Quantum-chemical substantiation of the reactivity and regioselectivity of nitration of biphenyl derivatives

Article abstract of DOI:10.1023/A:1013285331225

Quantum-chemical calculations were used to substantiate the reactivity and regioselectivity of nitration of benzene, biphenyl, and carboxy-substituted biphenyls.

Full text of DOI:10.1023/A:1013285331225

Russian Journal of General Chemistry, Vol. 71, No. 8, 2001, pp. 1269 1271. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 8, 2001,
pp. 1342 1344.
Original Russian Text Copyright
2001 by Kofanov, Sokolov, Kolobov, Ovchinnikov.
Quantum-Chemical Substantiation of the Reactivity
and Regioselectivity of Nitration of Biphenyl Derivatives
E. R. Kofanov, A. V. Sokolov, A. V. Kolobov, and K. L. Ovchinnikov
Yaroslavl State Technical University, Yaroslavl, Russia
Received January 17, 2000
Abstract Quantum-chemical calculations were used to substantiate the reactivity and regioselectivity of
nitration of benzene, biphenyl, and carboxy-substituted biphenyls.
We earlier obtained experimental evidence showing
that 2-biphenylcarboxylic acid (I) in CH3COOH is
nitrated faster than benzene (II) and 3,4-biphenyldi-
carboxylic acid (III) but slower than biphenyl (IV)
(Table 1).
To check and substantiate the proposed model, we
performed quantum-chemical for nitration of benzene,
biphenyl, and carboxy-substituted biphenyls. The
calculations were peformed by the AM1 method in
the gas-phase approximation [2]. As the model nitrat-
ing agent we chose the nitronium cation NO+2. The
resulting potential energy surface profile is represen-
ted in Fig. 1.
COOH
COOH
It is known that the limiting stage of an SEAr reac-
tion is -complex formation [3]. Therefore, it is this
stage which we deal with in further discussion.
I
NO2
COOH
We performed direct calculations of transition
states for the chosen objects. The heats of transition-
state formation (activation barriers) fairly fit the ex-
perimental reactivities of the compounds (correlation
coefficient 0.96, Fig. 2). Consequently, the calculation
method used provides a noncontradictory explanation
of the experimental results.
+
NO2
The ortho-to-para nitration ratios for compound I
in certain solvents attained 8.
To explain these facts, we proposed a model of the
reaction, according to which the attacking species
coordinates at the carboxy group of the substrate
(structure A).
To explain the reactivity of the compounds in
question, we attempted to find parameters descriptive
of the reactivity. Relying on the linear-free energy
principle, for such a reactivity index one can take re-
action heat [heat of -complex formation ( N )]
(Fig. 1).
O
HO
.
.
+
.
NO2
C
Figure 3 represents the plot of the calculated activa-
tion energies vs. the heats of -complex formation.
A
Table 1. Apparent partial rate constants of nitration of
compounds I IV
ko 104,
kp 104,
Ea
H
E
Substrate
1
1
l mol 1 s
l mol 1 s
I
5.8
3.4
II
III
IV
2.5
Reaction coordinate
0.14
13.0
0.37
24.0
Fig. 1. Potential energy surface profile of an SEAr
reaction.
1070-3632/01/7108-1269$25.00 2001 MAIK Nauka/Interperiodica

