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Part 7.
The chemistry of
AROMATIC COMPOUNDS
Doc
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
K12 grade 11 grade 12 organic chemistry
Part 7.6
Electrophilic substitution -
ring halogenation of benzene & methylbenzene, properties & uses of
chloro-aromatics and other aryl halides
Sub-index for this page
7.6.1
Halogen ring substitution reaction
(Cl and Br),
reagents, conditions and equations for synthesising aryl halides
7.6.2
The electrophilic substitution
mechanism yielding aryl halides
7.6.3
The physical properties of aryl halides
- halogenated arenes
7.6.4
Some chemical reactions of aryl halides
and theoretical aspects
7.6.5
The uses of aryl halides
(x-ref phenols 7.9 when
written !!!)
INDEX of AROMATIC CHEMISTRY
NOTES
All Advanced A Level Organic
Chemistry Notes
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7.6.1 Halogen substitution
reaction, reagents, conditions and equations for synthesising aryl
halides
Examples of aromatic
chlorination/bromination substitution reactions
The arene hydrocarbon is mixed with
anhydrous aluminium chloride (catalyst) and chlorine passed through the mixture at
room temperature, substitution takes place.
The aluminium chloride catalyst is
referred to as the halogen carrier and generates the attacking electrophile.
Iron(III) chloride can also be used as
the catalyst, and can be made in situ by adding iron filings to the mixture
before bubbling chlorine through:
2Fe + 3Cl2 ===> 2FeCl3
All reagents and glassware must be dry
- anhydrous aluminium chloride hydrolyses with water.
The exothermic reaction is quite rapid,
even at room temperature and clouds of hydrogen chloride or hydrogen bromide
are given off - hopefully in a fume cupboard.
(a)
+ Cl2 == AlCl3 ==>
+ HCl
benzene + chlorine
===> chlorobenzene + hydrogen chloride
-
(b)
+ Cl2 ===> 
 
+ HCl
methylbenzene +
chlorine ===> chloro-2-, chloro-3 or chloro4-methylbenzene + hydrogen chloride
Three positional structural isomers of
C7H7Cl
formed in different proportions.
-
 You
do NOT get (chloromethyl)benzene, (shown on the right), which is a
side-chain substitution product.
(chloromethyl)benzene is formed if uv light is used
instead of aluminium chloride.
(c)
+ Cl2 ===>
+ HCl
chlorobenzene + chlorine ==>
1,2- or
1,3- or
1,4-dichlorobenzene + hydrogen chloride
Three positional structural isomers of
C6H4Cl2 formed in different
proportions.
-
The reagents and conditions for bromination
The arene is mixed with anhydrous iron(III) bromide
(catalyst) and liquid bromine in a flask fitted with a reflux condenser -
with gentle heating.
Iron(III) bromide can be made in situ by adding excess
bromine to iron filings.
2Fe + 3Br2 ===> 2FeBr3
The iron(III) bromide catalyst is
referred to as the halogen carrier and generates the attacking electrophile
(see
mechanisms in next section).
All reagents and glassware must be dry
- iron(III) bromide hydrolyses with water.
The equations for bromination are
identical, except just swap Br for Cl e.g.
(d)
benzene + bromine
===> bromobenzene + hydrogen bromide
C6H6 + Br2 ===>
C6H5Br + HBr
(e)
methylbenzene + bromine
===> bromo-2/3/4-methylbenzene + hydrogen bromide
C6H5CH3 + Br2 ===>
CH3C6H4Br + HBr
Three positional structural isomers of
CH3C6H4Br formed in different
proportions.
TOP OF PAGE and
sub-index
7.6.2 The electrophilic substitution
mechanism yielding aryl halides
Mechanism diagram 21 -
A general electrophilic substitution by halogen in a benzene ring.
For Al and Cl you can substitute
Fe and Br i.e. using an FeBr3 catalyst (halogen
carrier).
[mechanism
21 above] When R = H, benzene forms
chlorobenzene.
Step
(1) The non-polar and uncharged
chlorine molecule is not a strong enough an electrophile to disrupt
the
pi electron system
of the benzene ring.
