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Aromatic compounds
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Introduction
Organic compounds
Aliphatic cpds Aromatic cpds
Aromatic compound an organic compound
that contains a benzene ring in its molecule. Benzene is an example of aromatic ring
with the molecular formula, C6H6.
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Aromatic compounds
Sometimes called arenes.
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Benzene
Colourless liquid with a characteristicsmell
Highly flammable and burns with a smoky
yellow flame Insoluble in water but soluble in all organic
solvents
Molecular formula: C6H6 The presence of three carbon-carbon
double bonds per molecule
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Types of aromatic
compounds
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Sidechain Substituted Compounds
the functional group is present in thesidechain of the ring.
Named as the phenyl derivatives of the
corresponding aliphatic compounds.
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Naming Benzene Derivatives
Only one kind of monosubstituted derivativesare possible in benzene rings as all six
hydrogen atoms are equivalent. For example,
there is only one toluene. It does not matterwhere the methyl group is attached because
all the following arrangements are equivalent.
When two same or different monovalent
substituents, are present on a benzene ring,
the following three arrangements are possible.
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For the same substituent (A)
ortho (or, 1,2-)
meta-(or, 1,3-)
para- (or, 1,4)
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For different substituents (A and B)
ortho (or, 1,2-)
meta-(or, 1,3-)
para- (or, 1,4)
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These arrangements are named as follows:
The compound containing the two groups onthe adjacent sites is called 'ortho'; it is denoted
as 'o-'. In the IUPAC system, the ortho
position is designated as 1,2-.
The compound containing the two groups on
alternate sites is called 'meta': it is denoted as'm-'. In the IUPAC system,the meta position is
designated as 1,3-.
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The compound containing the two groups
diagonally opposite to each other is called'para': denoted as 'p-'. In the IUPAC system,
the para position is designated as 1,4-.
For example, the three xylenes are named as,
o-xylene
(1,2-dimethylbenzene)
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m-xylene
(1,3-dimethylbenzene)
p- xylene
(1,3-dimethylbenzene)
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In the case of trisubstituted derivatives, the
nature of the substituted groups determine the
number of arrangements. When the three
substituent groups are identical (say, A), three
arrangements are possible. These are termed
as follows.
vicinal (vic-) unsymmetrical (unsym-)
symmetrical (sym-)
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For a trisubstituted product, if the two
substituents are identical and the third
different, then six products are possible. When
all the three groups are different, ten products
are possible.
Since, naming each individual compound is
not possible, it was found convenient to
indicate the position of any substituent by the
numeral indicating the serial number of thecarbon atom in the ring, to which that
substituent is attached.
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Numbering the Carbon Atoms in the Ring
The numbering of carbon atoms in the ring (ornucleus) is done as follows.
When there is only one substituent on thering, there is only one compound possible.
Thus, numbering of the carbon atoms of the
nucleus does not arises.
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If there are two or more substituents, then
numbering is in the alphabetical order of the
substituents on the carbon atoms. The
prefixes such as 'di', 'tri', 'cyclo', 'iso', etc., are
ignored while arranging the substituents
alphabetically.
When two or more functional groups are
present, then the principal functional group is
assigned the number 1. The order of priority ofthe functional groups is the same as done for
aliphatic polyfunctional compounds.
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For the sake of convenience, the ring is
oriented in such a way that position 1 is at the
top and numbering is done in a clockwise or
anticlockwise manner whichever gives lower
numbers to the other substituents.
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This is illustrated through the following example.The IUPAC names are written in bold letters.
(o-xylene) (m-xylene)
1,2-dimethylbenzene 1,3-dimethylbenzene
(p-xylene)
1,4-dimethylbenzene
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Names of some typical aromatic compounds are
given below:
Aromatic hydrocarbons (arenes)
toluene o-xylene
Methylbenzene 1,2-dimethylbenzene
m-xylene
1,3-dimethylbenzene
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p-xylene 1,3,5-trimethylbenzene
1,4-dimethylbenzene
2-phenylpropane (cumene) phenylethene (styrene)
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Anthracene Phenonthrene
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Halogen derivatives
2-chlorotoulene 1,2-dichlorobenzene
phenyl chloromethane
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Hydroxy derivatives:
Phenols and aromatic alcohols
2-methyl phenol phenylmethanol
1,4-dihydroxy benzene 2,4,6-trinitrophenol
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Aldehydes and Ketones
Nuclear substituted
Benzaldehyde Methyl phenyl ketone
Diphenyl ketone
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Carboxylic acids
Nuclear substituted
benzoic acid 2-methylbenzoic acid
2-hydroxybenzoic acid
1,4-benzenedicarboxylic acid
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Acid derivatives
Benzoyl chloride Benzamide
Phenyl benzoate
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Benzoic anhydride
Ethyl-4-bromobenzoate
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Alkoxy derivatives
Methoxy benzene 4-nitrobenzene
1-methoxy-phenoxybenzene
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Amines
Aminobenzene 2-Amino toluene
Benzyl amine
Nit d i ti
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Nitro derivatives
nitrobenzene 1,3-dinitrobenzene
2,4,6-trinitrotoluene
Nit il d C b l i
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Nitriles and Carbylamines
Benzonitrile Phenylcarbylamine
S l h i id
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Sulphonic acids
benzenesulphonic acid
benzene-1,3-disulphonic acid
N i A ti C d
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Naming Aromatic Compounds
Aromatic compounds are those carbocyclic
compounds that contain at least one benzenering.
