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
7.14 Electrophilic substitution and
explaining the orientation of products on
putting a 2nd substituent into a previously monosubstituted benzene derivative
(perhaps aspects of 7.14 go beyond what is normally required in pre-university courses?)
Sub-index for this page
(please read the notes below this sub-index before studying 7.14.1)
7.14.1
Substitution in a
previously monosubstituted benzene derivative
7.14.2
The effect of an
activating group on substitution orientation in a monosubstituted benzene
derivative - 2, 4 and 6 position directing groups (ortho and para
substitution positions)
7.14.3
The effect of a
moderately deactivating group on substitution orientation in a monosubstituted benzene
derivative - 2, 4 and 6 position directing groups (ortho and para
substitution positions)
7.14.4
The effect of a
strongly deactivating group on substitution orientation of a monosubstituted benzene
derivative - 3 and 5 position directing groups (meta substitution
positions)
An
important note on old nomenclature and terms used:
With respect to an initial single
substituent in a benzene ring,
By convention, this is given position
1 (for the moment ignoring the conventions on naming).
Ortho
referred to positions 2 and 6,
meta referred to positions 3 and 5,
and
para referred to position 4.
This convention is still widely used
in textbooks, the internet and research papers.
Also the 'terms':
'Activate/activated'
means the substituent group of the monosubstituted aromatic
compound, makes it more susceptible to electrophilic attack i.e. an
increased reaction rate with an electrophile compared to benzene
itself - the electron density of the benzene ring is increased
compared to benzene itself.
'Deactivate/deactivated'
means the substituent group of the monosubstituted aromatic
compound, makes it less susceptible to electrophilic attack i.e. a
decreased reaction rate with an electrophile compared to benzene
itself - the electron density of the benzene ring is decreased
compared to benzene itself.
Here an
inductive effect
(+/- I) refers to the increase or decrease of the electron density of
the benzene ring due to the influence of the substituent group (G)
already present in the benzene ring C6H5-G.
Sub-sub-index of reactions covered to
give a disubstituted benzene derivative
7.14.2 Activating substituent groups (2 and 4
substitution)
nitration of methylbenzene
sulfonation of methylbenzene
benzene ring
chlorination of methylbenzene
benzene ring bromination of methylbenzene
nitration of phenol
benzene
ring bromination/chlorination of phenol
Electrophilic substitution of the benzene ring of an aromatic ether
7.14.3 Moderately deactivating groups (2 and 4
substitution)
nitration of chlorobenzene
nitration of bromobenzene
ring chlorination of bromobenzene
7.14.4 More strongly deactivating groups (3
substitution)
nitration of nitrobenzene
nitration of benzoic acid
INDEX of AROMATIC CHEMISTRY
NOTES
All Advanced A Level Organic
Chemistry Notes
[SEARCH
BOX]
TOP OF PAGE and sub-indexes
7.14.1
Substitution in a
previously monosubstituted benzene derivative
There are five possible substitution positions in a
monosubstituted benzene derivative.
So, what happens when this benzene derivative undergoes a
second substitution reaction i.e. where does the second substituent go in
the benzene ring? and why does the substituent end up in that position on
the benzene ring?
The diagram below illustrates the situation, where the
initial substituent group is shown as
G
(and for the rest of this page).
Obviously there are five possible positions in the benzene
ring for an electrophile to attack - denoted by the carbon ring numbers 2 to
6, for the possible 2nd substituent position (the original substituent is by
convention, position 1.)
If we assume for a 2nd substituent into the benzene
ring from an electrophilic substitution reaction:
(i) the electron density is the same for positions 2 to
6, i.e. assuming group G has no effect on the π
orbitals,
(ii) completely random attack collisions of the
electrophile i.e. equal probability of hitting any of the π
electron cloud adjacent to one of carbon atoms 2 to 6,
(iii) positions 2 and 6 are equivalent and positions 3
and 5 are equivalent too (molecular symmetry, and the only assumption
which is actually true!),
(iv) there is no steric hindrance of the nucleophilic
attack by the pre-existing substituent group G e.g. most likely to
affect benzene ring positions 2 and 6 closest to the substituent G group.
On this hypothetical basis you would expect an electrophilic substitution
reaction of a monosubstituted benzene derivative to yield the following:
40% substitution at ring carbon atom 2 (= 6),
40% substitution at ring carbon atom 3 (= 5), and,
20% substitution at ring carbon atom 4.