The aluminium chloride reacts with a chlorine
molecule to form a positive chlorine ion Cl+
which is a much stronger electron pair accepting
electrophile and a tetrachloroaluminate(III) ion (either this or an
Cl2-AlCl3 complex acting as a halogen carrier-
see version 3 on mechanism diagram 80C further down the page).
Step
(2) When the
electrophile attacks, an electron pair from the
delocalised
pi electrons of the
benzene ring forms a C-Cl bond with the electron pair accepting
positive chlorine ion forming a highly unstable carbocation.
It is
very unstable because the stable electron arrangement of the benzene
ring is partially broken to give a 'saturated' C (top right of
ring).
Step
(3)
The
tetrachloroaluminate(III) ion, formed in step (1), abstracts a
proton from the highly unstable intermediate carbocation to give the
chloro-aromatic product, hydrogen chloride gas and reforms the
aluminium chloride catalyst.
The mechanism is similar for:
C6H5CH3
+ Cl2 ===> ClC6H4CH3
+ HCl
where R = CH3,
methylbenzene forms a mixture of chloro-2/3/4-methylbenzene (3
positional isomers).
chloro-3-methylbenzene is the minority product and the mechanism above
would show the formation of chloro-2-methylbenzene when R =
CH3.
FURTHER COMMENTS
The overall halogenation
reaction is the substitution of -H by -Cl
Bromination
can be carried in a similar way, so, you can write out the
mechanism in exactly the same way, but putting in Br instead of Cl and
FeBr3 instead of AlCl3.
REMINDER: Why do
aromatic
compounds tend to react by
electrophilic substitution BUT
alkenes tend to react by
electrophilic addition?
They both
interact with electrophiles because they both have 'electron
rich' electron pair donating bonding systems i.e. the >C=C<
double bond in alkenes and the delocalised π electrons of
the benzene ring.
But the benzene ring has a particularly high
stability which is preserved on substitution rather than
addition.
For the same
reason alkenes are generally more reactive than arenes.
Don't confuse this
electrophilic substitution in the benzene ring with what happens
if methyl benzene
is reacted with chlorine in the presence of uv light, substitution
takes place in the alkyl side chain.
In other words it behaves like
an alkane and undergoes a free radical substitution reaction.
The initial product is chloromethylbenzene, C6H5CH2Cl,
and further substitution products can be formed C6H5CHCl2
and C6H5CCl3.
For more see
side-chain chlorination
substitution of the methyl group of methylbenzene
Some specific mechanism diagrams based on
the general mechanism diagram 21
Mechanism diagram 80A
The electrophilic substitution of a
chlorine atom into the ring of benzene
The chlorine and aluminium chloride
generate the attacking electrophile Cl+ and the
tetrachloroaluminate ion.
The Cl+ attacks the pi
electron cloud of the benzene ring and combines with benzene to form the
unstable intermediate cation (partially, and temporarily, saturated).
The AlCl4- ion
then abstracts a proton from the carbocation to yield the desired
product chlorobenzene, hydrogen chloride ('waste') and the re-formed
AlCl3 catalyst.
Mechanism diagram 80C
Three versions of how an
electrophile is generated to attack benzene (and other aromatics) to
yield a carbocation
Version 1:
This is the simplest approach to expressing this electrophilic
substitution reaction and one I am using on this page at the moment.
Version 1 involves a pre-stage
step that generates the hypothetical electrophile X+ ,
from an X2 + MX3 collision, which attacks the
pi electron system of the benzene ring.
Version 2:
This is found in older textbooks and shows all the electron shifts
necessary, but is not considered the best
representation of the 'real' mechanism - it implies the simultaneous
collision of three particles, which is highly improbable.
Version 3:
This the best correct version that would be presented at university
level courses.
Version 3 also involves a
pre-stage step that generates the hypothetical electrophile, but not
a simple X+ ion.
The electrophile is a 'molecular'
combination of the halogen X2 and the catalyst MX3.
and it is this 'halogen carrier' that attacks the pi electron system
of the benzene ring and delivers the halogen atom into combination
with the benzene ring.
