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Part 7.11
The chemistry of
AROMATIC COMPOUNDS
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
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Part 7.11
The physical and chemical
properties of benzoic acid and some of its derivatives
R-COOH
where R is aromatic and the COOH functional group is directly attached to a benzene ring
Sub-index for this page
(split?)
7.11.1
The naming, structure and physical properties of
selected aromatic
carboxylic acids
7.11.2
The synthesis and preparation of
aromatic carboxylic acids
7.11.3
The structure of
aromatic acids and strength as
weak carboxylic acids
7.11.4
The reactions of
benzoic acid (or any aromatic acid) with alkalis and carbonates/hydrogencarbonates
7.11.5
The reduction of aromatic acids to
aliphatic primary alcohols e.g. benzoic acid
7.11.6
Aromatic acid chlorides - preparation,
properties and reactions
7.11.7
Examples of electrophilic substitution
in the benzene ring of benzoic acid - halogenation and nitration
7.11.8
Esters of aromatic carboxylic acids -
preparation, properties and reactions
7.11.9
Brief notes on aromatic
amides - physical and chemical properties of benzamide
7.11.10
Brief notes on
aromatic nitriles - physical and chemical properties of benzonitrile
7.11.11
Brief notes on
aromatic aldehydes - benzaldehyde
For basic notes e.g. GCSE level see
Carboxylic acids - molecular structure, chemistry and uses
TOP OF PAGE and sub-index
7.11.1 The structure and physical properties of selected aromatic
carboxylic acids
Note: quite a few of the 'old' names are still used and
accepted
The name ends in
...carboxylic acid or ...oic
acid, in salts and esters the suffix is ...oate
The derivative names are similar for
aliphatic carboxylic acids and their derivatives e.g.
acid chlorides -COCl, are named
...oyl chloride, amides -CONH2, are named ...amide
and esters are named in the same
way e.g. alkyl/aryl ...oate
Extra notes on the data table of aromatic
carboxylic acids
(a) Most aromatic carboxylic
acids are white crystalline
solids, non are liquid at room temperature.
The instantaneous dipole -
induced dipole force, plus permanent dipole - permanent dipole
(including hydrogen bonding), ensures the
intermolecular bonding forces are too great for any of these aromatic acids
to be a liquid at room temperature -
all solids.
Like aliphatic carboxylic acids,
aromatic acids form 'dimers' via hydrogen bonding e.g. benzoic acid
above. It is
not a true dimer in the sense that the two benzoic acid molecules are covalently
bonded together, but the hydrogen bonding is strong enough to hold
the two molecules together, not only in the crystalline state, but when dissolved in most
non-aqueous solvents (e.g. benzene, hexane, trichloromethane) i.e.
solvents other than water - in which benzoic acid is mainly bonded to
water molecules (see (b) below).
The 'dimerisation' effectively doubles the size of the
benzoic acid molecule and roughly doubles the instantaneous dipole - induced
dipole intermolecular forces between the neighbouring dimers.
A greater kinetic energy is needed to melt/vapourise the
'dimer' compared to the 'monomer', hence the higher than expected
melting points for aromatic carboxylic acids.
(b) Aromatic carboxylic acids
usually have a relatively low
solubility in water, but greater than arene hydrocarbons
  If
we compare benzoic acid (left) with 2-phenylpropane (right). Both have a similar
size, shape, molecular mass and numbers of electrons (64 and 66), so they will have similar
instantaneous dipole - induced dipole forces. Methylbenzene is non-polar and
only weakly bonds with water and not sufficient to disrupt the hydrogen bonds of
water, so is ~insoluble in water because there are no permanent dipole -
permanent dipole attractions possible, only weak instantaneous dipole - induced
dipole attractive forces. No hydrogen bonding possible compared to benzoic acid.
In organic solvents benzoic acid usually exists mainly as the
dimer molecule e.g. you have the equilibrium.
2C6H5COOH
(C6H5COOH)2
Kdimer
= [(C6H5COOH)2] / [C6H5COOH]2
= 167 (benzene) and 78 (trichloromethane) mol-1dm3
These association equilibrium constant values (from the
internet) show the benzoic acid dimer predominates in organic solvents, BUT
....
.... in aqueous solution, carboxylic acids can also hydrogen bond
directly with water - which accounts for why the lower members are so
soluble in water (compared to arene hydrocarbons) and benzoic acid mainly
exists as the hydrated monomer in aqueous solution (diagram below).
