and orbital structure.
Figure 18.1.11 shows benzene with normal covalent sigma bonds ( bonds)
between its carbon and hydrogen atoms. Each carbon atom uses three of its
electrons to form three bonds with its three neighbours. This leaves each
carbon atom with one electron in an atomic p orbital.
1
Benzene: Structure
Benzene consists of six carbon atoms arranged in a regular hexagon, each
Figure 18.1.11 Sigma bonds in benzene,
p orbital
joined to a hydrogen atom
and Hto it’s neighbouring
carbons
on
either
side
H
with one electron per carbon atom
remaining in a p orbital.
by σ bonds.
H
H
H makes three
H
Carbon in benzene only
bonds instead of four. The p-oritals in
σ bond The remaining p-orbital is non-hybridised
the bonds are sp2 hybridised.
and has an unpaired electron which forms part of the central delocalised
These six p electrons do not pair up to form three carbon–carbon double
delocalised electrons
electron
bonds
(consistingcloud.
of a bond plus a bond) as in the Kekulé structure.
Instead, they are shared evenly between all six carbon atoms, giving rise to
circular clouds of negative charge above and below the ring of carbon atoms
(Figure 18.1.12). This is an example of a delocalised electron system, which
occurs in any molecule where the conventional structure shows alternating
double and single bonds. Within the electron system, the electrons are free
to move anywhere.
Molecules and ions with delocalised electrons, in which the charge is spread
H
C
H
C
C
C
C
H
C
H
benzene
H
2
Benzene: Structure
p orbital
The six spare p orbitals, one on each carbon atom, are all parallel to each other
Aromatic
and phenol
andhydrocarbons
perpendicular
to the plane of the ring.
(a) σ-bonded skeleton
(C and H atoms
omitted for clarity)
e greater stability compared with
are localised between individual
ilisation energy. In benzene, its
p orbital
The result agrees reasonably closely with the delocalisation energy of benzene
obtained using enthalpy changes of hydrogenation. (See question 3 at the end
of this chapter.)
Electron delocalisation in benzene
The extra stability of benzene and the fact that its C − C bonds are all of equal
length can be explained using the following model.
itten in a formula is to draw a The carbon atoms in the benzene ring have sp hybrid orbitals (Section 3.5).
(b) σ-skeleton
with
π-bonds
Figure
10.9 Overlap
of
(a) σ-bonded skeleton
They
are
bonded
to
one
another
and
to
their
hydrogen
atoms
by
σ-bonds.
This
of that system
—
for
benzene,
(C and H atoms
Fig 26.2
Bonding
in benzene.
Note that
the
leaves
one
unused
p
orbital
on
each
carbon,
each
containing
a
single
electron.
p
-orbitals
in
benzene
z with both its neighbours,
Each
p
orbital
overlaps
equally
forming
a
delocalised
omitted
for
clarity)
of
the
molecule
is perpendicular
to the
ng the six carbon atoms (Figure These p orbitals are perpendicular to theplane
plane of the ring,
with one lobe
above
paper
and one below this plane (Figure 26.2 (a)). Each p orbital overlaps sideways
at A six-centre
level and atmolecular
university. πThe
orbital.
with the two neighbouring orbitals to form a single π-bond extending as a ring
, is drawn as in Figure 10.10b.
of charge above and below the plane of the molecule (Figure 26.2 (b)).
2
s
to two other carbon atoms, and one hydrogen atom. This leaves each
atom with
one electron in a p orbital.
3 carbon
Benzene:
Structure
H
H
C
H
C
C
C
C
H
P orbitals overlap
sideways
C
H
H
H
C
C
H
C
C
H
Sigma bond
Delocalised electrons above
and below the plane of the
carbon atoms
Figure 29.9 The formation of the delocalised electron structure of benzene.
H
Synoptic link
4You will
Benzene:
Structure
need to know
the covalent
bonding studied in Topic 3.2,
This,
therefore,
results
in
each
carbon
having
a
bond
angle
of
120˚and
a
Covalent bonding, and bonding
trigonal
planar
shape.
All
carbon-carbon
bonds
are
equal
in
length
and
in alkenes studied in Topic 14.1,
strength,
as
are
all
carbon-hydrogen
bonds.
Alkenes.
The benzene structure is normally represented by the skeletal formula
C
120°
C
C
C
120°
C
H
H
—
C
C
C
120° H
—
—
H
—
H
H
—
—
—
H
H
a
b
5
Benzene: Structure
The six-centre delocalised π bond is responsible for the following physical
and chemical properties of benzene.
•
It causes all C—C bond lengths to be equal, creating a planar, regular
hexagonal shape.
•
It prevents benzene undergoing any of the normal addition reactions that
alkenes show.
•
The π bond also makes benzene more stable than expected.
m
the carbon–carbon single bond in alkanes while the other three are s
in
length
to
the
carbon–carbon
double
bond
in
alkenes.
X-ray
diffr
6 Benzene: Structure
studies show that the carbon atoms in a benzene molecule are at the c
Carbon–carbon
lengths(Figure
in ethane,
ethene and
of a regularbond
hexagon
18.1.5).
All benzene.
the bonds are the same l
shorter than single bonds but longer than double bonds (Figure 18.1.6
C
C
0.154 nm
C
C
0.134 nm
0.139 nm
ethane
ethene
benzene
Figure 18.1.6 Carbon–carbon bond lengths in ethane, ethene and benzene.
The resistance to reaction of benzene
An inexperienced chemist looking at the Kekulé structure might
7
Aromatic Compounds
Compounds that contain rings of delocalised electrons are called aromatic
compounds.
The name was originally applied to certain natural products that had strong,
pleasant aromas, such as vanilla-bean oil, clove oil, almond oil, thyme oil and
oil of wintergreen.
8
Or
the
two
rings
could
share
two
carbon
atoms
in
common
(with
their
π
e
Or the two rings could share two carbon atoms in common (with their π el
as
in
naphthalene:
as in naphthalene:
Aromatic Compounds
Arenes are hydrocarbons, such as benzene, methylbenzene and
naphthalene. They are ring compounds in which there are delocalised
electrons.
The
simplest
arene
is
benzene.
Or the two rings could share two carbon atoms in common (with their π electrons),
as in naphthalene:
naphthalene
More
rings
can
fuse
together,
giving
such
compounds
as
anthracene
and
p
More rings can fuse together, giving such compounds as anthracene and py
anthracene
pyrene
anthracene
pyrene
More rings can fuse together, giving such
compounds
as
anthracene
and
pyrene:
anthracene
pyrene
Notice
that
with
each
successive
ring
fused
together,
the
hydrogen-to-carb
Notice that with each successive ring fused together, the hydrogen-to-carbo
decreases,
from
1
:
1
in
benzene
to
5
:
8
in
pyrene.
Eventually,
as
many
mo
decreases, from 1 : 1 in benzene to 5 : 8 in pyrene. Eventually, as many more
together,
a
sheet
of
the
graphite
lattice
(graphene)
would
result
(see
Topic
together, a sheet of the graphite lattice (graphene) would result (see Topic
Many
of
the
multiple-ring
arenes,
such
as
pyrene,
are
strongly
carcinoge
Many of the multiple-ring arenes, such as pyrene, are strongly carcinogen
substituted products of benzene or as compounds containing the phen
group, C6H5 –. The names and structures of some derivatives of benzene ar
9shown
Naming
Arenes
in Table 18.1.1.
The
names
and
structures
of
some
derivatives
of
benzene
are
shown
below.
Table 18.1.1 The names and structures of some derivatives of benzene.
