Hydrogenation of Substituted Benzenes Containing Nitro and Azo Groups over Skeletal Nickel in Aqueous Solutions of 2-Propanol

Article abstract of DOI:10.1134/S0036024420030164

Abstract: An analysis is made of the kinetic characteristics of the hydrogenation of 4-nitro- and 2-nitro-2'-hydroxy-5'-methylazobenzenes, 4-nitroaniline and 4-aminoazobenzene over skeletal nickel in neutral azeotropic 2-propanol–water mixture and in the same solvent in the presence of acetic acid or sodium hydroxide. It is found that the selectivity of the hydrogenation of these isomers to the intermediate products depends on the composition and nature of the solvent, and is determined by the rate of reactive group conversion. Compared to the process in the presence of sodium hydroxide, which suppresses the route leading to the predominant hydrogenation of the nitro group, the contribution from the transformation of azo group is considerably greater in the hydrogenation of 4-nitro-2'-hydroxy-5'-methylazobenzene in the presence of acetic acid. Adding a base to the solvent during the hydrogenation of 2-nitro-2'-hydroxy-5'-methylazobenzene accelerates the rate of nitro group conversion and the intramolecular cyclization of the intermediate compound, increasing the selectivity towards the products containing the benzotriazole cycle (particularly 2-(2-hydroxy-5-methylphenyl)benzotriazole-N-oxide). The almost linear correlation between the selectivity of the catalytic hydrogenation of isomers of nitro-2'-hydroxy-5'-methylazobenzene and the kinetic characteristics of the hydrogenation of nitro and azo groups in compounds containing a single reactive substituent at different values of medium’s pH is estimated.

Full text of DOI:10.1134/S0036024420030164

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 4, pp. 720–724. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Fizicheskoi Khimii, 2020, Vol. 94, No. 4, pp. 545–549.
CHEMICAL KINETICS
AND CATALYSIS
Hydrogenation of Substituted Benzenes Containing Nitro and Azo
Groups over Skeletal Nickel in Aqueous Solutions of 2-Propanol
a,
a
a
a
O. V. Lefedova *, Khoang An’ , A. V. Belova , and M. P. Nemtseva
a
Ivanovo State University of Chemistry and Technology, Ivanovo, 153000 Russia
*
е-mail: physchem.606@yandex.ru
Received April 19, 2019; revised April 19, 2019; accepted May 14, 2019
Abstract—An analysis is made of the kinetic characteristics of the hydrogenation of 4-nitro- and 2-nitro-2'-
hydroxy-5'-methylazobenzenes, 4-nitroaniline and 4-aminoazobenzene over skeletal nickel in neutral azeo-
tropic 2-propanol–water mixture and in the same solvent in the presence of acetic acid or sodium hydroxide.
It is found that the selectivity of the hydrogenation of these isomers to the intermediate products depends on
the composition and nature of the solvent, and is determined by the rate of reactive group conversion. Com-
pared to the process in the presence of sodium hydroxide, which suppresses the route leading to the predom-
inant hydrogenation of the nitro group, the contribution from the transformation of azo group is considerably
greater in the hydrogenation of 4-nitro-2'-hydroxy-5'-methylazobenzene in the presence of acetic acid. Add-
ing a base to the solvent during the hydrogenation of 2-nitro-2'-hydroxy-5'-methylazobenzene accelerates the
rate of nitro group conversion and the intramolecular cyclization of the intermediate compound, increasing
the selectivity towards the products containing the benzotriazole cycle (particularly 2-(2-hydroxy-5-methyl-
phenyl)benzotriazole-N-oxide). The almost linear correlation between the selectivity of the catalytic hydro-
genation of isomers of nitro-2'-hydroxy-5'-methylazobenzene and the kinetic characteristics of the hydroge-
nation of nitro and azo groups in compounds containing a single reactive substituent at different values of
medium’s pH is estimated.
