Advanced Higher Chemistry
Unit 3
Aromatics
Aromatics
Aromatics are hydrocarbons containing the
benzene ring (C6H6).
The systematic name for the family of alkyl
substituted aromatic hydrocarbons is the
arenes.
Bonding in Benzene
Structure (a) was the first sturcture proposed by Kekule in 1865
however this structure was proposed incorrect with x-ray diffraction
results and the fact that benzene is resistant to addition reactions. (b)
and (c) are resonance structures and differ only in the
arrangement of the electrons but do not actually exist . The real
structure of benzene is best represented by (d)
The six carbons in benzene are sp2 hybridised.
The carbons are bonded to each other and the
hydrogen atoms by sigma bonds.
Each carbon has a p orbital containing a single
electron that can overlap with neighbouring
orbitals to form pi bonds.
This area of negative charge is above and
below the ring (delocalised p electron system).
The six electrons become
delocalised and occupy two
continuous doughnutshaped electron clouds
above and below the planar
sigma framework.
The delocalisation of the electrons help to bond
the atoms more tightly together. The result is a
completely symmetrical molecule, with
considerable stability.
The term ‘aromatic’ can be defined to describe
any system that contains a ring of atoms
stabilised by delocalised pi electrons.
Molecules with straight or branched chains are
described as aliphatic.
Reactions of benzene
Benzene tends to undergo electrophilic
substitution reactions.
The areas of high electron density attract positive
reagents.
The electrophile attacks one of the carbon atoms
forming an intermediate ion with a positive
charge.
The intermediate ion then loses a hydrogen ion.
The aromatic nature of the hydrocarbon is then
restored.
Reactions with bromine or chlorine
Benzene can undergo electrophilic substitution by
bromine in the presence of a catalyst (e.g. iron (III)
bromide, iron (III) chloride, aluminium (III) chloride).
There are parallels in the catalysed reaction of bromine
with benzene with the reaction of bromine to an alkene.
Since benzene is less reactive than alkenes, the catalyst
is needed to polarise the bromine molecule creating an
electrophilic centre.
The partially positive bromine atom can then
attack the benzene ring. Heterolytic fission of the
Br-Br bond occurs to form a carbocation, just like
the bromination of an alkene.
However, the similarity ends there and, by losing
a hydrogen ion in the final step, the aromatic
system is restored and the catalyst regenerated.
The overall effect is that one of the hydrogen
atoms of the benzene molecule has been
replaced by a bromine atom i.e. an electrophilic
substitution reaction has taken place.
In the dark, chlorine undergoes a similar
reaction with benzene to form chlorobenzene.
In the light, chlorine adds to benzene by a
different mechanism to form 1,2,3,4,5,6hexachlorocyclohexane (one of the very few
addition reactions of the benzene ring).
Nitration of benzene
Benzene reacts with a mixture of conc.nitric acid and
conc. sulphuric acid to from nitrobenzene.
This reaction is of great importance to industry since
the nitrobenzene can be reduced to produce
phenylamine, C6H5NH2, also known as aniline, which
is an important intermediate in the manufacture of
dyes.
The mechanism for nitration involves
electrophilic attack by the very reactive nitronium
cation NO2+, which is produced when conc. nitric
acid reacts with conc. sulphuric acid
The nitronium cation is a powerful electrophile
and attacks the benzene molecule as shown.
Further substitution can occur to produce
di- and tri- compounds.
Further substitution can occur to produce
di- and tri- compounds.
Sulphonation of benzene
Benzenesulphonic acid can be produced by
electrophilic substitution if benzene is heated with
conc. sulphuric acid under reflux.
The same product is formed in the cold when using
fuming sulphuric acid, which is a solution of sulphur
trioxide in conc. sulphuric acid.
The electrophile in sulphonation is believed to be the
SO3 molecule, either free or in combination with acid,
which helps to turn it into a strong enough electrophile
to attack the benzene molecule.
The SO3 molecule is electron deficient and carries a
partial positive charge on the sulphur atom.
The mechanism is similar to that for nitration.
This reaction is important in the production of
synthetic detergents.
Alkylation of benzene : Friedel-Crafts Reaction
As for the reaction of benzene with bromine, a
halogenalkane is made polar by the action of a
catalyst (e.g. aluminium (III) chloride).
The C-Cl bond is already polar. The catalyst increases
the polarity and may even cause the bond to break
heterolytically to form a carbocation.
In either case, the power of the electrophile is
increased to allow it to attack the benzene ring to
form a monoalkylbenzene
This reaction is used industrially in the
manufacture of synthetic dyes, the production of
phenol and propanone, and in the manufacture of
nylon.
Although electrophilic substitution is the typical
reaction of the benzene ring, the presence of
substituents on the ring has an influence on the
outcome of this reaction.
Some substituents increase the
susceptibility to electrophilic attack while
others decrease it.
On the other hand, the benzene ring also
has an influence on the behaviour of such
substituents.
In the next slides phenol and phenylamine
(aniline) will be considered.
Phenol
Phenols are aromatic compounds with a hydroxyl
group attached to the benzene ring.
The OH bond is polar and can be broken
heterolytically to produce H+ ions, under certain
circumstances. The OH group is therefore potentially
acidic.
Phenol has a higher Ka value than ethanol therefore is
more acidic.
When phenol acts as an acid, it produces the
phenoxide ion in which the negative charge on the
oxygen atom can be partly delocalised into the
aromatic pi system.
This delocalisation can be illustrated using possible
resonance structures for the phenoxide ion.
The resonance structures show how the negative
charge on the oxygen atom is decreased, thus
stabilising the phenoxide ion and making it less
likely to accept a proton.
This effect is confirmed by phenol’s Ka
value lying between an alcohol and an
alkanoic acid i.e. phenol is more acidic
than an alcohol but less acidic than an
alkanoic acid.
Phenylamine
Phenylamine (aniline) is an aromatic, basic amine.
The lone pair of electrons on the nitrogen atom can
accept a hydrogen ion and form the
phenylammonium ion.
Phenylamine is a weak base as the Ka of the resulting
conjugate acid is high.
Base
Conjugate acid
Ka at 25oC
ammonia
Ammonium ion
5.5. x 10-10
ethylamine
Ethylammonium
ion
phenylamine Phenylammonium
ion
1.9 x 10-11
2 x 10-3
Phenylamine is less basic than both ammonia and
ethylamine.
The decrease in basicity can be explained by looking
at its resonance structures.
The lone pair of electrons on the nitrogen atom are
delocalised into the aromatic pi system therefore the
nitrogen atom is less able to act as a base.
Such delocalisation stabilises the phenylamine ion in
a way that is not possible for the
phenylammoniumion.
Exercise
Now try the exercise on page 57 of your
Unit 3(b) notes.