1270
KOFANOV et al.
10
2
2
6
11
1
Ea, kcal mol
ln k
10
3
3
9
8
1
8
7
0
6
1
10
4
4
2
3
5
4
10
20
10
0
10
12
1
H , kcal mol
10
0
10
1
Ea, kcal mol
Fig. 2. Plot of the logarithm of the experimental partial
nitration rate vs. activation barrier. (Dark circles) para-
attack and (light circles) ortho attack (the activation
barriers are related to that for benzene). (1) Benzene (II),
(2) biphenyl (IV), (3) 2-biphenylcarboxylic acid (I),
and (4) 3,4-biphenyldicarboxylic acid (III).
Fig. 3. Plot of activation energy vs. heat of -complex
formation (relative to benzene). (1) Benzene, (2, 3) bi-
phenyl, (4, 5) 2-biphenylcarboxylic acid (I), (6, 7) 3-bi-
phenylcarboxylic acid (V), (8, 9) 4-biphenylcarboxylic
acid (VI), and (10, 11) 3,4-biphenyldicarboxylic acid
(III) [(circles) para attack and (triangles) ortho-attack].
There is a correlation dependence between these
values, but the points are considerably scattered,
implying that the behavior of the corresponding com-
pound fails to fit the linear free-energy principle. The
deviations may result from the influence of a certain
factor (specific interaction), which shows up either in
the saddle point or in the -complex region (i.e. affects
Ea or on N ). We suggest that this factor is
Coulomb interaction. In the transition state the charge
on the attacking species is still preserved (Table 2),
while in the
complex the positive charge is fully
delocalized on the benzene ring, and the residual
charge on the nitro group is ca. 0.01 0.02 e; therefore,
in the latter case the Coulomb interaction between
NO+2 and the carboxy group can be neglected. For the
baseline in the plot (Fig. 3) we chose the line that
joins the points for unsubstituted benzene and bi-
phenyl. The points lying above the baseline relate to
Coulomb repulsion between the substituent in the
substrate and the attacking species and those lying
below the baseline, to attraction. Table 2 lists the
calculated energies of the Coulomb repulsion between
NO+2 and the substituent (COOH) in the second
benzene ring. The calculations were performed using
a procedure for energy separation, that allows estima-
tion of the energy of interaction of chemically non-
bonded groups.
Table 2. Contribution of the energy of NO+2 COOH inter-
action into the total energy of the transition state (by the
results of energy separation)a
Parameter
I
V
VI
III
ortho Attack
Etot, eV
0.245
0.238
0.002
0.399
0.619
0.091
0.091
0.043
0.375
0.579
0.074 0.188
0.075 0.190
0.047 0.072, 0.045
0.381 0.395
E
Q
Coul, eV
COOH, e
This interaction is a charge dipole interaction and,
consequently, the strength of the interaction is much
dependent on the orientation of the carboxy group.
The carboxy group is almost neutral, the atomic
charges are +0.3 (C), 0.3, 0.3 (O), and +0.2 (H),
the positive charge of the attacking nitronium cation
is localized on the nitrogen atom, and it is still high in
the transition state (Table 2, Fig. 4).
QNO ,
e
2
QN, e
0.581 0.591
para Attack
E
Q
QNO ,
QN, e
Coul, eV
COOH, e
0.159
0.003
0.398
0.591
0.065
0.038
0.411
0.581
0.050 0.130
0.042 0.070, 0.041
0.414 0.422
e
2
0.583 0.592
a
(E
)
Total energy, (E
)
Coulomb contribution, and
Table 2 and Fig. 4 show that the reaction with
tot
Coul
(Q) charge.
2-biphenylcarboxylic acid (I) is the only that may
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 8 2001

QUANTUM-CHEMICAL SUBSTANTIATION OF THE REACTIVITY
1271
additional (compared with complex) Coulomb inter-
action in the transition state (Fig. 3).
O
O
O
EXPERIMENTAL
+
N
O
Nitration was performed at a 1:47:115 substra-
te:HNO3 :CH3COOH molar ratio at 70 C. Analysis
of methyl biphenylcarboxylates and nitration products
was performed on a Chrom-5 chromatograph with a
flame-ionization detector on a 1500 3-mm column
packed with 5% SE-30 on Chromaton N-AW. Injector
temperature 260 C. Oven temperature 200 (acid I) and
230 C (acid III). Carrier gas nitrogen, flow rate 6.3
10 m3/s. Analysis of biphenyl and its nitration pro-
7
Fig. 4. Structure of the transition state for nitration of
2-biphenylcarboxylic acid (I) (ortho attack).
ducts was performed on an LKhM-80 chromatograph
with a flame-ionization detector on a 2500 3-mm
column packed with 10% SKTF-50kh on Chromaton
N-AW-DMCS. Injector temperature 260 C. Oven
temperature 204 C. Carrier gas helium, flow rate
occur with energy reduction occur. In the case of the
ortho attack, when the carboxy group is turned by the
negatively charged oxygen atoms to the nitronium
cation and the distance between the reacting species
is smaller than in the case of the para attack, this
effect is the strongest (Etot 0.245 eV). As a result,
the relative reactivity of compound I is enhanced
(Table 1). This model also explains the anomalously
high ortho/para mononitration ratio with 2-biphenyl-
carboxylic acid.
5 10 m3/s.
7
REFERENCES
1. Kolobov, A.V., Sokolov, A.V., Krasovskaya, G.G., Ko-
fanov, E.R., Mironov, G.S., and Ovchinnikov, K.L.,
Kinet. Katal., 1997, vol. 38, no. 3, pp. 367 370.
2. Stewart, J.J.P., J. Computer-Aided Molec. Design, 1990,
With the other substrates, the Etot value is positive,
and the corresponding points lie above the baseline.
Thus, the fact that the behavior of the substrates fails
to fit the linear free-energy principle is explained by
vol. 4, no. 1.
3. Ingold, C.K., Structure and Mechanism in Organic
Chemistry, Ithaca: Cornell Univ. Press, 1969.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 8 2001