BUT, this is not what you find in the products from
aromatic electrophilic substitution reactions.
You can see this
from the data table below of TYPICAL YIELDS from further substitution of
monosubstituted benzene derivatives.
KEY to
three important abbreviations with reference to electrophilic substitution
in monosubstituted benzene derivatives:
(A24) In these molecules the benzene ring is
activated by substituent group G compared to benzene and tends to direct the
2nd substituent to the 2 or 4 position in the benzene ring.
See section 7.14.2
(D24) In these molecules the benzene ring is
moderately deactivated by substituent group G compared to benzene, but still tends to
direct the 2nd substituent to the 2 or 4 position in the ring.
See section 7.14.3
(D3) In these molecules the benzene ring is
more strongly deactivated by substituent group G compared to benzene, and tends to direct
the 2nd substituent to the 3 position in the ring.
See section 7.14.4
Substituent
group
G in C6H5-G |
Electrophilic
substitution reaction
(details of some reactions given in other sections) |
% yield
position 2
(ortho-product) |
% yield
position 3
(meta-product |
% yield
position 4
(para-product |
If
G has no
effect |
All 5 substitution position rates equal |
40 |
40 |
20 |
–O–CH3
(A24) |
Nitration of an ether |
35 |
1 |
64 |
–O–CH3
(A24) |
Friedel-Crafts acylation
of an ether |
7 |
3 |
90 |
–NO2
(D3) |
Nitration of nitrobenzene |
7 |
90 |
3 |
–CH3
(A24) |
Nitration of methylbenzene |
58 |
2 |
40 |
–CH3
(A24) |
Sulfonation of methylbenzene |
32 |
6 |
62 |
–CH3
(A24) |
Friedel-Crafts acylation of
methylbenzene |
11 |
4 |
85 |
–Br
(D24) |
Nitration of bromobenzene |
39 |
2 |
59 |
–Br
(D24) |
Ring chlorination of bromobenzene |
40 |
6 |
54 |
-COOH
(D3) |
Nitration of benzoic acid |
22 |
78 |
2 |
-C≡N
(D3) |
Nitration of benzonitrile |
17 |
81 |
2 |
-CO-CH3
(D3) |
Nitration of 1-phenylethanone |
26 |
72 |
2 |
-COOCH2CH3 |
Nitration of ethyl benzoate |
28 |
66 |
6 |
-CHO
(D3) |
Nitration of benzaldehyde |
19 |
72 |
9 |
-F
(D24) |
Nitration of fluorobenzene |
13 |
1 |
86 |
-Cl
(D24) |
Nitration of chlorobenzene |
35 |
1 |
64 |
-I
(D24) |
Nitration of iodobenzene |
45 |
1 |
54 |
-OH
(A24) |
Nitration of phenol |
63 |
3 |
34 |
-NHCOCH3
(A24) |
Nitration of
N-phenylethanamide |
19 |
2 |
79 |
If
G has no
effect |
All 5 substitution position rates equal |
40 |
40 |
20 |
The reason being that, the 1st substituent group G has a
profound effect on the π orbitals
(in particular) i.e. the electron density and susceptibility to
electrophilic attack of the other five carbon atoms of the benzene ring
e.g.
If the carbon atom that is bonded by
a sigma bond to the benzene ring also has a pi bond (e.g. >C-C=O), the
electron density of the ring is reduced and substitution is more difficult,
but less so in the 3 and 5 positions, where electrophilic substitution is
more likely to take place.
If the atom that is bonded to
the benzene ring does not have any pi bonding, but has at least one
non-bonding pair of electrons, the electron density may be either increased
(methylbenzene) or decreased (chlorobenzene) BUT the electron density
is relatively greater at the 2, 4 and 6 positions where electrophilic
substitution is more likely to take place,
This in turns leads to the orientation of products
(positions of 2nd substitution) that you might not expect.
The influence of the substituent group G on product
orientation can be due to:
(a) the charge distribution can depends on the polar
effect of G, +/- inductive effect.
Strongly electronegative groups reduce the
reactivity (deactivate) the benzene ring.
However their effect is varied, the most
electronegative groups often strongly deactivate the benzene ring,
and orientate substitution to the 3 position.
But, less strongly electronegative groups reduce
reactivity less and direct substituents to the 2 and 4 positions.
Also note that non-polar alkyl groups give a
small +I effect to increase the electron density and hence the
reactivity of the benzene ring.
