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Part 7.10
The chemistry of
AROMATIC COMPOUNDS
Doc
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK GCE IB advanced level organic chemistry students US grade 11 grade 12 organic chemistry
of aromatic amines like phenylamine synthesising azo dyes from preparation
of diazonium salts base character of phenylamine
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All my advanced level
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Part 7.10 The physical and chemical
properties of aromatic amines e.g. phenylamine, selected derivatives
including diazonium ions and azo dyes
Sub-index for this page
(split?)
7.10.1
The structure and physical properties of
aromatic amines like phenylamine
7.10.2
Methods of preparing phenylamine (in principle for other aromatic amines)
7.10.3
Relating the structure of phenylamine to
electrophilic substitution reactions and orientation
of products - synthesis restrictions
7.10.4
Electrophilic substitution reactions of
phenylamine (and other aromatic amines)
7.10.5
Reactions of phenylamine as a
nucleophile e.g. formation of amides with acid chlorides/anhydrides, but a
useful intermediate is made for further electrophilic substitution
7.10.6
Preparation
of the benzenediazonium ion (diazotisation) and their coupling reactions to synthesise dyes
7.10.7
The acid-base chemistry of aromatic
amines - weak bases and relative strength
See also 8.4
Acid-base chemistry of aliphatic amines, their comparative strength as bases
and reactions with acids
7.10.1
The structure and physical properties of aromatic amines like
phenylamine
A note of classifying amines, using aromatic
(aryl) amines as examples
In aromatic (aryl) amines, the
amino/amine group is directly attached to the benzene ring,
| Functional group of
the homologous series |
PRIMARY |
SECONDARY |
TERTIARY |
Comments |
| aromatic amine examples |
phenylamine |
diphenylamine |
triphenylamine |
Aromatic amine examples
with 1, 2 or 3 aryl groups attached to the nitrogen of the amine
functional group |
Physical properties of selected
aromatic amines and selected
derivatives
Abbreviations used: mpt =
melting point; bpt = boiling point; sub. = sublimes, dec.= thermally decomposes
Many aromatic amines have two functional groups e.g. -Cl,
-OH, -COOH, NO2 as well as -NH2.
Extra notes on the structure and
data table on aromatic (aryl) amines
(a) A few notes on naming aromatic
amines
(i) Alternative names: e.g. 2-methylphenylamine
is methyl-2-phenylamine.
(ii) The old, and still widely used name
for phenylamine is 'aniline' and many names of its derivatives
use 'aniline' too - I'm afraid lots of older 'historic'
names are still used and accepted, particularly in universities and the
chemical industry e.g. acetanilide C6H5NHCOCH3,
whose IUPAC systematic name is N-phenylethanamide!
(iii) Carboxylic acid and phenol are
higher ranking groups, so in this case the amine group is named as a
substituent group.
(iv) Aminophenols can also be named
as hydroxyphenylamines e.g. 2-aminophenol is 2-hydroxyphenylamine.
(b) A comparison of methylbenzene and phenylamine
| Property |
Methylbenzene |
Phenylamine |
| Melting point oC |
-95 |
-6 |
| Boiling point oC |
111 |
184 |
| Solubility in water |
insoluble |
slightly
soluble |
| Hydrocarbon solvents |
very
soluble |
very
soluble |
(i) Melting points and boiling points
Methylbenzene and phenylamine
molecules have similar shapes, sizes, molecular masses and numbers
of electrons, but they have quite different melting points and
boiling points.
The Pauling electronegativities
are: H = 2.1, C = 2.5 and N = 3.0
Methylbenzene does not have a
significantly polar bond and hence a relatively non-polar molecule.
There the intermolecular force is
almost entirely due to instantaneous dipole - induced dipole forces
(Van der Waals dispersive forces).
However, due to a more
significant difference in electronegativity, phenylamine does have a
significant polar bond in the amine group
:Nδ--Hδ+,
and in the liquid or solid state, you get hydrogen bonding (N-Hδ+llllδ-N-H
shown in the diagram below).
Therefore for phenylamine, there
will be the same instantaneous dipole - induced dipole force plus
permanent dipole - permanent dipole attraction, including the
directional hydrogen bonding.
The increases in intermolecular
bonding forces leads to higher melting points and boiling points of
phenylamine compared to benzene.
(ii) Solubility in water
Carrying the polar bond arguments
from (i):
The lack of a polar bond means
that methylbenzene will not dissolve in water, but phenylamine will
dissolve, albeit a fairly low solubility, but still significantly
higher than for an aromatic hydrocarbon.
The hydrophobic methylbenzene
disrupts the hydrogen bonds without compensating solute - solvent
intermolecular bonding and so is virtually insoluble in water.
The reason for the difference in
water solubility is due to the hydrogen bonding between phenylamine and
water molecules (the hydrogen bonding
N-Hδ+llllδ-O-H
or O-Hδ+llllδ-N-H
diagram above).