Systematic name
Substituent group
Structure
Chlorobenzene
Chloro, – Cl
C6H5– Cl
Nitrobenzene
Nitro, – NO2
C6H5– NO2
Methylbenzene
Methyl, – CH3
C6H5– CH3
Phenol
Hydroxy, – OH
C6H5– OH
Phenylamine
Amine, – NH2
C6H5– NH2
The names used for compounds with a benzene ring can be confusing. Th
10 Naming Arenes
When more than one hydrogen atom is substituted, numbers are used to
indicate the positions of substituents on the benzene ring. The ring is
numbered to get the lowest possible numbers.
Cl
Cl
Cl
NH2
OH
Cl
Cl
CH3
Br
NO2
Cl
1,2-dichlorobenzene
1-chloro-3-methylbenzene
1-bromo-3-chlorobenzene
Test yourself
7 Why is the middle compound in Figure 18.1.13 named
1-bromo-3-chlorobenzene and not 1-chloro-3-bromobenzene?
3-nitrophenylamine
2,4-dichlorophenol
Figure 18.1.13 Naming disubstituted
products of benzene, phenylamine and
phenol.
11 Naming Arenes
Cl
methylbenzene
(no number is required
as there is only one hydrogen
substituted and all positions
are equivalent)
Cl
1,3-dichlorobenzene
(count anticlockwise to get
the lowest possible numbers)
1-chloro-3-methylbenzene
(if the two substituents are different
alphabetically chloro gets precedence over methyl)
OH
Cl
Arenes
CH3
Cl
CH3
Cl
CH3
Cl
Cl
2,4,6 -trichlorophenol
(phenol takes precedence and
does not need a number: it is
assumed the OH is at position 1)
H3C
Br
NH2
3,5-dimethylphenylamine
(phenylamine takes precedence and
does not need a number: it is
assumed the NH2 is at position 1)
Figure 7.7 A range of aromatic compounds, illustrating naming conventions.
Cl
2,5 -dichloro-1-bromobenzene
(alphabetically bromine takes position1
and the Cl are numbered to give the lowest
numbers)
Worked example
Draw out all possible positional isomers of C6H3Br2OH and name them.
Draw out all possible positional isomers of C6H3Br2OH and name them.
Answer of C6H3Br2OH
12 Isomers
Answer
there of
e,there
C6H2of
Cl 3(CH3)?
e, C6H2Cl 3(CH3)?
There are six isomers. Their names and formulae are as follows:
There areOH
six isomers. Their names and
formulae are as follows:
OH
OH
Br
Br
Br
Br
2,3-dibromophenol
2,3-dibromophenol
OH
OH
Br
Br
Br
Br
OH
Br
Br
Br
Br
2,4-dibromophenol
2,4-dibromophenol
OH
OH
Br
Br
2,6-dibromophenol
2,6-dibromophenol
Br
Br
3,4-dibromophenol
3,4-dibromophenol
OH
OH
Br
Br
Br
Br
2,5-dibromophenol
2,5-dibromophenol
OH
OH
Br
Br
Br
Br
3,5-dibromophenol
3,5-dibromophenol
13 Naming
If the benzene ring is a ‘substituent’ on an alkyl or alkenyl chain, it is given the
If the benzene ring is a ‘substituent’ on an alkyl or alkenyl chain, it is given the na
name phenyl:
phenyl:
CH2CO2H
C
C
H
phenylethanoic acid
Cl
2,2-diphenylchloroethene
14 Physical Properties
Benzene and most alkylbenzenes are strongly oily-smelling colourless
liquids.
They are non-polar, and the only intermolecular bonding is due to the
induced dipoles of van der Waals’ forces.
Because they are non-polar in nature, they do not mix with water but are
soluble in non-polar solvents such as cyclohexane.
15 Physical Properties
Their boiling points are similar to those of the equivalent cycloalkanes, and
increase steadily with relative molecular mass as expected.
Benzene and methylbenzene are liquids at room temperature, while
naphthalene is a solid as it has stronger Vander Waals forces.
16 Reactivity
In a similar way to the π bond in alkenes, the delocalised π bond in benzene
is an area of high electron density, above and below the six-membered ring.
Hence it attracts electrophiles and reacts with them.
But because of the extra stability of the delocalised electrons, however, the
species that react with benzene have to be much more powerful
electrophiles than those that react with ethene.
Bromine water and aqueous acids, thus, have no effect on benzene.
17
Reactivity
The electrophiles that react with benzene are all positively charged, with a strong
electron-attracting tendency.
The other major difference between benzene and alkenes is what happens after the
electrophile has attacked the π bond.
In alkenes, an anion ‘adds on’ to the carbocation intermediate.
In benzene, on the other hand, the carbocation intermediate loses a proton, so as to reform the ring of π electrons.
This demonstrates how stable the delocalised system is.
Y¥
**I
Tip
t.ae#.l*F
Brst
s, the18 Electrophilic
|
•
Mechanisms
In these intermediate cations, the positive charge is delocalis
:
t.rs
:
other carbon, the one at which substitution
occurs, is attache
%
.
Alkenes react by electrophilic addition. Arenes react by electrophilic
saturated and therefore not part of the electron delocalisatio
substitution.
with
This
intermediate
then
breaks
down
to
form
bromob
A
Aion is
are returned from the C–H bond to the system and
ring is restored (Figure 18.1.19). At the same time,
t.ae#.l*F
+
from the Brintermediate cation. This
Br HBr ion immedia
−
Br% ion released in stage 1 to form hydrogen bromid
Y¥ retail
**I
,
+
Fz¥
:
;
Began
.
,
.
o the
+
e Br Atrons
ation.
H
H
retail
Fz¥
,
+
+
Br
Br
;
go.gg#.itn:fq.E*s.::kI*s.
¥
Began
A
+
H+
.
,
+
Br
H Br
H
+
OH
+
Br Brions
I breaks down to form b
A
FigureBr 18.1.18 ElectrophilicA.Figure 18.1.19
Bruse
The intermediate cation
.
.
.
−
The nitronium ions are formed by removal of OH ions from nitric acid by
sulfuric acid. In this reaction, HNO3 is acting as a base and H2SO4 is acting as
19
Electrophilic
Substitution:
Step
1
an acid.
+
The NO2 ion is a reactive+ electrophile which is strongly attracted to the
The powerful electrophile (E ) becomes attracted to the π bond of benzene.+
delocalised electrons in benzene. As it approaches the benzene ring, the NO2
Ition
eventually
breaks
the
ring
of
electrons
and
forms
a
σ
bond
to
one
of
the
forms a covalent bond to one of the carbon atoms using two electrons
carbon
atoms
of the(Figure
ring. 1.18).
from the
π system
H
H
H
C
C
C
C
H
H
C
C
H
H
+
H
+
+
E
NO
C
C
C
+
2
H
C
C
C
H
NO
E 2
H
H
intermediate cation
The formation of a covalent bond in an intermediate cation disrupts the
Figu
Elec
delo
an i
−
emoval of OH ions from nitric acid by
is
acting
as
a
base
and
H
SO
is
acting
as
3
2
4
The bromine cation that is formed is a powerful electrop
the π bond of benzene. It eventually breaks the ring of e
to one of the carbon atoms of the ring:
20 Electrophilic Substitution: Intermediate
H
Br
ophile which is strongly attracted to the
Br
Two
of
the
six
π
electrons
are
used
to
form
the
(dative)
bond
to
the
+
s it approaches the benzene ring, the NO2
electrophile.
other
π electrons are spread
over
the
remaining
five
f the carbon
atomsThe
using
two four
electrons
Two of the six π electrons are used to form the (dative)
O2
The other four
π electrons are spread over the remaining
carbon atoms of the ring, in a five-centre delocalised
orbital.
ring, in a five-centre delocalised orbital (see Figure 25.10
H
H
+
Figure 25.10 The four π electrons are
delocalised over five carbons
C
C
C
+
H
C
C
C
H
NO
E 2
H
H
intermediate
cation
n an intermediate cation disrupts the
Figure 1.18 !