(b) Sometimes a lone pair of electrons of
the atom of G, or a pi bond system in G, which is directly attached to the benzene ring, gives rise
to a resonance effect that can decrease or increases the electron densities in
the 2, 4 and 6 positions.
These two factors can work in opposition or complement
each other, and detailed explanatory arguments are beyond the scope of
this page, neither have I gone into too detail on hyperconjugation and mesomeric effects.
However, I have used the concept of 'inductive' effect
(+/- I), delocalisation of electrons, and drawn some
resonance structures for selected molecules to:
(i) help understand the
relative reactivity of monosubstituted benzene derivatives compared to
benzene itself and,
(ii) why certain positions on the benzene ring are
most favoured for substitution, which usually means carbon atoms 2,
4, 6 or 3,5 sets of positions.
Examples of
substituent group G and their orientation effect on the substitution
products - three types of effect.
Orientation and reactivity effects of the initial ring
substituent
group G (R = alkyl)
|
Activating
substituent G
directing to positions
2, 4 and 6
(ortho and para-orientation) |
Strongly
deactivating substituent G directing to positions
3 and 5
(meta-orientation) |
Moderately deactivating
substituents directing to
positions 2, 4 and 6
(ortho and para-orientation |
–O–
(phenoxide)
–OH (phenol)
–OR (ether)
–OC6H5
(ether)
–OCOCH3
(ester) |
–NH2
(amine)
–NR2
–NHCOCH3
–R (alkylbenzene)
–C6H5 |
–NO2
(nitrobenzene)
–SO3H
(benzenesulfonic acid)
–SO2R |
–CO2H
(benzoic acid)
–CO2R
(benzoate ester)
–CONH2
(benzamide)
–CHO (benzaldehyde)
–COR (ketone)
–CN (benzonitrile) |
–F
(fluorobenzene)
–Cl
(chlorobenzene)
–Br
(bromobenzene)
–I
(iodobenzene)
–CH2Cl
((chloromethyl)benzene) |
For
details of some examples see section 7.14.2 |
For
details of some examples see section 7.14.4 |
For details of some examples see section 7.14.3 |
Data adapted from
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/chapt15.htm
and other 'thick' textbook sources!
(i) 2, 4 and 6
directing groups
If the atom of group G is directly bonded to the benzene
ring does not have any π
bonding the ring is usually activated compared to benzene
itself.
The atom/group G tends to increase the electron density of
the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5
positions.
Therefore the 2 and 4 positions become the preferred
substitution points in the benzene ring e.g. -CH3, -CH2CH3,
-OH, -NH2, OCH3, groups usually promote 2- and 4-
position substitution.
The small electron density shift is sometimes described as a
plus inductive shift (+I effect), but this does not necessarily coincide
with an atom of electronegativity higher than carbon e.g. Cl, N and O.
The reason being the lone pairs of Cl, N and O interact with
the ring to increase the electron density and this electron pair donation
often overrides the difference in electronegativity effect and this is all
about conjugation and possible resonance hybrid structures - see sections on
this page for further details.
(ii) 3 and 5
directing groups
If the atom of group G is directly bonded to the benzene
ring does have any π
bonding the ring is usually deactivated compared to benzene
itself.
In this case the atom/group G tends to decrease the electron
density of the ring and more so at the 2, 4 and 6 positions, compared to the
3 and 5 positions.
Therefore the 3 position become the preferred substitution
point in the benzene ring e.g. -NO2, -COOH, -COOR, -C≡N,
COCH3, -SO2OH, groups usually promote 3- position
substitution.
The small electron density shift is sometimes described as a
minus inductive shift (-I effect), and often coincides with an atom of
electronegativity higher than carbon e.g. N and O.
BUT, π
bonding is involved too, and can facilitate electron pair withdrawal
from the benzene ring (again this is all about conjugation and possible
resonance hybrid structures - see sections on this page for further details.
The following
three sections now look at the three types of substituent groups and their
effect, with examples explained in more detail, of the effect of the 'G'
group.
TOP OF PAGE and sub-indexes
7.14.2
The effect of an
activating group on substitution orientation in a monosubstituted benzene
derivative - 2, 4 and 6 position directing groups (ortho and para
substitution positions)
Certain substituent groups in a benzene ring can increase
the electron density of the benzene ring and make the aromatic compound more
reactive towards electrophiles.
