Kinetics and mechanism of p-nitrochlorobenzene nitration with nitric acid

Article abstract of DOI:10.1023/A:1013408229563

Kinetics of homogeneous nitration of p-nitrochlorobenzene with 85-95% nitric acid was investigated. An introduction of a nitro group into a chlorobenzene molecule results in 1600 times deceleration of nitration. It was presumed from comparison of kinetic parameters and correlations of log k_eff for the mono- and dinitration with the acidity functions of nitric acid that the limiting stage in p-nitrochlorobenzene nitration was the transformation of diffusion pairs into reaction products, whereas in chlorobenzene nitration the limiting stage consisted in diffusion pairs formation.

Full text of DOI:10.1023/A:1013408229563

Russian Journal of Organic Chemistry, Vol. 37, No. 10, 2001, pp. 1451 1454. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 10, 2001,
pp. 1520 1523.
Original Russian Text Copyright
2001 by Veretennikov, Lebedev, Tselinskii.
Kinetics and Mechanism of p-Nitrochlorobenzene Nitration
with Nitric Acid
E. A. Veretennikov, B. A. Lebedev, and I. V. Tselinskii
St. Petersburg State Technological Institute, St. Petersburg, 198013 Russia
Received June 23, 2000
Abstract Kinetics of homogeneous nitration of p-nitrochlorobenzene with 85 95% nitric acid was
investigated. An introduction of a nitro group into a chlorobenzene molecule results in 1600 times decelera-
tion of nitration. It was presumed from comparison of kinetic parameters and correlations of log keff for the
mono- and dinitration with the acidity functions of nitric acid that the limiting stage in p-nitrochlorobenzene
nitration was the transformation of diffusion pairs into reaction products, whereas in chlorobenzene nitration
the limiting stage consisted in diffusion pairs formation.
2,4-Dinitrochlorobenzene is widely used as an
initial compound in the organic synthesis. The most
common method of its production is a nitration of
nirtochlorobenzene with mixtures of nitric and
sulfuric acids [1]. The kinetics of homogeneous
nitration of nitrochlorobenzene by nitric acid in
sulfuric acid medium is sufficiently well studied. It
was demonstrated that like the behavior of nitrobenz-
ene, p-nitrotoluene, 2,4-dinitrotoluene and the other
aromatic compounds the bimolecular rate constant of
nitration of o- and p-nitrochlorobenzenes into 2,4-di-
nitrochlorobenzene went through a maximum at 25 C
in 89 90% sulfuric acid, and its decrease at further
increase in sulfuric acid concentration was apparently
due to diminishing activity coefficient of the non-
ionized aromatic substrate [2, 3].
Typical curves of 2,4-dinitrochlorobenzene ac-
cumulation and their semilog plots are given on
Fig. 1.
The dependence of nitration rate on the concentra-
tion of the arising dinitrochlorobenzene in all studied
ranges of nitric acid concentrations and of tempera-
ture (Fig. 2) is linear in bilog coordinates, and the
slope is close to unity (n 0.98 1.08), and thus the
assignment to the selected conditions of the pseudo-
first order is valid [5].
Effective rate constant of the pseudofirst order
k eff corresponding to accumulation of dinitrochloro-
benzene under experimental conditions does not
change in the course of the process.
No publications appeared on kinetics of homo-
geneous nitration of nitrochlorobenzene isomers with
nitric acid only.
We studied [4] kinetics of chlorobenzene nitration
with nitric acid under homogeneous conditions into
a mononitrocompound. In order to compare the
mechanism of the mono- and dinitration, of their
limiting stages, it was necessary to measure the rate
of dinitrochlorobenzene formation in this system.
The nitration was carried out with p-nitrochloro-
benzene that furnished a single reaction product,
2,4-dinitrochlorobenzene [3].
We established that due to the deactivating effect
of a nitro group the nitration of the p-nitrochloro-
benzene into 2,4-dinitrochlorobenzene occurred with
a measurable rate only in nitric acid of concentration
greater than 85%.
Fig. 1. Kinetic curves of 2,4-dinitrochlorobenzene accu-
mulation during nitration of 4-nitrochlorobenzene with
90.5% nitric acid, and their semilog plots. (1) 65, (2) 75,
(3) 85 C.
1070-4280/01/3710-1451$25.00 2001 MAIK Nauka/Interperiodica

