Sales de Diazonio -Colorantes
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Lecture 16 Aromatic Diazonium Salts
7.1.1 The Formation of Diazonium Salts
Addition of aqueous solution of NaNO2 to a solution of amine hydrochloride in presence
of excess of HCl which is cooled by an ice-bath such that the temperature of the reaction
remains below 5 ◦C diazotization of primary aromatic amine occurs.
NH2
NaNO2
HCl,< 5 oC
N2 Cl
Mechanism
NH2
NO+
H2N NO
H
HN NO HN NOH
N NOH N N
OH2-H2O
N N
-H
-H H
HO NO H2O NO-OH2
NOH
If electron withdrawing groups are attached to aromatic nucleus then the aromatic amines
are difficult to diazotize because the nucleophilicity of the amino-nitrogen is reduced by
the partial withdrawal of the unshared electron pair into the nucleus.
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NH2
NO O N
NH2
O O
7.1.2 The Reaction of Diazonium Salts
7.1.2.1 Reactions of nucleophiles at nitrogen
Nucleophiles react with diazonium ions to give covalent diazo-compounds. For example,
phenol via phenoxide ion couples with diazonium salt at pH 9-10 to afford para-
azophenols in good yield.
ON N Ar
OH
N N
Ar
tautomerizesHO N
N Ar
7.1.2.2 S N1 Reaction
Diazonium salts decompose on warming into nitrogen and aryl cation which is highly
reactive and could be attacked by any nucleophile in its vicinity.
N2
warm
-N2 H2OOH2 -H
OH
7.1.2.3 One Electron Reductions
Diazonium ions could be reduced by single electron transfer to give an aryl radical and
nitrogen. Copper(I) is frequently used for this purpose and the aryl radical is highly
reactive capable of abstracting a ligand from the transition metal ion or a hydrogen atom
from a covalent bond.
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-N2
CuCl -CuCl.Cl Cu Cl+
ClN2 Cl
7.2 Reactions in which N itr ogen Eliminated
7.2.1 Replacement by Hydroxyl
Diazonium salt on warming in water gives phenol via SN1 mechanism. The reaction is
generally performed in acidic solution to preserve phenol in its unionized form.
N2 ClH2O/H
warm
OH
7.2.2 Replacement by Halogen
7.2.2.1 Schiemann Reaction
Treatment of an aqueous solution of diazonium salt with fluoroboric acid under cold
conditions gives diazonium fluoroborate as precipitate, which could be dried and gently
heated to afford the flurobenzene by decomposition. The reaction involves S N1
mechanism.
NN
Cl Aq. MBF4, cold
M = Na, H, NH4
NN
BF4
- MCl salt precipitates
dry ppt
heat
BF4
F B
FF
F
F-BF3
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Examples:
S
NH2
CO2Me
NaNO2, HCl
HPF6 S
F
CO2Me
47%
A. Kiryanov, A. Seed, P. Sampson, Tetrahedron Lett. 2001, 42, 8797.
N NH2
OH NaNO2
F BOH
OH
N F
OH
F. Dolle, L. Dolei, H. Valette, F. Hinnen, F. Vaufrey, H. Guenther, C. Fuseasu, C. Coulon, M. Buttalender,C. Crouzel, J. Med. Chem. 1999, 42, 2251.
MeNH2
Me
Br
NaNO2, H
HBF4
MeF
Me
Br
H. Hart, J. F. Janssen, J. Org. Chem. 1970, 35 , 3637.
7.2.2.2 Sandmeyer Reaction
This method provides an effective route for the preparation of aromatic bromides and
chlorides. Addition of cold aqueous solution of diazonium chloride to a solution of CuCl
in HCl medium gives a sparingly soluble complex which is separated and heated to give
aryl chloride or bromide by decomposition.
NH2
HONON2
HX
XCuX .
+ X-Cu-X + N2
-N2
-CuX
X
X = Br, Cl, CN
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Examples:
S
NNH2
Me
NaNO2, HCl
CuSO4S
NCl
Me
53%
N. Zou, J.-F. Liu, B. Jiang, J. Combin. Chem. 2003, 5 , 754.
Me
NH2
CuCN
HONO
Me
CN
H. T. Clarke, R. R. Reed, Org. Synth. 1941, 1, 514.
F NH2
NO
HNO2, CuBr
HBr
F Br
N
O 65%
B. Mallesham, B. M. Rajesh, P. R. Reddy, D. Srinivas, S. Trehan, Org. Lett. 2003, 5 , 7963.
7.2.3 Replacement by Aromatic Carbon
A number of methods lead to arylation of aromatic carbon by diazonium salts.
7.2.3.1 Pschorr Reaction
This reaction describes the synthesis of phenanthrene and its derivatives via diazotation
followed by intramoecular cyclization using copper powder.
G
NH2
HNO2, H
Cu(0)
G
G = CH=CH, CH2-CH2, CO, NH, CH2, others
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Mechanism
The reactive intermediate depends on the reaction conditions. Under acidic conditions,
the diazonium salt is believed to decompose into aryl cation and nitrogen. The aryl cation
is highly reactive and attacked by the aryl ring that leads to cyclization. On the other
hand, under neutral and basic conditions, the diazonium salt is reduced by single electron
transfer to give aryl radical which proceeds reaction intramolecularly with benzene ring
to give the cyclized product.
G
N2
-N2
H
GG
H
-HH
G
In acid solut ion :
In neutral or basic solu t ion:
G
N2 -N2
GG
H
Cu(0)
.
+Cu(I)
.Cu(I) + H
Cu(0) + H
.