The effect is 'partial', and seems to enhance the reactivity at the 2 and 4
carbon atom substitution positions more than the 3 substitution position for
a 2nd hydrogen substituted group put into a benzene ring.
Groups
that increase reactivity are e.g. methyl -CH3,
hydroxy (phenol) -OH, amino/amine -NH2, acylated amine -NHCOCH3,
which greatly
favour substitution at the 2 and 4 positions (often >90%
combined and <10% for the 3 position).
This group of substituents all,
by some means, have a small, but significant, electron
donating (+I inductive effect) on the ring of
pi electrons.
This is shown in the diagram below where I've indicated the
1st substituent group as
G and its effect on the pi
electron clouds - the subsequent effect of the susceptibility of the
other 5 ring CH groups to electrophilic attack i.e. their relative
reactivity - the activating or deactivating effect of
G.
The diagrams above and below show the increase in electron
density of C6H5-G compared to C6H6
and below a more specific diagram for methylbenzene showing the increased
electron density at positions 2, 4 and 6 - the sites more susceptible to
electrophilic attack.
(a)
The nitration of methylbenzene
+ HNO3 ===>


+ H2O
Typical results for nitrating methylbenzene with conc.
nitric and sulfuric acids (electrophile NO2+).
The theoretical percentage yields are quoted as if the
methyl group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from nitrating methylbenzene |
Name of the isomer |
methyl-2-nitrobenzene |
methyl-3-nitrobenzene |
methyl-4-nitrobenzene |
Actual yield of each isomer |
58% |
4% |
38% |
Actual yield of 2- and 4- isomers |
96% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
The methyl group has a small, but significant +I
inductive effect that increases the electron density of the benzene
ring, particularly at the 2, 4 and 6 positions.
So, methylbenzene is significantly more reactive
than benzene and when nitrated, 96% of the products
are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.
If the G group is -NO2, you get deactivation.
The diagram below shows the 2, 4 and 6 activated sites
on the methylbenzene molecule.
The nitration reactivity order is: methylbenzene >
benzene > nitrobenzene
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(b)
The sulfonation
of methylbenzene
Benzene and methylbenzene are insoluble in sulfuric
acid, but the resulting sulfonic acids are soluble.
Therefore the sulfonation reaction is complete when the hydrocarbon
layer disappears.
Using fuming sulfuric acid (extra SO3
electrophile
available), at room temperature, benzene reacts in 20-30 minutes, but
methylbenzene only takes 1-2 minutes.
+ H2SO4 == ==>

 +
H2O
or
+ SO3 == ==>


The yield with fuming sulfuric acid at 35oC
are shown in the table below.
The electrophile is actually sulfur trioxide (sulfur(VI)
oxide, SO3).
The theoretical percentage yields are quoted as if the
methyl group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors - equal
chance of electrophilic attack by SO3.
The actual typical and theoretical
yields of products from sulfonating methylbenzene |
Name of the isomer |
methyl-2-benzenesulfonic acid |
methyl-3-benzenesulfonic acid |
methyl-4-benzenesulfonic acid |
Actual yield of each isomer |
32% |
6% |
62% |
Actual yield of 2- and 4- isomers |
94% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
What you see is the vast majority of the product involves
substitution at the 2 and 4 positions of the benzene ring in
methylbenzene. There is very little substitution at the 3 position.
This is due, as argued via the diagram above, because of the
increased electron density at the 2, 4 and 6 benzene ring positions from the
plus inductive effect of the methyl group of methylbenzene.
(c)
Benzene
ring chlorination of methylbenzene
+ Cl2 == FeCl3 ==>

+ HCl
The reagents chlorine and iron(III)/aluminum chloride
substitute a hydrogen atom with a chlorine atom in the benzene ring (NOT
in the methyl group).
The actual typical and theoretical
yields of products from ring chlorination of methylbenzene |
Name of the isomer |
chloro-2-methylbenzene |
chloro-3-methylbenzene |
chloro-4-methylbenzene |
Actual yield of each isomer |
58% |
<1% |
42% |
Actual yield of 2- and 4- isomers |
>99% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
What you see is the vast majority of the product involves
substitution at the 2 and 4 positions of the benzene ring in
methylbenzene. There is only a trace of substitution on ring carbon 3.
This is due, as argued via the diagram above, because of the
increased electron density at the 2, 4 and 6 ring positions from the
plus inductive effect of the methyl group of methylbenzene.