However, the hydrophobic benzene ring
does limit the solvation of the phenylamine, so the solubility in water
is moderate at ~3.6g/100g water = ~(10 x 3.6/93 = 0.39 mol/dm3).
Phenylamine, like most aromatic
compounds is much more soluble in organic solvents like ethoxyethane
('ether'), ethanol and propanone,
(c) Don't confuse aromatic side chain
aliphatic amines with 'true' aryl amines.
 left is a primary aliphatic
amine with the amine group -NH2 in a side-chain off the benzene
ring,
and right, the isomeric aryl amine
with the -NH2 directly attached to the benzene ring.
(d)
TOP OF PAGE
and sub-index
7.10.2
The
preparation
of phenylamine and other aromatic amines
Method 1. Using
lithium tetrahydridoaluminate(III) as the reducing agent
LiAlH4
is a more
powerful reducing agent than NaBH4 and in ether solvent readily
reduces nitro–aromatics to primary aromatic amines, the simplified equation
for the reduction of nitrobenzene to phenylamine is:
C6H5NO2 + 6[H] ===> C6H5NH2
+ 2H2O
and methylnitrobenzenes would
be reduced to methylphenylamine primary aromatic amines, i.e.
CH3C6H4NO2 + 6[H] ===> CH3C6H4NH2
+ 2H2O
as will any aromatic
compound with a nitro group (–NO2) attached directly to the benzene
ring.
This is an example of the reduction of a polar N-O pi bond via
the hydride ion (H-) acting as a nucleophile generated by the
tetrahydridoaluminate(III) ion, which does not reduce the non-polar pi bond C=C
you find in alkenes.
Method 2. Using an
appropriate metal and acid as a reducing agent
Reduction of
nitro–aromatics with tin and concentrated hydrochloric acid
In the
laboratory, reacting a nitro–aromatic with a mixture of tin and conc.
hydrochloric acid by heating under reflux will reduce it to a primary aromatic amine (–NH2
directly attached to benzene ring). In industry a cheap metal like iron
powder and acid are used or a direct reduction in the gas phase with
hydrogen/transition metal catalyst, but here in the school laboratory e.g.
2 cm3 of
nitrobenzene, 4g of tin and 10 cm3 of conc. hydrochloric
acid that is slowly and carefully added to the mixture.
In the 'laboratory' preparation, the mixture may need
cooling with a beaker of cold water if the reaction is too vigorous, then
gentle heating with a beaker of boiled water to complete the reaction (the
condenser causes avoids loss of product) when hydrogen will stop being
evolved
The formation of phenylamine (aniline) from
nitrobenzene can be summarised as
C6H5NO2 + 6[H]
===> C6H5NH2
+ 2H2O
but the 'real' equations
are rather more complicated, the simplest redox equation I can come up with is
2C6H5NO2(aq)
+ 14H+(aq) + 3Sn(s) ==> 2C6H5NH3+(aq)
+ 3Sn4+(aq) + 4H2O(l)
which shows the formation of
the phenylammonium cation because the amine is a base and
formed in an acid medium.
You should appreciate
that any phenylamine formed, will immediately react with hydrochloric
acid and dissolve to form the salt
C6H5NH2(l)
+ HCl(aq) ===> C6H5NH3+(aq)
+ Cl–(aq)
and then conc.
aqueous sodium hydroxide is added to free the amine (immiscible with water) from its arylammonium
cation
C6H5NH3+(aq)
+ OH–(aq) ===> C6H5NH2(l) + H2O(l)
The flask is cooled and the apparatus assembled to perform a
steam distillation (diagram below).
The primary aromatic amine must be extracted by steam
distillation because the reaction mixture is quite messy to deal with in
any other way!
For more on the theory of steam
distillation see Equilibria Part 8.5
 On addition of conc.
sodium hydroxide (a strong soluble base) the amine separates out as an oily
layer and the mixture is heated with the steam input via the procedure known
as steam distillation. The addition of an alkali is necessary because the
phenylamine base is soluble in the excess acid and must be freed to steam
distil over
A mixture of the amine and water 'steam
distils' into the condenser and separates into two layers in the
collection flask. Steam distillation is the most efficient way of
extracting the phenylamine from the reaction mixture, leaving behind all the
inorganic residues. The inorganic residues are soluble, so can't be
filtered of. It would prove difficult to extract the phenylamine from the
reaction mixture by direct distillation OR trying to use an extracting
solvent directly on the reaction mixture. Its best leave as much of the
reaction residues behind before attempting the final purification
procedures.
2g of salt can be added to
the immiscible liquid mixture to help the two layers separate out. and
reduce the solubility of phenylamine in the water layer.