Electrophilic nitronium ions attack the
delocalised electrons
in benzene
to
form
5
6
H
an intermediate cation.
1
4
3
2
Br
E+
The distribution of the four π electrons is not even. They
carbon atoms 3 and 5 than with atoms 2, 4 and 6. The p
25.10
JM/Chem.4over
AS&A2
distributed
atoms 2, 4 and 6. This is best represente
delocalised ring. A large input of energy is needed to do this and the reaction
has a fairly high activation energy.
The Electrophilic
unstable intermediate
cation quickly Step
breaks down
21
Substitution:
2 producing
nitrobenzene. This involves the return of two electrons from a C−H bond to
the
electron system.
The stability
the delocalised
is restored
and of π
Theπintermediate
carbocation
thenofloses
a proton, toring
re-form
the sextet
energy
is
released
(Figure
1.19).
electrons.
H
H
H
C
C
C
+
H
C
C
C
H
NO
E 2
H
H
intermediate cation
cation
intermediate
Halogenation
H
H
C
C
C
C
C
C
H
nitrobenzene
nitrobenzene
Figu
The
form
NO
E 2
+
H
H
+
or bromine
CH3
22 Bromination
Benzene will react with bromine in the presence of an
anhydrous iron(III) bromide catalyst. The catalyst can be
Benzene willmade
react
non-aqueous
onfilings
warming
in with
the reaction
vessel by bromine
adding iron
to thein the presence
benzene
and
bromine.toThe
substitution
reaction is:
of anhydrous
iron(III)
bromide
form
bromobenzene.
+ 2Cl2
Br
anhydrous
FeBrby
Electrophilic substitution
bromine can occur (Figure 18.1.17) but, as
3 catalyst
+
HBr
+
Br
2
in all electrophilic substitution reactions of benzene, the first step of the
reaction involves use of a catalyst to produce a stronger electrophile.
H the electrophile that starts the attack
Br
At first sight
on
benzene
isC not obvious.
The electrophile
when
C
H
H
H is created
H
C
C
C
C
warmpolarises
with
an iron(III) bromide
a bromine
+ molecule
Br2
+ HBr
Fe or FeBr3
C
C
C
molecule. The Br2 molecule forms a dativeC (co-ordinate)
H
H
H
H
C
C
bond with iron(III) bromide by donating a lone pair of
H
H
electrons from one bromine atom into an empty 3d orbital
bromobenzene
in the
iron.
This
draws
electrons
from
the
other
bromine
Figure 18.1.17 The reaction of benzene with bromine.
Key terms
If excess chlorine g
Electrophilic
substitu
form 1-methyl-2,4
in arenes involve the r
dichlorobenzene
a
hydrogen atom followi
electrophile. that th
(Remember
are Chemists
equivalent.)
sometimes
halogen
carrier
to
de
The carbon–ha
such as iron(iii) brom
stronger
the e
chloridethan
which catalys
benzeneone
with of
chlorine
because
the
overlaps slightly w
+ two electrons from a C−H bond to
nitrobenzene.
This
involves
the
return
of
delocalised electrons in benzene. As it approaches the benzene ring, the NO2
electron
system.
ion forms a covalent bond to one of the π
carbon
atoms
usingThe
two stability
electronsof the delocalised ring is restored and
energy is released (Figure 1.19).
from the π system (Figure 1.18).
23 Bromination: Electrophilic Substitution
H
H
H
H
C
C
C
C
C
C
H
H
H
+
Br+2+
NO
C
C
C
C
+
H
H
H
C
C
H
C
C
H
NO
Br 2
H
H !
Figure 1.18
Electrophilic nitronium ions attack the
C electrons
Hdelocalised
NO
2 benzene to form
Br in
C
C cation.
an intermediate
+
+ H
C
C
H
H
C
H
H
intermediate cation
nitrobenzene
The formation
of
a
covalent
bond
in
an
intermediate
cation
disrupts
the
The bromine cation is attracted to the delocalised π bond of benzene. It
delocalised ring. A large input of energy is needed to do this and the reaction
Halogenation
eventually
breaks
the
ring
of electrons and forms a σ bond to one of the
has a fairly high activation energy.
is warmed
with bromine in the presence of iron filings, the
The unstable intermediate cation When
quicklybenzene
breaks down
producing
carbon
atoms
of
the
ring.
bromine
reactsfrom
withathe
iron
to form
nitrobenzene. This involves the return
of two first
electrons
C−H
bond
to iron(III) bromide:
the π electron system. The stability of the
delocalised
ring
is
restored
and
2FeBr
2Fe(s)
+
3Br2(l) →
The
other
four
π
electrons
are
spread
over
the
3(s)remaining five carbon atoms of
energy is released (Figure 1.19).
then The
acts asintermediate
a catalyst for thecarbocation
reaction of bromine with
iron(III) bromide
the ring, in a five-centreThe
delocalised
orbital.
δ+ ! δ−
H
H
Figure
1.19
benzene by polarising further bromine molecules as Br −Br (Figure 1.20).
then loses a proton, to re-form the sextet of π electrons.
H
C
H
H
C
Br
NO2
3+
–
Br + Fe (Br )3
The intermediate cation breaks down to
δ–
formδ+
nitrobenzene.
3+
–
.....
Br
Br
Fe (Br )3
R
h
is
C
R
+
+
CI +
C
–
AICI
AICI34
R
saturated
and
therefore
not
part
of
the
electron
delocalisatio
de
derivatives
(Chapter
19).
+
–
C
+
AICI4
This
intermediate
then
breaks
down
to
form
bromo
This intermediate
then breaks down to form bromob
24
Bromination:
Electrophilic
Substitution
O O
O
are
returned
from
the
C–H
bond
to
the
system
a
are returned from the C–H bond to the system an
an acylium ion
an acylium ion
ringis isrestored
restored
(Figure
18.1.19).
Atthe
thesame
sametime
tim
Benzene will react with non-aqueous
bromine(Figure
on
warming
in theAtpresence
ring
18.1.19).
+
Figure
18.1.27
Formation
of
an
acylium
ion.
an
acylium
ion.
+
ionimmedia
immed
from
theintermediate
intermediatecation.
cation.This
ThisHHion
of anhydrous iron(III) bromide
to form
bromobenzene.
from
the
−
−
ionreleased
releasedininstage
stage1 1totoform
formhydrogen
hydrogenbromid
brom
BrBr ion
e
+
ns
n.
R
R
H
+
+
C
O
+
R
R
Br
HCH H
+ ++ C
OR +
BrBrBr
+
Br
H
+
C
BrBr
+
+
+
+
H
O
+ +H H
O
ions
use
Figure 18.1.18 Electrophilic Figure
Figure
18.1.19
The
intermediate
cation
breaks
down
to
form
18.1.19
The
intermediate
cation
breaks
down
to
form
b
Figure
18.1.28
Electrophilic
substitution
mechanism
for
Friedel–Crafts
acylation
of
substitution
mechanism
for
Friedel–Crafts
acylation
of
two of the delocalised electrons in benzene
A
similar
reaction
occurs
when
benzene
is
warm
benzene.
to form an intermediate cation.A similar reaction occurs when benzene is warme
Tip
presence
of
iron,
iron(iii)
chloride
or
aluminium
presence of iron, iron(iii) chloride or aluminium
are
often
referred
to
as
halogen
carriers.
are often referred to as halogen carriers.
comp
deriv
25 Bromination:
of
the
Electrophile
+ Formation
–
+ AICI3
R C + AICI4
Anhydrous iron(III) bromide contain electron-deficient atoms. They can
O
react with the bromine molecule by accepting one of the lone pairs of
an
acylium
ion
electrons on bromine.
