fatty alcohols is that derived from the paraffin series of hydrocarbons, by the substitution of OH for H. The general formula for this series of alcohols therefore is C21122+2.(OH); it includes methyl alcohol, ethyl alcohol, and, in fact, nearly all of the more familiar alcohols. The monohydric alcohols of the olefine series of hydrocarbons have the gen eral formula Gdisi-,.(OH), but they include only a single familiar example— allyl alcohol, CsHm.OH. Those of the acetylene series have the general formula Gliss-3.(OH), but they do not include any that are of importance save to the theoretical chemist.
The fatty acids are so called because their higher members occur in the natural fati which are esters of the fatty acids with gly cerin. They are exceedingly numerous, and play a highly important part in the processes of animal and vegetable life. A complete classifi cation of them cannot be here attempted, but three of the known series must be mentioned. The series derived from the paraffins has the general formula C.H..0, and its members may be derived, by oxidation, from the alcohols of the paraffin series that contain the same number of carbon atoms. The best-known examples of this series are as follows: Formic Acid CH2O, Acetic Acid C2I-1402 Propionic Acid Butyric Acid Capric Acid C2I-12.02 Laurie Acid Ci2112202 Palmitic Acid C22I-1,20, Stearic Acid C2112202 Melissic Acid Colf-1,202 The alcohols corresponding to these acids have not all been prepared, but many of them have been and the chemical deportment of the remaining members of the group leaves no doubt of their relation with the normal paraffins from which they are assumed to be derived. All of the foregoing acids are monobasic, containing hut one atom of replaceable hydrogen ; and these acids, together with the others that belong in the same series, are the ones to which the name fatty acids' is often applied, by writers on chemistry, to the exclusion of all other organic acids. According to the definition of afatty com pounds' adopted above, however, all organic acids that do not contain closed carbon chains would appear to be logically included under this title.