The ring of six carbons in benzene or any of
its derivatives is termed benzene ring or
nucleus. A benzene ring may be represented
in any of the following ways.
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Any aliphatic group attached to the benzene
ring is termed as the sidechain as shown
alongside.
In this structure R may be CH3, CH2Cl, CH2OH
or CH2NH2.
However, the functional groups such as NO2,OH, NH2, etc., and any other non-carbon
substituent, linked directly to the nucleus are
called as the sidechain.
A ti d b l ifi d
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Aromatic compounds may be classified as
follows.
Monocyclic Aromatic Compounds Monocyclic aromatic compounds are those
that contain only one six-membered benzene
ring.
The following compounds are some examples
of monocyclic aromatic compounds.
M lti li A ti C d
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Multicylic Aromatic Compounds
Aromatic compounds containing two or morebenzene rings, condensed with each other,
are known as multicyclic aromatic compounds
or polynuclear aromatic compounds.
Some typical examples of polynuclear
aromatic compounds are,
A l G
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Aryl Group
Aromatic hydrocarbons are called arenes. When one or more hydrogens are removed
from the aromatic hydrocarbons, the residues
left behind are termed as aryl groups (similarto alkyl groups in the case of aliphatic
hydrocarbons). The aryl groups are
represented by Ar.
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Some typical aryl groups are:
Aromaticity: Importance Of Aromatic
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Aromaticity: Importance Of AromaticCompounds
Aromatic compounds are important in industry.Key aromatic hydrocarbons of commercialinterest are benzene, toluene, ortho-xylene andpara-xylene.
About 35 million tonnes are produced worldwideevery year.
They are extracted from complex mixtures
obtained by the refining of oil or by distillation ofcoal tar, and are used to produce a range ofimportant chemicals and polymers, includingstyrene, phenol, aniline, polyesterand nylon.
http://www.servinghistory.com/topics/aromatic_hydrocarbonshttp://www.servinghistory.com/topics/benzenehttp://www.servinghistory.com/topics/toluenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/styrenehttp://www.servinghistory.com/topics/phenolhttp://www.servinghistory.com/topics/anilinehttp://www.servinghistory.com/topics/polyesterhttp://www.servinghistory.com/topics/nylonhttp://www.servinghistory.com/topics/nylonhttp://www.servinghistory.com/topics/polyesterhttp://www.servinghistory.com/topics/anilinehttp://www.servinghistory.com/topics/phenolhttp://www.servinghistory.com/topics/styrenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/toluenehttp://www.servinghistory.com/topics/benzenehttp://www.servinghistory.com/topics/aromatic_hydrocarbons7/30/2019 5.Aromatic Compounds Whole
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Other aromatic compounds play key roles in
the biochemistry of all living things.
Three aromatic amino acids phenylalanine,
tryptophan, and tyrosine, each serve as one
of the 20 basic building blocks of proteins.
http://www.servinghistory.com/topics/phenylalaninehttp://www.servinghistory.com/topics/tryptophanhttp://www.servinghistory.com/topics/tyrosinehttp://www.servinghistory.com/topics/tyrosinehttp://www.servinghistory.com/topics/tryptophanhttp://www.servinghistory.com/topics/phenylalanine7/30/2019 5.Aromatic Compounds Whole
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Further, all 5 nucleotides adenine, thymine,
cytosine, guanine, and uracil that make upthe sequence of the genetic code in DNA
and RNA are aromatic purines or
pyrimidines.
As well as that, the molecule haem
contains an aromatic system with 22 electrons. Chlorophyll also has a similar
aromatic system.
http://www.servinghistory.com/topics/purineshttp://www.servinghistory.com/topics/pyrimidineshttp://www.servinghistory.com/topics/haemhttp://www.servinghistory.com/topics/Chlorophyllhttp://www.servinghistory.com/topics/Chlorophyllhttp://www.servinghistory.com/topics/haemhttp://www.servinghistory.com/topics/pyrimidineshttp://www.servinghistory.com/topics/purines7/30/2019 5.Aromatic Compounds Whole
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Reactions of Aromatic
Compounds
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Unlike alkenes, which undergo addition
reactions, aromatic compounds usually
undergo substitution reactions.
A group Y substitutes for one of the
hydrogen atoms on the aromatic ring
without changing the ring itself.
