Electrochemical studies of protonation reaction of anion radicals of some dinitroaromatics in dichloromethane

Article abstract of DOI:10.14233/ajchem.2015.18291

Protonation of anion radicals of 1,2-,1,3- and 1,4-dinitrobenzenes in dichloromethane in the temperature range 0, 5, 10 and 15 °C has been investigated by cyclic voltammetric method. Glassy carbon electrode and hanging mercury drop electrode are used as working electrodes. Benzoic acid and salicylic acid are used as protonating agents. Homogeneous rate constant is calculated by using Nicholson and Shain equation. The position of nucleophilic attack in dinitrobenzenes has been investigated by calculation of charge densities using MNDO and SCF-UHF molecular orbital methods. The heterogeneous rate constant k_s,h for the first reduction process in dinitrobenzenes is determined by digital simulation of the cyclic voltammograms.

Full text of DOI:10.14233/ajchem.2015.18291

Asian Journal of Chemistry; Vol. 27, No. 8 (2015), 2924-2932
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18291
Electrochemical Studies of Protonation Reaction of Anion
Radicals of Some Dinitroaromatics in Dichloromethane
1
2,*
2
2
FARZANA HANIF , GHAZALA YASMEEN , SURYYIA MANZOOR and MUHAMMAD AAMIR
1Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
2Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan
*Corresponding author: Tel: +92 300 6345628; E-mail: ghazala31pk@yahoo.com
Received: 11 June 2014;
Accepted: 13 September 2014;
Published online: 27 April 2015;
AJC-17164
Protonation of anion radicals of 1,2-,1,3- and 1,4-dinitrobenzenes in dichloromethane in the temperature range 0, 5, 10 and 15 °C has
been investigated by cyclic voltammetric method. Glassy carbon electrode and hanging mercury drop electrode are used as working
electrodes. Benzoic acid and salicylic acid are used as protonating agents. Homogeneous rate constant is calculated by using Nicholson
and Shain equation. The position of nucleophilic attack in dinitrobenzenes has been investigated by calculation of charge densities using
MNDO and SCF-UHF molecular orbital methods. The heterogeneous rate constant ks,h for the first reduction process in dinitrobenzenes
is determined by digital simulation of the cyclic voltammograms.
Keywords: Anion radicals, Cyclic voltammetry, Protonation reactions, Dinitrobenzene, Dichloromethane.
perchlorate from Merck chemicals. The prepared salt is washed
with deionized water and dried under vacuum in a desiccator
for about 5 h and then used. The blank cyclic voltammetry
run is used to determine impurity if any (mp found 212 °C:
reported 212 °C). Protonating agents benzoic acid (Chemapol)
and salicylic acid (Fluka) were purified by vacuum sublimation
and melting points are determined (mp found 122 °C: reported
122 °C) and (mp found 159 °C: reported 159-160 °C), respec-
tively. All of the above chemicals are purified as described by
Perrin15. Dichloromethane (CH2Cl2) of Aldrich is purified by
drying it over CaCl2 and then distilled from CaH2. It is then
stored in a brown bottle with Linde type 4 Å molecular sieves15.
Electrochemical cells, electrodes and instrumentation:
For current-voltage measurements EG& G princeton applied
research model 174A polarographic analyzer is used. This
instrument offers complete flexibility in potential or current
control for electrochemical applications16.
A double walled electrochemical cell of EG & G prin-
ceton applied research is used for all electrochemical studies.
The cell cap contains five 14/20 standard taper holes in which
working electrode, counter electrode, reference electrode and
nitrogen inlet etc. are embedded. The electrochemical cell is
connected to a circulating thermostat Lauda Model K-4R for
maintaining constant temperature during measurements. Two
working electrodes are used in the present study.
INTRODUCTION
Integrated study of the dinitrobenzene electroreduction
mechanism by electroanalytical and computational methods
in DMF has already been done1,2. The electroreduction mecha-
nism of aromatic nitroso compounds, particularly of nitroso-
benzene, was the focus of a number of researches3,4. The eletro-
analytical methods in combination with other experimental
(e. g., controlled potential electrolysis) and theoretical methods
such as digital simulation have been employed for the studies
on the mechanism of electrochemical processes and develop-
ment of their theoretical models5-14
.
The objective of this research is to investigate the proto-
nation of the first reduction product of dinitrobenzenes and to
measure the heterogeneous electron transfer rate constants in
dichloromethane solvent at different temperatures. The subs-
trates are the three isomers of dinitrobenzenes and the proto-
nating agents are benzoic acid and salicylic acid.
EXPERIMENTAL
Dinitroaromatic compounds such as 1,2-dinitrobenzene
(mp found 116 °C: reported 116.5 °C) of BDH chemicals and
1,3-dinitrobenzene (mp found 90.5 °C: reported 90.5 °C) of
Hopkin & Williams Ltd. were purified by crystallizing it from
ethanol. Tetra n-butyl ammonium perchlorate (TBAP) used
as supporting electrolyte throughout is prepared using tetra n-
butyl ammonium bromide from BDH chemicals and magnesium
The glassy carbon electrode (GCE) having an area of
0.485 cm2 is used as a working or test electrode. This is a very