Formation of an acylium ion.
This causes strong polarisation of the Br—Br bond, weakening it, and
eventually leading to its heterolytic breaking.
R
C
H
R
+
C
R
O
+
O
Electrophilic substitution mechanism for Friedel–Crafts acylation of
H
+
18.1.28).
spectromet
compounds
derivatives
26 Bromination:
Catalyst
Regneration
+
We can think+of the electrophile
as a Br cation:
–
CI + AICI3
R
δ+
π bonding
+
C
O
Br
δ–
Br
an acylium ion
AICI4
FeBr3
Br
+
+ [FeBr4]
–
+
Br
cation
and the ‘electron-rich’ benzene ring are
8.1.27
Formation The
of an acylium
ion.
electrons
attracted to each other, as the mechanism below shows.
The final stage
regenerates
the
catalyst,
by
the
reaction
between
the
proton
Remember that the curly arrows show the movement
of a
R
ed
+
−
(H ) formedpair
withofthe
[FeBr4] .
electrons.
nding +in
C
?
f:
O
C
H
R
H+
++
—
C 4R
FeBr
1
+ stage
O
Br ⎯⎯⎯→
H —> Br HBr
+
+
stage 2
⎯⎯⎯→
–
[FeBr4]
BrO
FeBr3 +
H
+
+ HBr
(+FeBr3)
8.1.28 Electrophilic substitution mechanism for Friedel–Crafts acylation of
.
A similar reaction happens when chlorine gas is bubbled
section. It is used industrially to manufacture nitrobenzene, from which
enylamine (aniline), C6H5NH2, is produced by reduction. Phenylamine is
Chlorination
d to 27
manufacture
dyes (see Chapter 27).
A similar electrophilic substitution reaction occurs when chlorine gas is
logenation
bubbled through benzene at room temperature in the presence of a catalyst
nzene does not react with chlorine, bromine or iodine on their own in the
such
as
iron(III)
chloride
or
aluminium
chloride.
k. This is because the non-polar halogen molecule has no centre of positive
arge to initiate electrophilic attack on the benzene ring. However, in the
The
catalysts
in
these
reactions,
i.e.
FeBr
3, AlCl3 and FeCl3, are known as
sence of a catalyst such as iron filings, iron(III) bromide or aluminium
halogen
carriers.
oride, benzene is substituted by chlorine or bromine:
−
+ Cl2
anhydrous
AlCl3
Cl
+
HCl
chlorobenzene
hen iron filings act as the catalyst, they first react with the halogen, forming
n(III)chloride or iron(III) bromide.
28 Alkylation
When benzene is heated under reflux with a chloroalkane in the presence of
aluminium chloride, the alkyl group attaches to the benzene ring.
This reaction is an important method for substituting an alkyl group for a
hydrogen atom in an arene.
+
CH3Cl
CH3
AlCl3 catalyst
+
heat
HCl
methylbenzene
Figure 18.1.24 Friedel–Crafts alkylation of benzene with chloromethane forming
methylbenzene.
catalytic rolebenzene
in creating
the electrophiles
attack benzene.
So, w
molecules
to form anwhich
intermediate
cation, which
+
−
δ+CH –Clδ− + AlCl
+
→
CH
+
AlCl
with
aluminium
AlCl3 molecules
3chloromethane
3 is mixed
3
4and chloride,
18.1.26). rem
producing
methylbenzene
H ions (Figure
−
δ+
δ−
electrophile
29 Alkylation
Cl ions from polar CH3 –Cl molecules, allowing reactive carbocatio
+
ions,
to
act
as
electrophiles.
CH
+
CH
3
3
system of
These reactive CH electrophiles then attack the delocalised
3
+
+
−
δ+
δ−
H
benzene molecules
to
form
an
intermediate
cation,
which
CH3 –Cl + AlCl
→
CH
+
AlCl
+ 3 CH3
3
4
+
+
producing methylbenzene and H ions (Figure 18.1.26).
breaks down
C
electrophile
+
CH3
electrophiles thenintermediate
attack the delocalised methylbenze
system
These reactive
CH3
cation
benzene molecules to form an intermediate cation,
CH3which breaks do
+
+
Figure
18.1.26
The
reaction
of
CH
electrophiles
with
benzene
in
the
Fr
+
H
3
+
ions
(Figure
18.1.26).
producing
methylbenzene
and
H
+ CH
+ H
+
3
alkylation reaction to produce methylbenzene.
+
intermediate
Finally,
the
aluminium
cation
+
CHelectrophilic
the
3
+
CH3
methylbenzene
+
CH
3
chloride catalyst is regenerated as H i
H
+
−
+
+
H
substitution
react with AlCl4 ions.
Figure 18.1.26 The reaction of CH3 electrophiles with benzene in the Friedel–Crafts
+
+ AlCl4 → HCl + AlCl3
H methylbenzene.
alkylation reaction to produce
intermediate
cation
methylbenzene
+
H
+
Finally, the
aluminium
chloride
catalyst
regenerated
ions
in
Figure 18.1.26 The reaction of CH3 iselectrophiles
withasbenzene
in released
the Friedel–Crafts
methylbenzene
Figure
18.1.24
Friedel–Crafts
alkylation
of
benzene
with
chloromethane
forming
30 Acylation
methylbenzene.
A similar reaction occurs when benzene is refluxed with the acyl chloride,
A similarchloride,
reactionplus
occurs
when benzene
is refluxed
ethanoyl
aluminium
chloride as
a catalyst.with the acyl chloride,
ethanoyl chloride, plus aluminium chloride as a catalyst. This time, the product
is phenylethanone,
also
known as methylphenylketone
This
time, the product
is phenylethanone,
also known as(Figure 18.1.25).
methylphenylketone.
CH3
O
+
CH3C
Cl
AlCl3 catalyst
C
heat
O
+
HCl
phenylethanone
Figure 18.1.25 Friedel–Crafts acylation of benzene with ethanoyl chloride forming
31
benzene molecules to form an intermediate cation, which
with aluminium
chloride
to
from
an
acylium
ion
(Figure
18.1.27).
This
io
+
(Figure 18.1.28).
spectromete
producing methylbenzene and H ions (Figure 18.1.26).
acts as the electrophile in the two-step electrophilic substitution
reactio
compounds
Acylation
(Figure 18.1.28).
derivatives (
+
–
R
C
O
CI + AICI3
R
R
+
C
+
O
+ 3 CH3
R
CI + AICI
an acylium ion
C
Figure 18.1.27 Formation
of an acylium ion.
O
CH3
AICI4
+
+
C
C
R
H
O
intermediate
an acylium ion
cation R
Figure 18.1.27 Formation of an acylium ion.
+
–
+ 4
AICI
C
methylbenze
+
Figure 18.1.26 The
reaction
of
CH
electrophiles
with
benzene
in
the
Fri
3
C
H
alkylation reaction
to
produce
methylbenzene.
+ R
+
O
C
O
O
R
+
H
H
C
+
H
Finally, the aluminium
chloride
catalyst
is
regenerated
as
io
+ for Friedel–Crafts
O + H
C
R
R
Figure 18.1.28 ElectrophilicCsubstitution
mechanism
acylation of −
the electrophilic substitution react with AlCl4 ions.
benzene.
+
O
+
AlCl
→
HCl
+
AlCl
4
3
Figure 18.1.28 Electrophilic substitution mechanism
for Friedel–Crafts acylation of
+
H
Tip
O
benzene.
In Friedel–Crafts alkylation, the initial product contains an alkyl group attached to a
32 Skill Check
Draw the structural formulae of the products you would expect from the
reaction of benzene and aluminium chloride with
(a) CH3CH2Cl
(b) (CH3)2CHCOCl.