The phenylamine layer is
separated out with a separating funnel
Some methods use ether
solvent to extract the phenylamine from the steam distilled mixture.
Can't say I like this idea since a fractional distillation is done
later, ether is very volatile and very flammable!
Either way, the 'damp' phenylamine or mixture with ether, can be dried with
anhydrous solid sodium hydroxide or anhydrous potassium carbonate.
The drying agent is filtered
off
and the dried liquid fractionally distilled to obtain pure phenylamine liquid
- the fraction boiling between 180 and 185oC should be
reasonably pure phenylamine.
If ether isn't used, the
phenylamine is distilled with an air condenser (just a tube, not Liebig
style), though some texts say distil under reduced pressure because
phenylamine can decompose at its boiling point.
Phenylamine distils over
between 180-185oC at normal pressure, quite a high
boiling point, and a water condenser can crack of very hot
phenylamine vapour.
Method 3. Using
hydrogen gas and catalyst - industrial hydrogenation of nitro group
In the chemical
industry aromatic
nitro–compounds are more efficiently reduced with hydrogen gas using a Ni or
Cu catalyst at elevated temperatures, rather than a 'laboratory style'
preparation. The resulting
primary aromatic amines are very important intermediate compounds in dye and drug
manufacture e.g.
C6H5NO2 + 3H2 ===> C6H5NH2
+ 2H2O (nitrobenzene ==> phenylamine)
CH3C6H4NO2
+ 3H2 ===> CH3C6H4NH2
+ 2H2O
(nitromethylbenzenes ===>
aminomethylbenzenes/methylphenylamines)
CH3C6H3(NO2)2
+ 6H2 ===> CH3C6H3(NH2)2
+ 4H2O
(dinitromethylbenzenes ===>
diaminomethylbenzenes)
TOP OF PAGE
and sub-index
7.10.3
Relating the structure of phenylamine to
electrophilic substitution reactions and orientation
of products - synthesis restrictions
The diagrams below give an 'impression' of an electron
density 'map' of the delocalised pi electrons of the benzene ring and the
non-bonding pair of electrons on the nitrogen atom.
The amine group has a plus inductive effect (+I
electron shift) which increases the electron density of the ring compared
to benzene, so you expect phenylamine to be more reactive than benzene,
particularly to electrophilic attack at the 2, 4 an 6 substitution positions
where the electron density is highest.
So, the lone pair of electrons on the nitrogen atom
directly attached to the ring activates the benzene ring compared to benzene
itself in terms of the potential for electrophilic substitution.
The +I effect and partial delocalisation of the lone pair of
electrons on the nitrogen reduces their availability to act as a proton
acceptor, so phenylamine and other aromatic amines are usually weaker
bases than aliphatic amines.
Discussed further in 7.10.7
The acid-base chemistry of aromatic
amines and their relative strength
To some extent the lone pair of electrons on the
nitrogen atom interact with pi orbital electrons of the benzene ring
i.e. the lone pair of the nitrogen atom become part of the delocalised
system and are more or less in the same plane as the pi electrons of the
benzene ring (this has been shown experimentally and I've tried to
indicate this with my 'fuzzy' diagram!).
The resonance structures of phenylamine and the three on
the right correspond to the increase in density at the 2, 4 and 6
substitution positions.
BUT, there are three problems in synthesising derivatives
of phenylamine:
(i) It is difficult to get just one extra
substituent group into the benzene ring of phenylamine because
of its activated benzene ring - see the
reaction with bromine water in section 7.10.4.
(ii) Any synthesis involving an acid as a reagent,
an acid catalyst or an acid produced in the reaction, will
immediately cause problems because the amine group acts as a base,
accepting a proton.
C6H5NH2 + H+
C6H5NH3+
The protonated amine group deactivates the ring,
inhibiting electrophilic substitution.
Therefore you cannot nitrate phenylamine directly or
perform a Friedel-Crafts style synthesis.
(iii) Phenylamine is quite easily oxidised
e.g. nitric acid produces a dark mass of complex composition!
All three of these potential problems can be avoided
by a technique known as group protection.
This is described in
section 7.10.5.
TOP OF PAGE
and sub-index
7.10.4 Electrophilic substitution reactions of
phenylamine (and other aromatic amines)
Phenylamine is so reactive (argument above in
7.10.3) it rapidly reacts with bromine water at room
temperature to form a precipitate of 2,4,6-tribromophenylamine, without catalyst!
A reaction similar to phenol. See
7.9
The chemical
properties of phenol
A similar reaction happens with chlorine water to
rapidly give a similar precipitate of 2,4,6-trichlorophenylamine.
As already mentioned, you can't halogenate, acylate, alkylate
with Friedel-Crafts reactions or nitrate phenylamine because of the
basicity of the amino group in any controlled way because
(i) the amine group reacts with any acid present as a
product or reactant of the reaction
(ii) The ensuing positive phenylammonium ion
group strongly deactivates the benzene ring with respect o electrophilic
substitution.