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Examples:
O
NMe2Me2N NH2
NaNO2, H2SO4
Cu-Sn, Na2SO4
O
NMe2Me2N
S. G. R. Guinot, J. D. Hepworth, M. Wainwright, J. Chem. Res. Synop. 1997, 183,.
65%
N
N
O
Bn
H2N
HNO2 N
N
O
Bn
P. Tapolcsanyi, B. U. W. Maes, K. Monsieurs, G. L. F. Lemiere, Z. Riedl, G. Hajos, B. Van den Driessche, R. A.Dommisse, P. Maytyus, Tetrahedron 2003, 59, 5919.
7.2.3.2 Reduction by Copper(I) Ammonium Ion
Diazotized anthranilic acid with copper(I) ammonium hydroxide gives diphenic acid in
90% yield.
CO2H
N2
2Cu(NH3)2OH
CO2H CO2H
The process probably takes place via one-electron reduction of the diazonium ion
followed by dimerization of the resulting aryl radicals.
7.3 Reactions in which Ni trogen is Retained
The reaction of the terminal nitrogen of a diazonium salt with nucleophile affords a
covalent azo-compound.
7.3.1 Reduction to Arylhyrazones
The reduction of aromatic diazonium salts can be accomplished with sodium sulfite or
SnCl2 or electrolysis to provide arylhydrazines. In case of sodium sulfite based reduction,
the diazonium ion with a sulfite anion may give a covalent azo-sulfite that, having a
double bond conjugated to an electron-accepting group, may add to a second nucleophilic
sulfite ion which on hydrolysis provides the hydrazine.
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N N
O
SO O
NN
S
O
OO
O
SO O
NN S
O
OO
SO3
NN S
O
OO
SO3
H
N
HN S
O
OO
SO3
2H
-2SO3 NH
NH2
7.3.2 Coupling Reactions
Diazonium ions are weak electrophiles, however, they undergo coupling with activated
aromatic nuclei such as aryl amines, phenols and aromatic heterocyclic compounds. For
example, N,N-dimethylaniline reacts with diazonium ion almost at the para-position.
However, the careful control of the pH of the reaction medium is necessary for the
success of the process.
Me2N.. N N-Ar
Me2NH
N N
Ar
-HN
N Ar
Me2N
In case of primary and secondary aromatic amines, the reaction preferentially takes place
at the nitrogen atoms of the diazonium ions. For example, aniline adds to the aromatic
diazonium salt to give diazoaminobenzene.
N N-PhNH2
..
-HNH
N N
Diazoaminobenzene
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7.4 The Synthetic Value of Diazo-Coupling
7.4.1 Dye-stuffs
Aromatic azo-compounds are coloured. Several of those compounds synthesized by the
diazo-coupling are employed as dye-stuffs. These compounds can be classified into three
groups.
First group of azo-compounds are neutral which are used as azoic combination (or
ingrain) dyes. An example is para red which is prepared by coupling of 2-napthol
with p-nitrobenzenediazonium salt.
O-
NN
OH
NN
NO2
NO2
Para red
+
The second group of azo-compounds posses either a sulfonic acid group or an
amino group which are generally adsorbed directly on the fiber from aqueous
solution. Examples are orange II (an acidic dye) and Bismarck brown R (a basic
dye).
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O-
NN
OH
NN
SO3-
SO3H
orange II (acidic dye)
+
+
Me
NH2
NH2
2 +
Me
N2
N2
-2H
Me
N
N
N
N
H2N NH2
Me
H2N
NH2
Me
Bismarck brown R
Azo-compounds that contain chelating groups to bind with a metal ions such as
Al(III) are used a mordant dyes. An example is alizarin yellow R, which contains
phenolic and carboxyl chelating groups.
OH
CO2H
+
N2
NO2
OH
CO2H
N N
NO2
Al(III)
O
N N
NO2
AlO
O
alizarin yellow R
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7.4.2 Indicators
Azo-compounds that contain both an acidic and a basic group can be utilized as
indicators since the colours of the conjugate acid and the conjugate base are different.
Examples are methyl orange and methyl red which are prepared by coupling of
dimethylaniline with diazotized sulfanilic acid and diazotized anthranilic acid,
respectively.
NN
NMe2
HO3S
Methyl orange
NN
NMe2Methyl red
CO2H
7.4.3 Synthesis of Amines
Azo-compounds are susceptible to hydrogenolysis to give amines. For example, the
coupling of 1-naphthol with benzenediazonium chloride gives azo-compound which
could be reduced to 4-amino-1-naphthol in the presence of dithionite.
O- OH
NN
Ph
OHNa2S2O4
NH2
+ PhNH2
PhN2
7.4.4 Synthesis of Quinones
The ortho and para diamines and aminophenols readily oxidize to give quinones. Both
classes of compounds can be readily prepared by diazo-coupling followed by reduction.
O- PhN2+
NN
OHNa2S2O4
OHNH2
O
OFeCl3/O2
Ph
Quinone
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Problems:
NN
NMe2
HO3S
NN
NMe2
CO2H
O
O
NN
OH
A. How would you obtain the following compounds?
B. Complete the following reactions.
O
N2 Cl
Me1.
CuI
2.
CHO
+CO2H
Ac2O
3. CO2Et
ONaOAc
PhN2
Text Books:
R.O.C. Norman and C. M. Coxon, Principles of Organic Synthesis, CRC Press, New
York, 2009.
B. P. Mundy, M. G. Ellerd, F. G. Favaloro Jr, Name Reactions and Reagents in Organic
Synthesis, Wiley Interscience, New Jersey, 2005.