Keywords: hydrogenation, skeletal nickel, 4-nitro-2'-hydroxy-5'-methylazobenzene, 2-nitro-2'-hydroxy-5'-
methylazobenzene, 4-aminoazobenzene, 4-nitroaniline
DOI: 10.1134/S0036024420030164
INTRODUCTION
the selectivity of 2-nitro-2'-hydroxy-5'-methylazo-
benzene hydrogenation towards products containing
Products of the hydrogenation of substituted nitro- benzotriazole cycle.
benzenes can be used as both end products and pre-
cursors for the synthesis of a wide range of intermedi-
EXPERIMENTAL
ate products and dyes [1]. The stepwise nature of the
transformations of substituted nitro- and azobenzenes
containing single groups reactive with the adsorbed
hydrogen has been studied quite well [2–7]. However,
In this work, we studied the hydrogenation of
-nitroaniline (4NA), 4-aminoazobenzene (4AAB),
-nitro-2'-hydroxy-5'-methylazobenzene (4NAB),
4
4
and
2-nitro-2'-hydroxy-5'-methylazobenzene
insufficient attention has been given to investigating (2NAB). The hydrogenation was conducted at atmo-
spheric pressure at 288 K in a reactor under intense
the kinetics of the hydrogenation of compounds con-
taining two groups that can actively interact with
hydrogen under the corresponding conditions.
3
stirring (3200 rpm) of the liquid phase (100 cm ). A
neutral aqueous solution of 2-propanol of azeotropic
composition (х = 0.68) and a solution containing
2
The aim of this work was to compare the rate con- sodium hydroxide (0.01 M) or acetic acid (0.01 M)
were used as solvents. The catalyst (m ) load in each
stants of nitro and azo group conversion in com-
pounds containing a single reactive substituent and the
selectivity of the hydrogenation of isomers of substi-
tuted nitroazobenzenes in aqueous solutions of 2-pro-
panol containing different additives, and to determine
the reasons for the effect the composition of the sol-
cat
experiment was 0.625 ± 0.01 g; the amount of the ini-
tial compound was varied from 0.79 ± 0.05 to 1.950 ±
0
.05 mmol. A direct proportional dependence of the
observed rate of the process on the amount of catalyst
was observed at this ratio between the catalyst and
hydrogenated compound [3, 4]. Hydrogen was con-
vent (particularly the pH value) has on the change in tinuosly supplied to the reactor and its consumption
720

HYDROGENATION OF SUBSTITUTED BENZENES
721
during the reaction was monitored using a system of The total mass balance according to the analytical
gas burettes connected to the hydrogen source. The data was at least 98% in each experiment; this testi-
selected conditions of hydrogenation allowed us to
exclude the influence of external mass transfer on
the kinetic characteristics and precisely determine
the observed rate of the reaction (the error did not
exceed 5%).
fies to the high reliability of our results and allows
objective conclusions on the stepwise nature of the
transformation of hydrogenated compounds during
the process.
The skeletal nickel catalyst (Raney nickel) was pre-
pared according to the standard technique in [8] from
nickel–aluminum alloy containing 50% Ni, 48.8% Al,
and 0.2% Fe.
RESULTS AND DISCUSSION
In a a number of works [1, 2, 4, 6, 7, 9], it was
shown that adding an acid or a base to a solvent
strongly influences the rate of hydrogenation of differ-
ent unsaturated groups (particularly nitro and azo
groups) in substituted benzenes over skeletal nickel.
The content of 4NA and 4-amino-2'-hydroxy-5'-
methylazobenzene (4AAB), which are intermediate
products of 4NAB hydrogenation, also changes appre-
ciably with these substances present in aqueous solu-
tions of aliphatic alcohols [10, 11].
The reaction mixture was sampled during hydroge-
nation to determine the content of the initial com-
pounds and products of 2NAB and 4NAB hydrogena-
tion via spectrophotometry in combination with thin-
layer and liquid chromatography.
Ascending thin-layer chromatography was per-
formed using Sorbfil plates. A binary toluene : ethyl
acetate mixture (7 : 3 volume ratio) was used as the
eluent, and nitrogen oxides were for developing the
plates. The resulting chromatograms were compared
to those of the individual pure substances to identify
the products of hydrogenation.
The possibility of the intramolecular rearrange-
ment of the reaction’s intermediate products with
the subsequent formation of the benzotriazole cycle
A
Shimadzu LC-6A liquid chromatograph is a specific feature of 2NAB hydrogenation, com-
equipped with a spectrophotometric detector contain-
ing deuterium and tungsten lamps for the UV and vis-
ible spectral regions and a stainless steel packed col-
umn containing Lichrosorb RP-18 stationary phase
was used for quantitative analysis of the samples.