1452
VERETENNIKOV et al.
Table 1. Dependence of pseudofirst order rate constant
of 4-nitrochlorobenzene nitrationa on nitric acid
concentration and reaction temperature
k
104, 1/s
75 C
E,
kJ
mol
eff
[HNO3],
%
55 C
65 C
85 C
85.0
90.5
95.1
0.31
0.43
0.74
0.90
1.20
1.95
2.6
3.4
5.2
5.5
7.1
11.7
93 2
90 2
88 2
a
In all experiments the concentration of 4-nitrochlorobenzene
was 1.06 mol l .
1
Fig. 2. Dependence of nitration velocity of 4-nitrochloro-
benzene with nitric acid (V = C / ) on concentration
(c ) of arising 2,4-dinitrochlorobenzene at 85 C. [HNO3],
%: (1) 85.0 (n 1.01), (2) 90.50 (n 0.98), (3) 95.10 (n 1.08).
NO2
+
+
Ar
ArH NO2
ArNO2 + H+ ,
(3)
H
where ArH NO+2 is a diffusion pair [8].
Regarding the nitronium concentration as sta-
tionary we obtain the following expression fr the
reaction rate:
k1k2k3[ArH] aHNO aH
+
aHn
O
2
3
d[ArH]
=
. (4)
d
k
a2H anH O(k2 + k3)+ k2k3[ArH]
1 O
2 2
At low concentrations of the aromatic compound
in excess nitric acid
k
a2H anH O(1+ k / k3)>> k2[ArH]
(5)
1
O
2
2
2
Fig. 3. Arrhenius plot for chlorobenzene nitration at various
concentrations of nitric acid [HNO3], %: (1) 85.0, (2) 90.5,
(3) 95.1.
and expression (4) can be written as follows:
k1 k2 aHNO aH+ [ArH]
The dependence of nitration velocity on the
temperature fits to Arrhenius equation (Fig. 3). The
obtained values of pseudofirst order rate constant k eff
are presented in Table 1.
3
d[ArH]
=
.
(6)
d
k
a2H O (1+ k / k3)
1 2
2
At nitration of aromatic hydrocarbons of high reactivity
Taking into account the significant difference in
nitration rates of chlorobenzene and its nitro deriv-
ative that will be demonstrated further, and also the
participation of two water molecules in the reversible
reaction of the nitronium cation formation in the
aqueous nitric acid [6, 7], we suggest the following
scheme for the nitration of nitrochlorobenzene:
k3>> k , and the pseudofirst order constant is
2
Kp1 k2 aHNO aH
+
3
keff
=
(7)
aH2
O
2
where Keq1 is the equilibrium constant of reaction (1).
k1
HNO3 H2O+ H+ (H2O)n
(n + 2)H2O+ NO+2 (1)
At nitration of aromatic hydrocarbons of low
k
1
reactivity k3 << k , and the stage (3) of transforma-
2
k1
k
tion of diffusion pair into reaction products becomes
the rate limiting one. In this case the pseudofirst
order constant is
HO2 + ArH
ArH NO+2
(2)
1
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

