a) ethanoyl chloride
b) bromine
(1,2)
c) nitric acid
1 Phenols
Phenol, C6H5OH
CH 3
+
CH 3
3[O]
+ Br2
33
1
50
1
6
93.5
Br
OH
Phenol
is an group
aromatic is
alcohol.
It contains
an attached
−OH groupdirectly
bonded directly
to a
A phenol
an —OH
group
to a benzene
ring.
+ HNO
carbon atom in a benzene ring (Figure 10.33).
OH
NO 2
3
24.3 Phenol
(1,3
Thelone
C—O
bond
in phenol
is veryonstrong,
asatom
a result
of with
the the
delocalisation of
the oxygen
interacts
The
pair of
electrons
in the pz -orbital
delocalised π-electrons in the ring (Figure 10.34). This increases the electron density + HNO
NO
the
lone
pair
of
electrons
on
oxygen
over
the
arene
ring.
There
are
no
inside the ring, making it easier for phenol to be attacked by an electrophile. This
6
5
CO H
CO H
explains
the relative
ease with
which phenol
reactsthe
in situation
electrophilic with
substitution
reactions
in which
it breaks,
unlike
the alcohols.
reactions compared with benzene. It also decreases the δ− charge on the oxygen + HNO
NO
atom, making it less reactive as an alcohol.
NO 2
NO2
3
2
Phenol, C H OH, is a crystalline solid wh
43 °C. It is used to manufacture a wide ran
However–OH group in phenol undergoes
a
variety
of
reactions.
(Figure 24.5). It’s structure is:
2
2
3
19
80
2
The data in Table 25.5 can be interpreted as follow
●
H
O
Figure 10.34 Interaction of the oxygen electrons with
H
O
Figure 25.12 Delocalisation of the lone pair
the inaromatic
ring
2,4-directing substituents
●
The orientation of the incoming group (NO2 or B
already in the ring, and not on the electrophile.
Some substituents favour both 2- and 4-substituti
favour 3-substitution, at the expense of both 2- a
OH
If we look closely at the types of substituents tha
either they are capable of donating electrons to t
they have a lone pair of electrons on the atom jo
can be incorporated into the π system by sideway
Figure 25.12).
On the other hand, all those substituents that fav
2 formed,
Phenol With
Sodium
The salt
sodium
phenoxide, is soluble in water.
Phenol
alsowith
reacts
sodium
metal,
Phenol reacts
sodiumvigorously
metal, formingwith
a phenoxide
and giving
off giving
hydrogen
gas.
off hydrogen gas and again forming sodium phenoxide:
2
OH + 2Na
⎯→ 2
–
+
O Na + H2
Substitution into the benzene ring
of phenol
xide.
nol
3
The
derivatives of phenol are named in a similar fashion to those of benzene,
Nomenclature
by numbering the carbon atoms in the benzene ring starting from the −OH
group (Figure 18.1.33).
OH
OH
O2N
OH
1
6
2
5
4
NO2
3
CH3
OH
1
2
6
3
4
5
NO2
NO2
phenol
2,4,6-trinitrophenol
Test yourself
Br
2-methyl-5-nitrophenol
(not 6-methyl-3-nitrophenol)
4-bromophenol
4
Reaction With Sodium
Phenol reacts with sodium in a similar way to ethanol.
Bubbles of hydrogen form when a small cube of sodium is added to molten
phenol or to a solution of phenol in an inert solvent.
The main product of the reaction is sodium phenoxide.
5
Ring Reactions With Phenols
At first sight we might expect the –OH group to be electron-withdrawing
due to the high electronegativity of O.
ring more negative, i.e. mor
the reaction occurs more quic
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 might expect
withdrawing due to the high e
group also possesses a lone pai
pair into the ring activates
This π donation into the rin
effect (due to the electronegat
donation of electron densit
attract electrophiles more s
NO 2
6
Ring Reactions With Phenols
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
7
Phenols With Nitric Acid
Phenols are more susceptible to electrophilic attack than benzene, owing to
the delocalisation of the lone pair of electrons on oxygen.