Aqueous solutions of acetonitrile were used as the elu-
ents. The concentration of acetonitrile was 70 vol %
pared to that of 4NAB. For example, 2-nitro-
2
'-hydroxy-5'-methylhydrazobenzene (2NHB), a
product of azo group hydrogenation, can rearrange
into 2-(2-hydroxy-5-methylphenyl)benzotriazole-
N-oxide (NO) in a homogeneous reaction in solu-
tion. It is known that the rate of intramolecular
upon analyzing the nitro and aminoazocompounds, cyclization increases in the presence of bases [12].
and a 30 vol % acetonitrile aqueous solution was used The catalytic hydrogenation of 2NAB can thus be
in our analysis of nitroanilines and 2-amino-4-meth-
ylphenol (2AMP). The error in determining the con-
centration of all the analyzed substances did not
exceed 4%, and the reproducibility of results was 96–
followed by the formation of the compounds that
contain no benzotriazole cycles (2NHB) or ones
that do contain it (NO and the product of its subse-
quent hydrogenation, 2-(2-hydroxy-5-methylphen-
yl)benzotriazole (BT)). In this work, we considered
the total selectivity relative to that of NO and BT
towards the products of 2NAB hydrogenation con-
taining the benzotriazole cycle (PBTC).
99%.
A combination of thin-layer chromatography and
spectrophotometry was used to determine the content
of 2- and 4-phenylenediamines (2PDA and 4PDA)
formed during the hydrogenation of 2NAB and 4NAB,
respectively. After chromatography, the spot corre-
sponding to phenylenediamine was washed with 2-pro-
panol from the Sorbfil plate in a vial. The concentration
of substances in the resulting solution was measured
using a CARY 50 Scan UV–Vis spectrophotometer
Hydrogenation of the azo group of 2NAB and
4
2
2
NAB results in the formation of 2AMP and thus
-nitroaniline (2NA) and 4-nitroaniline (4NA). The
NA and 4NA are then reduced to corresponding
diamines 1,2-phenylenediamine (2PDA) and
,4-phenylenediamine (4PDA) under the conditions
−
5
(
sensitivity, (1–3) × 10 M or higher). The error in
1
determining concentration did not exceed 3–5%.
of hydrogenation.
Concentrations of 2PDA and 4PDA were addition-
ally calculated as the difference between the content of
The schemes given below show the main stages and
2
AMP and the corresponding nitroaniline isomer in sequence of the transformations of the products of
an analyzed sample.
2NAB (I) and 4NAB (II) hydrogenation [10]:
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 94 No. 4 2020

7
22
LEFEDOVA et al.
I)
(
HO
N N
NO2
HO
Heterogeneous catalytic stage
CH3
N N
H H
CH3
NO2
Heterogeneous
catalytic
Heterogeneous
Homogeneous
stage
catalytic
stage
stage
HO
HO
HO
N
N
O
N
N
N
NH + NH
2
2
N
CH3
NO2
CH3
CH3
(
II)
HO
N N
HO
Heterogeneous
catalytic stage
CH3
N N
CH3
O N
2
H N
2
Heterogeneous catalytic
stage
Heterogeneous catalytic
stage
HO
NH2
H N
2
NO2
H2N
NH2
CH3
Heterogeneous catalytic stage
Data illustrating the dependence of rate constants group reduction and slows azo group hydrogenation.
of hydrogenation of 4NA and 4AAB in aqueous solu- The selectivity of 4NAB therefore changes: the con-
tion of 2-propanol on medium’s pH are given in tent of 4NA rises while that of 4AAB falls when there
Table 1. The observed rate of 4NA and 4AAB hydro- is acid present, and adding a base has the opposite
genation was determined using the slope of the initial effect. The effects liquid phase’s pH has on the rate
portion of the kinetic curve plotted as the amount of constants of 4NA and 4AAB hydrogenation, and on
consumed hydrogen versus time. The rate constant selectivity towards the products formed during 4NAB
was calculated as the ratio between the observed rate of hydrogenation, are therefore similar. With 2NAB
the reaction and the solubility of hydrogen in our sol- hydrogenation, the increase of medium’s pH increases
vent, adjusted for the density of the catalyst. The ratio the selectivity of hydrogenation towards PBTS, while
between the maximum amount of reaction products its selectivity towards 2NHB is reduced. It is worth
and that of the initial compound consumed at a given noting the similarity between the dependences of
moment in time was considered the reaction’s selec- selectivity to PBTC and 4AAB and the selectivity
tivity (S).
