Aromatic amines are much weaker bases than the aliphatics. 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
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 thus obtained from acetic acid and aniline. 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. p-Chloroaniline, a chlorinated aniline at 4 position, is a slightly
yellow crystalline solid; melts at 70 C, boils at 232 C; soluble in hot
water and readily soluble
in most organic solvents. The vapour pressure and octanol/water partition
coefficient are moderate. It decomposes in the presence of light and air and at
elevated temperatures. p-Chloroaniline is used as an intermediate in the
production of a number of products, including agricultural chemicals (e.g.
monuron, diflubenzuron, monolinuron), azo dyes and pigments (e.g., acid red 119,
pigment red 184, pigment orange 44), cosmetics (as an antimicrobial agent but
banned in some coumtries), bactericide or biocide (e.g., chlorohexidine,
triclocarban) and pharmaceuticals.
When substituted benzene molecules undergo electrophilic substitution reactions,
substituents on a benzene ring can influence the reactivity.
substituents that activate the benzene ring toward electrophilic
attack can alter the reaction rate or products by
electronically or sterically affecting the interaction of the two reactants.
deactivating substituents removes electron density from the benzene ring, making
substitution reactions slower and more difficult than benzene itself. For example, a hydroxy or methoxy substituent in
phenol and anisole increases the rate of
electrophilic substitution, while a nitro
substituent decreases the ring's reactivity. Electron donating
substituents activate the benzene ring toward electrophilic
attack, and electron withdrawing substituents deactivate the ring, making it less reactive to electrophilic attack.
The strongest activating substituents are the amino
(-NH2) and hydroxyl (-OH) groups.
Toluene, aniline and phenol
are activated aromatic compounds. Examples of deactivated aromatic compounds
are nitrobenzene, benzaldehyde and halogenated benzenes.
-COOH, -COOR, -COH, -COR
-Cl, -Br, -I
generally direct substitution to the ortho and para positions
where substitutions must
take place. With some
exceptions, deactivating substituents direct to the meta position. Deactivating substituents
which orient ortho
and para- positions are the halogens (-F, -Cl, -Br, -I) and -CH2Cl,
When disubstituted benzene molecules undergo electrophilic substitution reactions,
a new substituent is directed depends on the orientation of
the existing substituents and their individual effects; whether the groups have cooperative or antagonistic directing effects.
Ortho position is the most reactive towards electrophile
due to the highest electron density ortho positions.
But this increased reactivity is countervailed by steric hindrance between substituent and
A nucleophilic substitution is a substitution reaction which the nucleophile
displaces a good leaving
group, such as a halide on an aromatic ring. This
mechanism is called SNAr
( the two-step addition-elimination mechanism), where electron withdrawing substituents activate
the ring towards nucleophilic attack. Addition-elimination reactions usually
occur at sp2 or sp
hybridized carbon atoms, in contrast to SN1 and SN2
Chloro and bromobenzene reacts with the very
strong base sodium amide (NaNH2) to give good yields of aniline.
Other nucleophilic aromatic substitution mechanisms
include benzyne mechanism and free radical
(SRN1) mechanism. Common
reactions of substituent groups on benzene ring include:
- Conversion of halogens
into other various substituents
- Modifying activating substituents
- Oxidative degradation of
- Reduction of
nitro or carbonyl substituents
- Reversibility of the aromatic sulfonation reaction