Vol. 27, No. 8 (2015)
Electrochemical Studies of Protonation Reaction of Anion Radicals of Some Dinitroaromatics 2925
hard vitreous carbon with good conductivity, very rugged and
can usually be cleaned simply by wiping with tissue paper
or a soft emery paper. The hanging mercury drop electrode
(HMDE) is prepared by sealing a platinum wire of about 0.012
mm diameter. Then this electrode is electroplated with mercury
using 0.1 M mercurous nitrate solution17.A platinum wire elec-
trode is used as counter electrode. It is made by sealing a
platinum wire about 1 mm diameter and 0.5 cm in length at
the end of a capillary glass tube.
A silver wire is used as a Quasi Reference Electrode (QRE)
and it is made by sealing a wire of about 0.069 mm diameter
and about 1 cm in length at the end of the capillary glass tube.
This electrode is cleaned every day by rubbing it with a soft
emery paper to prevent it from being converted toAgS (surface
layer)18.
RESULTS AND DISCUSSION
Electrochemical measurements (triangular wave cyclic
voltammetry) are carried out at two different working elec-
trodes i.e., glassy carbon electrode and hanging mercury drop
electrode. A platinum wire is used as counter electrode while
a silver wire (Ag) as a reference electrode. All the measure-
ments are carried out in aprotic solvent dichloromethane
(CH2Cl2) at four different temperatures 0, 5, 10 and 15 °C.
Tetra n-butyl ammonium per chlorate (TBAP) is used as
supporting electrolyte.
The reduction process for a reversible reaction can be
represented as
O + n
R
(1)
e
kf
R + [HA]
Z
(2)
All the analyses are carried out under nitrogen atmosphere.
Nitrogen drying apparatus consists of six traps or bubblers.
Nitrogen from the cylinder is first passed through two traps
filled with chromous chloride solution (over zinc amalgam
and HCl) to remove the traces of oxygen. The third bottle is
kept empty to avoid mixing of chromous chloride solution
and concentrated sulphuric acid in the fourth trap (used to
absorb water from the nitrogen gas). The nitrogen gas leaving
the fourth trap is then passed through fifth bottle containing
silica gel for complete drying. The gas is then passed through
the sixth trap containing the purified (pertinent) solvent. Finally
the gas is bubbled through the solution for 15 min and after
that the tip is drawn up to create the inert atmosphere.
A 0.1 M solution of tetra n-butyl ammonium perchlorate
(TBAP) in solvent (CH2Cl2) is prepared in a 10 mL volumetric
flask. The solution is transferred into the electrolytic cell and
the cable connections are made to Ag wire as reference elec-
trode, Pt wire as counter electrode and glassy carbon/HMDE
as working electrode. The purified nitrogen gas is passed
through the solution so as to deoxygenate the solution. After
10-15 min. deareation of the solution, it is subjected to current
- voltage scanning to check any impurity in the blank solution.
The blank run showed that the solution is free of impurities.
After running the blank, an appropriate amount of compound