The hydrogen bonding solvation of
benzoic acid in aqueous solution OR dimer formation liquid ethanoic acid
or dissolved in an organic solvent
However, benzoic acid and other aromatic
carboxylic acids have a hydrophobic C6H5-
benzene ring that disrupts the hydrogen bonding between water molecules, but
also a hydrophilic -COOH
group.
Although water - water hydrogen bonds
are disrupted (Oδ--Hδ+ǁǁǁ:Oδ-),
some
new carboxylic acid - water hydrogen
bonds are formed e.g. (C-Oδ--Hδ+ǁǁǁ:Oδ--Hδ+)
OR (H-Oδ--Hδ+ǁǁǁ:Oδ-=Cδ+) partly compensate for this,
allowing limited solvation with low solubility.
(ǁǁǁ
hydrogen bonds in diagram above, allowing some solvation - but low
solubility)
Although benzoic acid is only slightly soluble
in cold water, but is much more soluble in hot water - in fact its a
good solvent to purify it by recrystallisation.
C6H5COOH(s)
+ aq
C6H5COOH(aq)
The solvation of benzoic acid
when dissolving in water is endothermic, therefore increase in
temperature drives the equilibrium to the right (energy absorbing
direction, Le Chatelier's Rule).
The presence of a 2nd polar group
in the benzene ring e.g. amino -NH2 or hydroxy -OH
can increase the solubility in water, but not by a great deal despite
the scope for more permanent dipole - permanent dipole (including
hydrogen bonding) intermolecular attractions - see the solubilities of
hydroxybenzoic acids and aminobenzoic acids.
BUT things are complicated,
solubility is a complex subject e.g. two of the benzendicarboxylic acids
are almost insoluble, despite the fact of having two hydrogen bonding
hydrophilic groups attached to the benzene ring!
(c) Some aromatic acids exhibit
intramolecular hydrogen bonding
e.g. 2-hydroxybenzoic acid,
2-aminobenzoic acid and 2-nitrobenzoic acid.
See
2-nitrophenol on my phenols page
TOP OF PAGE and sub-index
7.11.2 The synthesis and preparation of
carboxylic acids
Oxidation of aromatic hydrocarbons to make aromatic
acids
Acidified potassium dichromate(VI) might
oxidise alkyl benzene compounds to benzoic acid, but if it does, it will be
much slower than with the alkaline manganate(VII) method
described below - NaOH/KMnO4 is a much stronger oxidising agent.
 Overall change is
represented by the equations for the conversion:
== oxidation ==>
C6H5CH3 + 3[O]
===> C6H5COOH
+ H2O
Aromatic
are not easily oxidised and longish reflux times are necessary (illustrated,
fig. PD2 right).
Hydrocarbons are
difficult to oxidise with typical organic oxidising agents compared to
compounds like alcohols.
However, aromatic hydrocarbons with an alkyl side
chain can be oxidised with strong reagents such as aqueous potassium
manganate(VII)/sodium hydroxide.
Whatever the length of
the alkyl group on a benzene ring it gets whittled down to carbon of
the carboxylic acid group e.g. propylbenzene ends up as benzoic acid, the
same product obtained by oxidising the shorter methylbenzene.
The
more stable aromatic benzene ring is left intact.
The overall process for
producing benzoic acid from methylbenzene can be summarised ..
C6H5CH3
(l) + NaOH(aq) + 3[O]
===> C6H5COO-Na+
(aq)
+ 2H2O(l)
After removing the
excess KMnO4/MnO2 with a reducing agent, the weak
acid, benzoic acid, is freed from
its sodium salt by adding dilute hydrochloric acid.
C6H5COO-(aq) + H+(aq)
===>
C6H5COOH
and you can oxidise the three
isomeric dimethylbenzenes to make the three isomeric aromatic
dicarboxylic acids via the same procedure:
(i)
Synthesis of benzene-1,2-dicarboxylic acid from 1,2-dimethylbenzene
 +
6[O]
 + 2H2O
(ii)
benzene-1,3-dicarboxylic acid from 1,3-dimethylbenzene
+ 6[O]
 + 2H2O
(iii)
benzene-1,4-dicarboxylic acid from 1,2-dimethylbenzene
 + 6[O]
+ 2H2O
In the chemical industry, many of
these oxidations can be done directly with oxygen at raised temperatures
and suitable catalyst.
