Hydrocarbons belong to the department of organic chemistry, and may be shortly defined as compounds of carbon and hydrogen, and nothing else. Despite their apparently simple nature, they are frequently very complex, and exist in such numbers as to bewilder the beginner in chemical study. Fortunately, they can be gathered into groups, each having distinctive characters, and the members of which are closely related to each other. The chief of these are the Paraffins, the Olefines, the Acetylenes, the Benzene (q.v.) series, and the Anthracenes (q.v.) group.
The Paraffins are found in natural petroleum as well as in the products of the destructive distillation of coal, and are known as saturated hydrocarbons. By this is meant that the carbon present is already saturated (so to speak) with hydrogen, and has no tendency to unite with other elements or molecules. Graphically, carbon may be represented as —i.e. with four arms, each one of which is capable of being united to one atom of hydrogen; and when all four arms are so united, a paraffin is produced; thus, . But, instead of the single atom of hydrogen, one arm (or all the arms) may be engaged by such a group as , so that we get another paraffin, . Thus we go on forming a series, each member of which differs from the preceding one in having an extra . Thus:
and so on indefinitely. It will, however, be noticed that when we pass to a higher member than propane, by replacing an atom of hydrogen by , we may do so in two ways, according as the atom replaced is in the group at either end, or the group in the centre. The result is that two hydrocarbons are possible—viz. , and also . In like manner, as we proceed further, wider scope is given to us, the result being that when Tridecane, , is reached, it is theoretically possible to recognise 802 such bodies, all having the same percentage composition, but differing more or less in characters. Many of these isomers are already known.
The paraffins are characterised by their indifference to chemical action, being unacted on by caustic potash, sulphuric or nitric acid. The general formula of the paraffins is , where represents the number of atoms in the formula.
The Olefines, —The chief of these are Ethylene, , Propylene, , Butylene, , Amylene, , &c.; and it will be observed that in all of them the percentage composition is identical, and that each member differs from the lower one by the addition of . When acted on by chlorine, bromine, or iodine they readily form oily liquids, such as Dutch liquid, and, generally speaking, they markedly differ from the paraffins in the readiness with which they unite with other bodies.