Amines are organic derivatives, either in principle or in practice, of ammonia, NH3, in which one or more of the hydrogen atoms is replaced by an organyl group (a substituent having a single bond from a carbon atom, represented here as R). Amines are classified as primary, secondary, or tertiary depending on whether organic groups have replaced one, two, or three of the hydrogen atoms of ammonia. Amines are classified as primary when only one hydrogen is replaced (RNH2), secondary when two are replaced (R2NH), and tertiary when three are replaced (R3N). A fourth category consists of quaternary ammonium compounds, which are obtained by replacement of all four hydrogen atoms of the ammonium ion, NH4+; an anion is necessarily associated (R4N+X - ).Diamines, triamines, and polyamines contain two, three, or more nitrogen groupings. The older and most widely used system for naming amines is to identify each group that is attached to the nitrogen atom and then add the ending -amine, as in methylamine, CH3NH2; N-ethyl-N-propylamine (or ethyl(propyl)amine), CH3CH2NHCH2CH2CH3; and tributylamine, (CH3CH2CH2CH2)3N. Amines are alkaline, although the intensity of this property varies, and are either aliphatic, having only aliphatic groups attached, or aromatic, having one or more aryl groups attached. They may be open-chain, in which the nitrogen is not part of a ring, or cyclic, in which it is a member of a ring (generally aliphatic). Naturally-occurring amines include the alkaloids, which are present in certain plants; the catecholamine neurotransmitters (i.e., dopamine, norepinephrine, and epinephrine); and a local chemical mediator, histamine, that occurs in most animal tissues. Aniline, ethanolamines, and several other amines are major industrial commodities. These are employed in making rubber, dyes, pharmaceuticals, and synthetic resins and fibers and in an array of other applications. Amines form salts with acids; with many organic acids or acid anhydrides, primary and secondary amines undergo further reaction to form amides.