The theoretical percentage yields are quoted as if the
methyl group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors - equal
chance of attack from the Cl+ electrophile.
See also
Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides
(d)
Benzene ring bromination of methylbenzene
Using bromine and iron(III) bromide you can substitute a
hydrogen atom for a bromine atom in the benzene ring (NOT in the methyl
group).
The theoretical percentage yields are quoted as if the
methyl group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from ring bromination of methylbenzene |
Name of the isomer |
bromo-2-methylbenzene |
bromo-3-methylbenzene |
bromo-4-methylbenzene |
Actual yield of each isomer |
33% |
<1% |
67% |
Actual yield of 2- and 4- isomers |
>99% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
What you see is the vast majority of the product involves
substitution at the 2 and 4 positions of the benzene ring in
methylbenzene and only trace of substitution at the 3 position.
This is due, as argued via the diagram above, because of the
increased electron density at the 2, 4 and 6 ring positions from the
plus inductive effect of the methyl group of methylbenzene.
See also
Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides
(e)
The nitration of phenol
The presence of the hydroxy group (OH) directly attached
to the benzene ring making the molecule a 'phenol', increase the
electron density of the pi orbitals of the ring and greatly increases
the reactivity of phenol towards electrophiles, particularly at the 2, 4
and 6 positions.
The lone pairs on the oxygen atom can 'merge'
(overlap) to a small extent increasing the electron density of the pi
orbitals so these benzene hydroxyl (phenol) derivatives and more
reactive towards electrophilic reagents than benzene itself.
Phenol is so reactive, that even at low temperature (<
20oC), with dilute nitric acid, electrophilic substitution
readily takes place at the 2, 4 and 6 positions on the benzene rings.
The non-bonding electrons of the oxygen atom interact
with the pi orbitals of the benzene ring - which outweighs any minus
inductive effect from the more electronegative oxygen atom
(electronegativities: C = 2.5, oxygen 3.5).
The theoretical percentage yields are quoted as if the
hydroxy group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from low temperature nitration of phenol with
dilute nitric acid |
Name of the isomer |
2-nitrophenol |
3-nitrophenol |
4-nitrophenol |
Actual yield of each isomer |
53% |
<1% |
47% |
Actual yield of 2- and 4- isomers |
>99% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
What you see is the vast majority of the product involves
substitution at the 2 and 4 positions of the benzene ring in phenol.
This is due, as argued via the diagram above, because of the
increased electron density, specifically. at the 2, 4 and 6 ring positions from the inductive effect of the hydroxy group of phenol.
Another approach to explaining the extra reactivity at
the 2, 4 and 6 positions of the benzene ring is to consider the three
resonance structures of phenol, shown on the right of the diagram above.
You can draw structures that place a negative charge
on carbon atoms 2, 4 and 6 of the ring, but not on ring carbons 3
and 5, where little substitution occurs with phenol.
Although the contribution of these three resonance
structure is less than the first two, the effect is sufficient to
promote much more substitution on the 2, 4 and 6 positions compared
to 3 and 5.
The more negative the carbon atom, the more likely
it is to allow the attacking electrophile to substitute the hydrogen
atom.
With conc. nitric/sulfuric acids phenol gives 2,4,6-trinitrophenol
(diagram above) - a very explosive
product, but note the substitutions take place at the carbon atoms of
highest electron density in the ring.
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(f)
Benzene ring bromination/chlorination of phenol
With excess chlorine water or bromine water room
temperature, phenol gives an immediate white precipitate of
2,4,6-trichlorophenol or 2,4,6-tribromophenol, with almost zero substitution at the 3 and 5 positions
of the benzene ring (structures in diagram below).
This further illustrates the much greater reactivity of
phenol towards electrophiles compared to benzene itself/
See also
Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides
(g)
Electrophilic substitution of the benzene ring of an aromatic ether
The ether group, like the hydroxy group, increases the
electron density of the benzene ring, particularly at the 2, 4 and 6
positions. The lone pairs on the oxygen atom can 'merge' (overlap) to a
small extent increasing the electron density of the pi orbitals,
particularly at the 2, 4 and 6 positions. So these benzene ether
derivatives and more reactive towards electrophilic reagents than
benzene itself.
You can also, like with phenol, construct three
resonance structures with a negative charge on benzene ring carbon atoms
2, 4 and 6.
