types of Electrophilic substitution reaction.

1. Electrophilic
substitution
reactions

2. Diazo coupling
 The coupling of diazonium salt to suitable aromatic substrate is
an example of an aromatic electrophilic substitution reaction.
 When primary aromatic amines (𝑨𝒓𝑵𝑯 𝟐 )are treated with
nitrous acid diazonium salts are formed.
 The rex also occurs with aliphatic amines but the diazonium
salts are extremely unstable, even in solution.
 Aromatic diazonium ions are more stable, because of the
resonance interaction b/w nitrogen and the ring.
N
+
N N NH N
+
N
-
+
1 2 3

3.  1 contributes more to hybrid than 2 because of bond distance
measurements. In benzene diazonium chloride, the C—N
distance is ~1.42 Å and the N—N distance is ~1.08 Å which
values closely to a single and triple bond than to two double
bonds.
 Diazonium salts are stable at low temperature usually below at
5°C.
 Diazotization can takes place in acid solution. In dilute acid the
attacking species is 𝑵 𝟐 𝑶 𝟑 which acts as a carrier of 𝑵𝑶+ This
reaction is second order.
NH2
0°C
HNO2 + HCl
N
+
N

4. NaNO2 + HCl Na
+
N OO
-
H Cl
N OOH
HCl
N OOH2
+
NO+OH2 +
Formation of nitrosonium ion:

5. Attack of nitrosonium ion:
NH2
NO+
+
N
+
H
H
N O
base
N N
H
O
N
+
N OH
H
B-N N OH
H-Cl
N N OH2
+N
+
N

6. The synthetic value of diazo-coupling
a) Dye-stuffs
Azo compounds are strongly coloured and many of these are used as dyes-
stuffs. Three classes of dyes may be distinguished:
1) Neutral azo-compounds are used in azoic combination dyes, they are
formed in situ in the fibre. An example is para red, from 2-napthol and
p-nitrobenzenediazonim ion.
O
-
+ N
+
N N
+
O
-
O OH
N
N
N
+ O
-
O
para red

7. 2) Azo compounds which contains sulfonic acid group or an
amino group are adsorbed directly onto the fibre from
aqueous solutions. Examples are orange 2 an acidic dye
and Bismarck brown R, a basic dye.
O
-
+ N
+
N S
-
O
O
O
OH
N
N
SO O
OH
orange 2
CH3
NH2
NH2
CH3
N
+ N
N
+
N
CH3
N
N
NH2 NH2
CH3
N
N
NH2
NH2
CH3
-2H+
+
Bismarck brown R

8. 3) Azo compounds which contain
groups capable of chelation with
metal are used as mordant dyes
the metal ion is adsorbed on the
fibre and binds the dye-stuff. An
example is alizarine yellow R,
which forms chelates by means of
its phenolic and carboxyl groups.
Aluminum and chromium oxides
are commonly used as mordants.

9. Indicators
Azo compounds that contain both acidic and basic
group may be used as an indicator because of different
colours of conjugate acid and conjugate base. Examples
are methyl red, made from diazotized anthranilic acid and
dimethylaniline.

10. Formylation
 The addition of formyl group (—CHO) is known as formylation.
 Aromatic formylation reactions via electrophilic substitution include:
1. Gatterman Koch reaction
2. Vilsmeier Haack formylation
3. Reimer-Tiemann formylation
Gatterman Koch reaction
 The introduction of an aldehyde group into a certain aromatic nuclei by means of
CO, HCl and supportive catalyst is known as Gatterman Koch reaction. It can be
applied to many alkylbenzenes, phenols and their ethers and heterocyclic
compounds.
 The reaction was first reported in 1897.

11. Reimer-Tiemann formylation
 Treatment of phenol with chloroform (𝑪𝑪𝒍 𝟑 ) and aqueous hydroxide induces aldehyde group –CHO Ortho to
–OH.
 This is conducted in basic solution.
 The rex occurs through dichlorocarbene, which is generated from chloroform and alkali and being
electrophile attacked by strongly nucleophile phenoxide ion.

12. Mechanism

13. Carboxylation
 The reaction in which carboxylic group is introduced in a substrate to form carboxylate or carboxylic
acid.
Kolbe-Schmitt carboxylation
 Sodium phenoxides can be Carboxylated mostly in the ortho position by carbon dioxide. Phenoxides are
reactive enough but the are weak electrophile and reaction carried out under pressure at about 100°C.
 The ortho product predominates because of the stabilizing influence of chelation on the transition state
for ortho substitution.
 The mechanism is not clear but a sort of complex is formed b/w the reactants making the 𝐶𝑂2.