A substitution reaction
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The most common reaction of aromatic
compounds is the electrophilic aromatic
substitution. An electrophile (E ) reacts with an
aromatic ring
No addition reactions of alkenes:no decolorisation of bromine or potassium
manganate(VII) solution
no reaction with HCl, HBr or concentratedH2SO4
+
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Substitution reaction
1. Halogenation
2. Nitration
3. Friedel-Crafts reactioni. Alkylation
ii. Acylation
4. Sulphonation
H l i
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Substitution of a bromine or chlorine(Br/Cl) for a ring hydrogen occurs when
benzene reacts with Br2 or Cl2.
a hydrogen atom a halogen atom Catalyst: FeBr3 or FeCl3
Bromination
Halogenation
BenzeneBromobenzene
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Chlorination
Benzene Chlorobenzene
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Substitution of a nitro group ( NO2) for a
ring hydrogen occurs when benzene
reacts with nitric acid.
a hydrogen atom a nitro group, NO2
Catalyst: sulphuric acid, H2SO4
Nitration
Benzene Nitrobenzene
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i. Alkylation: the attachment of an alkyl
group to the benzene ring.
a hydrogen atom an alkyl group from
haloalkane to form an alkylbenzene
Benzene ring can be alkylated by
reaction with an alkyl chloride.
Catalyst: aluminium chloride
Friedel-Crafts reaction
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For example:
The Friedel- Crafts
alkylation ofbenzene with methyl
chloride
MethylbenzeneBenzeneMethyl
chloride
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ii.Acylation: a hydrogen atom an acyl
group from an acyl halide to form an
aromatic ketone
An acyl group, -COR is introduced onto
the ring when an aromatic compound
reacts with a carboxylic acid chloride,
RCOCl.
Catalyst: AlCl3
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For example
Reaction of benzene with acetyl chloride
yields the ketone, aceptophenone.
Benzene Acetyl chloride Aceptophenone
AlCl3
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Aromatic rings can be sulphonated by
reaction with fuming sulfuric acid, a
mixture of H2SO4 & SO3.
a hydrogen atom an SO3H group
the only reversible aromatic substitution.
Sulphonation is favored in strong acid;
desulphonation is favored in hot, dilute
aqueous acid.
Sulphonation
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Benzenesulfonic acidBenzene H2
SO4
+ SO3
For example
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Properties of aromatic
compounds
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Physical characteristic
Benzene is a colourless liquid with a
characteristic smell.
Both the liquid and the vapour of benzene
are highly poisonous.
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Density
Aromatic compounds are more dense
than other hydrocarbons, but less dense
than water
It has a density of 0.88g cm-3.
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Boiling points
In benzene, the only attractions between
neighbouring molecules are van der Waals
dispersion forces. There is no permanent
dipole on the molecule.
Benzene boils at 80C - rather higher than
other hydrocarbons of similar molecular
size (pentane and hexane, for example).
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This is presumably due to the ease with
which temporary dipoles can be set up
involving the delocalised electrons
It is highly flammable and burns with asmoky yellow flame.
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Melting points
Melting point of benzene is higher thanmethylbenzene.
Benzene melts at 5.5C while
methylbenzene melts at -95C. Molecules must pack efficiently in the
solid if they are to make best use of their
intermolecular forces. Benzene is a tidy, symmetrical molecule
and packs very efficiently.
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Solubility
The arenes are insoluble in water. Benzene is quite large compared with a water
molecule. In order for benzene to dissolve it would have
to break lots of existing hydrogen bondsbetween water molecules.
Energy would also needed to break the quitestrong van der Waals dispersion forcesbetween benzene molecules. Both of thesecost energy.
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Benzene is soluble in all organic solvents.
It is a very good solvent for fats, organic
compounds with low relative molecular
mass and inorganic substances such asiodine and sulphur.
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Methylbenzene
Methylbenzene is a colourless liquid which
smells like benzene.
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Boiling Point
Methylbenzene boils at 111C. It is a bigger
molecule and so the van der Waals
dispersion forces will be bigger.
Methylbenzene also has a small permanentdipole, so there will be dipole-dipole
attractions as well as dispersion forces. The
dipole is due to the CH3
group's tendency to
"push" electrons away from itself. This also
affects the reactivity of methylbenzene.
M lti P i t
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Melting Point Methybenzenes melting point (-95C) is
lower than that of benzene (5C) eventhough its relative molecular mass is
larger than benzene.
This because the planar molecules ofbenzene can pack close together and the
intermolecular forces of attraction are
therefore stronger.
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In contrast, the methyl group in
methylbenzene prevents such close packing.
Thus, the melting point of methylbenzene islower than that of benzene.
If the molecules aren't as closely packed, theintermolecular forces don't work as well and
so the melting point falls.
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Uses of aromatic
compounds
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Nitration
- Nitration of aromatic rings is a key step in
the synthesis of explosives such as TNT
(trinitrotoluene) & many importantpharmaceutical agents.
- Nitrobenzene is a starting material for
preparing many of the brightly coloureddyes used in clothing.
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Halogenation
- chlorination of an aromatic ring is a step
used in the synthesis of numerous
pharmaceutical agents, such as theantianxiety agent diazepam, marketed as
Valium.
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Sulphonation
- aromatic sulphonic acids are valuableintermediates in the preparation of dyes
and pharmaceuticals. Eg: the sulfa drugssuch as sulfanilamide were among the firstuseful antibiotics. These drugs areprepared commercially by a process that
involves aromatic sulphonation as the keystep.