Draw the structural formulae of the products you would
benzene and aluminium chloride with
33 Skill Check
a CH3CH2Cl
Now try this
b (CH3)2CH—COCl.
What
compounds
are needed to synthesise the following
What organochlorine
organochlorine compounds
are
Answer
needed
to
synthesise
the
following
compounds from benzene?
1
C(CH3)3
O
a
compounds from benzene?
O
C
b
2
O
C
O
substitution reactions of benzene involve electrophilic substitution.
high density of negative charge in the delocalised electron system of the
zene34
ring Nitration
tends to attract electrophiles.
nsiderWhen
as anbenzene
example
the
nitration
of
benzene.
Benzene
reacts
with
a
is added to a mixture of concentrated nitric and sulphuric
tureacid
of concentrated
nitric
acid
and
concentrated
sulfuric
acid
(called
a
o
and heated under reflux to around 50 C nitrobenzene is formed.
rating mixture) at 50 °C. The product is nitrobenzene:
−
+ HNO3
conc. H2SO4
50 °C
NO2
+
H2O
nitrobenzene
(a yellow oil)
s is a substitution reaction. A hydrogen atom on the benzene ring has been
stituted by a nitro group, − NO2.
reaction of benzene with concentrated nitric acid alone is slow, whilst pure
HNO3 + H2SO4 → NO2
++
HSO4 + H2O
35
Nitration:
Mechanism
+
The NO2 ion is a reactive electrophile. It replaces a hydrogen in the benzene
ring in a two-step electrophilic substitution mechanism (Figure 18.1.23)
The mechanism of the electrophilic substitution is:
similar to that which occurs in the bromination of benzene.
+
NO2
NO2
H
+
NO2
+
H
+
Figure 18.1.23 Electrophilic substitution mechanism for the nitration of benzene.
2 H2SO4 + HNO3 ⟶
Test yourself
－
2 HSO4
+
+ H3O
+
+
NO2
electrophile
17 Explain why dilute nitric acid does not react with benzene.
18 Write an overall equation for the formation of the nitronium ion in
36 Nitration: Mechanism
Formation of the electrophile occurs in 3 steps:
HNO3 ⟶
－
+
NO2
+ OH－
H2SO4 ⟶
－
HSO4
+ H2O
H2O + H2SO4 ⟶
－
HSO4
+ H3O+
OH
+
2 H2SO4 + HNO3 ⟶
－
2 HSO4
+
+ H3O
+
+
NO2
en
type of reaction retains the delocalised π electron system wit
stability. However, addition reactions involving disruption of
37 Hydrogenation
system do occur. Benzene, like alkenes, will undergo addition
o
Benzene
reacts
with
hydrogen
gas
and
nickel
catalyst
at
200
C to form
in the presence of a nickel catalyst, but at considerably
highe
cyclohexane.
(Figure 18.1.29).
+
3H2
nickel
Raney
nickel
200 °C
cyclohexane
A higher temperature is needed with benzene in order to brea
electron system and allow addition to occur. A special finely
of nickel, called Raney nickel, is also used because this has an e
38 Skill Check
7
A
synthesis
of
1-phenylpropene
from
benzene
Give the reagants and conditions for steps 1, 2 and 3.
is shown below.
COCH2CH3
step 1
CH(OH)CH2CH3
step 2
step 3
CH=CHCH3
a) i)
Identify a reagent and a catalyst for
c) The benze
reactive wi
benzene.
Explain wh
more susce
than benze
d) In the hum
‘L-dopa de
carboxylic
produce th
i) State w
‘prima
ii) Draw
dopam
9 The following
this question.
39 Methyl Benzene
Methylbenzene reacts in the same way as benzene, via an electrophilicsubstitution mechanism.
The conditions for the reactions of methylbenzene are slightly milder than
those for the reactions of benzene as the methyl group is an activating
group.
than benzene).
If substitution were to occur at position 3, stabilisation o
charged intermediate is not possible by donation of the lon
O into the ring.
40 Methyl Benzene: Reactivity
The methyl group donates electron density into the benzene ring (positive
step inductive
in the reaction
effect). is the attack of the electrophile
activating group is present this step occurs more
This
increases
the
amount
of
electron
density
in
the
ring
so
that
it
is
more
e electron density in the ring so that an electrophile
attractive to electrophiles and reacts more readily.
gly. When the ring is deactivated by the withdrawal
electrophile is attracted less strongly and the
lowly.
methyl group has an electron-releasing effect and this
e reacts more readily than benzene due The
to the
the intermediate when substitution occurs at positions 2 an
ffect of the –CH3 group (positive inductive
The electron-releasing effect of the methyl group comes
oup activates the ring towards electrophilic
overlap of the electron density in the C–H bond with a p
adjacent
atom
(Figure
G43).
ng electron density into the ring. This makes the
The stabilising effect of the methyl group on the interm
41 Methyl Benzene: Reactivity
Methylbenzene reacts more readily than benzene due to the electronreleasing effect of the –CH3 group (positive inductive effect).
The methyl group activates the ring towards electrophilic substitution by
donating electron density into the ring.
This makes the ring more negative, i.e. more attractive towards electrophiles
and the reaction occurs more quickly than with benzene.
electron density into the benzene ring (positive inductive effect). This
increases the amount of electron density in the ring so that it is more
42 Methyl Benzene: Reactivity
attractive to electrophiles and reacts more readily.
The
methyl
group
is a 2,4-directing
group and
so theand
major
of products
The
methyl
group
is a 2,4-directing
group
soproducts
the major
substitution
are:
of substitution are:
Some of the electrophilic substitution reactions of methylbenzene are
shown in Figure G39.
ii The aromatic ring
43 Chlorination
of Methyl Benzene
If chlorine is bubbled through methylbenzene in the absence of sunlight
and in the presence of a halogen carrier such as AlCl3, the ring is substituted
If methylbenzene
withThis
chlorine
the presence
of a halogen
instead of is
thereacted
CH3 side group.
reactionin
proceeds
by the electrophilic
substitution mechanism described in Section 26.6. A mixture of two isomers
carrier catalyst
(AlCl
)
at
room
temperature
2-chloromethylbenzene
and
43
is obtained:
−
CH3
+ Cl2
AlCl3
Cl
2-chloromethylbenzene
58%
+
HCl
CH3
−
−
CH3
−
chloromethylbenzene are formed.
−
Cl
4-chloromethylbenzene
42%
Fig 26.8 Manufacturing
in 1940. TNT was an im
both world wars
ms
44
The properties
of aromatic
very different from those of aliphatic
Chlorination
ofcompounds
Methyl are
Benzene
ones. Methylbenzene’s molecule has an aromatic portion (the benzene ring)
− CHchlorine
an aliphatic portion
(the with
These
two portions
make
different
3 group).in
If and
methylbenzene
is reacted
the presence
of UV
light
then
contributions to the properties of methylbenzene and have a modifying effect
side-chain
substitution
occurs
where
a
hydrogen
atom
in
the
methyl
group
on each other.
is substituted by a Cl atom.
i The — CH3 group
− CH3 group shows some reactions we would expect of an alkyl group. For
The
This involves a free radical substitution mechanism as for alkanes.
example, its H atoms can be substituted by chlorine when chlorine is bubbled
into boiling methylbenzene in sunlight:
CH2Cl
−
CH3
−
s
Reactions of methylbenzene
+ Cl2
sunlight
+ HCl
The reaction has a free-radical mechanism similar to the reaction of methane
with chlorine described in Section 14.6.