Mechanism diagram 80F
The reaction progress profile for the
electrophilic substitution of a chlorine atom into the ring of benzene
Reaction profiles usually start with
the reactants, but here I've started with the arene (e.g. benzene) and
the carbocation.
The first activation energy is high
because it involves breaking open the stable pi orbital rings of
electrons and is the slower of the two steps shown in the diagram.
The final step is faster with a lower
activation energy as the proton is removed and the strong C-X bond is
formed.
Mechanism diagram 80B
The electrophilic substitution of a
bromine atom into the 2 position of the benzene ring of methylbenzene
The bromine and iron(III) bromide
generate the attacking electrophile Br+ and the
tetrabromoiron(III) ion.
The Br+ attacks the pi
electron cloud of the benzene ring and combines with methylbenzene to
form the unstable intermediate cation, which is partially, and
temporarily saturated - at carbon atom 2 in terms of aromatic ring
nomenclature.
The FeBr4- ion
then abstracts a proton from the carbocation to yield the desired
product bromo-2-methylbenzene, hydrogen chloride ('waste') and the
re-formed FeBr3 catalyst.
Mechanism diagram 80D
The electrophilic substitution of a
chlorine atom into the 4 position of the benzene ring of methylbenzene
The chlorine and aluminium chloride
generate the attacking electrophile Cl+ and the
tetrachloroaluminate ion.
The Cl+ attacks the pi
electron cloud of the benzene ring and combines with methylbenzene to
form the unstable intermediate cation, which is partially, and
temporarily saturated - at carbon atom 4 in terms of aromatic ring
nomenclature.
The AlCl4- ion
then abstracts a proton from the carbocation to yield the desired
product chloro-4-methylbenzene, hydrogen chloride ('waste') and the
re-formed AlCl3 catalyst.
Mechanism diagram 80E
The electrophilic substitution of a
chlorine atom into the 4 position of the benzene ring of chlorobenzene
The chlorine and aluminium chloride
generate the attacking electrophile Cl+ and the
tetrachloroaluminate ion.
The Cl+ attacks the pi
electron cloud of the benzene ring and combines with chlorobenzene to
form the unstable intermediate cation, partially, and temporarily,
saturated - at carbon atom 4 in terms of aromatic ring nomenclature.
The AlCl4- ion
then abstracts a proton from the carbocation to yield the desired
product 1,2-dichlorobenzene, hydrogen chloride ('waste') and the
re-formed AlCl3 catalyst.
TOP OF PAGE and
sub-index
7.6.3 The physical
properties of aryl halides - halogenated arenes
Abbreviations used: mpt =
melting point oC; bpt = boiling point oC; sub. = sublimes;
dec. = thermally decomposes
| Name |
Structure |
Mpt/oC |
Bpt/oC |
Comments |
| chlorobenzene |
|
-45 |
132 |
|
|
1,2-dichlorobenzene |
|
-17 |
179 |
3 positional
structural isomers of C6H4Cl2 |
|
1,3-dichlorobenzene |
-24 |
172 |
|
1,4-dichlorobenzene |
53 |
174 |
|
chloro-2-methylbenzene |
|
|
|
3 positional
structural isomers of CH3C6H4Cl |
|
chloro-2-methylbenzene |
|
|
|
chloro-2-methylbenzene |
|
|
| bromobenzene |
|
-31 |
156 |
|
|
1,2-dibromobenzene |
|
7 |
221 |
3 positional
structural isomers of C6H4Br2 |
|
1,3-dibromobenzene |
|
-7 |
220 |
|
1,4-dibromobenzene |
|
87 |
218 |
| iodobenzene |
|
-31 |
189 |
|
|
1,2-diiodobenzene |
|
27 |
286 |
|
|
1,3-diiodobenzene |
|
40 |
285 |
|
|
1,4-diiodobenzene |
|
129 |
sub. 285 |
|
| |
|
|
|
|
Notes
(a) All the above aryl halides are insoluble in
water.
Generally, they are not highly polar and cannot
hydrogen bond with water.