This allows phenol to react with reagents that are more dilute, and also to
undergo multiple substitution with ease.
of concentrated nitric and sulfuric acids to reflux with
at about
°C forAcid
nitration to take place (see
8 benzene
Phenols
With55Nitric
page 385). However, the activated ring in phenol
When treated with dilute aqueous nitric acid (no sulfuric acid is needed)
readily undergoes nitration with dilute nitric acid at
phenol gives a mixture of 2- and 4-nitrophenols.
room temperature:
OH
OH
dil. HNO3
OH
NO2
+
NO2
If we use concentrated nitric acid we get
2,4,6-trinitrophenol formed, shown below:
of concentrated nitric and sulfuric acids to reflux with
benzene atWith
about
55 °CAcid
for nitration to take place (see
9 Phenols
Nitric
page 385). However, the activated ring in phenol
If we use conc. nitric acid with phenol, we get 2,4,6-trinitrophenol.
readily undergoes
nitration
with
dilute
nitric
acid
at
If we use concentrated nitric acid we get
room temperature:
2,4,6-trinitrophenol formed, shown below:
OH
OH
conc.
dil.nitric
HNO3acid
O 2N
OH
OH
NO2 NO2
+
NO2
If we use concentrated nitric acid we get
2,4,6-trinitrophenol formed, shown below:
NO2
NO2
26.11
ring in phenol
10 Bromination of Phenols
Chapter 25: Benzene and its
en aqueous
is added
to abromine
solutionat
ofroom
phenol,
the bromine
is a white
Phenolbromine
decolorises
aqueous
temperature,
giving
mediately
decolorised
and
a
white
precipitate
is
formed.
This
is
a
substitution
precipitate of 2,4,6-tribromophenol.
tion and the white precipitate is
2,
4,
6-tribromophenol.
BROMINATION OF PHENOL
OH
Br
+ 3HBr
However, bromine water reacts readily with phenol,
decolorising the orange solution and forming a white
precipitate of 2,4,6-tribromophenol (see Figure 25.6):
−
+ 3Br2
−
Br
−
−
Phenol undergoes similar reactions to benzene,
but phenol does so under milder conditions. For
example, bromine water will not react with benzene
at room temperature. To produce bromobenzene we
need pure bromine (not a solution) and an iron(III)
bromide catalyst.
−
OH
Br
OH
OH
2,4,6-tribromophenol
Br
Br
white precipitate
+ 3HBr
+ 3Br2 →
In th
• Des
phe
rea
mo
ben
• Tes
com
• Sum
phe
zene does not react with bromine except in the presence of a halogen
Figure 25.6 Bromine water is added to aqueo
Br
ier catalyst (Section 26.6). In general, phenol is more readily attacked
KEY
P
n benzene by electrophiles. This is because the lone pairs of electrons
Br
Br
11 Chlorination3Br
of2Phenols
(aq)
3HBr
A similar product is formed by the action of chlorine water on phenol.
Br
PhenolAreacts
with
aqueous
chlorine
in
water
at
room
temperature,
giving
a
similar product, formed by the action of chlorine water on phenol, is used in
white precipitate
solution as of
the2,4,6-trichlorophenol.
antiseptic TCP (see page 436).
OH
OH
Cl
Cl
3Cl2(aq)
3HCl
Cl
white precipitate
2,4,6-trichlorophenol
(TCP)
phenol as the ligand.
Figure 26.14 summarises some of the important reactions of phenol.
12 Phenol Reactions Summary
OH
NO2
NO
Br −
CH
6 C
5 O
C
Br2(aq)
OH
−
−
NO2
NO2
Na
O
a
or N
−
Br
Br
l
OOCC6H5
−
−
−
OH
O3
N
H
.
c
con H 2SO 4
.
c
n
co
OH
−
3
)
q
a
H(
−
−
dilu
te H
NO2
O2N −
O–Na+
−
+
−
−
−
OH
Which is the stronger acid, ethanol or phenol?
13 Acid Nature
Phenol behaves as a weak acid. The aqueous solution has pH of around 5 or
—10
6.
(K
of
the
acid
is
1.3
x
10
).
onsider athe equilibrium:
−
phenol
−
H2O +
OH
–
O
+ H3O
+
phenoxide ion
see The
whyposition
phenol
is
a
stronger
acid
than
aliphatic
alcohols
such
as
ethanol
of this equilibrium lies well over to the left-hand side. However,
phenoxide ion, the negative charge on the O atom can be partly delocalise
phenol is still a stronger acid than water or an alcohol.
+
the ring. This reduces the tendency of the phenoxide ion to attract H .
r words it reduces its strength as a base. This increases the strength of
14 Acid Nature
Phenol is more acidic than water, with ethanol being the least acidic of the three
compounds. (phenol > water > ethanol in terms of acidity).