However, you can get round this
problem with group protection i.e. see use of
N-phenylethanamide in section
7.10.5 next.
If the atom of the original group directly bonded to the
benzene ring does not have any π
bonding the ring is usually activated compared to benzene
itself. The -NH2 group increases the electron density of the
ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5
positions.
Therefore the 2, 4 and 6 positions become the preferred
2nd substitution point in the benzene ring. The small electron density
shift is sometimes described as a plus inductive shift (+I effect), but
this does not necessarily coincide with an atom of electronegativity
higher than carbon e.g. N. The reason being a lone pairs of the N
interact with the ring to increase the electron density and this
electron pair donation often overrides the difference in
electronegativity effect (this is all about conjugation and possible
resonance hybrid structures - see
section 7.14 for more details).
TOP OF PAGE
and sub-index
7.10.5
The reactions of phenylamine as a
nucleophile e.g. formation of amides with acid chlorides/anhydrides, but a
useful intermediate is made for further electrophilic substitution
When primary aromatic amines are reacted with an acid
chloride (RCOCl) you do not get electrophilic substitution, even with a
catalyst like AlCl3 or FeCl3.
Instead, the amine acts as a nucleophile and forms an
N-substituted amide.
e.g. phenylamine reacts rapidly with at room temperature
with ethanoyl chloride to yield ...
N-phenylethanamide (N-phenylacetamide, acetanilide)
C6H5NH2 + CH3COCl
===> C6H5NHCOCH3 + HCl
In industry, it is cheaper to use ethanoic anhydride
employing an acid catalyst e.g.
C6H5NH2 +
(CH3CO)2O
===> C6H5NHCOCH3 + CH3COOH
Both reactions are illustrated below using structural
formulae.
Note the term 'N-substituted' i.e. the
substitution involves replacing a hydrogen of the amino group and NOT a
hydrogen in the benzene ring.
N-phenylethanamide is less reactive than phenylamine, but
more than benzene and more controllable than phenylamine.
Electrophilic substitution reactivity order:
C6H5NH2
> C6H5NHCOCH3 > C6H6
Note that N-phenylethanamide is hydrolysed to the
free amine by aqueous sodium hydroxide
N-phenylethanamide + sodium hydroxide
===> phenylamine + sodium ethanoate
C6H5NHCOCH3 +
NaOH ===> C6H5NH2 +
CH3COO-Na+
In my examples here, it is the final step to synthesise a desired
derivative of phenylamine and other aryl amines.
We are now ready to look at 'group protection'
synthesis!
Group protection in aromatic synthesis
Starting with 'previously prepared' N-phenylethanamide, I'll
use four electrophilic substitution reactions to illustrate the idea
of group protection - in this case protecting the amine group in
phenylamine.
You protect this group by converting phenylamine to
N-phenylethanamide, carry out the electrophilic substitution reaction
and then obtain the desired 'orientated' product by hydrolysing the
substituted N-phenylethanamide.
(1) Mono-nitration of the benzene ring to make 4-nitrophenylamine
Using a cold mixture of conc. nitric and sulfuric acid.
C6H5NHCOCH3 +
HNO3 ===> O2NC6H4NHCOCH3 +
H2O
The structural formula equation for the formation of the
majority product is shown below.
Comments on equation 14B:
(i) The intermediate is called
N-(4-nitrophenyl)ethanamide and is hydrolysed with aqueous sodium
hydroxide solution, so the full synthetic sequence from phenylamine is
phenylamine ==> N-phenylethanamide ==>
N-(4-nitrophenyl)ethanamide ==> 4-nitrophenylamine
(you can even start the synthesis sequence from
benzene ===> nitrobenzene ==> phenylamine etc.)
The product can be reduced to form
1,4-diaminobenzene and important intermediate in the production of
aramid fibres and Kevlar polymers.
See section 7.12
The structure, properties and uses of
polyamides
(ii) Typical orientation yields of the substitution
products are: 2% in the 2 position, 8% in the 3 position and 90% in the
4 position of the benzene ring (2 + 4 substitution = 98%).
(iii) The typical % yields fit in with the activation of
the benzene ring theory and favoured substitution orientation,
particularly the 4 position of the benzene ring of the
N-phenylethanamide.
(iv) Substitution in position 4 is also enhanced by the
steric hindrance of the 2 position by the -NHCOCH3 group, so
this synthesis route is very good at giving high yields of substituted
products in the 4 position of the benzene ring of phenylamine e.g. to
synthesise 4-nitrophenylamine.
These comments also apply to reactions (2) to (4)
described next
(2) Mono-chlorination of the benzene ring
Using a cold mixture of phenylamine, pure ethanoic acid
and chlorine bubbled in.
