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 7.2
Proof of benzene structure, aromaticity and introduction to electrophilic substitution reactions
INDEX of AROMATIC CHEMISTRY
NOTES
All Advanced A Level Organic
Chemistry Notes
Sub-index for this page
7.2.1
Comparison of cyclic hydrocarbon
molecules
7.2.2
Enthalpy of hydrogenation evidence for
the 'real' structure of a benzene ring
7.2.3
X-ray crystallography - shape and bond
lengths evidence for the 'real' structure of a benzene ring
7.2.4
Resonance structures and lack of 'triene' isomers in disubstituted benzene compounds
as more evidence for benzene's structure
7.2.5
Influence
of the stability of the benzene ring on the chemistry of aromatic compounds -
more evidence - electrophilic substitution rather than addition
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7.2.1 A comparison of cyclic
hydrocarbon molecules
REMINDERS
Aliphatic compounds are
considered to be relatively simple compounds that have straight or
branched chains or rings of carbon atoms e.g. alkanes, alkenes,
halogenoalkanes, alcohols, ketones, carboxylic acids and amines.
The study of their chemical
properties is called aliphatic chemistry, and their chemistry
is not based on the presence of a benzene ring.
Aromatic compounds contain at
least one benzene ring, a ring of 6 carbon atoms with three
σ bonds for each carbon atom between
adjacent carbon atoms and a hydrogen atom.
The 4th
electron from each carbon atom becomes part of a delocalised π
bond ring system - all of this described and explained on this page
along with the experiment evidence for the structure of a benzene
ring.
This pi bond system gives these
molecule a particular chemical character known as aromatic
chemistry - aromaticity, which you need to be able to
distinguish from the functional group chemistry of aliphatic
compounds.
Aromatic compounds can
incorporate all of the functional groups you find in aliphatic
chemistry.
BUT, when these functional groups
are attached directly to a benzene ring, although the chemistry is
essentially the same, chemical differences can show up e.g.
reactivity or a reaction has become impossible for some reason.
The chemistry of natural products
often involves more complex molecules e.g. chlorophyll, DNA,
flavourings, glyceride ester fats/oils, hormones, pheromones, proteins.
Three types of relevant
cyclic hydrocarbon molecules
are illustrated below to illustrate the difference between cyclic
aliphatic compounds (alicyclic) and aromatic compounds with a benzene ring.
1. Aliphatic
saturated ring hydrocarbon compounds
cyclohexane, C6H12
or
AND methylcyclopentane, C6H12
or
For more details see
Molecular Structure and naming of ALKENES
2. Aliphatic
cycloalkene hydrocarbons e.g. with one or two C=C double bonds.
cyclohexene, C6H10
or
AND cyclohexa-1,3-diene, C6H8
or
Examples 1. and 2. are also referred
to as alicyclic compounds because they are cyclic aliphatic compounds.
For more details see
Molecular structure and naming of ALKENES
3. Arenes -
aromatic hydrocarbon compounds with one or more benzene rings.
benzene C6H6
or
AND methylbenzene C7H8
or
These represent the real structure of benzene
and methylbenzene (as I put the cart before the horse!).
All the C-C bond lengths are the same
with a C-C-C bond angle of exactly 120o.
The
cyclic structure of benzene was first proposed by the German chemist
Kekule in 1865, but he assumed that the ring consisted of alternate
single (C-C) and double bonds( C=C).
Representations of Kekule
structures of benzene are shown on the right AND they are still
widely used in aromatic chemical equations and mechanisms, so take
care!
The arguments for the real arene structures are
presented in the next two sections 7.2.2 and 7.2.3 on this page.
For more on naming see
Molecular structure and
naming of AROMATIC COMPOUNDS
TOP OF PAGE and
sub-index
7.2.2
The evidence from a comparison of enthalpies of hydrogenation
Below are five hydrogenation equations, together with the
enthalpies of hydrogenation.
I've used the latest enthalpy values from
https://webbook.nist.gov/chemistry/
The 5th equation assumes that benzene really is a 'triene',
so we will show in sections 7.2.2 and 7.2.3 that this is not the real
structure of benzene.