(b) For the size of molecule (e.. in terms of
electrons in molecule, they have relatively low melting/boiling
points.
Most of the intermolecular force is due to
instantaneous dipole - induced dipole interactions, plus a
smaller contribution from the permanent dipole - permanent
dipole forces (
Cδ+-Clδ-) of the polar bond.
(c)
TOP OF PAGE and
sub-index
7.6.4 Some chemical reactions of aryl halides
and theoretical aspects
(a) Hydrolysis of aryl halides to phenols
- with difficulty!
Bond enthalpies quoted from
https://www2.chemistry.msu.edu/courses/cem850/handouts/Ellison_BDEs.pdf
Bond lengths quoted from
https://onlinelibrary.wiley.com/doi/pdf/10.1002/9783527616091.app1
Data table
| Molecule |
Specific C-Cl
Bond |
Bond enthalpy/ kJ/mol |
C-Cl Bond length/ nm |
Comments |
| chloroethane |
CH3CH2-Cl |
355 |
0.179 |
Pure saturated aliphatic
σ
C-Cl bond, no delocalisation connection. Bond order 1.0,
longer than chloroethene. |
| chloroethene |
CH2=CH-Cl |
382 |
0.173 |
σ
C-Cl bond plus a 'partial
C-Cl π bond' from the
partial overlap of Cl's non-bonding electron orbitals with the
delocalised π C=C
alkene bond. Bond order >1.0 <1.5 |
|
(chloromethyl)benzene |
C6H5CH2-Cl |
310 |
0.179 |
Pure saturated aliphatic
σ
C-Cl bond, no delocalisation connection. Bond order 1.0,
longer than chlorobenzene. |
| chlorobenzene |
C6H5-Cl |
406 |
0.170 |
σ
C-Cl bond plus a 'partial
C-Cl π bond' from the
partial overlap of Cl's non-bonding electron orbitals with the
delocalised aromatic π
bond. Bond order >1.0 <1.5 |
There is a pattern from the table which
makes two points as to why chloro-aromatic compounds hydrolyse more slowly
than aliphatic halogenoalkanes.
(i) The C-Cl bond in the benzene ring
(and chloroethene) is
shorter than the C-Cl in aliphatic halogenoalkanes.
(ii) The C-Cl bond in the benzene ring
(and chloroethene) is stronger than the C-Cl in aliphatic halogenoalkanes.
(iii) We can also add the point that
the C-Cl bond in the benzene ring (and chloroethene) is
less polar than the C-Cl in aliphatic halogenoalkanes because the
redistribution of the charge compared to the full effect of the more
electronegative chlorine atom on the C-Cl bond..
The cause of the stronger C-Cl bond
in chlorobenzene and other chloro-aromatics is the non-bonding pairs of
electrons on the chlorine atom form a partial pi C-Cl bond making it
stronger (diagram below).
The same thing seems to happen
with chloroethene, which is also much harder to hydrolyse compared
to chloroethane.
compare with
So we can contrast the
slow hydrolysis of aromatic chlorobenzene or isomeric
chloro-2/3/4-methylbenzenes (aryl halides) with (chloromethylbenzene),
with the relatively much faster hydrolysis an aliphatic halogenoalkane on the basis of the three points above.
Another approach to understanding the
lack of reactivity of aryl halides towards nucleophiles is to consider the
possible resonance structures that contribute to a 'conceptual' resonance
hybrid structure.
Industrial hydrolysis of aryl
halides
If heated under pressure, to increase
reaction temperature, chlorobenzene can be made to hydrolyse to
phenol.
Typical industrial reaction
conditions are a temperature of 350oC, 300 atm pressure
and using aqueous sodium hydroxide as the hydrolysing agent.
These more extreme conditions
compared to ordinary 'laboratory' reflux preparations are required
because of the lesser reactivity of chlorobenzene towards
nucleophiles compare to aliphatic halogenoalkanes.
Initially, (i) sodium phenoxide
(sodium phenate) is formed in the hydrolysis reaction, and (ii) the
phenol released by adding a stronger acid than phenol itself (a very
weak acid) - cheap hydrochloric acid will do.