C6H5NHCOCH3 +
Cl2 ===> ClC6H4NHCOCH3 +
HCl
The structural formula equation for the formation of the
majority product is shown below.
Comments on equation 14C:
(i) The intermediate is called
N-(4-chlorophenyl)ethanamide and is hydrolysed with aqueous sodium
hydroxide solution, so the full synthetic sequence from phenylamine is
phenylamine ==> N-phenylethanamide ==>
N-(4-chlorophenyl)ethanamide ==> 4-chlorophenylamine
(ii) Typical orientation yields of the substitution
products are: >80% in the 4 position of the benzene ring and smaller %
for the 2 and 3 positions).
(iii) The typical % yields fit in with the activation of
the benzene ring theory and favoured substitution orientation,
particularly the 4 position of the benzene ring of the
N-phenylethanamide.
(iv) Substitution in position 4 is also enhanced by the
steric hindrance of the 2 position by the -NHCOCH3 group, so
this synthesis route is very good at giving high yields of substituted
products in the 4 position of the benzene ring of phenylamine e.g. to
synthesise 4-chlorophenylamine.
(3) Mono-bromination of the benzene ring
Using a cold mixture of phenylamine, pure ethanoic acid
and bromine.
C6H5NHCOCH3 +
Br2 ===> BrC6H4NHCOCH3 +
HBr
The structural formula equation for the formation of the
majority product is shown below.
Comments on equation 14D:
(i) The intermediate is called
N-(4-bromophenyl)ethanamide and is hydrolysed with aqueous sodium
hydroxide solution, so the full synthetic sequence from phenylamine is
phenylamine ==> N-phenylethanamide ==>
N-(4-bromophenyl)ethanamide ==> 4-bromophenylamine
(ii) Typical orientation yields of the substitution
products are: >80% in the 4 position of the benzene ring and smaller %
for the 2 and 3 positions).
(iii) The typical % yields fit in with the activation of
the benzene ring theory and favoured substitution orientation,
particularly the 4 position of the benzene ring of the
N-phenylethanamide.
(iv) Substitution in position 4 is also enhanced by the
steric hindrance of the 2 position by the -NHCOCH3 group, so
this synthesis route is very good at giving high yields of substituted
products in the 4 position of the benzene ring of phenylamine e.g. to
synthesise 4-bromophenylamine.
(4) Mono-acylation of the benzene ring
Using ethanoyl chloride (or ethanoic anhydride in
industry) and Lewis acid catalyst.
C6H5NHCOCH3 +
CH3COCl ===> CH3COC6H4NHCOCH3 +
HCl
The structural formula equation for the formation of the
majority product is shown below.
Comments on equation 14E:
(i) The intermediate is called
N-(4-ethanoylphenyl)ethanamide and is hydrolysed with aqueous sodium
hydroxide solution, so the full synthetic sequence from phenylamine is
phenylamine ==> N-phenylethanamide ==>
N-(4-ethanoylphenyl)ethanamide ==> 1-(4-aminophenyl)ethanone
(ii) Typical orientation yields of the substitution
products are: >80% in the 4 position of the benzene ring and smaller %
for the 2 and 3 positions).
(iii) The typical % yields fit in with the activation of
the benzene ring theory and favoured substitution orientation,
particularly the 4 position of the benzene ring of the
N-phenylethanamide.
(iv) Substitution in position 4 is also enhanced by the
steric hindrance of the 2 position by the -NHCOCH3 group, so
this synthesis route is very good at giving high yields of substituted
products in the 4 position of the benzene ring of phenylamine e.g. to
synthesise 1-(4-aminophenyl)ethanone.
TOP OF PAGE
and sub-index
7.10.6
Preparation
of benzenediazonium ions and their coupling reactions to synthesise dyes
See also
uv-visible absorption
spectra - index of examples: uses, applications, more chemistry of colour
(a) The structure
of benzenediazonium compounds
Primary aromatic amines form a diazonium salts with nitrous
acid, a cation balanced by an anion e.g.
the salt benzenediazonium chloride has the formula
C6H5N2+Cl-(aq)
The benzenediazonium ion from phenylamine has the
structure C6H5N+≡N:
The positive charge of any diazonium ion is on the nitrogen
atom directly attached to the benzene ring.
(b) The
preparation of a benzenediazonium salt solution
Phenylamine (or any aromatic amine) is dissolved in
hydrochloric acid and (ideally) cooled to ~5oC.
A solution of sodium nitrite (NaNO2)
previously cooled to ~5oC is slowly added to the amine
solution.
The mixture should be kept cool at ~5oC.
At 0oC the ensuing reactions are too slow
and above 10oC the diazonium ion decomposes evolving nitrogen gas.