Resonance structures for aromatic ethers
e.g.
for methoxybenzene (R = CH3), ethoxybenzene (R = CH2CH3)
and phenoxybenzene (R = C6H5)
So you would expect the 2nd substituent to go onto
benzene ring carbons 2 or 4, and this is born out by the data below and
comparable with phenol in terms of very little 3- substitution.
Substituent
group
G in C6H5-G |
Electrophilic
substitution reaction |
% yield
position 2
(ortho-product) |
% yield
position 3
(meta-product |
% yield
position 4
(para-product |
–O–CH3 |
Nitration of an ether |
35 |
1 |
64 |
–O–CH3 |
Friedel-Crafts acylation
of an ether |
7 |
3 |
90 |
-OH |
Nitration of phenol |
61-63 |
1-3 |
32-34 |
If G has no effect |
Assuming 5 rate factors are all the same |
40 |
40 |
20 |
The theoretical percentage yields are quoted as if the
RO- group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors in terms
of electrophilic attack from the electrophiles NO2+
or RC=O+.
The major products from methoxybenzene are:
e.g. from nitration: 2-methoxy-1-nitrobenzene and
2-methoxy-1-nitrobenzene
7.14.3
The effect of a
moderately deactivating group on substitution orientation in a monosubstituted benzene
derivative - 2, 4 and 6 position directing groups (ortho and para
substitution positions)
Certain
groups, already present in a monosubstituted benzene derivative, can
moderately decrease the electron density of
the benzene ring and make the aromatic compound less reactive
towards electrophiles.
These groups usually have just one electronegative atom e.g.
a halogen atom that interacts with the pi orbitals of the benzene ring.
The electronegative substituent group results in a lowering of electron density, deactivating
the benzene ring towards electrophiles, but
deactivates more at the 3 and 5 positions so the 2, 4 and 6 positions are
favoured by electrophilic substitution
(a) The
nitration of chlorobenzene
+ HNO3 ===>


+ H2O
The theoretical percentage yields are quoted as if the
chloro group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from nitrating chlorobenzene |
Name of the isomer |
chloro-2-nitrobenzene |
chloro-3-nitrobenzene |
chloro-4-nitrobenzene |
Actual yield of each isomer |
30% |
<1% |
70% |
Actual yield of 2- and 4- isomers |
>99% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
Chlorobenzene is less reactive than benzene, but, as you
can see from the yield data, the deactivation is almost complete for the
3 position of the benzene ring (see the diagram below).
Although the overall electron density is reduced in the
benzene ring of chlorobenzene, it is still the highest at ring carbons
2, 4 and 6, so those positions are more susceptible to electrophilic
attack.
The 2nd diagram above, shows on the right, three resonance
hybrids of chlorobenzene with a negative charge on carbon atoms 2, 4 and 6
on the benzene ring, favouring electrophilic attack by a positive
electrophile, and less so on the 'neutral' carbon atoms 3 and 5.
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(b) The
nitration of bromobenzene
This is done by treating bromobenzene with a mixture of
conc. nitric and sulfuric acids.
There is very little substitution at the 3 position of
the benzene ring.
The theoretical percentage yields are quoted as if the
bromo group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from nitrating bromobenzene |
Name of the isomer |
bromo-2-nitrobenzene |
bromo-3-nitrobenzene |
bromo-4-nitrobenzene |
Actual yield of each isomer |
39% |
2% |
59% |
Actual yield of 2- and 4- isomers |
98% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
The diagram below indicates the change in electron
density around the benzene ring of bromobenzene caused by the bromine atom.
Although the overall electron density is reduced in the
benzene ring of bromobenzene, it is still the highest at ring carbons 2,
4 and 6, so those positions are more susceptible to electrophilic
attack.
The deactivation of the ring by the electronegative bromine
atom, greatly affects the 3 position of the benzene ring, so the major
products are bromo-2-chlorobenzene and bromo-4-chlorobenzene and the
resonance hybrid structures contribute to the effect too.
The 2nd diagram above, shows on the right, three resonance
hybrids of bromobenzene with a negative charge on carbon atoms 2, 4 and 6 on
the benzene ring, favouring electrophilic attack by a positive electrophile,
and less so on the 'neutral' carbon atoms 3 and 5.
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(c) The
ring chlorination of bromobenzene
This can be accomplished with chlorine and the halogen
carrier catalyst aluminium chloride.
Although the overall electron density is reduced in the
benzene ring of bromobenzene, it is still the highest at ring carbons 2,
4 and 6, so those positions are more susceptible to electrophilic attack
from Cl+.