See also
hydrolysis of
nitriles, esters and amides to yield carboxylic acids
(These reactions also
have a brief mention on this page too)
TOP OF PAGE and sub-index
7.11.3 The structure of aromatic acids and behaviour as
weak carboxylic acids
See Section 6.4
The acidic nature of carboxylic acids
for a more detailed discussion based on this diagram.
The R-C-O and O-C-O bond angles
are 120o - a trigonal planar sigma bond arrangement, plus the
delocalised pi bond system in which two electrons reside in the pi
orbitals above and below the plane of the O-C-O bonds.
The result of the possible 'resonance hybrids' is
that the theoretical pi bond pair of electrons from the C=O group
actually form a delocalised system across the two C-O bonds (blue arc in
the diagram above).
This means both of the C-O
bonds in the carboxylate ion are identical.
The diagram above gives an impression of how the
delocalised pi electrons are distributed around the benzene ring and
connected to the delocalised electrons of the two C-O bonds -
illustrating the dispersion, to some extent, of the negative charge of
the carboxylate group.
For a 'higher level' approach, the diagram of the
resonance structures of the benzoate ion illustrate how the negative
charge of the anion is partially further distributed around the benzene
ring. This further stabilisation of the benzoate ion explains why
benzoic acid is a stronger acid than phenol.
For more on the last point see chemical
properties of phenol
Expressing the
ionisation, both chemically and mathematically
In aqueous solution, carboxylic acids are weak acids,
that is only ionised or dissociated to a small extent.
RCOOH(aq)
+ H2O(l)
H3O+(aq) + RCOO-(aq)
RCOOH(aq)
H+(aq) + RCOO-(aq) (useful when doing pH and Ka calculations)
e.g.
(aq) + H2O(l)
(aq) + H3O+(aq)
The ionisation constant (dissociation constant, acidity
constant), that is
the equilibrium constant (Ka)
for the above equilibrium, is given by the expression:
|
Ka =
|
[H+(aq)]
[RCOO–(aq)]
|
|
––––––––––––––––––
mol dm-3 |
|
[RCOOH(aq)] |
Like pH, the range of Ka is so
great, it is often quoted on the logarithmic scale where
pKa
= -log10(Ka/mol dm-3) and
Ka = 10-pKa
The stronger the
acid, the greater the Ka and the lower the pKa value.
In the data table below
I've quoted both values and a few comments what affects the value of Ka.
See equilibrium calculations section 5c
for calculations involving pH, Ka
and pKa
Dissociation of
aromatic dicarboxylic acids in aqueous solution
The ionization of an aromatic dicarboxylic acid requires two
equations e.g. in 'shorthand' for benzenedicarboxylate acids
(i)
HOOC-C6H4-COOH(aq)
HOOC-C6H4-COO-(aq) + H+(aq)
(ii)
HOOC-CH2-COO-(aq)
-OOC-C6H4-COO-(aq) + H+(aq)
Although they are
generally weak acids, the range of ionisation is quite wide and greatly
affected by substituent groups in the 'alky' or 'aryl' hydrocarbon part
of the molecule, so, like with pH, a logarithmic (base 10) scale is used ...
... for examples see the relative strengths of carboxylic
acids in the data table below.
If the R group contains electronegative, electron
cloud withdrawing atoms, the electron shift affects the δ-O-Hδ+
bond (-I induction effect) and promotes the donation of a proton to a base
(left of bottom diagram).
On the other hand, other groups like alkyl groups, can
have a +I inductive electron shift effect and make the proton less available
to a base.
Reminders of mathematical
connections:
Ka =
[H+(aq)] [RCOO-(aq)] /
[RCOOH(aq)]
(R = H, alkyl, aryl, the
latter may be substituted with other atoms)
pKa = - log10(Ka/moldm-3)
and Ka = 10-pKa
Like the pH scale, a base 10
logarithmic scale means each unit
of pH is equal to a factor of 10 in the
hydrogen ion concentration [H+(aq)] and
each unit of pKa
represents a factor of 10 in the value of the ionization constant
(dissociation constant) in aqueous solution.
