Amine is a group
of basic organic compounds derived from ammonia (NH3) by replacement
of one (primary amines), two (secondary amines), or three (tertiary amines)
hydrogen atoms by alkyl, aryl groups or organic radicals. Most
low mole weight aliphatic amines are liquids with characteristic ammonia-like
odor. But methylamines are gases and long chain alkylamines (from C12) or higher
mole weight amines are
solid. The water solubility is decreasing if molecular weight increase. They
are freely soluble in common organic solvents such as methanol, acetone,
toluene, and ether except aliphatic hydrocarbons. 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 ester ) to form amides. They react with halogenoalkanes to
form longer chains.
Many amines are not only bases but also nucleophiles that form a variety of
electrophile compounds. They are important intermediates for chemical syntheses
due to the basic functionality of the nitrogen atom and electrophilic
substitution at nitrogen. Some examples of compounds obtained by reaction of
amines are:
- Amides (by reaction with acyl halides or ammonium
carboxylate salts)
- N-Alkyl amines (by reaction with halogenoalkanes)
- Isocyanates (by reaction with phosgene)
- Carbamoyl chlorides or Urea
derivatives (by reaction with phosgene)
- Alkoxylated amines (by reaction
alkylene oxide)
- Quaternary ammonium compounds (by reaction with alkyl halides
and dialkyl sulfates)
- N-Alkylcarbamic acids or N,N'- Dialkyl ureas (by
reaction with carbon dioxide)
- Urea derivatives (by reaction with
isocyanates)
- Schiff bases (by reaction with aldehydes or
ketones)
- Aminopropionitriles (by reaction of 1° and 2° amines with
acrylonitrile)
- N-Alkylamino acids (by reaction of 1° and 2° amines with
monochloroacetic acid or with unsaturated acids)
- Amine oxides (by reaction of
3° amines with hydrogen peroxide)
- Sulfonamide derivatives (by reaction of 1°
and 2° amines with benzenesulfonyl chloride)
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
functional 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
reactions are condensation products with aldehydes; forming anilides; forming
alkyl thioureas; yielding isonitriles with alcoholic potash and chloroform.
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. Quaternary ammonium salts are used as
corrosion inhibitor, emulsifying and antiseptic agents. 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 form
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. Short chain alkyl amines
are used as raw materials of solvent, alkyl
alkanolamines, and ingredients of rocket fuels. They are used to make other
organic chemicals including rubber vulacanization accelerators, pesticides,
quaternary ammonium compounds, photographic chemicals, corrosion inhibitors,
explosives, dyes and pharmaceuticals. They are used in rayon and nylon industry
to improve the tensile strength. Allylamines are used as intermediates for
ion exchange resins, pharmaceuticals, water soluble
polymers, herbicide softeners, rubber chemicals, polymerization initiators and
cross-linking agents. Amines are used as reducing
agents for the recovery of precious metals. They are versatile intermediates.
They have active applications in organic synthesis for polymerization catalyst,
chain extender in urethane coatings, agrochemicals, pharmaceuticals,
photographic, heat stabilizers, polymerization catalysts, flame-retardants,
blowing agents for plastics, explosives, and colorants. Long chain alkyl
amines are used for the synthesis of organic
chemicals and surfactants used as a corrosion inhibitor, detergent, ore floating
agent, fabric softener, anti-static agent, germicide, insecticide, emulsifier,
dispersant, anti-caking agent, lubricant and water treatment agent. Alkyl
tertiary Amines are used as fuel additives and preservatives. They have similar
applications with long chain alkyl amines. 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.
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