towards 2NHB an 4NA on the solvent’s pH. We may
The presented data show that adding sodium therefore conclude that using solvents which ensure
hydroxide or acetic acid to an aqueous solution of acceleration of nitro group reduction or the slowing of
alcohol greatly altered the rate of hydrogenation of the azo group hydrogenation enhances the contribution
compound with a single reactive group. The reduction from the stage resulting in the formation of NO upon
in the medium’s acidity ensures acceleration of nitro 2NAB hydrogenation.
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HYDROGENATION OF SUBSTITUTED BENZENES
723
Table 1. Rate constants of hydrogenation (k ) of 4NA and 4AAB and selectivity (S) of hydrogenation of 2NAB and 4NAB
H2
over skeletal nickel in aqueous solutions of 2-propanol (х = 0.68) containing various additives
2
−1
k , s
S, %
H2
Additives
рН
4NAB
2NAB
k4NA
k4AAB
4NA
4AAB
2NHB
PBTC
СН СООН, 0.01 М
Without additives
NaOH, 0.01 М
4.6
8.0
13.4
39 ± 3
71 ± 2
91 ± 5
115 ± 10
46 ± 2
23 ± 3
30 ± 1
26 ± 1
4 ± 1
2.5 ± 0.2
6 ± 1
15 ± 1
76 ± 4
48 ± 4
32 ± 3
20 ± 2
35 ± 2
70 ± 2
3
Analysis of the experimental results allowed us to genation towards PBTC (S
) and the ratio of the
PBTC
estimate the correlation between the kinetic parame- rate constants of 4NA and 4AAB hydrogenation
ters of the hydrogenation of compounds containing (k /k
). These data show that the selectivity
4NA 4AAB
only nitro or azo groups, and the selectivity of the towards the benzotriazole derivative increases in the
hydrogenation of nitroazobenzenes isomers. Accord- solvents that ensure higher rates of nitro group hydro-
ing to our data, the resulting dependences are almost genation than that of the azo group.
linear. Figure 1a illustrates the correlation between the
The increase in selectivity towards PBTC in the
ratios of selectivities towards 4NA and 4AAB
presence of a base can be explained by the increased
(
S
/S
) that form during 4NAB hydrogenation
4
NA 4AAB
contribution from the heterogeneous catalytic stage
and the rate constants of hydrogenation of the com-
(
i.e., rearrangement of the intermediate products of
pounds that contain either a nitro or an azo group: the
2
NAB hydrogenation during the initial stage of the
4
NA and 4AAB (k4NA/k4AAB). S /S
ratio
4NA 4AAB
reaction directly in the surface layer, in addition to the
cyclization of 2NHB in the volume of the solution).
The considerations below do not contradict this
changes from 0.3 to 12 upon moving from the solvent
containing base to the one containing acid. The higher
the rate constant of nitro group transformation during hypothesis. The selectivity towards PBTC is 70% upon
4
NAB hydrogenation in the corresponding solvent, 2NAB hydrogenation with sodium hydroxide; this
the lower the selectivity towards NA. Figure 1b shows value exceeds by 35% the one obtained in a neutral
the correlation between the selectivity of 2NAB hydro- aqueous solution of 2-propanol. If we believe the for-
S
PBTC, %
S
4NA/S4AAB
80
1
4
2
0
8
6
4
2
(a)
(b)
NaOH
70
60
50
40
30
1
1
CH COOH
3
Without additives
CH COOH
Without additives
20
3
10
NaOH
0
1
2
3
4
0
1
2
3
4
k
4NA/k4AAB
k
4NA/k4AAB
Fig. 1. Correlation of the ratio between (a) the selectivity of 4NAB hydrogenation towards 4NA and 4AAB (S4NA/S4AAB) and
b) the selectivity of 2NAB hydrogenation towards PBTC (SPBTC), along with the ratio between the rate constants of the hydro-
(
genation of the compounds containing nitro or azo group (k4NA/k4AAB).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 94 No. 4 2020

7
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LEFEDOVA et al.