KINETICS AND MECHANISM OF p-NITROCHLOROBENZENE NITRATION
1453
Kp1 Kp2 k3 aHNO aH
+
These results are unlike those obtained in the study
on kinetic and mechanism of the chlorobenzene
mononitration. We showed in [4] that in the chloro-
benzene nitration with aqueous nitric acid the logkeff
depended linearly on the acidity functions H0, HR
3
keff
=
(8)
aH2
O
2
where Keq2 is the equilibrium constant of reaction (2).
and (HR + logaH O), and the slope of the plot was
To elucidate the nitration mechanism of nitro-
chlorobenzene we studied the dependence of the reac-
tion rate on the acidity of the medium. Since for
temperature higher than 25 C are no data on activity
of the aqueous nitric acid in the calculation of the
2
close to unity. The comparison of kinetic parameters
and dependencies thereof on the acidity of medium in
nitration of chlorobenzene and nitrochlorobenzene
suggests that in the chlorobenzene nitration with the
aqueous nitric acid the process rate is determined by
the stage of diffusion pairs formation (2). Apparently
under conditions where the rate of diffusion pairs
transformation into reaction products is considerably
higher than the rate of their dispersion the nitration
kinetics is described by equation (7), and the bi-
molecular rate constant keff characterizes the equilibr-
ium process of nitronium cation formation along
reaction (1) that is affected by the medium acidity.
This reasoning is supported by the fact that the for
bimolecular rate constant along the equation keff
=
k eff/aHNO were used k eff values extrapolated to
3
25 C. The values of keff obtained are given in
Table 2.
The transformation of equation (8) gives the fol-
lowing expression for the bimolecular rate constant:
Kp1 Kp2 k3 aH
+
keff
=
(9)
aH2
O
2
a linear correlation logkeff = f(HR + logaH O) which
2
we have calculated from the data of [8] on nitration of
mesitylene and p-xylene with aqueous nitric acid the
slope was equal to 1.1. The rate of nitration for these
compounds was limited by the stage of diffusion pairs
formation [8].
and consequently
logkeff
=
logKp1 Kp2 k3 (HR + logaH O).
(10)
2
The dependence of logkeff on acidity functions H0,
HR and (HR + logaH O) is described by equations
(11 13) respectively. 2
The weak dependence of the nitration rate of the
nitrochlorobenzene on the medium acidity may show
that due to the low reactivity of the substrate the
dispersion rate of diffusion pairs is higher than the
rate of their transformation into the reaction products.
Therefore the stage (3) becomes limiting, and its rate
constant is not directly dependent on the medium
acidity and is determined solely by reactivity of the
substrate.
2
logkeff
=
(9.77 0.12) 10 H0 (7.90 0.02)
r 0.99, s 0.014.
(11)
2
logkeff
=
(4.74 0.19) 10 HR (7.74 0.01)
(12)
r 0.99, s 0.075.
2
logkeff = (3.06 0.16) 10 (HR + logaH2
(7.68 0.02)
)
O
(13)
r 0.98, s 0.098.
It is seen from the data of Table 2, that keff grows
only insignificantly with increasing concentration of
the nitric acid. It is presumable that as an additional
factor reducing the influence of the medium acidity
on the reaction rate operates the decrease in activity
coefficient of the substrate in the more concentrated
It is seen from these equations that the slopes of
all these linear dependencies are very small, i.e., the
nitration rate is weakly affected by the change in
acidity of the aqueous nitric acid expressed by these
functions.
Table 2. Values of nitration rate constant for 4-nitrochlorobenzene nitration with nitric acid, and the acidity functions
of the latter, 25 C
[HNO3],
%
k
106,
keff 108,
logkeff
logaHNO
H0
logaH2
HR
(HR+log aH2 )
O
eff
O
3
1
1
1 s
l mol 1 s
85.0
90.5
95.1
1.06
1.59
2.93
3.80
3.89
4.57
7.42
7.40
7.34
1.45
1.65
1.81
4.96
5.00
5.75
1.61
2.05
2.58
6.57
7.05
8.33
8.18
9.10
10.91
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

1454
VERETENNIKOV et al.
nitric acid as was observed for nitration of nitrobenz-
ene isomers in sulfuric acid medium [4].
content of mono- and dinitrochlorobenzene isomers.
The effective constants of pseudofirst order were
calculated by the formula:
The data in Table 1 indicate that in nitrochloro-
benzene nitration the experimental value of activation
energy E only slightly decreases with growing con-
centration of nitric acid; apparently the activation
energy is weakly affected by H of the process of
nitronium cation formation along reaction (1).
(c c0)
k
=
1 ln
,
(c c )
where
c0,
c ,
c
is
dichlorobenzene
1
concentration, mol l at the start of reaction, at the
moment of measurement, and at the end of the experi-
ment respectively.
The comparison of keff values for mono- and di-
nitration of chlorobenzene shows that introduction of
a nitro group in the para-position with respect to
chlorine results in 1600 times reaction rate decrease,
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The reaction was carried out in a flask kept at
constant temperature and equipped by stirrer, thermo-
meter, and sampling device. A weighed portion of
p-nitrochlorobenzene was charged to the flask at stir-
ring into the nitric acid of a given concentration. The
reaction progress was monitored by sampling.
Temperature in the course of reaction was controlled
within 0.1 C. The samples of the reaction mixture
were diluted with excess 2-propanol and analyzed for
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001