(1)
+ H2 ===>
or
+ H2 ===>
hydrogenation of
cyclohexene to cyclohexane
ΔHhydrogenation(cyclohexene)
= -118 kJ mol-1
This is a typical value for the hydrogenation of an
alkene with one C=C bond.
(2a)
+ 2H2 ===>
or
+ 2H2 ===>
Hydrogenation of a
1,3-cyclohexadiene to cyclohexane
ΔHhydrogenation(cyclohexa-1,3-diene)
= -229 kJ mol-1
This is just below the expected value for hydrogenating
two C=C bonds (2 x -118 = -236 kJ mol-1).
(2a)
+ 2H2 ===>
or
+ 2H2 ===>
Hydrogenation of a
1,4-cyclohexadiene to cyclohexane
ΔHhydrogenation(cyclohexa-1,4-diene/)
= -233 kJ mol-1
This is just below the expected value for hydrogenating
two C=C bonds (2 x -118 = -236 kJ mol-1).
(3)
+ 3H2 ===>
or
+ 3H2 ===>
hydrogenation of
the 'real' benzene to cyclohexane
ΔHhydrogenation(benzene)
= -208 kJ mol-1
This is well below the expected value for
hydrogenating three C=C bonds (3 x -118 = -354 kJ mol-1).
(4)
+ 3H2 ===>
or
+ 3H2 ===>
hydrogenation of
methylbenzene to methylcyclohexane
ΔHhydrogenation(methylbenzene)
= -205 kJ mol-1
This is considerable below the expected value for
hydrogenating three C=C bonds (3 x -118 = -354 kJ mol-1).
It should be noticed that this 'addition' reaction
requires a heated nickel catalyst because of the stability of
the benzene ring.
The hydrogen molecules are adsorbed on the Ni
surface and split into atoms.
These atoms are effectively very reactive
hydrogen radicals (H•)
which can break open the pi bond system and form new C-H
bonds until the saturated cycloalkane is formed.
(5)
+ 3H2 ===>
or
+ 3H2 ===>
This is a
'fictitious' theoretical equation
The enthalpy of
hydrogenation for this is theoretically about -354 kJ
mol-1.
If we assume (incorrectly) that benzene has a
cyclotriene structure
with alternating single and double bonds.
Discussion
of the above enthalpy of hydrogenation values:
Apart
from the first and simplest alkene, ethene, most enthalpies
of hydrogenation are about -120±6
kJ mol-1 (for 1 mole H2 per mole
alkene) and in the case of dienes about double that for two
moles of hydrogen per diene, which is what you might
reasonably expect.
The special case of
benzene - the aromatic ring structure of arenes -
aromaticity
However, on the basis
of these trends, the expected value for the complete hydrogenation of
benzene and other aromatic compounds with a single benzene ring would be
around 3 x -118 = ~-354 kJ mol-1, but not so!
In
fact the energy released on hydrogenating benzene (208
kJ/mole) is even less than hydrogenating a diene!
So,
something must be different about benzene but it can be
explained with the enthalpy level diagram shown below and an
examination of possible molecular structures.
Also note the comparison
of equations 8. and 10. from above.
+ 3H2 ==>
(actual) and theoretical for a cyclotriene
+ 3H2 ==>
('fictitious')
The
'top' molecule in the diagram shows the theoretical
structure of a triene with the same molecular formula of
benzene (C6H6) and, if it existed in
this form it would be called cyclohexa-1,3,5-triene or
1,3,5-cyclohexatriene.
This
molecular structure assumes there are simple alternate
single (C-C) and double (C=C) carbon-carbon bonds.
BUT,
according to the actual thermochemical data calculated and
derived from e.g. enthalpies of hydrogenation, benzene is
already more stable by ~149 kJ mol-1, so, whatever
its structure, it cannot have this 'triene' structure.
This lowering of the potential energy of the benzene
molecules is referred to as the resonance stabilisation energy.
The concept of resonance structures is discussed in section
7.2.4
What
happens in reality, is that the equivalent of 3 double bonds
(C=C) and 3 single bonds (C-C) 'merge' to form a six equal
bonds each involve an average of 3 shared electrons.
Two of
the electrons for each bond are concentrated between the two
carbon atoms OR between a carbon atom and hydrogen atom,
both equivalent to a single bond known as a sigma
(σ) bond.