Compound
Dissociation in water
pKa
phenol
C6H5OH(aq) ⇌ C6H5O−(aq) + H+(aq)
10.0
water
H2O(l) ⇌ H+(aq) + OH−(aq)
14.0
ethanol
C2H5OH(aq) ⇌ C2H5O− (aq) + H+(aq)
16.0
act that it
or, showing structures:
by
the
fact
that
it
om sodium
15
Acid
Nature
from
sodium
CO
ions,2i.e. it
ate
solutions,
i.e.
it
bonicThe
acid.
dissociation of phenol may be shown as:
than carbonic acid.
n sodium
oluble
inItsodium
in
water.
ydroxide
on
than in water. It The dissociation of ethanol is:
−
+
H5O Na
)
odium
hydroxide
The dissociation of ethanol is:
−
+
tronger
The
dissociation
of
ethanol
is:
H
O
Na
)
salt (C
6 5
molecules.
ere are stronger
water molecules.
Phenol is more acidic than water, with ethanol being the
the three compounds.
We this
can by
explain this by
least acidicleast
of theacidic
threeofcompounds.
We can explain
–10
–3
Phenol
is
more
acidic
than
water,
with
ethanol
being
the
–10
–3
looking
at
the
conjugate
bases
formed
on
the
right-hand
K
=
1.3
×
10
mol
dm
–10
–3
a
O
H
H
O
looking
at
the
conjugate
bases
formed
on
the
right-hand
K
=
1.3
×
10
mol
dm
a 1.3 × 10
Ka =
mol dm
Nature
enol is16
moreAcid
acidic than
water,
OO
HH with ethanol
OObeing
HH the
side
of theinequations
in Table
24.3. Ththis
eion,
phenoxide
ion,
least acidic side
of the
three
compounds.
We
cane phenoxide
explain
by
of the
equations
Table 24.3.
Th
− by
st acidic of the three compounds. We can explain
this
−
C
H
O
(aq),
has
its
negative
charge
spread
over
the
whole
C
H
O
(aq),
has
its
negative
charge
spread
over
the
whole
6
5
looking
at
the
bases
formed
the
right-hand
The negative charge
of the anion
can6−beconjugate
the benzene
ring.on
Figure
25.17
5 delocalised over
The
ion,
C
O
(aq),
has
its
negative
charge
spread
over
6H5on
king at
thephenoxide
conjugate
bases
formed
the
right-hand
shows various ways in which this ion
can be
represented.
ion
as
one
of
the
lone
pairs
on
the
oxygen
atom
overlaps
as
one
of
the
lone
pairs
on
the
oxygen
atom
overlaps
side of the equations in Table 24.3. The phenoxide ion,
the
whole
ion
as
one
of
the
lone
pairs
on
the
oxygen
atom
overlaps
eldite
of
the
equations
in
Table
24.3.
Th
e
phenoxide
ion,
with
the
delocalised
π
bonding
system
in
the
benzene
ring.
with
the
delocalised
π
bonding
system
in
the
benzene
ring.
−
adhesive,
compact
discs
and TCP
are all
ve, compact
discs
and TCP
areantiseptic
all
–antiseptic
O
O
C6H5O (aq),
has its negativeOcharge spread over
the whole
ing
phenol
asmaterial.
ahas
starting
material.
H
(aq),
negative charge
spread
oversystem
the whole
as5aO
starting
with
theitsdelocalised
π
bonding
in
the
benzene
ring.
–
–
–
O
O oxygen atom
ion as one of– the lone pairs
on the
overlaps
as one of the lone pairs on the oxygen atom overlaps
with
the
delocalised
π
bonding
system
in
the
benzene
ring.
–
This
makes
the ionsystem
moreinstable
than ring.
an ethoxide ion.
TCP
antiseptic
are all π bonding
–
h the
delocalised
the benzene
O
CH CH
ons of the
− phenol
O
3
–
2
CH3CH2
O
–
O–
–
(–)
O
O
ol
was
the
fi
rst
antiseptic
to
be
used
in
ntury phenol was the first antiseptic to be used in
phenoxide ion,phenoxide ion, ethoxide ion, with
ethoxide
ion,
with
ManyLister.
people
usedpeople
to die when
their
(–)
(–)
seph
Many
used to
die when their
negative
charge
with
negative
– negative charge
with
negative
or
or
O
O
CH
CH
dme
after
an
operation.
However,
when
Lister
3
2
–
over spread over concentrated onconcentrated on
infected after an operation. However, when Lister
O spreadcharge
CH3CH2 charge
the
oxygen
the
whole
ion
eatre
and
recovery
wards
with
phenol
(–)
the oxygen
the whole ion
perating theatre and recovery wards with phenol
ically
reduced.