Note: For dicarboxylic acids or
tricarboxylic acids, the ionisation decreases from the 1st, 2nd
ionisation etc.
so that:
Ka1 > Ka2
etc. and pKa1
< pKa2 etc.
| Aromatic acid |
Abbreviated formula |
pKa |
Ka/mol
dm-3 |
Comments |
| Benzoic acid |
C6H5COOH |
4.20 |
6.31 x 10-5 |
pH of 0.10
moldm-3 solution is 2.66 |
| (CH3COOH) |
(CH3COOH) |
4.76 |
(1.74 x 10-5) |
(Aliphatic acid
for comparison) (pH of 0.10 moldm-3
solution is 2.88) |
|
2-methylbenzoic acid |
 |
3.91 |
1.23 x 10-4 |
Methyl group slightly increases strength. |
|
3-methylbenzoic acid |
4.24 |
5.75 x 10-5 |
Little effect on strength. |
|
4-methylbenzoic acid |
4.34 |
4.57 x 10-5 |
Decreases strength. |
|
2-hydroxybenzoic acid |
  
Note the extra acidic phenol group, but it's
pKa will be much higher, a much smaller Ka. |
2.99 |
1.02 x 10-3 |
The
presence of the extra electronegative oxygen atom (-I effect), increases the
strength of the 2- and 3- acid compared to benzene.
The effect decreases as the electronegative OH is
further from the COOH (same for -Cl and - NO2 substituted
acids below. |
|
3-hydroxybenzoic acid |
4.08 |
8.32 x 10-5 |
|
4-hydroxybenzoic acid |
4.58 |
2.63 x 10-5 |
| 2-chlorobenzoic
acid |
|
2.94 |
1.48 x 10-3 |
The
presence of the electronegative chlorine atom (-I effect), increases the
strength of the acid compared to benzene.
The effect decreases as the electronegative Cl is
further from the COOH (same for NO2 acids below) |
| 3-chlorobenzoic acid |
3.83 |
1.48 x 10-4 |
| 4-chlorobenzoic acid |
3.99 |
1.02 x 10-4 |
| 2-nitrobenzoic acid |
|
2.17 |
6.76 x 10-3 |
The
presence of the strongly electronegative nitro group -NO2
(-I effect) increases the strength of the acid compared to benzene, more so than
the a single chlorine atom. |
| 3-nitrobenzoic acid |
3.45 |
3.55 x 10-4 |
| 4-nitrobenzoic acid |
3.43 |
3.72 x 10-4 |
| benzene-1,2-dicarboxylic acid |
|
pKa1 = 2.98 pKa2 =
5.41 |
Ka1
= 1.05 x 10-3 Ka2
= 3.89 x 10-6 |
Two ionisations
possible to yield hydrated protons H3O+.
Note the order
Ka1 > Ka2
and
pKa1 < pKa2
for dibasic acids.
The extra electronegative group ensures the
1st ionisation is more so than in benzene, but the 2nd ionisation
constant is smaller than for benzoic acid. |
| benzene-1,3-dicarboxylic acid |
|
pKa1 = 3.46 pKa2 =
4.60 |
Ka1
= 3.47 x 10-4 Ka2
= 2.51 x 10-5 |
| benzene-1,4-dicarboxylic acid |
 |
pKa1 = 3.51 pKa2 =
4.82 |
Ka1
= 3.09 x 10-4 Ka2
= 1.51 x 10-5 |
| |
|
|
|
|
TOP OF PAGE and sub-index
7.11.4 The reactions of aromatic acids with
alkalis and carbonates/hydrogencarbonate
(a) Reaction with alkalis
(i) benzoic acid + sodium hydroxide
===> sodium benzoate + water
(aq) + NaOH(aq) ===>
(aq) + H2O(l)
This reaction ensures benzoic acid is much
more soluble in alkali than water because of the salt formation -
ions much readily solvated in water than the original acid - see
Aspirin next.
(ii) Making soluble aspirin
from:
(aq)
+ NaOH(aq) ===>
(aq)
+ H2O(l)
sodium 2-acetoxybenzoate (sodium
acetylsalicylate)
A couple of more complicated examples
- using simplified equations
(iii) Neutralisation of aromatic
dicarboxylic acids (dibasic acids)
This will be a 2 stage
neutralisation and both salts can be crystallised from aqueous
solution
HOOC-C6H4-COOH(aq)
+ NaOH(aq) ===>
HOOC-C6H4-COO-Na+(aq) + H2O(l)
HOOC-C6H4-COO-Na+(aq)
+ NaOH(aq) ===> Na+-OOC-C6H4-COO-Na+(aq) + H2O(aq)
(iv) The neutralisation of
hydroxybenzoic acids
Again, there is a two stage
neutralisation, but the carboxylic acid group will be neutralised
first, because the phenol group is a weaker acid - but strong
enough to react will alkali, but contrast with reaction with
carbonates in (b).