The actual typical and theoretical
yields of products from chlorinating bromobenzene |
Name of the isomer |
bromo-2-chlorobenzene |
bromo-3-chlorobenzene |
bromo-4-chlorobenzene |
Actual yield of each isomer |
43% |
2% |
55% |
Actual yield of 2- and 4- isomers |
98% |
Theoretical yield of each isomer |
40% |
40% |
20% |
Theoretical yield of 2- and 4- isomers |
60% |
The theoretical percentage yields are quoted as if the
bromo group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
Again, the deactivation of the ring by the
electronegative bromine atom, greatly affects the 3 position of the
benzene ring, so the major products are bromo-2-chlorobenzene and
bromo-4-chlorobenzene and the resonance hybrid structures contribute to
the effect too.
See diagrams above for the nitration of bromobenzene to
illustrate the orientation effect
See also
Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides
TOP OF PAGE and sub-indexes
7.14.4
The effect of a
deactivating group on substitution orientation of a monosubstituted benzene
derivative - 3 and 5 position directing groups (meta substitution
positions)
Certain
groups, already present in a monosubstituted benzene derivative, can
strongly decrease the electron density of
the benzene ring and make the aromatic compound less reactive
towards electrophiles.
These groups have at least one strongly electronegative atom and
a double or triple bond that interacts with the pi orbitals of the benzene ring.
This results in a lowering of electron density, deactivating, but
deactivates more at the 2, 4 and 6 positions so the 3 and 5 positions are more
favoured by electrophilic attack and consequent substitution.
Examples of groups
that considerably decrease reactivity, compared to benzene, by some means
or other, are nitro
-NO2, carboxylic acid COOH, aldehyde -CHO, and
sulfonic acid -SO2OH
These favour
electrophilic substitution at the 3 position (often > 70%) and their
effect does fit in with them all being strongly
electronegative groupings giving a
-I inductive effect and/or theoretically, resonance hybrids
with a positive charge on ring carbon atoms 2, 4 and 6.
These points are illustrated in the diagram below.
(a) The
nitration of
nitrobenzene
1,3-dinitrobenzene is the favoured substituent product
when nitrobenzene is treated with a concentrated nitric acid and
sulfuric acid mixture.
The theoretical percentage yields are quoted as if the
nitro group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from nitrating nitrobenzene |
Name of the isomer |
1,2-dinitrobenzene |
1,3-dinitrobenzene |
1,4-dinitrobenzene |
Actual yield of each isomer |
6% |
93% |
1% |
Theoretical yield of each isomer |
40% |
40% |
20% |
As you can see, there is only a total of 7%
substitution at ring carbons 2 and 4.
The nitro O-N-O group is shown as delocalised system and
merges with the pi bond system of the benzene ring, but it has, overall,
an electron withdrawing effect, deactivating the ring.
On the right are three resonance structures of
nitrobenzene with a positive charge on ring carbon atoms 2, 4 and 6
making them less favourable to attack from an incoming positive electrophile
like NO2+.
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(b) The nitration of benzoic
acid
The 3- substitution is highly favoured i.e. the major
product is 3-nitrobenzoic acid.
The theoretical percentage yields are quoted as if the
carboxylic acid group has no effect on the orientation of the product i.e. all 5
positions on the benzene ring have equal partial rate factors.
The actual typical and theoretical
yields of products from nitrating benzoic acid |
Name of the isomer |
2-nitrobenzoic acid |
3-nitrobenzoic
acid |
4-nitrobenzoic
acid |
Actual yield of each isomer |
19% |
80% |
1% |
Theoretical yield of each isomer |
40% |
40% |
20% |
The yield of 3-nitrobenzoic acid is double what you
expect if the carboxylic acid group had no influence on the rate of
nitration at the different ring carbon atoms.
The >C=O delocalised and merges with the pi orbitals
of the benzene ring, but the net effect is a large minus inductive
effect, deactivating the ring, particularly at the 2, 4 and 6 positions.
See also
Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
Future additional work
Images for future work for meta
directed substitution - add them plus note
Resonance structures for aromatic ketones R = CH3, 1-phenylethanone , R = C6H5
diphenylmethanone
Resonance structures for benzenesulfonic acid
and implication of direction orientation for
synthesis e.g. how to make 4-nitrobenzoic acid or 4-aminobenzoic acid
TOP OF PAGE and sub-indexes
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