Amine is a group of basic organic compounds derived by replacement of one
(primary amines), two (secondary amines), or three (tertiary amines) of the
hydrogen atoms in ammonia (NH3) by alkyl or aryl groups. Tertiary amines combine
with one molecular proportion of an alkyl iodide to form quaternary ammonium
salts in which a central nitrogen atom is joined to four organic radicals and
one acid radical used as a corrosion inhibitor, emulsifying and antiseptic
agents. Amines, like ammonia, are weak bases because the unshared electron pair
of the nitrogen atom can form a coordinate bond with a proton. Amines react with
acids to give salts and with acid anhydrides (or or ester ) to form amides. They
react with halogenoalkanes to form longer chains. Low molecular amine names are
formed by adding '-amine' as a suffix to the name of the parent compound. In
substitutive nomenclature, the prefix 'amino-' is placed before the name of the
parent compound to denote the fuctional group in high molecular amines.
Synthetic amines are made mostly by reaction of alcohols with ammonia, catalyzed
by metals( nickel or copper) or metal oxide at high temperature. Many methods
have been devised for the synthesis of the amines; reacting ammonia with an
alkyl halide and neutralizing the resulting alkyl ammonium salt with an alkali,
e.g., sodium hydroxide. This procedure yields a mixture of primary, secondary,
and tertiary amines that is easily separated into its three components by
fractional distillation; boiling methyl isocyanate with caustic potash, heating
the alkyl iodides with ammonia; reduction of nitriles with alcohol and sodium;
heating the esters of nitric acid with alcoholic ammonia; reducing on
nitro-paraffms; action of zinc and hydrochloric acid on aldehyde ammonias;
reduction of the phenylhydrazones and oximes of aldehydes and ketones with
sodium amalgam in the presence of alcohol and sodium acetate; action of dilute
hydrochloric acid on the isonitriles; heating the mustard oils with a mineral
acid, by the hydrolysis of the alkyl phthalimides. Primary amines contain the
functional group -NH2 (called amino group) and are converted into secondary and
tertiary amines if heated with alkyl or aryl iodides. Primary amines form
various oxidation products violently with concentrated nitric acid. If the
amines are acetylated, they form nitro derivatives with concentrated nitric
acid. Primary amines form diazonium salts with nitrous acid in cold solution in
the presence of excess of mineral acid. Or a diazoamine is obtained in absence
of excess of acid. Other ractions; forming condensation products with aldehydes;
forming anilides; forming alkyl thioureas; yielding isonitriles with alcoholic
potash and chloroform. Aliphatic amines which have the lowest carbon content are
water-soluble gases or liquids of low boiling point also readily soluble in
water in case of the next low carbon content. But aliphatic amines which have
the high carbon content are odourless solids of high boiling point and are
insoluble in water. They are all bases and easily forms salts with the mineral
acids and solid salts with the halogenoalkanes. Amine Salts are crystalline
substances that are readily soluble in water. Many insoluble alkaloids (e.g.
quinine and atropine) are used medicinally in the form of soluble salts. If
alkali (sodium hydroxide) is added to solutions of such salts the free amine is
liberated. Hexamethylenediamine used in the manufacture of nylon-6,6 is prepared
by catalytic addition of hydrogen to nitriles. Aromatic amines also exist, such
as phenylamine, which are important for the production of diazonium salts. They
dissociate in water (some very weakly). Aromatic amines are much weaker bases
than the aliphatics. One of the most important aromatic amines is aniline; pale
brown liquid boiling at 184 C, melting at -6 C. Aniline is obtained commercially
from chlorobenzene by heating with ammonia in the presence of copper catalyst
or from a product of coal tar (nitrobenzene) through the reduction reaction.
Aniline is the starting material in the dye manufacturing industry and as in
the manufacture of others. Aniline is converted into sulfanilic acid which is
the parent compound of the sulfa drugs. Aniline is also important in the
manufacture of rubber-processing chemicals, antioxidants and varnishes. Amines
take part in many kinds of chemical reactions and offer many applications
include in agrochemicals, dyestuffs (the best known being aniline),
pharmaceuticals, and corrosion inhibitors.
|