HOC-C6H4-COOH(aq)
+ NaOH(aq) ===>
HOC-C6H4-COO-Na+(aq) + H2O(l)
HOC-C6H4-COO-Na+(aq)
+ NaOH(aq) ===> Na+-OC-C6H4-COO-Na+(aq) + H2O(aq)
(b) The reaction of aromatic
carboxylic acids with carbonates and hydrogencarbonates
(i) Benzoic acid will readily react
with sodium carbonate solution to give sodium benzoate and carbon
dioxide gas.
2C6H5COOH(s/aq)
+ Na2CO3(aq) ===> 2C6H5COO-Na+(aq)
+ H2O(l) + CO2(g)
(ii) Watch out with hydroxybenzoic
acids - the phenol group is not strongly acidic enough to
liberate carbon dioxide from sodium hydrogencarbonate, so the
equations can only be a single step neutralisation reaction, as with
benzoic acid itself.
C6H5COOH(s/aq)
+ NaHCO3(aq) ===> C6H5COO-Na+(aq)
+ H2O(l) + CO2(g)
HOC6H4COOH(s/aq)
+ NaHCO3(aq) ===> HOC6H4COO-Na+(aq)
+ H2O(l) + CO2(g)
The salts formed are sodium benzoate
and sodium 2-/3-/4-hydroxybenzoate
TOP OF PAGE and sub-index
7.11.5 The reduction of aromatic acids
Sodium
tetrahydridoborate(III) NaBH4, (sodium borohydride), is not a powerful enough reducing agent to reduce
carboxylic acids.
Lithium
tetrahydridoaluminate(III), LiAlH4, (lithium aluminium tetrahydride), is a
much more
powerful reducing agent than NaBH4, and in a DRY ether solvent
(ethoxyethane), readily
reduces carboxylic acids to primary alcohols.
The LiAlH4 effectively
releases a hydride ion,
:H-, a
powerful nucleophile - electron pair donor, which can attacks the
δ+ carbon of polarised carbonyl bond
δ+C=Oδ-.
(electronegativity of O > C).
The reaction must be carried with a
dry solvent because LiAlH4 reacts rapidly with water
(and ethanol too).
The reaction is complex and goes
through various stages and can be summarised
as:
RCOOH + 4[H]
===> RCH2OH + H2O (R = H, alkyl or aryl)
The initial product must then be
hydrolysed with water to release the primary alcohol.
e.g. benzoic acid, an aromatic
carboxylic acid, is reduced to
phenylmethanol, a primary aliphatic alcohol:
C6H5COOH + 4[H] ==> C6H5CH2OH
+ H2O
+ 4[H] ===>
+ H2O
TOP OF PAGE and sub-index
7.11.6 Aromatic acid/acyl chlorides - preparation,
properties and reactions
(a) Preparation with
thionyl chloride from benzoic
acid
RCOOH + SOCl2
===> RCOCl + SO2 + HCl
+ SOCl2 ===>
+ SO2 + HCl
Benzoyl chloride is a colourless
liquid, boiling point 197oC
This method has the advantage of
producing gaseous waste products and just leaving the acid chloride
behind for further purification.
(b) Hydrolysis
by water
Aromatic acid chlorides are more
table to nucleophilic attack (e.g. from water), but they will slowly
hydrolyse in contact with water e.g. benzoyl chloride slowly
hydrolyses to benzoic acid.
+
H2O ===>
+ HCl
benzoyl
chloride + water ===> /benzoic acid + hydrogen
chloride (fuming in damp air)
or
C6H5COCl(l) + 2H2O(l)
===> C6H5COOH(aq) +
H3O+(aq) + Cl-(aq)
(c) The reaction of aromatic
acid chlorides with phenols
The acyl chloride
and alcohol usually readily react at room temperature, especially if
both are aliphatic.
However, phenols may require the presence of aqueous
sodium hydroxide to facilitate the reaction, especially if the acyl
chloride itself is itself a less reactive (than aliphatic) aromatic
e.g. C6H5COCl.