(c) The diazotisation
equations
The equations for the diazotisation of phenylamine or a
substituted primary amine are:
C6H5NH2(aq) +
HNO2(aq) + H+(aq) ==> C6H5N2+(aq)
+ 2H2O(l)
XC6H4NH2(aq) +
HNO2(aq) + H+(aq) ==> XC6H4N2+(aq)
+ 2H2O(l)
X can be OH, CH3, NO2, Cl, Br etc.
The diazonium cation R-N+≡N:
is stabilised by the interaction of the positive charge of -N2+
group with the electron rich pi orbitals of the benzene ring -
delocalisation of the positive charge.
Even so, diazonium salt solutions decompose above 5oC
and the solid salts are explosive!
(d) Coupling
reactions of diazonium compounds (ions) with phenols and aromatic amines to
form dyes
As mentioned, one nitrogen on the diazonium ion carries
a positive charge and since nitrogen is quite an electronegative
element, the diazonium ion can act as a powerful electrophile.
So the benzenediazonium ions can substitute into a
benzene ring activated by an OH group (phenol) or an NH2
group (aromatic amine).
These 'coupling reactions' produce coloured
precipitates, many of which are useful dyes ('dyestuffs').
These dye compounds can form hydrogen bonds via e.g.
N-Hδ+llllδ-O-H, O-Hδ+llllδ-O-H
or O-Hδ+llllδ-N-H
with groups on the molecules of cotton and wool fibres, which strongly
bind the dye molecules to the fabric.
Dye
preparation
The coupling reactions often involve two aromatic
compounds, a benzenediazonium salt solution and mixing its
solution with a phenol in alkaline solution
or an aromatic (aryl) amine in neutral solution - the latter two
molecules are called the coupling agents.
Some coupling
reaction equations to form the dye assuming you start with the diazonium
chloride salt solution
The aromatic amine that is diazotised and the substrate aromatic amine or
phenol coupling agent used to synthesise the dye molecule.
The formation of the -N=N-
grouping connecting the benzene rings identifies the molecule as an
azo dye.
phenylamine + phenol:
+
===>
+ HCl
phenylamine + phenylamine:
+
===>
+ HCl
4-methylphenylamine + phenol:
+
===>
+ HCl
4-methylphenylamine + phenylamine:
+
===>
+ HCl
Four examples are now described in more detail (including
two of the above)
DYE formation diagram A
A cooled sodium hydroxide solution of phenol (phenoxide
ion C6H5O- formed) is slowly added to
the diazotised solution of phenylamine and a yellow precipitate forms.
DYE diagram A
(i) The diazotisation of phenylamine to
form the benzenediazonium ion
(ii) Formation of the phenoxide ion
followed by its coupling with the benzenediazonium ion to give a yellow
dye precipitate of 4-hydroxybenzene.
(iii) A shorthand version of (ii)
omitting the formation of the phenoxide ion.
DYE formation diagram B
Phenylamine is slowly added to the diazotised solution
of phenylamine and a yellow precipitate forms.
DYE diagram B
(i) The diazotisation of phenylamine to
form the benzenediazonium ion
(ii) The coupling of the benzenediazonium
ion with phenylamine to give a yellow precipitate of 4-aminoazobenzene
('Aniline yellow' dye).
DYE formation diagram C
A cooled sodium hydroxide solution of 2-naphthol
(2-naphthoxide ion formed) is slowly added to the diazotised solution of
phenylamine and a red precipitate forms.
DYE diagram C
(i) The diazotisation of phenylamine to
form the benzene diazonium ion
(ii) Formation of the 2-naphthoxide anion
followed by its coupling with the benzenediazonium ion to give a red dye
precipitate.
(iii) A shorthand version of (ii)
omitting the formation of the 2-naphthoxide ion.
DYE formation diagram D
A cooled sodium hydroxide solution of 2-naphthol
(2-naphthoxide ion formed) is slowly added to the diazotised solution of
2-nitrophenylamine and a red precipitate forms.
DYE diagram D
(i) The diazotisation of 4-nitrophenylamine to form the
4-nitrobenzenediazonium ion
(ii) The coupling of the
4-nitrobenzenediazonium ion with alkaline 2-naphthol to give a red dye precipitate
'Para red'.
Origin of the
colour in azo-compounds
Azo-compounds are coloured because the -N=N- grouping
attached to benzene rings absorbs visible light - the colour of the dye
is what isn't absorbed.
The group responsible for the 'dye' colour is called a
chromophore.
The particular colour depends on the substituents
present in the benzene ring.
TOP OF PAGE
and sub-index
7.10.7 The acid-base chemistry of aromatic
amines
See also 8.4
Acid-base chemistry of aliphatic amines, their comparative strength as bases
and reactions with acids
Reminder 1. What is a base?