The 4th electron per
carbon atom is located in a ring
orbital, of two sections, above and below the plane of the
hexagonal ring of carbon atoms.
These are known as pi (π)
orbitals and each one contains 3 pi (π) electrons which are
delocalised around the ring.
This is indicated by the ring in the centre of the ring
either in skeletal formula or structural formula and
is a symmetrical planar hexagonal molecule.
Whenever charge is
delocalised or 'spread out' the potential energy of the system is lowered
and in the case of benzene about 149 kJ per mole compared to the triene
structure of alternate single C-C and double C=C bonds.
TOP OF PAGE and
sub-index
7.2.3 X-ray crystallography - electron density maps, shape and bond lengths
as
evidence for the 'real' structure of a benzene ring
Apart
from the thermochemical evidence argued above, X-ray
crystallography has shown:
The actual bond
length measurements show that there are six carbon -
carbon bonds all of equal length and intermediate between
single and double bonds.
The electron density map shows a symmetrical
distribution of the electron clouds that fitted a symmetrical hexagonal
planar ring of six carbon atoms.
BUT, the electron density map also showed
significant electron density in a ring above and below the plane of
the ring of 6 carbon atoms.
X-ray crystallography
also showed all the bond angles are 120o, that is
C-C-C or C-C-H due to 3 groups of bonding electrons around
each carbon atom of the benzene ring
It was also shown that benzene is a planar molecule
giving the 'benzene or
aromatic ring' a symmetrical hexagonal shape.
Summary of types of carbon - carbon bonds.
σ = single sigma bond;
π
= delocalised pi bond system
There term bond
order refers to the 'electronsworth' of the bond i.e. the
average number of shared electrons involved in that
particular bond.
Bond description |
Bond order |
Bond length/nm |
Bond enthalpy/kJmol-1 |
σ
single bond, C-C |
1.0 |
0.154 |
348 |
σ plus π,
 |
1.5 |
0.139 |
518 |
σ plus π double
bond,
C=C |
2.0 |
0.134 |
612 |
σ plus π, triple
bond,
C≡C |
3.0 |
0.120 |
837 |
Typical
bond lengths: single bond
C-C
is 0.154 nm (bond order 1)
e.g. in typical alkanes.
An aromatic ring bond
is
0.139
nm in length (bond order 1.5) e.g. in arenes like
benzene and methylbenzene. X-ray diffraction has shown this bond is shorter
than a single C-C bond, but not as short as an alkene C=C bond.
A double bond
C=C
is 0.134 nm in length (bond order 2)
e.g. in typical alkenes.
Incidentally the triple bond in alkynes has a
typical length of 0.120 nm in length (C≡C, bond order 3).
There is a clear trend of decreasing bond length with
increasing in bond order and increasing bond strength shown by the
increasing bond enthalpy.
How do not always think a high bond strength is always
associated with low reactivity e.g. compare the reactivity of alkanes
and alkenes.
TOP OF PAGE and
sub-index
7.2.4
Resonance structures
and lack of 'triene' isomers in disubstituted benzene compounds - more evidence
for benzene's structure
Resonance structures and aromaticity
Another approach to understanding the structure and chemical
behaviour of benzene is to consider the possible, but alternating resonance
structures (shown in the top half of the diagram above).
You can consider the two resonance structure as the only
two possible extremes of a 'triene' structure.
So, for benzene, the real aromatic structure is a
resonance hybrid of these two resonance structures.
You can think of the real structure of benzene as a sort
of 'blurred' fusion of the two resonance structures.
Other evidence against the Kekule structure - the lack of isomers
In this case
consider the Kekule structures of a 1,2-disubstituded benzene compound
If benzene was a triene (previous diagram) with alternate
C-C and C=C bonds, then structures 1 and 2 would be genuine positional
structural isomers of 1,2-dichlorobenzene.
Structure 1 has the two chlorine atoms across a
C=C double bond.
Structure 2 has the two chlorine atoms across a
C-C single bond.
They are clearly different structures, but no evidence
of there existence has ever been found.
Structure 3 is however, the proven structure of
1,2-dichlorobenzene and can only exist as a one unique structure.