The alkali generates a negative
phenoxide ion (e.g. C6H5O–
from phenol C6H5OH), which is a more powerful
nucleophile than the original neutral phenol molecule.
benzoyl chloride + phenol
===> phenyl benzoate + hydrochloric acid
+
 ===>
+ H+ + Cl-
Notes:
Phenols are distinguished
from alcohols by having the OH hydroxy group directly to the benzene ring.
You can use aqueous conditions
because benzoyl chloride is more stable in water than the aliphatic
acid chlorides like ethanoyl chloride.
If you shake benzoyl
chloride with phenol dissolved in sodium hydroxide, you get an immediate
white precipitate of the ester phenyl benzoate.
The reaction is particularly fast because
the sodium hydroxide reacts with phenol (a very weak acid) to form the negative phenate ion - a much more powerful nucleophile than a neutral
alcohol OR water molecules. The structure of sodium phenoxide is shown on
the left.
(d) Amide formation
from conc. ammonia solution or amine
(i) benzoyl chloride + ammonia
===> benzamide + hydrogen chloride
+ 2NH3 ===>
+ NH4Cl
This equation illustrates the
formation of the primary aromatic amide, benzamide
(ii) benzoyl chloride
+ phenylamine ===> N-phenylbenzamide +
hydrogen chloride
+
===>
+ HCl
This equation illustrates the
formation of the secondary aromatic amide, N-phenylbenzamide
(e)
The
reduction of acid/acyl chlorides - conversion to primary alcohol
Acyl/acid chlorides, like carboxylic
acids esters, are reduced by the powerful reducing agent lithium
tetrahydridoaluminate(III), LiAlH4, giving the corresponding
primary alcohol.
LiAlH4 reacts with water and
ethanol, therefore the reaction must be carried out in a dry
inert solvent e.g. ethoxyethane ('ether') and NOT water or ethanol.
The general equation is:
RCOCl + 4[H] ===>
RCH2OH + HCl
The LiAlH4 effectively
generates the equivalent of a hydride ion (:H-)
which is a powerful nucleophile - lone pair of electrons donor.
e.g. benzoyl chloride + hydrogen ===>
phenylmethanol ('benzyl alcohol') + hydrogen chloride
+ 4[H] ===>
+ HCl
(f) The reaction of acyl/acid chlorides with
aromatic hydrocarbons - ketone formation
Acid chlorides will react with aromatic hydrocarbons like
benzene and methylbenzene to form ketones.
This is an example of a electrophilic substitution Friedel-Crafts acylation reaction
The reaction must be carried out in dry conditions using an
aluminium chloride catalyst.
You reflux the mixture of the acid chloride, aromatic
hydrocarbon and aluminium chloride in a fume cupboard.
Fumes of hydrogen chloride are given off. e.g.
(ii) benzene + benzoyl chloride == AlCl3
==> diphenylmethanone + hydrogen chloride
+
===>
+ HCl
The product is also called 'diphenyl ketone'. You can describe this product as a completely aromatic
ketone.
TOP OF PAGE and sub-index
7.11.7 Examples of electrophilic substitution
in the benzene ring of benzoic acid
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 - illustrated in the 'electron density'
picture above.
The same orientation argument is backed
up by considering the secondary resonance structures on the diagram below.
The three on the right show the electron density is more reduced on carbon
atoms 2, 4 and 6 of the benzene ring.
Therefore the main
product in electrophilic substitution of benzoic acid is the 3- ....benzoic
acid if one more substituent is placed in the benzene ring of benzoic acid -
see the three examples described below.
See also 7.14
Explaining orientation of products
of a 2nd substituent into a monosubstituted benzene derivative
Examples of electrophilic
substitution in the benzene ring of benzoic acid
Introduction
If the atom of the original group (-COOH) 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 -COOH group 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. 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. 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
section 7.14 for more details).
(a)
The ring halogenation of aromatic benzoic acid
You can replace the hydrogen
atoms in the benzene ring using chlorine and a catalyst such as aluminium
chloride (AlCl3) or iron(III) chloride (FeCl3).
+ Cl2 == AlCl3 or FeCl3 ==>
+ HCl
The principal product is 3-chlorobenzoic
acid, with much smaller amounts of 2-chlorobenzoic acid and 4-chlorobenzoic acid.
Note the major product is
substitution at the 3 position of the benzene ring.