(1,2- DNB and 1,3- DNB) is added in the TBAP solution to
make different concentrations ranging from 1 × 10-4 M to 1 ×
10-3 M. Then after necessary deareation of this solution in the
cell cyclic voltammograms are recorded at required scanning
rates i.e., 20, 50, 100 and 200 mv/sec. at 25 °C.
Thus the reduction of dinitrobenzene (DNB) could be
expressed as
DNB +
DNB−•
(3)
e
The reaction of the anion radical of dinitrobenzene with
the protonating agent (HA) may be written as
DNB−• + [HA]
Z
(4)
kf
The homogeneous pseudo first order rate constant (kf)
for the above process is calculated using the equation of
Nicholson and Shain19
kf
a
RT
nF
Ep = E1/2
−
[(0.78 − ln
)]
(5)
RT
nF
where
a =
v
E1/2 is the reversible half wave potential (potential correspon-
ding to 85 % of the peak current in above process), EP is the
peak potential after the addition of protonating agent and v is
the scan rate (mv/sec.). Since large excess ofA is used for pseudo
first order conditions, [A] represents large excess of A.
Cyclic voltammograms of substrates (dinitrobenzenes) are
recorded in two steps as in the absence of protonating agent
and after addition of the protonating agent under pseudo first
order conditions.
On addition of large excess of the protonating agent, the
anodic peak disappeared and the cathodic peak shifted
anodically. The rate constant kf could be calculated from this
shift (Ep-E1/2). The second order rate constant (k2) is calculated
kf
For protonation, the aliquots of 0.4, 0.6, 0.8, 1.0 and 1.4
mL of 0.1 M benzoic acid solution are added in succession.
After each addition cyclic voltammograms are recorded at scan
rates mentioned above.
from the relation k2 =
, [HA] being the concentration
[HA]
of the protonating agent which is present in large excess.
Results are given in Tables 1 and 2.
In order to optimize the experimental conditions, it is
observed from the rate constant values that out of five
concentrations (1 × 10-3 M, 8 × 10-4 M, 5 × 10-4 M, 2.5 × 10-4
M, 1 × 10-4 M) of dinitrobenzenes used in this study, 5 × 10-4
M concentration showed the best reproducible results in
CH2Cl2 at scan rate 50 mv/sec. Concentration of the protonating
agent is kept at 1 × 10-2 M for the protonation studies to
maintain pseudo first order conditions for the reaction. Higher
concentrations of protonating agent are avoided due to its
dimerization. Scan rate higher than 50 mv/sec is not used
because of IR (potential) drop problem.
Heterogeneous rate constant ks,h for the first reduction
process is calculated from digital simulation method. In this
method, experimental parameters (reduction potential, kf and
scan rate) are given from the voltammetric studies as input for
simulation. The dimensionless standard rate constant (RKS in
the simulation program) is a variable parameter and its value
is given as input also for simulation. The program calculates
current and potential and draws the cyclic voltammogram for
the first reduction process for the given RKS. The value of
dimensionless standard rate constant RKS is continuously