However, don't confuse this argument with the three
genuine isomeric positional isomers of dichlorobenzene!
1,2 or
1,3 or
1,4-dichlorobenzene, C6H4Cl2
and 
TOP OF PAGE and
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7.2.5 Influence of the stability of the benzene ring on the
chemistry of aromatic compounds
In chemistry, aromaticity is a property of cyclic,
planar structures with pi bonds in resonance that gives increased stability
compared to other geometric or connective arrangements with the same set of
atoms, bonding with the same valencies.
Aromatic rings e.g. the benzene ring are very
stable and do not readily change in a completed reaction.
The hexagonal aromatic ring of carbon atoms e.g.
in arenes like benzene and methyl benzene tends to undergo electrophilic
substitution reactions, rather than electrophilic addition reactions
like
alkenes.
It is the very great stability of the benzene ring in aromatic
compounds means
that, unlike alkenes, they are reluctant to undergo addition reactions, and
this is best understood by considering the basics of electrophilic
substitution reactions undergone by aromatic compounds..
Addition of a pair of atoms to a benzene ring means the
stable aromatic ring is destroyed - this can be done, but not easily e.g. uv
light and chlorine or hydrogen, catalyst and a high temperature.
Therefore aromatic compounds tend to undergo electrophilic
substitution reactions involving the generation of a powerful electrophile
(electron pair acceptor) which subsequently attacks the electron rich
π (pi) electron system of the double bond.
In other words, arenes like benzene and methylbenzene tend
to undergo substitution, rather than addition, because substitution in the
ring allows
the very stable benzene ring to remain intact.
With the model portrayed above we can now consider the attack of an
electrophile on the pi bond electrons.
The general mechanism for electrophilic substitution
is shown above, though the source and/or generation of the electrophile is
not shown, so it is often, but not always, a 3-step mechanism.
The electrophile E+ is an electron pair acceptor.
The benzene ring is capable of donating a pair of
electrons from the pi orbitals of the aromatic ring to the incoming
electrophile.
The electrophile E+ attacks the pi orbitals (the
electron rich clouds - high electron density) above and below the plane of the benzene ring.
A C-E bond is formed and yielding a
carbocation - but notice that the delocalised system only extends
across five of the six carbon atoms of the benzene ring - you must draw
this accurately in exams.
It is effectively an electrophilic addition, just like
with alkenes, but here the similarity ends.
Instead of a further addition of a negative ion, as with
electrophilic addition to alkenes, the mechanistic pathway follows a
course that reforms the very stable delocalised electron system of
the benzene ring, which is at a lower energy than a saturated system
- think of the hydrogenation data and argument.
Since, in the carbocation, the stable delocalised system
of the benzene ring is broken, a proton (H+) is expelled and
the associated C-H bond pair of electrons rejoin the delocalised system,
so, the complete benzene ring of pi orbitals is re-formed (aromaticity
restored), yielding the stable substituted aromatic molecule.
e.g. a substituted benzene molecule C6H5E
or a substituted methylbenzene molecule EC6H4CH3.
In the above diagram for methylbenzene I've just
assumed that the substituent is on carbon atom 2 in aromatic
nomenclature, but other positions are available!
Overall the mechanism = an electrophilic
addition + an elimination (expulsion) ===>
substitution
Technically, the final step of this electrophilic
substitution mechanism involves the expelled proton adding to a base
(not shown in the above diagram)
The base is an electron donor, so one of two things
can happen.
H+ + :B ===> HB-
(anion formed) OR
H+
+ :B- ==> HB (neutral molecule
formed).
The diagram above shows a general reaction progress profile
for the electrophilic substitution mechanism for aromatic compounds e.g.
arenes like benzene and methyl benzene.
I've omitted the source and formation of the
electrophile and just used benzene for simplicity.
The dip is decreases in potential energy for the
formation of the carbocation.
Ea1
= the much larger initial activation energy for the attacking
electrophile to form a C-E bond from benzene's pi electrons.
Ea2
= the much smaller activation energy for the expulsion of a proton (that
will combine with a base) and the re-formation of the stable aromatic
ring to complete the substitution reaction.
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INDEX of AROMATIC
CHEMISTRY NOTES
All
Advanced Organic Chemistry Notes
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