You can brominate i.e. substitute bromine
into the benzene ring using bromine and a catalyst of aluminium bromide
(AlBr3) or iron(III) bromide (FeBr3) to yield
mainly 3-bromobenzoic acid - equation as above, just swap Br for Cl.
Note again, that the major
product is substitution at the 3 position of the benzene ring.
(b) Nitration with a mixture of
concentrated nitric and sulfuric acids
+ HNO3 ===>
+ H2O
The aromatic acid (benzoic acid) is
heated with a mixture of concentrated nitric and sulfuric acids.
The principal product is
3-nitrobenzoic acid (78%), with much smaller amounts of 2-nitrobenzoic acid
(22%)
and 4-nitrobenzoic acid (2%).
Again, the major product is
substitution at the 3 position of the benzene ring.
3-nitrobenzoic acid is an important
intermediate in the production of certain dyes.
3-nitrobenzoic is reduced to
3-aminobenzoic acid which can then be coupled with a phenol to make an
azo dye.
+ 6[H] ===>
+ 2H2O
TOP OF PAGE and sub-index
7.11.8 Esters of aromatic carboxylic acids -
preparation, properties and reactions
Brief descriptions of the methods
with equations
 (a) Esters from carboxylic acid plus alcohol using an acid
catalyst
Concentrated
sulphuric acid acts as a catalyst in this reaction.
(i) Esterification using an
acid catalyst
Refluxing benzoic acid with
methanol in the presence of conc. sulfuric acid yields the ester
methyl benzoate.
General word equation for esterification:
carboxylic acid + alcohol == acid catalyst ==> ester + water
e.g.
benzoic acid +
methanol
methyl benzoate + water
+
CH3OH
+ H2O
sometimes more simply written as
C6H5COOH + CH3OH
C6H5COOCH3 + H2O
The reaction is reversible and
the mixture reaches equilibrium, and only about 2/3rds
of the carboxylic acid and alcohol have been converted to the ester.
Without a strong acid catalyst
e.g. conc. sulfuric acid, the reaction is very slow and the mixture is
heated to further increase the rate of reaction (see details of the
method below).
(ii) Esterification reaction between an acid chloride and an alcohol/phenol
The acyl chloride
and alcohol usually readily react at room temperature, especially if
both are aliphatic.
Examples of
nucleophilic addition of an alcohol to acid/acyl chlorides, followed by elimination to give the ester and
hydrogen chloride.
+
CH3OH ===>
+ HCl
Yields methyl benzoate
+
===>
+ H+ + Cl-
Yields phenyl benzoate, rapid if sodium hydroxide
added to the mixture - phenoxide ion is a stronger nucleophile
Phenyl benzoate C6H5COOC6H5 is a white solid, insoluble in
water, melting point 71oC and boiling point
314oC.
(b) Hydrolysis of an ester with acid or alkali - the
opposite of esterification
Esters can be hydrolysed by refluxing
with strong mineral acids or alkali, it is very slow with pure water.
(i)
Hydrolysis of an ester with dilute
hydrochloric or sulfuric acid
RCOOR' + H2O
RCOOH + R'OH
This reaction cannot go to
completion because it is a reversible reaction, despite the use of a
catalyst and heating the mixture under reflux.
You can go to a high % yield to the
right using an aqueous solution with lots of excess water.
e.g.
+ H2O == H+ ==>
+ CH3CH2OH
(ii)
Hydrolysis of an ester with aqueous
sodium hydroxide
Hydrolysing an ester with strong alkali e.g.
aqueous or ethanolic sodium/potassium hydroxide is called saponification,
i.e. its a specific name for a particular type of hydrolysis reaction.
RCOOR' + NaOH
===> RCOONa + R'OH
RCOOR' + OH-
===> RCOO- + R'OH
e.g.
+ NaOH ===>
+ CH3CH2OH
or
+ OH- ===>
+ CH3CH2OH
This reaction is not reversible and
goes to 100% completion forming the sodium salt of the acid.
The carboxylic acid is freed by
adding excess of a strong hydrochloric or sulfuric mineral acid.
e.g.
+ H+ ===>
TOP OF PAGE and sub-index
7.11.9
Brief notes on aromatic amides - physical and chemical properties of benzamide
Benzamide
amide of the parent benzoic acidAromatic amide -CONH2 group directly attached to the benzene
ring, slightly soluble in water.
