compounds of the fatty series, or aliphatic compounds; organic substances (hydrocarbons and their derivatives) whose molecules do not contain rings but rather are “open” chains.
In the compounds with so-called normal structure (for example, n-butane CH3—CH2—CH2—CH3) the atoms of the carbon skeleton of the molecule are arranged linearly. The arrangement of the carbon atoms in the form of branching chains is called the isostructure—for example, isobutyl alcohol:
The length of the chains in acyclic compounds is extremely variable—for example, a hydrocarbon has been obtained with a linear chain containing 100 atoms of carbon: CH3(CH2)98CH3.
Acyclic and cyclic compounds are connected to each other by mutual transitions. Thus, from acetylene CH ≡ CH under certain conditions it is possible to obtain acyclic vinyl acetylene CH2 ≡ CH—C≡CH, and under different conditions the cyclic hydrocarbon benzene C6H6 is obtained. The transformation of saturated into aromatic acyclic hydrocarbons such as benzene and its homologues has great practical significance. Many methods for opening rings are also known. Thus, ethylene glycol is formed from ethylene oxide under the influence of water:
Many different classes of compounds (alcohols, ketones, acids, amines, and others) which are derivatives of hydrocarbons with open chains—saturated as well as unsaturated—belong to the acyclic compounds.
The principal source for obtaining acyclic compounds is petroleum and petroleum products. Ethylene and its homologues, acetylene, and carbon monoxide are the raw material for large-scale industrial production of many acyclic compounds. For example, synthetic rubber, polymers, ethyl and isopropyl alcohols, and acetone are obtained from olefins; from acetylene—acetic acid and chloroprene rubber; from carbon monoxide and hydrogen—methanol and synthetic gasoline; and from carbon monoxide and acetylene—acrylic acid.