Nowadays,
derivatives
ere
c todramatically
be used inreduced. Nowadays, derivatives This delocalisation reduces the charge density of the
Th
is
delocalisation
reduces
the
charge
density
of
the
use
phenol
itself
is
toxic
and
caustic.
ethoxide
ion,
with
phenoxide
ion,
used
because
phenol
itself
is
toxic
and
caustic.
ethoxide
ion,
with
phenoxide
ion,
ie
when
their
negative
charge
on
the
phenoxide
ion
compared
with
®
6-trichlorophenol, found in TCP and
negative
charge
on
the
phenoxide
ion
compared
with
negative
charge
with
negative
®
−
−
+
such as 2,4,6-trichlorophenol,
found
in
TCP
and
negative
charge
with
negative
26_17 Cam/Chem AS&A2
−H5O (aq). Therefore
−
+ not
OH
(aq)
or
C
H
(aq)
ions
are
wever,
when
Lister
2
without the nasty side-effects of phenol.
concentrated
on
charge
spread
over
OH
(aq)
or
C
H
O
(aq).
Th
erefore
H (aq) ions are not
2
5
ill bacteria
without
the
nasty
side-effects
of
phenol.
433
concentrated on
charge spread over
Barking Dog Art
as strongly attracted to the phenoxide ion as they are to
The negative charge of the anion can be delocalised over the benzene ring. Figure 25.17
shows various ways in which this can be represented.
17 Acid Nature
–O
We will consider the stability of the anion/conjugate base.
17 Representations of the phenol
O
O
O
–
–
The lone pair on the O— in the phenoxide ion overlaps with the delocalised
–
system in the benzene ring. This spreads out the negative charge in the ion.
–
or
O
(–)
O
or
(–)
(–)
( –)
26_17 Cam/Chem AS&A2
Barking Dog Art
433
18 Acid Nature
The O is thus less negative and the H+ is not attracted back as strongly. The
anion is thus stabilised and phenol is a stronger acid than ethanol i.e. the
backward reaction is favoured less.
In ethanol this delocalisation does not occur and indeed the negative charge
on the O in the ethoxide ion is increased by the electron-donating alkyl
group.
19 Acid Nature
The phenoxide ion (C6H5O−) is more stable than the ethoxide ion
(C2H5O−) and hence its formation is favoured.
In the phenoxide ion, the charge is delocalised and is stable. Thus phenol
reacts with alkalis unlike alcohols.
Compound
Dissociation in water
pKa
phenol
C6H5OH(aq) ⇌ C6H5O−(aq) + H+(aq)
10.0
water
H2O(l) ⇌ H+(aq) + OH−(aq)
14.0
ethanol
C2H5OH(aq) ⇌ C2H5O− (aq) + H+(aq)
16.0
an acid, phenol reacts with alkalis such as sodium hydroxide. Hydroxide
20
Phenol
In
Alkalis
+
+
om sodium hydroxide remove H (H3O ) ions, displacing the above
–
brium
to
the
right
and
forming
a
solution
of
sodium
phenoxide,
C
H
O
6
5
Although phenol is only slightly soluble in water, it dissolves well in an alkali.
Because
of
this
reaction,
phenol
is
much
more
soluble
in
aqueous
sodium
Phenol will react with an alkali to give a salt plus water:
xide than it is in water.
−
−
NaOH +
OH
–
O Na
+
+ H2O
sodium phenoxide
ol was once known as ‘carbolic acid’, and it was used as one of the earliest
ptics. The Edinburgh doctor, Joseph Lister first used it in the 1860s to
nt wounds going septic after surgery. Unintentionally, it had been used
antiseptic even before this, because the old-fashioned way of treating
21 Acidic Strengths
Table 10.2 Acidity of alcohols, phenol and carboxylic acids
Add blue litmus
Add sodium hydroxide
Add sodium carbonate
Alcohols
Stays blue
No reaction
No reaction
Phenol
Goes red
Salt formed
No reaction
Carboxylic acids
Goes red
Salt formed
CO2 evolved
Phenol as an alcohol
Phenol does not react as an alcohol as readily as aliphatic alcohols, such as ethanol
or propan-2-ol. For example it does not react with carboxylic acids to form esters.
This is because the lone pair of electrons on the oxygen atom is drawn into the ring,
making the oxygen less susceptible to electrophilic attack.
Reaction with acyl chlorides