C6H5CONH2 : melting
point 132oC; boiling point
290oC; both quite high due to intramolecular force of
hydrogen bonding via C=O and -NH2 groups.
(a) Preparation of benzamide
Reaction between benzoyl chloride and concentrated
ammonia solution.
C6H5COCl
+ 2NH3 ===> C6H5CONH2
+ NH4Cl
(b) Hydrolysis of benzamide
(i) Refluxing with strong mineral acid e.g.
hydrochloric acid yielding benzoic acid and ammonium chloride
C6H5CONH2
+ H2O + H+ ===>
C6H5COOH + NH4+
(ii) Refluxing with strong alkali e.g. sodium
hydroxide yielding sodium benzoate and ammonia
C6H5CONH2
+ OH- ===> C6H5COO-
+ NH3
You then add strong mineral acid to free
the weak benzoic acid.
C6H5COO-
+ H+ ===> C6H5COOH
7.12
The structure, properties and uses of
polyamides involving aromatic monomers
7.13
Examples of aromatic compounds from the
pharmaceutical industry including paracetamol
TOP OF PAGE and sub-index
7.11.10
Brief notes on aromatic nitriles - physical and chemical properties of
benzonitrile
Aromatic nitriles e.g. benzonitrile
Benzonitrile, C6H5CN, is a colourless liquid at room
temperature, mpt.
13oC and bpt. 191oCAn aromatic nitrile has the
functional group -C≡N
(abbrev. -CN) directly
attached to the benzene ring.
Reaction (a) Reduction
Sodium tetrahydridoborate(III), NaBH4 (sodium borohydride),
is not a powerful enough reducing agent to reduce nitriles to primary
amines.
LiAlH4
is a more powerful reducing agent than NaBH4 and reacts
violently with water (and reacts with ethanol too), so the reaction must be carried out in an inert
non-aqueous solvent
like dry ethoxyethane ('ether').
The reduction reaction of nitriles can be summarised
as:
RC N + 4[H]
===> RCH2NH2 (R = H, alkyl or aryl)
e.g.
+ 4[H] ===>
The product benzylamine,
C7H9N,
an aliphatic primary
amine, NOT aromatic, compare with one of its isomers
2-methylphenylamine
Reaction (b) The
hydrolysis of an aromatic nitriles
If the nitrile is refluxed with dilute
hydrochloric/sulfuric acid or sodium hydroxide (strong base - alkali) the
corresponding carboxylic acid or its sodium salt is formed.
The hydrolysis with pure water is to
slow, but the reaction is speeded up by a strong acid or strong alkali. Strictly speaking all the reactants and products
should be suffixed by (aq)
(i) Equations for the dilute mineral acid hydrolysis of a nitrile to give
the free acid
In this case converting
benzonitrile to benzoic acid.
+ 2H2O + H+ ===>
+ NH4+
Here the free acid and an ammonium
ion are formed.
(ii) Equations for the dilute mineral acid hydrolysis of a nitrile to give the
sodium salt of the acid
In this case converting
benzonitrile to sodium benzoate.
+ 2H2O + OH- ===>
+ NH3
You then add strong mineral acid to free the benzoic
acid from the sodium salt solution
+ OH+ ===>
TOP OF PAGE and sub-index
7.11.11 Brief
notes on aromatic aldehydes
Benzaldehyde is the simplest aromatic
aldehyde where the -CHO aldehyde functional group is directly attached to
the benzene ring.
At room temperature it is a colourless
liquid, boiling point 179oC, insoluble in water and has a strong smell of
almonds and is synthetically produced as a food flavouring and a fragrance
component in perfumes.
Note the oxidation sequence
== [O] ==>
== [O] ==>
Phenylmethanol (aliphatic primary alcohol)
==> benzaldehyde (aromatic aldehyde) ==> benzoic acid (aromatic carboxylic
acid)
You normally come across
disproportionation in the redox reactions of transition metal ion chemistry.
The Cannizzaro Reaction (shown below) is
an organic example of disproportionation involving the three molecules
above.
2
+ OH- ===>
+
When benzaldehyde is mixed with a strong
base like potassium hydroxide, one molecule is reduced to
phenylmethanol and another is oxidised to benzoic acid.
The benzoic acid can be freed by
adding hydrochloric acid, filtered off and recrystallised from hot
water.
The phenylmethanol can be extracted
with ether and purified by fractional distillation.
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