45
Methyl Benzene: Reactions
46 Skill Check
b Explain why phenol re
c Nitrobenzene reacts w
a compound with the m
Compound X, shown below, can be formed from benzene in a two-step
d
Compound
X,
shown
reaction sequence. Design a reaction pathway showing all reagents and
Design a reaction pathw
conditions and the intermediate compound for the conversion of benzene to
for the conversion of b
X.
CH 2 CH 3
CHClCH 3
UV light
UV light
Cl
HCl
47 Oxidation
of
the
Side
Chain
or boiling
or boiling
Cl2
2
HCl
> 90%
> 90%
When alkylbenzenes are treated with hot acidified potassium
Oxidation
of
the
side
chain
Oxidation
of the side
chain
manganate(VII),
oxidation
of the whole side chain occurs, leaving the carbon
When alkylbenzenes are treated with hot alkaline potassium manganate(VII),
When alkylbenzenes are treated with hot alkaline potassium manganate(VII),
atom closest oxidation
to the ring
a carboxylic
acid leaving
group:the carbon atom closest to the ring
of theas
whole
side chain occurs,
oxidation of the whole side chain occurs, leaving the carbon atom closest to the ring
as a carboxylate or carboxylic acid group:
as a carboxylate or carboxylic acid group:
CH 2CH 3
CH 2CH 3
KMnO4
heat with OH–
CH 3
CH 2CH 3
Worked example
CH 3
KMnO4
heat with OH–
COKMnO
2K 4
acidified KMnO
1 4
1 H (aq)
–
heat with OH
heat
CO
K
KMnO
2
acidified 4KMnO4
CH 2CH 3
Worked example
1
1
H
(aq)
–
heat with OH
heat
CO2K
CO2K
CO2K
CO2K
CO21HH1(aq)
CO21HH1(aq)
CO2H
CO2H
CO2H
CO2H
heat with OH
Worked example
48 Skill Check
Three hydrocarbons A, B and C with the formula C9H12
manganate(VII).
CO2K
CO2H
CH 3
Three hydrocarbons A, B and CKMnO
with the formula C9H12 were
oxidised
by
hot
1 H (aq)
● Hydrocarbon A gave benzoic acid,
–
potassium manganate(VII). heat with OH
C H CO H.
1
4
CH 2CH 3
6 5
2
CO
2K
Hydrocarbon B gave benzene-1,2-dioic
Hydrocarbon A gave benzoic acid, C6H5CO
2H.
acid:
●
Hydro
acid:
CO2H
●
CO2
Worked example
CO2 H
Three
A, B and C with the
formula C9H12 were oxidised by hot potassium
Hydrocarbon
B hydrocarbons
gave benzene-1,2-dioic
acid:
manganate(VII).
CO2 H
● Hydrocarbon C gave benzene-1,2,4-trioic CO2
● Hydrocarbon A gave benzoic acid,
acid:
C6H5CO2H.
Suggest the structures of A, B and C.
Hydrocarbon
B
gave
benzene-1,2-dioic
CO
H
2
Now
try
this
Hydrocarbonacid:
C gave benzene-1,2,4-trioic
acid:
Answer
CO2 H
Since A gave benzoic acid, all three ‘extra’ carbon atoms m
Suggest structures for theCO
aromatic
H
2
carboxylic
acids which will
CH2CH2CH3
CH(CH3 )2
Suggest
the structures
ofbeA,produced
B and C.
when the following compounds are
CO
H
2
oxidised by hot potassium manganate(VII).
CO2 H
or
(All these compounds are isomers with
●
Answer
CO2 H
Suggest
the
structures
of
A,
B
and
C.
Since
A
gave
benzoic
acid,
all
three
‘extra’
carbon atoms must be in the same side chain. So A is:
matic
ructures of A, B and C.
roduced
CH(CH3 )2
Answer CH2CH2CH3
s are
Since
A
gave
benzoic
acid,
all
three
‘extra’
carbon
atoms
must
be
in
the
same
side
chain.
So
A
is:
matic
enzoic
acid, all three ‘extra’ carbon atoms must be in the same side chain. So A is:
ganate(VII).
produced
or
CH
CH
CH
CH(CH
)
2
2
3
3
2
H2CH
CH(CH
)
ers
with
3
3 2
ds are
nganate(VII).A is:
Compound B must contain
two side chains, since two carboxylic acid groups are left after
or
ers with or
oxidation. What is more, the chains must be on adjacent carbons in the ring, as a
1,2-dicarboxylic acid is formed. So B is:
B since
must two
contain
two side
chains,
since
acid groups are left after
must contain twoCompound
side chains,
carboxylic
acid
groups
aretwo
left carboxylic
after
CH
3
oxidation.
What
is
more,
the
chains
must
be
on
adjacent
carbons
in
the
ring,
as
a
at is more, the chains must be on adjacent carbons in the ring, as a
1,2-dicarboxylic
acid
is
formed.
So
B
is:
CH
CH
2
3
c acid is formed. So B is:
49 Answer
B is:
CH3
CH2CH3
CH2CH3
By similar reasoning, C must be:
CH3
CH3
CH3
CH3
CH3
CH3
By
similar
reasoning,
C
must
be:
oning, C must be:
C is:
CH3
CH3
CH3
CO2 H
50 Skill Check
Now try this
CO2 H
Suggest the structures of A, B and C.
Answer
Since
A gave benzoicacids
acid, allwhich
three ‘extra’
atoms must be in the sam
Suggest structures
for the aromatic
Suggest
structures
for the aromatic
carboxylic
willcarbon
be produced
carboxylic acids which will be produced
CH CH CH
CH(CH )
when
the
following
compounds
are
oxidised
by
hot
potassium
when the following compounds are
oxidised by hot potassium
manganate(VII).
(Allmanganate(VII).
these compounds are isomersorwith the molecular
(All these compounds are isomers with
formula
C11formula
H14.) C11H14.)
the molecular
2
1
2
3
3 2
Compound B must contain two side chains, since two carboxylic acid g
oxidation. What is more, the chains must be on adjacent carbons in the
1,2-dicarboxylic acid is formed. So B is:
CH3
2
CH2CH3
CH3
CH3
By similar reasoning, C must be:
3
CH3
51 Reactions of Substituted Benzene Rings
Substituted benzene rings undergo basically the same reactions as a
benzene ring, i.e. electrophilic substitution.
The nature of the substituent determines the position of further
substitution and the rate of the reaction relative to unsubstituted
benzene.
52 Reactions of Substituted Benzene Rings
Substituents on a benzene ring may be divided into two groups: those
which cause substitution predominantly at positions 2 and 4 (and 6) (ortho
and para positions) and those that cause substitution at position 3 (and 5)
(the meta position).
The orientation of the incoming group (NO2 or Br) depends on the
substituent already in the ring, and not on the electrophile.
53 2,4-Directing
If substitution were to occur at position 3, stabilisati
charged intermediate is not possible by donation of th
O into the ring.
If we look closely at the types of substituents that are 2,4-directing, we find
that either
• they are capable of donating electrons to the ring by the inductive effect, or
• they have a lone pair of electrons on the atom joined to the ring. This lone
pair can be incorporated into the π system by sideways overlap of p
orbitals.
methylbenzene
ring
the r
At fi
with
grou
The methyl group has an electron-releasing effect and
pair
the intermediate when substitution occurs at positions
Th
The electron-releasing effect of the methyl group co
eff
ec
overlap of the electron density in the C–H bond wit
dona
adjacent atom (Figure G43).
phenol
attra
The stabilising effect of the methyl group on the int
The most dramatic difference in basicities to be seen in Table 27.3 is between that of
CO 2 H
phenylamine (Kb ≈ 10−10) and the alkyl amines (Kb ≈ 10−3). Taking two compounds of
about
the same relative
27_06 Cam/Chem
AS&A2 molecular mass and shape, we see that phenylamine is about
a million times less basic than cyclohexylamine:
54
2,4-Directing
25
NH2
25.4 Halogenoarenes
NH2
●
phenylamine
cyclohexylamine
–10
– chlorobenzene
4
We saw
on
pages
423–424
how
bromobenzene
and
can be made from H
Kb = 4.2 × 10
K b = 3.3 × 10
O
benzene. The reactions of the ring in halogenobenzenes are similar to those of benzene.
