Chloroazobenzenes: studies on syntheses.

Full text of DOI:10.1515/znb-1969-0811

994
H. A. B. LINKE. R. BARTHA. AND D. PRAMER
Chloroazobenzenes: Studies on Syntheses
H. A. B. L in k e , R. B a r t h a , and D. P ra m e r *
Department of Biochemistry and Microbiology. Rutgers University,
New Brunswick, New Jersey 08903, USA
Dedicated to Prof. Dr. O t t O N e u N h O e f f e r on his 65-th birthday
(Z. Naturforschg. 24 b, 994—996 [1969] ; eingegangen am 19. Februar 1969)
Thirteen chloroazobenzenes were synthesized by reduction of chloronitrobenzenes or by oxidation
of their corresponding chloroanilines. Five of the compounds have not been previously prepared or
described. Reduction by LiAlH4 was the method of choice, but it proved inadequate for the syn-
theses of chloroazobenzenes from o-chloronitrobenzenes. This reaction eliminated chlorine if ortho to
the aniline nitrogen and an additional chlorine atom; in the other cases steric hindrance of azo
bond formation resulted in low yields. Chromatographic systems for the separation of pure chloro-
azobenzenes from the crude reaction mixtures were developed and are described.
Results and Discussion
chlorine atom was not eliminated, but the yield of
dichloroazobenzene was low ( < 5 % ) . Nitrobenzenes
with chlorine substitutents in meta and/or para posi-
tion produced azo compounds in yields as great as
95 per cent. These differences can be explained only
The synthesis of chloroazobenzenes depends in
part on the starting material. Some methods are
based on oxidation of the amino group of chloro-
anilines and others require the reduction of the nitro
group of chloronitrobenzenes. E d w a rd 1 oxidized
primary aromatic amines to azobenzenes using ben-
zoylperoxide (yield 5 —20% ). The oxidant used by
by steric hindrance. Apparently,
a halogen atom
ortho to an amino or nitro group interferes steri-
cally with the formation of an azo bond. D a in s and
K e n y o n 7 also noted halogen losses when sub-
stituted aromatic nitro and nitroso compounds were
reduced to azoxybenzenes with sodium alcoholates.
Losses amounted to as much as 62 per cent. They
were not a function of location of the halogen but
depended on the nature of the alcohol used.
W h e e le r and G o n z a le s 2 was M n 02 (yield
5 to
90% ); and they observed that halogenoanilines sub-
stituted in the p-position were especially reactive in
the order F > C l > B r > I . D a n ie ls and S a u n d e r s 3
produced 4 ,4 /-dichloroazobenzene (yield 20%) enzy-
matically in a system that contained p-chloroaniline,
H20 2 and horseradish peroxidase in acetate buffer
at pH 4.6. K rem er and B e n d ic h 4 reduced chloro-
nitrobenzenes to chloroazobenzenes (yield 40 —80%)
and other products with monoethanolamine in the
presence of anhydrous Na2C 03 , but reduction with
LiAlH4 in ether solution as described by C o r b e tt
and H o l t 5 proved to be a superior method for the
synthesis of chloroazobenzenes (yield up to 95% ).
However, present results indicate that this method
is not suitable for production of chloroazobenzenes
from o-chloronitrobenzenes. Chlorine that was ortho
to the nitro group and an additional chlorine atom,
was eliminated in the course of the reaction. Chlorine
ortho to the nitro group and without an adjacent
Some of the reactions yielded quantities of tar, so
that purification of chloroazobenzenes presented dif-
ficulties. The physical properties of chloroazoben-
zenes, chlorohydrazobenzenes and chloroazoxy-
benzenes are very similar, but they were separated
by chromatography on neutral aluminum oxide
(Woelm, Act. I) columns that were developed by
nonpolar solvents (ligroine, benzene, toluene). Ef-
fluents were fractionated and examined by gas chro-
matography for di-, tetra- and hexachloroazobenze-
nes (DCAB, TCAB. HCAB). As a final step in the
purification of all azobenzenes**, the compounds
were recrystallized from acetone and their homo-
genity was established by thin layer and gas chro-
matography.
* Dept, of Biochem. and Microbiol, and the Bur. of Conser-
vation and Environmental Sei., Rutgers-The State Univer-
sity, New Brunswick, New Jersey 08903, USA.
1 J. T. e d w a r d . J. chem. Soc. [London] 1956. 2 2 2 .
2 O. H. w h e e l e r and D. G O N z a l e z , Tetrahedron [London]
20. 189 [1964],
3
4
D. G. H. D a n ie ls and B. C. S a u n d e r s , J. diem. Soc. [L o n -
don] 1953,822.
C h. B. K re m e r and A. B e n d ic h , J. Amer. chem. Soc. 62,
1279 [1940],
5 J. F. C o r b et t and P. F. H o l t , J. diem. Soc. [L ondon]
1963, 2385.
Unauthenticated
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CHLOROAZOBENZENES: STUDIES ON SYNTHESES
995
The azobenzenes synthesized are listed in Table I.
