FATTY COMPOUNDS, or ALIPHATIC COMPOUNDS, in organic chemistry, are those compounds whose molecules do not include a closed chain of carbon atoms, as distinguished from the Aromatic Compounds in which this closed chain does obtain. (See AROMATIC COM POUNDS). The name °fatty') no longer im plies any necessary relation to the fats, but it was originally given to the group because many of its earliest-known representatives occur in the animal or vegetable fats, or are related to them in a simple manner. The name "methane compounds* has also been proposed for the group, but it has not been generally adopted, although it is a particularly happy designation, since all of the compounds of the group are theoretically derivable from methane, CH4, by addition or substitution. Generally they are grouped, particularly in works on chemistry, under the title "Hydrocarbons? The fatty compounds may be subdivided, in a general way, into those that are and those that are °unsaturated? Saturated compounds are those in which the atomic linkage is such that no more atoms can be attached to the carbon chain of the molecule without causing it to split; and unsaturated compounds are those in which this condition is not fulfilled. For example, ethane, GH., has the structural for mula: H H and it is therefore a saturated compound, be cause there is obviously no way to introduce more atoms without separating the carbon atoms or removing one or more of the hydro gen atoms. If we introduce (say) the group CH, we must place it between the two carbon atoms, or between a carbon atom and one of the hydrogen atoms now at tached to it. In either case the structural for mula of the resulting compound (known as "propane? GIL) is H H H I I H H so that the new compound is also saturated. By continued additions of CH., butane and pentane are formed. Compounds of such sim ple structure are termed °normal compounds,' but the same composition may be developed in a different structure, or, as it is called, a "branched chain,* and these are termed "isomers* (see ISOMERISM), such as CH, (methyl), Call. (ethyl& C,H, (propyl), etc. Theoretically, a very large number of such compounds are possible, and those containing up to 60 atoms of carbon in the molecule have already been recognized.
The majority of the saturated hydrocarbons are natural substances. The lower members of the series — those containing up to four atoms of carbon — are gases; they are produced during the dry distillation of many organic substances, as in making coal gas. They are also produced in the processes of digestion in animals. The middle members of the series— those containing from 5 to 16 carbon atoms are liquids, and are found naturally in crude petroleum, which consists of a mixture of satu rated hydrocarbons. The higher members of
the series are solids, as the natural mineral wax ozokerite and paraffin wax.
The saturated hydrocarbons have a peculiar odor. They are soluble in alcohol and ether, but not in water. They are all inflammable to a greater or less degree. The gaseous members ignite readily and burn with a non-luminous flame. Mixed with air they form explosive compounds. The liquid members in some cases ignite readily, in others after being heated, they burn with a more or less luminous flame. The solids burn with the aid of a wick, with a highly-luminous flame.
The unsaturated hydrocarbons are so-called because, as they contain less hydrogen in the molecule, the four valencies of the carbon atom are not satisfied by the hydrogen atoms. The unsaturated hydrocarbons bear the terminations ene and ine, as ethene (ethylene) and ethine (acetylene).
Ethylene (q.v.), C,H4 is an unsaturated com pound because a carbon nucleus containing two atoms of carbon is capable of fixing six atoms of hydrogen, as appears in the first of the foregoing structural formula:. It is, therefore, usual to consider that in this case two valencies in the carbon nucleus satisfy each other, so that the structural formula for ethylene is H 4 4 Here it is obviously possible to introduce two additional monad atoms or radicals, by break ing one of the two valencies uniting the carbon atoms, and attaching one of the monad atoms so introduced to each of the free ends of the valency so broken; the carbon atoms remaining constantly united by one of the bonds. If a pair of hydrogen atoms be introduced in this way, ethane is produced, as will be seen by inspecting the structural formula of that substance, given above. In acetylene (q.v.) three bonds are commonly assumed to unite the two carbon atoms, so that the structural formula of this substance is H — C=-C — H. Acetylene, there fore, is an unsaturated compound. Much at tention has been paid to the theory of these double and triple bonds between adjacent carbon atoms, and it has been conclusively established (contrary to what might be naturally assumed) that a double or triple linkage between two car bon atoms constitutes a weaker bond than the single linkage. When, for example, an unsat urated compound is broken up by the action of powerful chemical reagents, it is almost in variably the double (or triple) bond that is ruptured. A quantitative study of the heat energy liberated when compounds of carbon and hydrogen are burned also shows that the force required to break a double or triple bond is smaller than that required to break a single bond.