(i)
(aq) + 2NaOH(aq) ===>
(aq) + NaCl(aq) + H2O(l)
ionically:
C6H5Cl(aq)
+ OH-(aq) ===> C6H5OH(aq)
+ Cl-(aq)
(ii)
+ HCl ===>
+ NaCl
ionically:
C6H5O-(aq)
+ H+(aq) ===> C6H5OH(s)
(b) Electrophilic substitution - nitration
Aryl halides can be nitrated like
many other aromatic compounds e.g.
+ HNO3 ===>
+ H2O
using a mixture of concentrated
nitric and sulfuric acids.
Chlorobenzene is less reactive than
methylbenzene, so the reaction is slower and the reaction mixture needs
heating under reflux.
Also, what is the major product and
why?
Although the electronegative chlorine
atom reduces the reactivity of chlorobenzene compared to benzene, the
reduction is greater at the 3 and 5 positions on the benzene ring - the
yields quoted below bare this out.
The typical yields (from left to
right in the equation) are chloro-2-nitrobenze (~30%),
chloro-3-nitrobenzene (<1%) and chloro-4-nitrobenzene (~70%), so over
99% of the substitution occurs at the 2 (= 6) and 4 positions.
On a purely random basis for a
disubstituted aromatic, you would expect 40%, 40% and 20% (diagram
below).
(c) Electrophilic substitution -
further ring halogenation
This reaction has already been
discussed in detail in sections
7.6.1 and
7.6.2.
+ Cl2 == AlCl3 ==>
+ HCl
However, what wasn't discussed was,
what is the major product and why?
TOP OF PAGE and
sub-index
7.6.5 The uses of aryl halides - aromatic organo halogen
compounds
From section 7.6.4 (a) we can see that
chlorobenzene is used to manufacture phenol, from which a huge number of
other phenolic compounds are made.
See uses of
chloro-phenols
Aromatic chloro compounds (aryl halides) are
widely used as insecticides, herbicides, fungicides, and bactericides.
However their over-use has serious
environmental problems because they are non-biodegradable, soluble in
fatty tissue and accumulate up food chains - toxicity increasing,
causing genetic damage and death.
Their lack of biodegradability is
often due to the strong C-Cl bonds on the aromatic ring.
As we learn more and more about the
effect on organisms, more and more of aryl halides are being banned or
their use strictly controlled and limited.
For example,
hexachlorophene (structure below) is an external bactericide that was widely used in
cosmetic preparations such as soaps, deodorants, but evidence has emerged that it can be absorbed through
the skin in potentially dangerous amounts, affecting infants and small
children.
Note that apart from the C-Cl
group, hexachlorophene has two other functional groups i.e. primary
aliphatic halogenoalkane and phenol.
Other aryl halides have been used as pesticides
like DDT (dichlorodiphenyltrichloroethane, above) and the
herbicides 2,4-D (2,4-dichlorophenoxyethanoic acid, above) and 2,4,5-T
(2,4,5-trichlorophenoxyethanoic acid, above) have been partially banned for
environmental
reasons.
DDT still used to control
mosquito populations that spread malaria.
2,4-D and 2,4,5-T were used in 'Agent
Orange' defoliation missions by the US in the Vietnam War (to deny the
Vietcong soldiers cover), and their adverse poisoning effects are still
happening today.
Note these two molecules, apart from
C-Cl, have two other functional groups i.e. ether and carboxylic acid.
Aryl bromides are used as flame
retardants - compounds that inhibit the burning of combustible materials.
Overall use of aryl halides is in decline
(thank goodness!) out of concern for their impact on the environment - their
lack of biodegradability - persistence in the environment and building up
concentrations in food chains - bad for ecological systems.
BUT, they are important intermediates in the
manufacture of other useful compounds e.g. phenol from chlorobenzene.
They are rarely biodegradable and many
are carcinogenic, though some of the chloro-phenols can be used safely as
bactericides-disinfectants.
See uses of
chloro-phenols
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CHEMISTRY NOTES
All
Advanced Organic Chemistry Notes
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