2926 Hanif et al.
Asian J. Chem.
TABLE-1
(Ep-E1/2) VALUES AND THE BIMOLECULAR RATE CONSTANT (k2) FOR THE PROTONATION OF ANION RADICALS
OF ISOMERS OF DINITROBENZENES IN CH2Cl2 (SCAN RATE: 50 mV sec-1 AND PROTONATING AGENT:BENZOIC ACID)
Glassy carbon electrode
k2 (L mol-1 sec-1)
Hanging mercury drop electrode
Concentration of Temperature
Compound
1,2-DNB
k2 (L mol-1 sec-1)
(4.36 1.75) × 102
(3.84 2.44) × 103
(10.95 6.31) × 104
(26.53 17.71) × 104
(7.49 3.52) × 103
(1.78 1.04) × 104
(1.96 1.13) × 104
(6.08 2.73) × 104
(1.57 1.04) × 104
(4.45 2.06) × 104
(1.71 0.88) × 105
(2.88 1.92) × 105
benzoic acid (M)
(°C)
(Ep-E1/2) (V)
(-0.002 0.012)
(0.012 0.011)
(0.019 0.011)
(0.054 0.009)
(-0.021 0.007)
(0.015 0.006)
(0.031 0.007)
(0.053 0.009)
(0.032 0.010)
(0.045 0.009)
(0.057 0.006)
(0.068 0.005)
(Ep-E1/2) (V)
(1.35 1.04) × 103
(3.96 2.87) × 103
(6.70 4.81) × 103
(9.57 5.94) × 104
(2.00 1.10) × 102
(3.95 1.83) × 103
(1.46 0.76) × 104
(8.80 5.50) × 104
(2.14 1.48) × 104
(5.56 3.53) × 104
(1.19 0.54) × 105
(2.51 0.96) × 105
(-0.011 0.005)
(0.013 0.009)
(0.055 0.008)
(0.066 0.010)
(0.022 0.006)
(0.032 0.008)
(0.034 0.008)
(0.050 0.006)
(0.029 0.009)
(0.044 0.006)
(0.061 0.007)
(0.067 0.010)
0
5
10
15
0
1.0 × 10-2
5
1,3-DNB
1,4-DNB
1.0 × 10-2
1.0 × 10-2
10
15
0
5
10
15
TABLE-2
(Ep-E1/2) VALUES AND THE BIMOLECULAR RATE CONSTANT (k2) FOR THE PROTONATION OF ANION RADICALS OF
ISOMERS OF DINITROBENZENES IN CH2Cl2 (SCAN RATE: 50 mV sec-1 AND PROTONATING AGENT: SALICYLIC ACID)
Glassy carbon electrode
k2 (L mol-1 sec-1)
Hanging mercury drop electrode
Concentration of Temperature
Compound
1,2-DNB
k2 (L mol-1 sec-1)
(6.92 3.25) × 102
(2.55 1.74) × 104
(8.54 3.89) × 104
(2.88 1.92) × 105
(4.18 3.06) × 102
(5.54 3.23) × 103
(1.25 0.49) × 104
(5.16 3.66) × 104
(2.29 1.07) × 102
(2.03 1.47) × 103
(1.36 0.86) × 104
(7.43 3.80) × 104
benzoic acid (M)
(°C)
(Ep-E1/2) (V)
(0.005 0.009)
(0.042 0.006)
(0.048 0.007)
(0.063 0.006)
(-0.005 0.009)
(0.006 0.006)
(0.042 0.005)
(0.065 0.009)
(-0.002 0.007)
(0.015 0.010)
(0.024 0.009)
(0.037 0.010)
(Ep-E1/2) (V)
(2.04 1.31) × 103
(3.97 1.74) × 104
(5.89 3.06) × 104
(1.73 0.78) × 105
(8.70 5.60) × 102
(1.86 0.86) × 103
(3.35 1.30) × 104
(2.32 1.44) × 105
(1.02 0.54) × 103
(4.79 3.27) × 103
(9.04 5.68) × 103
(2.56 1.71) × 104
(-0.006 0.006)
(0.035 0.010)
(0.053 0.006)
(0.067 0.010)
(-0.015 0.011)
(0.018 0.008)
(0.030 0.005)
(0.045 0.011)
(-0.019 0.006)
(0.004 0.011)
(0.029 0.009)
(0.052 0.007)
0
5
10
15
0
1.0 × 10-2
5
1,3-DNB
1,4-DNB
1.0 × 10-2
1.0 × 10-2
10
15
0
5
10
15
changed until the simulated voltammogram exactly matches
with the experimental voltammogram. The standard hetero-
geneous rate constant values are given in Tables 3 and 4.
Disproportionation constant (KD) for the process is
calculated by the following equation
Activation energy, Ea, of protonating step is obtained by
1
plotting ln k2 vs
in accordance withArrhenius equation for
T
the reaction rate. Other activation parameters such as free
energy of activation (∆G*), enthalpy of activation and entropy
of activation (∆S*) are calculated from the following relations
∆E1/2
log KD =
(6)
2.303RT / nF
∆E1/2 = (E1/2)2 - (E1/2)1
−∆G* /RT
kT
h
e
where
k2 =
(10)
(E1/2)1 and (E1/2)2 are the half wave potential of the first and
second reduction processes, respectively.
Thermodynamic parameters ∆H°, ∆G°, ∆S° are calculated
from the following relations
where k is Boltzmann constant, R is gas constant, h is Planck's
constant
∆H* = Ea-nRT
∆G* = ∆H* - T∆S*
(11)
(12)
∆G° = -RT ln KD
(7)
Activation parameters are given in Tables 7 and 8.