Here a base
is defined as a proton acceptor (See
Lewis and Bronsted–Lowry acid–base theories)
The amine functional group is -NH2,
-NH- or >N-, and all acts as Lewis/Bronsted-Lowry bases via the
lone non-bonding pair of electrons on the nitrogen atom i.e.
R3N: + H+
[R3NH]+
(where, in this case, R = H, alkyl or
aryl)
Reminder 2. Defining an aromatic (aryl) amine
In aromatic (aryl) amines, the amino/amine group is
directly attached to a benzene ring,
Reminder 3. Effect of the benzene ring on the 'base'
strength of aromatic amines
The amine group has a plus inductive effect (+I electron
shift) which increases the electron density of the ring compared to
benzene ('fuzzy' diagram below).
The +I effect and delocalisation
of the lone pair of electrons on the nitrogen reduces their availability
to act as a proton acceptor (N:
less δ-), so phenylamine and other aromatic amines
are usually weaker bases than aliphatic amines.
Reminder 3. Here, the term weak base refers to one
that only ionises (dissociates) to a small extent in aqueous solution.
e.g. ammonia is a weak base in water:
NH3(aq) + H2O(l)
== ~2% ==>
NH4+(aq) + OH-(aq)
A strong base like sodium hydroxide ionises completely:
NaOH(s) + aq
== 100% ==> Na+(aq) + OH-(aq)
The equilibrium involved when a weak base B dissolves in
water:
(i)
:B(aq) + H2O(l)
BH+(aq) + OH-(aq)
(for any soluble base, note the lone pair on N)
(ii)
:NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
(for ammonia)
(iii)
RNH2(aq) + H2O(l)
RNH3+(aq) + OH-(aq)
(for an organic base, R = alkyl or aryl)
From equation (iii) for a weak organic base, the ionisation constant, the equilibrium constant (Kb)
for equilibrium (iii), is given by the expression (water is a subsumed
constant):
|
Kb =
|
[RNH3+(aq)]
[OH–(aq)]
|
|
––––––––––––––––––
mol dm-3 |
|
[RNH2(aq)] |
Like pH, the range of Kb is so
great, it is often quoted on the logarithmic scale where
pKb
= -log10(Kb/mol dm-3) and
Kb = 10-pKb
The stronger the
base, the greater the Kb and the lower the pKb value.
More on
Definition of a weak
base, theory and examples of Kb, pKb, Kw weak
base CALCULATIONS
In the data table below
I've quoted both values and a few comments what affects the value of Ka.
Neutralisation
equations - illustrating why relatively insoluble aromatic amines dissolve
in acids
C6H5NH2(aq)
+ HCl(aq) ===> C6H5NH3+(aq)
+ Cl-(aq)
C6H5NH2(aq)
+ H+(aq) ===> C6H5NH3+(aq)
(the phenylammonium ion/cation)
From the salt solutions you can crystallise the ionic
compounds e.g. and noting the name of the cation
Hydrochloric acid yields phenylammonium chloride:
C6H5NH3+Cl-
and sulfuric acid yields phenylammonium sulfate:
(C6H5NH3+)2SO42-
Comments on the above data table on primary
aliphatic and aromatic
amines and don't forget to appreciate the significance of the relative Kb
and pKb values
(a) Comparing
the base strength of ammonia and the primary aliphatic amine ethylamine
Equation
20A: As you can see from the Kb value, ammonia is a weak
base, with only a few % of the molecules ionising to give an
alkaline solution of ammonium ions and hydroxide ions (pH
<7).
Note (i) the ammonium ion is the conjugate
acid of the ammonia base.
(ii) Ammonium salts of strong acids (e.g. HCl, H2SO4)
in aqueous solution are acidic because when you dissolve them
in water the ammonium ion (the conjugate acid) protonates water, lowering
the pH.
i.e.
NH4+(aq) + H2O(l)
NH3(aq) + H3O+(aq)
Equation
20B: As you can see from the low Kb value,
ethylamine is a weak
base, with only a few % of the molecules ionising to give an
alkaline solution of ethylammonium ions and hydroxide ions.
The ethyl alkyl group has a +I effect electron shift
increasing the availability of the lone pair of electrons on the
nitrogen atom of the amine group to accept a proton.
Therefore ethylamine is a stronger base than ammonia
i.e. Kb(ethylamine) > Kb(ammonia)
Note (i) the ethylammonium ion is the conjugate
acid of the ethylamine base.
(ii) Ethylammonium salts of strong acids (e.g. HCl, H2SO4)
in aqueous solution are acidic because when you dissolve them
in water the ethylammonium ion (the conjugate acid) protonates water, lowering
the pH.
i.e.
CH3CH2NH3+(aq) + H2O(l)
CH3CH2NH2(aq) + H3O+(aq)
(b) Comparing
the base strength of ammonia and two aromatic amines
Equation
20C: The very low Kb value for phenylamine
means it is a very weak
base, with only a tiny fraction of the molecules ionising to give an
alkaline solution of phenylammonium ions and hydroxide ions.