●
The orientation of the in
already in the ring, and
Some substituents favou
favour 3-substitution, at
electrophilic
substitution,
can be nitrated:
ThisHalogenoarenes
is because inundergo
phenylamine,
the lone
pair ofand
electrons
on the nitrogen atom If
is we look closely at the
delocalised
Cl over the benzene ring. The bondsClaround the nitrogen atom can takeeither they are capable o
up a planar arrangement, with the nitrogen’s lone pair in a p orbital, so that extrathey have a lone pair of e
can
be incorporated into
conc. HNO3 + conc. H2 SO4
stability can be gained
by overlapping this
p
orbital
with
the
delocalised
π
bond
of
Figure 25.12 Delocalisation of the lone pair
Figure 25.12).
in
2,4-directing
substituents
the benzene ring (see Figure 27.7).
NO2
26_12 Cam/Chem AS&A2
However, unlike halogenoalkanes, halogenoarenes cannot be hydrolysed, even
Table 25.6 Substituents and their effects on
Barking Dog Art
by boiling in aqueous sodium hydroxide. Thethe
carbon–halogen
benzene ring bond is stronger
in halogenoarenes than it is in halogenoalkanes,
H possibly due to an overlap of p
electrons similar to that in phenol (see Figure 25.13, and compare it with
Figure 25.12).
–
+NH
NH
or
2
2
In addition to this, the carbon attachedNto the halogen is not accessible
to the usual
nucleophilic reagents that attack halogenoalkanes, since its δ+ charge is shielded by the
H
negative π cloud of the ring. This means that halogenoarenes are inert to all nucleophiles.
Certain halogenoarenes find important uses as insecticides and herbicides (see the
panel below).
phenyl amine
On the other hand, all t
joined directly to the ring
2- and 4-directing substitu
CH 3
Ar
etc.
H
•
•
phenol
25_13 Cam/Chem AS&A2
Figure 27.7 Delocalisation of the nitrogen
Barking Dog Art
lone pair in phenylamine
The data in Table 25.5 can
O
•
•
chlorobenzene
Figure 25.13 Delocalisation of the lone pair
in chlorobenzene
+ HNO 3
Arenes and phenols
Barking Dog Art
Cl
CO
This overlap, causing a drift of electron density from nitrogen to the ring, has twoH2 N
effects on the reactivity of phenylamine.
Ar
Ar
N
Ar
−
N
Ar
If we look closely at the types of substituents that are 2,4-directing, we fi
−
O
bstituents
3-directing
substituents
55
3-Directing
O they
either
are capable of donating electrons to the ring by the inductive
δ–
•
•
•
•
O of electrons on the atomOjoined to the ring. This lo
theyδ –have a lone pair
H the
δ+
On
hand, all those substituents that favour 3-substitution
have a
O OotherAr
can be incorporated
into
theAr
π system by sidewaysC overlap
δ+
Ar of p orbitals
N
δ+ atomCjoined Ar
directly
to
the
ring.
−
Figure 25.12).
O
H
On Hthe other hand, all those substituents that favour 3-substitution have a
δ–
δ+
δ–
O
joined
to the ring δ+
(see Table 25.6). N
C
Ar
δ+
δ – directly
HN
2N
Ar
C
Ar
C
Ar
2- and 4-directing substituents
H
δ–
O
CH 3
Ar
δ+
δ+
δ–
Ar
C
N
C
Ar
•
•
R
δ–
3-directing
substituents
O
δ+
R
N
−
δ–
O
O
δ+
C
Ar
δ–
Ar
C
O
O
Ar
56 Directing Groups
2- and 4-directing substituents
3-directing substituents
CH 3
O
Ar
N
−
Ar
H
•
•
O
O
δ–
Ar
O
δ+
C
Ar
H
•
•
ects on
Figure 25.12).
On the other hand, all those substituents that favour 3-substitution have a δ+ atom
joined directly to the ring (see Table 25.6).
H2 N
δ+
δ–
Ar
N
δ–
C
O
δ+
C
R
Ar
Ar
−
−
−
−
−
−−
−
−
−
−
−
−
−
NO
NO
− −
− −
− −
−−
− −
−−
−
−
−
−−
−
−
−
−
−
−
−
−
−
−
−
NO
−
−
−
−
−
−−
−
−
−
−
−
−
−
−
−
−
−
−
57 Directing Groups
−
of different
mononitration
(i.e. substitution by one nitro gr
of mononitration (i.e.
substitution by one nitro group) of
benzene
position?
• Predict which isomers would be
phenol
OH faster o
OH
derivatives,
and whetherOH
they are nitrated
derivatives, and whether
they
are
nitrated
faster
or
slower
than
benzene.
b Which
groups
to direct
given
as the
maintend
products
from
NO2
OH
OH
OH
−
substitution
to
the
3
position?
various
substitution
reactions
Table 26.4 Mononitration products
of benzene derivatives
Table 26.4 Mononitration products of benzene derivatives
NO2
−
c Is there any correlation
Compound
Main products of mononitration
Compound
Main products of mononitration
Rate of nitration relative
NO2
between the position to which
to benzene
NO
a
group
directs
substitution
2
Q
U
E
S
T
I
O
N
methylbenzene
methylbenzene
Faster
nitrobenzene
NO
NO
2
2
CH3
CH
CH
CH
CH3 and the CH
rate
at
which
it
causes
3
3
3
3
CH
CH
CH
CH
CH
CH3
NO
NO
3NO2
3
3
14 Look
Table
26.4
and
answer
3
3 the at
NO
2
2
2
ring
to
substitute?
−
−
NO
NO
2
2
these
questions:
−
−
−
NO2
−
a Which groups tend to direct
NO2
NO2
NO2
substitution
NO2
NO2to the 2 or 4
NH2
NH2
phenylamine NH2
position?
phenol
NO2
phenol
Faster
OH
OH
OH
OH
OH
OH
NH2
NH2
−NH2
b Which groups
tend
to
direct
NO
NO2
OH
OH
2
OH
OH
OH
OH
NO
−
− to the 3 position?
2
substitution
−
NO2
NO2
−
−
NO2
c Is there any correlation
NO
NO2to which
between the position
2
NO2
NO2
a group directs
NO2 substitution
benzoic
nitrobenzene
Slower acid COOH
nitrobenzene NO
COOH
NO
NO
NO
2
2
2
2
and the rate at which
it causes
NO2
NO2
COOH
COOH
NO2
NO
2
the ring to substitute?
−−
−
NO
NO2 2
NO2
−−
−
58 Directing Groups
One explanation is that an electron-releasing group stabilises the
intermediate by electron donation into the ring.
This stabilisation is only possible when substitution occurs at positions 2, 4
and 6.
An electron-withdrawing group destabilises the intermediate by
withdrawing electron density from the ring.
This destabilisation is greatest when substitution occurs at positions 2, 4
and 6, therefore substitution at position 3 is preferred.
3
d) Explain why Fe(s) is not regarded as a catalyst.
4 Under certain conditions benzene can be nitrated to form a mixture of
isomers each with molecular formula C6H4N2O4. Draw and name the
isomers.
5 Write equations and state the conditions for each of the following
Write
the
reagants
and
conditions
for
the
following
reactions:
conversions.