Compounds 2, 3, 4, 6, 7, 9, 10, and 12 were pro-
duced from their corresponding chloronitrobenzenes
dichloronitrobenzene with zinc powder in alkaline
methanol 6. Compounds 11 and 13 were produced
from their corresponding chloroanilines by oxida-
tion with M n02, using the method of W h e e le r and
G o n z a le s 2. The oxidation of 2,6-dichloroaniline
with lead tetraacetate yielded compound 8. The
chloroazobenzene yields obtained, and those re-
ported by other authors, are summarized in Table II.
mp [°C]
Refer.
1. 2, 9
5
Azobenzene
2,2'-Dichloro-
S.S'-Dichloro-
68
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
133-134
101-102
184-185
204-205
167-168
190-191
128
Experimental Section
5
1, 2, 3, 5
4,4'-Diehloro-
Gas chromatography. A F & M Model 700 gas chro-
matograph with flame ionization detection aided the
separation and purification of chloroazobenzenes from
the crude synthetic products. Columns: 1.8 m long,
3 mm o.d. stainless steel, packed with 5% UC-W98
on Chromosorb W. Carrier: helium, 30 cc/min., oven
temperature 250 °C. Retention times of azobenzenes:
azobenzene 20 sec., DCAB 55 —65 sec., TCAB 120 to
155 sec., and HCAB 270 —310 sec.
to, e,
2,2',3,3'-Tetrachloro-
2,2/,4,4/-Tetrachloro-
2,2',5,5'-Tetrachloro-
2,2',6,6'-Tetrachloro-
3,3',4,4'-Tetrachloro-
3,3',5,5'-Tetrachloro-
S^'-D ichloro^^'-dim ethyl-
2,2',4,4',5,5/-Hexachloro-
2,2/,4,4',6,6'-Hexachloro-
1
5
b
4, 5, 8
158
(9)
4, 5
b
b
(10)
193-194
132-133
216-217
193
(11)
(12)
b
(13)
Thin-layer chromatography for the separation of
DCAB, TCAB and HCAB was performed on MN-Poly-
gram® Cel 300 AC-10 sheets (Brinkman Instruments,
Inc., Westbury, N. Y.) developed with glacial acetic
acid:acetone (or pyridine) :methanol= 10:20:70(v/v).
Table I. Some azobenzenesa and their melting pointsd.
a The UV- and IR-spectra indicated that all compounds were
?rarcs-isomers.
New compounds. c See reference 6. d For
UV and IR characterization see reference 11.
by reduction with excess LiAlH4 in ether. Since it
was not possible the obtain compound 5 by this pro-
cedure, it was synthesized by mild reduction of 2,3-
Movement was in the order: Rf DCAB
R f HCAB. For the separation of DCAB from TCAB
and HCAB, the preferred solvent system was H20 :
>
R f TCAB
>
acetone:methanol
=
20:20:60 (v/v) and movement
was in the order: R f DCAB
R f TCAB
R f HCAB.
Yields obtained
Synthesis
Yields obtained by
other authorsb
Compounds. Activated MnOa: 59.4 g MnCl2’4 H 20
in 1000 ml H20 were mixed with 31.6 g KM n04 in
2000 ml H20 and the pH of the mixture was made
alkaline with KOH. The precipitated M n02 was col-
lected by filtration, washed with H20, activated at
120 °C and stored in a desiccator until used.
Azo- Reduct, of
ben-
zene
Synthesis
Reduct, of
chloronitro-
benzenes
by other
methods
by other
methods
chloronitro-
benzenes
with LiAlH4
with LiAlH4
_
_
_
40
7 5 -9 5
7 5 -9 5
—
1
20
—
2,2
,3,3-Tetrachloroazobenzene (5) : 15 g NaOH
2
—
—
—
40
80
were dissolved in 30 ml H20. To this solution 10 g
2,3-dichloronitrobenzene, 100 ml methanol and 10 g
zinc powder were added and the reaction mixture was
refluxed for 16 hours. After cooling, it was acidified
with HC1, diluted with H20, and partitioned with ether.
The ether extract was evaporated to dryness and the
residue dissolved in ligroin (60 —70 °C) and trans-
—
3
4
80
15
—
dehalog.
20
5
—
5
—
5
6
—
—
20
40
—
7
—
< 0.1
80
80
—
5
95
—
8
4 0 -7 0
4 0 -6 0
—
9
7 5 -9 5
10
11
12
13
7 5 -9 5
ferred to
a 30 cm chromatographic column packed
—
75
—
with neutral aluminium oxide (Woelm, Act. I) . The
column was eluted with ligroin and fractions of the
effluent were examined by gas chromatography for the
presence of 5. Fractions containing 5 were combined,
dried by evaporation and the compound was recrystal-
—
—
5
20
< 1
—
—
Table II. Yields of azobenzenes a. a In per cent of the theo-
retical yields. b For reference see Table I.
6 H. A. B . l iN k e , R. B a r t h a , and d . P r a m e r , Naturwissen-
schaften 55, 444 [1968].