mation of NO is determined only by the homogeneous
transformation of 2NHB, we would expect that selec-
tivity towards PHBC grows by a higher value (48%)
and there is no 2NHB in the alkali medium. It should
be noted that the rate constant of 2NHB cyclization in
the presence of sodium hydroxide in the solution
2. Theory and Practice of Liquid-Phase Hydrogenation of
Substituted Nitrobenzenes, Ed. by O. I. Koifman
(
KRASAND, Moscow, 2016) [in Russian].
3. I. V. Smirnova, M. P. Nemtseva, and O. V. Lefedova,
Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 53
(11), 4 (2010).
−3
−1
(
pH 13.4) is 3.8 × 10 s . This value is almost two
orders of magnitude lower than the rate constant of
NAB transformation upon hydrogenation in the
4. A. G. Krotov and O. V. Lefedova, Russ. J. Phys. Chem.
A 76, 935 (2002).
2
5. O. V. Lefedova, Nguen Tkhi Tkhu Kha, A. A. Koma-
−
2 −1
same solvent: 55 × 10 s [1, 2].
rov, and M. A. Budanov, Russ. J. Phys. Chem. A 86, 32
(2012).
6
. Nguen TkhiTkhu Kha, O. V. Lefedova, and A. A. Mer-
kin, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekh-
nol. 55 (10), 79 (2012).
CONCLUSIONS
Our experimental data show that high selectivity
towards PTBC can be achieved during 2NAB hydro-
genation at a certain optimum ratio between the
hydrogenation rates of nitro and azo groups over skel-
etal nickel in aqueous solutions of aliphatic alcohols. A
simultaneous hydrogen attack to nitro and azo groups
without the complete opening of bonds can result in
the formation of a mixed type of associated complex,
and the intramolecular rearrangement of this complex
can proceed. The possible cyclization of such an asso-
ciated complex is confirmed by our quantum chemical
calculations [13] and does not contradict the concepts
of the mechanism of intramolecular cyclizations [14,
7. I. V. Smirnova, M. P. Nemtseva, and O. V. Lefedova,
Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 52
(5), 25 (2009).
8. M. V. Ulitin, A. V. Barbov, V. P. Gostikin, and
V. A. Shalyukhin, Zh. Prikl. Khim. 66, 497 (1993).
9
. M. A. Budanov, O. V. Lefedova, M. V. Ulitin, and
Nguen Tkhi Tkhu Kha, Russ. J. Phys. Chem. A 84,
1901 (2010).
1
0. Anh Hoang, M. P. Nemtzeva, and O. V. Lefedova,
Russ. J. Phys. Chem 92, 669 (2018).
1
1. Anh Hoang, V. A. Kalashnikova, O. V. Lefedova, and
D. V. Filippov, Izv. Vyssh. Uchebn. Zaved., Khim.
Khim. Tekhnol. 61 (8), 66 (2018).
15].
Adding three moles of hydrogen only to the nitro
1
2. V. A. Pal’m, Principles of the Quantitative Theory of Or-
ganic Reactions (Khimiya, Leningrad, 1967) [in Rus-
sian].
groups results in the formation of 2-amino-2'-
hydroxy-5'-methylazobenzene incapable of intramo-
lecular rearrangement, while the consumption of
1
3. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scu-
seria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
ery, Jr., T. Vreven, K. N. Kudin, J. C. Burant,
J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, et al., Gaussian 03, Rev. B.04 (Gaussian
Inc., Pittsburgh, PA, 2003).
2
moles of hydrogen via azo groups produces 4NA and
AMP.
2
CONFLICTS OF INTEREST
The authors have no conflicts of interest to declare.
14. L. M. Gornostaev, Soros. Obraz. Zh., No. 4, 33 (1996).
1
5. G. Klopman, Chemical Activity and Reaction Paths (Wi-
ley, New York, 1974).
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Translated by P. Vlasov
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2020