59 Skill Check
(a)
(b)
(c)
requires two
separate reactions
CH3
NO2
60 Rate of Electrophilic Substitution
2,4-directing groups usually cause substitution faster than benzene and 3directing group normally cause electrophilic substitution to occur more
slowly than benzene (chlorine as a substituent is an exception to this – it is a
2,4-directing group and chlorobenzene reacts more slowly than benzene).
Substituents that cause substitution faster than with benzene are called
activating groups (2,4-directing groups except for chlorobenzene) and
those that cause substitution to occur more slowly than with benzene are
called deactivating group (3-directing group).
61 Rate of Electrophilic Substitution
The rate-determining step in the reaction is the attack of the electrophile on
the ring.
When an activating group is present this step occurs more quickly as there
is more electron density in the ring so that an electrophile is attracted more
strongly.
62 Rate of Electrophilic Substitution
When the ring is deactivated by the withdrawal of electron density, the
electrophile is attracted less strongly and the reaction occurs more slowly.
Activating groups donate electron density into the ring. This makes the ring
more negative, i.e. more attractive towards electrophiles and the
reaction occurs more quickly than with benzene.
63 Phenol Faster Than Benzene
At first sight we might expect the –OH group to be electron-withdrawing
ring
more
negative,
due to the high electronegativity of O.
the reaction occurs m
However, the –OH group also possesses a lone pair of electrons and overlap
of this lone pair into the ring activates the benzene ring.
At first sight we migh
withdrawing due to th
group also possesses a
pair into the ring a
This π donation int
effect (due to the elec
donation of electro
attract electrophiles
NO 2
64 Phenols Faster Than Benzene
NO2
+ HNO 3
CO H
This π donation into the ring is a bigger effect than the electron
withdrawing
effect (due to the electronegativity of O).
2
CO 2 H
+ HNO 3
Therefore there is net donation of electron density into the ring and the ring
will attract electrophiles more strongly.
The data in Table 25.5 can be
●
H
O
●
The orientation of the incom
already in the ring, and not
Some substituents favour bo
favour 3-substitution, at the
If we look closely at the type
either they are capable of do
they have a lone pair of elect
can be incorporated into the
O
65 Ring Reactions With Phenols
See section 26.3 for a fuller description of the conditions used for this reaction.
phenyl ethanoate
Phenols
are more susceptible
to electrophilic
attack than
Substitution
reactions
of the benzene
ringbenzene, owing to
As we mentionedof
onthe
pagelone
428, phenols
more susceptible
to electrophilic
attack phenol
the delocalisation
pair ofare
electrons
on oxygen.
This allows
than benzene, owing to the delocalisation of the lone pair of electrons on oxygen.
to react
with
reagents
that
are
more
dilute,
and
also
to
undergo
multiple
This allows phenol to react with reagents that are more dilute, and also to undergo
multiple substitution
with ease.
substitution
with ease.
Nitration
WhenWhen
treated
with
dilute
aqueous
nitric
acid
(no
sulfuric
acid
is
needed)
treated with dilute aqueous nitric acid (no sulfuric acid is needed) phenol gives
phenol
gives ofa 2mixture
of 2- and 4-nitrophenols.
a mixture
and 4-nitrophenols:
OH
OH
OH
NO2
dilute HNO 3 at room temperature
50%
NO 2
50%
26.11
66 Bromination of Phenols
Substitution reactions of th
Phenol decolorises a dilute solution ofring
bromine
water at room
inin phenol
temperature, giving a white precipitate of 2,4,6-tribromophenol. No
aluminium
bromide
is
needed.
When aqueous bromine is added to a solution of phenol, the bromine is
immediately decolorised and a white precipitate is formed. This is a substitution
Contrast
this
with
the
conditions
needed
for
the
bromination
of
benzene.
reaction and the white precipitate is 2, 4, 6-tribromophenol.
OH
−
−
Br
+ 3HBr
−
+ 3Br2
−
Br
−
OH
Br
2,4,6-tribromophenol
Benzene does not react with bromine except in the presence of a halogen
In th
• Des
phe
rea
mo
ben
• Tes
com
• Sum
phe
25.4 Halogenoarenes
67 WeChlorobenzene
saw on pages 423–424 how bromobenzene and chlorobenzene can be made from
Cl
benzene. The reactions of the ring in halogenobenzenes are similar to those25of benzene.
Halogenoarenes undergo electrophilic substitution, and can be nitrated:
Halogenoarenes undergo electrophilic substitution, and can be nitrated:
Cl
Cl
25.4 Halogeno
conc. HNO3 + conc. H2 SO4
Cl
e lone pair
We saw on pages 423–424 how
benzene. The reactions of the
Halogenoarenes undergo e
Cl
NO2
conc. HNO3 +
Figure 25.13 Delocalisation of the lone pair
in chlorobenzene
However, unlike halogenoalkanes, halogenoarenes cannot be hydrolysed, even
25_13 Cam/Chem AS&A2
by boiling in aqueous sodium hydroxide. The carbon–halogen bond is stronger
Barking Dog Art
in halogenoarenes than it is in halogenoalkanes, possibly due to an overlap of p
electrons similar to that in phenol (see Figure 25.13, and compare it with Figure 25.12).
In addition to this, the carbon attached to the halogen is not accessible to the usual
nucleophilic reagents that attack halogenoalkanes, since its δ+ charge is shielded by the
negative π cloud of the ring. This means that halogenoarenes are inert to all nucleophiles.
Certain halogenoarenes find important uses as insecticides and herbicides (see the
panel below).
However, unlike halogenoalkanes, halogenoarenes cannot be
hydrolysed, even by boiling in aqueous sodium hydroxide.
However, unlike halogenoalka
by boiling in aqueous sodium
in halogenoarenes than it is in
electrons similar to that in phe
In addition to this, the carbo
nucleophilic reagents that attac
negative π cloud of the ring. Th
Certain halogenoarenes find
panel below).
Organochlorine inse
Insecticides
Chlorobenzene used to be ma
production of the insecticide
68 Chlorobenzene
The carbon–halogen bond is stronger in halogenoarenes than it is in
halogenoalkanes, due to an overlap of p electrons similar to that in phenol.
The p orbitals from the Cl atom tend to overlap with the delocalised p
electrons in the benzene ring. This causes the C—Cl bond to be stronger,
and hydrolysis does not occur.
69 Electrophilic Substitution On Chlorobenzene
Chlorobenzene reacts with electrophiles more slowly than benzene does.
The chlorine is a deactivating group.
Chlorine is more electronegative than carbon, and so pulls the electrons in
the ring towards itself.That makes the electron density around the ring rather
less in chlorobenzene.
It becomes less attractive for electrophiles, and so the reaction is slower.
Halogenobenzenes are deactivating, yet are 2,4-directors. (The exception to
the rule).
70 Skill Check
Give the structure of the organic products formed when the following
8 Give the structure of the organic products formed when the
molecules are heated with excess aqueous sodium hydroxide:
following molecules are heated with excess aqueous sodium
hydroxide:
a
b
Organometallic compounds are organic molecules which also contain a
End of chapter questions
71 Skill Check
7 Predict the major products of the following reactions.
a
NO2
H2/Ni
300 °C
−
−
CH3
d
conc. H2SO4
conc. HNO3
− 120 °C
NO2
−
OH
b
−
CH3
−
c
CH3
Cl2(aq)
alkaline
KMnO4(aq)
warm
e
f
CH3CHClCH3
Al Cl3, warm
CH3COCl
Al Cl3, heat
9 a Write structural formulae
molecular formula C8H10
b For each of these compo
all the possible mononitr
ones you would expect a
(monosubstituted
product
only)
c For one of the compoun
mononitration products
produced in the majority
10 Consider two possible reac
benzene in the gas phase:
A
(g) + 2Cl2(g)