The benzene ring group has a -I effect electron
shift decreasing the availability of the lone pair of electrons on
the nitrogen atom of the amine group to accept a proton.
Therefore phenylamine is a weaker base than
ammonia i.e.
Kb(ethylamine) > Kb(ammonia)
> Kb(phenylamine)
Note (i) the phenylammonium ion is the conjugate
acid of the phenylamine base.
(ii) Phenylammonium salts of strong acids (e.g. HCl, H2SO4)
in aqueous solution are acidic because when you dissolve them
in water the phenylammonium ion (the conjugate acid) protonates water, lowering
the pH.
i.e.
C6H5NH3+(aq) + H2O(l)
C6H5NH2(aq) + H3O+(aq)
Equation
20D: The extremely low Kb value for
4-nitrophenylamine means it is an extremely weak
base, with only a minute fraction of the molecules ionising to give an
alkaline solution of phenylammonium ions and hydroxide ions.
The benzene ring group plus the very electronegative
nitro group has a big -I effect electron shift decreasing the
availability of the lone pair of electrons on the nitrogen atom of
the amine group to accept a proton.
Therefore 2-nitrophenylamine is even a much
weaker base than phenylamine i.e.
Kb(ethylamine) > Kb(ammonia)
> Kb(phenylamine) > Kb(4-nitrophenylamine)
Note (i) the 4-nitrophenylammonium ion is the
conjugate
acid of the 4-nitrophenylamine base.
(ii) 4-nitrophenylammonium salts of strong acids (e.g. HCl, H2SO4)
in aqueous solution are acidic because when you dissolve them
in water the phenylammonium ion (the conjugate acid) protonates water, lowering
the pH.
i.e.
O2NC6H4NH3+(aq) + H2O(l)
O2NC6H4NH2(aq) + H3O+(aq)
(c) More on
conjugate acids and their 'acidic' nature
Below are another series of equations illustrating
the formation of the conjugate acid from the protonation of the
base.
To free an organic amine (aliphatic or aromatic) you
add a strong alkali like sodium hydroxide which neutralises the
conjugate acid and reverses the reaction.
The proportions of free base and conjugate acid
present in the equilibrium are determined by the pH.
ammonia ===> ammonium ion
ethylamine ===> ethylammonium ion
To free ethylamine:
CH3CH2NH3+(aq)
+ OH-(aq) ===> CH3CH2NH2(aq)
+ H2O(l)
phenylamine ===> phenylammonium ion
To free phenylamine:
C6H5NH3+(aq)
+ OH-(aq) ===> C6H5NH2(aq/l)
+ H2O(l)
The phenylammonium ion is sufficiently acidic to
liberate carbon dioxide from sodium hydrogencarbonate solution e.g.
C6H5NH3+(aq)
+ HCO3-(aq) ===> C6H5NH2(aq/l)
+ H2O(l) + CO2(g)
4-nitrophenylamine ===>
4-nitrophenylammonium ion, and to free the 4-mitrophenylamine:
O2NC6H4NH3+(aq)
+ OH-(aq) ===> O2NC6H4NH2(aq/l)
+ H2O(l)
Again, the 4-nitrophenylammonium ion is more than
sufficient acidic to liberate carbon dioxide from sodium
hydrogencarbonate solution e.g.
C6H5NH3+(aq)
+ HCO3-(aq) ===> C6H5NH2(aq/l)
+ H2O(l) + CO2(g)
(d) Comparing
methylphenylamines and chlorophenylamines with phenylamine
Phenylamine has a pKb of 9.38, the methylphenylamines
have pKb values of 9.00 to 9.62, so the methyl
substituent doesn't to have any great effect on the strength of
these amines.
  
Phenylamine has a pKb of 9.38, the chlorophenylamines
have pKb values of 10.07 to 11.44, so the chloro
substituent does have an effect on the strength of these amines.
The
minus inductive effect (-I) electron shift of the electronegative chlorine atom
makes the lone pair of non-bonding electrons on the nitrogen less
available for protonation compared to phenylamine.
This makes them
weaker bases, though the chlorine -I effect is not as much as that cause by
the nitro group (NO2) discussed above with three very
electronegative atoms in the group.
(f) The aminophenols (or hydroxyphenylamines)
are amphoteric and react with acids or bases depending on which
functional group is active at a particular pH e.g.
(i) In acid media:
H2NC6H4OH +
H3O+ ===> +H3NC6H4OH
+ H2O
(+ on nitrogen atom)
(ii) In alkaline media:
H2NC6H4OH
+ OH- ===> H2NC6H4O-
+ H2O (- on oxygen atom)
See also 8.4
Acid-base chemistry of aliphatic amines, their comparative strength as bases
and reactions with acids
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