7 F. B . d a iN s and W. 0. k e N y O N , J. Amer. them. Soc. 53,
2357 [1931],
8 R. G a u d r y and K . F. k e ir s t e a d , Can. J. Research 27 B,
897 [1949],
** Unsubstituted azobenzene (1) was pm-chased from K & K
Laboratories, Inc., Plainview, N. J.
9 L. G a t t e r m a N N and H. w ie l a N d , „Die Praxis des organi-
schen Chemikers“, p. 163, Walther de Gruyter & Co., Ber-
lin 1956.
10 A. I. V O G e l , Textbook of Practical Organic Chemistry,
p. 199. Longmans Green, London 1948.
11 H. A. B. l iN k e and D. P r a m e r , Submitted for publication
in Z. Naturforschg., Teil b.
Unauthenticated
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CHLOROAZOBENZENES: STUDIES ON SYNTHESES
9 9 6
Anal. Calcd for C14H12C12N2: C 60.23; H 4.33; N
10.03; Cl 25.41. Found: C 60.53; H 4.54; N 9.27;
Cl 25.38.
lized from acetone. Red needles, mp: 204 —205 °C.
Yield: 5 per cent.
Anal. Calcd for C12H6N9C14: C 45.02; H 1.89; N
8,76; Cl 44.33. Found: C 45.09; H 1.85; N 8.91;
Cl 44.23.
2.2',4,4',5,5-Hexachloroazobenzene (12) : To an ice-
cooled solution of 12 g 2,4,5-trichloronitrobenzene in
150 ml anhydrous ether, 6 g LiAlH4 suspended in
150 ml of cold anhydrous ether were added dropwise
with constant stirring. After 2 hours at room tempera-
ture, the reaction mixture was refluxed for 15 minutes
and cooled. Excess LiAlH4 was destroyed by the drop-
wise addition of H20, and the ether phase was se-
parated and evaporated to dryness. The residue was
dissolved in ligroin (60 —70 °C) and transferred to a
30 cm neutral aluminium oxide column (Woelm, Act.
I ) . The column was developed first with ligroin and
then with benzene. Gas chromatographic analysis de-
monstrated that the benzene eluate contained the bulk
of 12. This material was evaporated to dryness and the
product was recrystallized from acetone. Orange need-
les. mp: 216 —217 cC. Yield: 5 per cent.
2,2',6,6'-Tetrachloroazobenzene (8) : 5 g 2,6-dichloro-
aniline and 15 g freshly prepared lead tetraacetate 10
were reacted in the cold (ice bath) with stirring for
1 hour. The precipitate was collected by filtration and
washed with benzene. The benzene washes were com-
bined with the filtrate, the solution was treated with
Na2C 03 and anhydrous Na2S 04 and refiltered. The
benzene filtrate was evaporated to dryness, and the
residue was dissolved in ligroin (60 —70 °C) and puri-
fied by column chromatography as described for com-
pound 5, with the addition of elution with benzene;
the bulk of 8 was eluted by the benzene. The eluate
was dried by evaporation, the residue was dissolved in
toluol and rechromatographed on neutral aluminium
oxide (Woelm, Act. I) column that was developed with
toluol. Fractions of the eluate containing 8 were com-
bined, evaporated to dryness and the compound was
recrystallized from methanol and acetone. Green plates,
mp: 128 °C. Yield: 5 per cent.
Anal. Calcd for C12H4N2C16: C 37.04; H 1.04; N
7.21; Cl 54.71. Found: C 37.13;
Cl 53.41. ^
H
1.27; N 7.61;
2.2 ,4,4,,6,6'-Hexachloroazobenzene
(13) : 4.9
g
2,4,6-trichloroaniline in 150 ml benzene were refluxed
for 10 hours with 10.8 g activated M n02. The reaction
product was purified by the procedures described for
compound 11 and recrystallized from an ether-acetone
mixture. Red-brown needles, mp: 193 °C. Yield: 20
per cent.
Anal. Calcd for C12H6N2C14: C 45.02; H 1.89; N
8.76; Cl 44.33. Found: C 45.09;
Cl 44.48.
H 1.84; N 8.84;
3,3-Dichloro-4,4-dimethylazobenzene (11) : To 3.5 g
3-chloro-4-methylaniline dissolved in 150 ml benzene,
10.8 g activated M n02 were added. The reaction mix-
ture was refluxed for 10 hours. The insoluble residue
was collected by filtration, extracted with hot benzene,
and the filtrate and washes were combined, concen-
trated by evaporation and compound 11 crystallized
when cold acetone was added. Recrystallized from ace-
tone. Orange needles, mp: 132 —133 °C. Yield: 75 per
cent.
Anal. Calcd for C12H4N,C16: C 37.04; H 1.04; N
7.20; Cl 54.72. Found: C 37.04;
Cl 54.38.
H 1.04; N 7.20;
We thank the Upjohn Company for the gift of
2,4,5-trichloroaniline. The work described in this paper
of the Journal Series, New Jersey Agricultural Experi-
ment Station, was supported in part by USPHS Re-
search Grant ES-16.
Unauthenticated
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