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 and benzylamine, 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. The term benzyl describes the radical, ion or functional group C6H5CH2-, derived
by removing hydrogen atom from methyl group in toluene, while phenyl is the term
for the monovalent radical C6H5-, derived by removal of hydrogen
from benzene.
The common name of phenylamine is aniline. Benzylamine is called aminotoluene or benzenemethanamine in methane nomenclature system. Benzylamine
functions in the way as primary aliphatic amines. Benzylamine is a clear liquid
boiling at 185 C. It functions in the same way as primary aliphatic amines.
Benzylamine and its derivatives are used as chemical intermediate for the
manufacture of dyestuffs, pigments, optical brighteners, textile auxiliaries,
agrochemicals, amino acids and other organic compounds.
One of the most
important aromatic amines is aniline, a primary aromatic amine replacing one
hydrogen atom of a benzene molecule with an amino group. It is a pale brown
liquid at room temperature; boiling at 184 C, melting at -6 C; slightly soluble
in water and freely soluble in ether and alcohol. It causes serious industrial
poisoning. The substance may have effects on the blood, resulting in formation
of methaemoglobin. Repeated or prolonged exposures may be carcinogenic.
Commercial aniline is obtained from nitrobenzene which is prepared from benzene
with nitric acid by electrophilic substitution reaction or from chlorobenzene by
heating with ammonia in the presence of copper catalyst. It is also obtained as
a by-product of coal tar. In commerce the term of aniline oil blue refers to the
pure one while aniline oil red indicates a mixture of aniline and toluidines with
equimolecular weights.
Considerable quantity of aniline is converted into 4,4กฏ-methylenedianiline (MDA)
by the condensation reaction of formaldehyde with aniline in the presence of
hydrochloric acid. MDA is is used as an epoxy curing agent, a corrosion
inhibitor and molded plastics, and as an intermediate to prepare organic
compounds used for polyurethane, spandex fibers, azo dyes, isocyanates and
poly(amide-imide) resins. Other important aromatic amine compound as the
starting material to produce polyurethane foam production is toluenediamine
(TDA). TDA is the mixture of 2,4-diaminotoluene and 2,6-diaminotoluene, usually
in a ratio of 80:20. Most of TDA is used in the manufacture of toluene
diisocyanate (TDI), which is the predominant diisocyanate in the flexible foams
and elastomers industries. TDI reacts with an alcohol to form urethane linkages.
Other applications of TDA include to produce dyes, polyamides, antioxidants,
hydraulic fluids, and fungicide stabilizers. Aniline is a starting moiety to
prepare plant protecting agents. Examples include fenuron (CAS RN: 101-42-8),
propham (CAS RN: 122-42-9), siduron (CAS RN: 1982-49-6), carboxin (CAS RN:
5234-68-4), fenfuram (CAS RN: 24691-80-3) and propachlor (CAS RN: 1918-16-7).
Aniline is processed to produce a series of compounds being used in the rubber
industry, e.g. diphenylguanidines, phenylenediamines mercaptobenzothiazoles,
aniline ketones and etc. There are three isomers of phenylenediamine: ortho-,
meta-, and para-phenylenediamine. They are low toxic diamines used as components
of plastic composites and engineering polymers. They are used to produce aramid
fibers, dyes including hair dyes, rubber chemicals (vulcanization accelerators
and antioxidants), and pigments.
Aniline is the starting material in the dye
manufacturing industry. It forms aniline colors when combined with other
substances, particularly chlorine or chlorates. Aromatic amines are weaker bases
reacting with strong acids to form amides. Anilide is an amide derived from
aniline by substitution of an acyl group for the hydrogen of NH2. Acetanilide is from acetic acid and aniline. Acetanilide is an odourless,
white flake solid or crystalline powder (pure form);
soluble in hot water alcohol, ether, chloroform,
acetone, glycerol, and benzene;; melting point 114 C and boiling point 304 C;
can undergo self-ignite at 545 C, but is
otherwise stable under most conditions. Acetanilide which can be obtained by
acetylation of aniline
undergoes
nitration at low temperature and yields highly the para-nitro products. Acetyl group can then be removed by acid-catalyzed hydrolysis to yield para-nitroaniline. Although the activating affection
of the
amino group can be reduced, the acetyl derivative remains an
ortho/para-orientation and activating substituent. 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, explosives,
plastics, antioxidants and varnishes. Amines take part in many kinds of chemical
reactions and offer many industrial applications.
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