(KD )1 ∆H°
1
1




ln
=
−
Voltammograms of dinitrobenzenes in aprotic solvent
CH2Cl2 show typically two reversible waves. The first and the
second waves correspond to the formation of radical anion
and dianion respectively. The three dinitrobenzenes i.e., 1,2-
DNB, 1,3-DNB and 1,4-DNB exhibit completely reversible
first reduction wave in CH2Cl2 at glassy carbon electrode. The
∆Ep = Epc – Epa values range between 65 to 75 mv and Ep – Ep/2
values are nearly 65 mv. The second reduction process is quasi
(8)
(KD )2
R
T2
T
1


∆Ho − ∆Go
∆S° =
(9)
T
where (KD)1 and (KD)2 are the disproportionation constants
corresponding to temperatures T1 and T2, respectively. These
results are given in Tables 5 and 6.

Vol. 27, No. 8 (2015)
Electrochemical Studies of Protonation Reaction of Anion Radicals of Some Dinitroaromatics 2927
TABLE-3
HETEROGENEOUS RATE CONSTANT FOR THE ISOMERS OF DINITROBENZENES FOR FIRST REDUCTION PROCESS
IN CH2CL2 CALCULATED FROM DIGITAL SIMULATION METHOD (PROTONATING AGENT : BENZOIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
(°C)
Eoa
(mv)
kfb
HKOd
Eoa
(mv)
kfb
HKOd
Compound
1,2-DNB
RKSc
RKSc
(s-1)
(cm s-1)
(s-1)
(cm s-1)
0
5
10
15
0
-0.820
-0.850
-0.850
-0.860
-0.835
-0.830
-0.830
-0.850
-0.635
-0.640
-0.640
-0.650
13.50
39.60
67.00
957.0
2.00
39.50
146.0
880.0
241.0
556.0
1190.0
2510.0
1.00
-
-
-
0.0105
-0.810
-0.825
-0.850
-0.850
-0.800
-0.810
-0.810
-0.815
-0.630
-0.640
-0.650
-0.660
4.36
38.40
119.5
265.0
74.90
178.0
196.0
608.0
157.0
445.0
1710.0
2880.0
11.00
0.1159
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
30.00
0.3162
5
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1,3-DNB
1,4-DNB
10
15
0
5
10
15
aReduction potential of dinitrobenzenes; bPseudo first order rate constant; cContinuously changing rate constant; dStandard heterogeneous rate constant
TABLE-4
HETEROGENEOUS RATE CONSTANT FOR THE ISOMERS OF DINITROBENZENES FOR FIRST REDUCTION PROCESS
IN CH2Cl2 CALCULATED FROM DIGITAL SIMULATION METHOD. (PROTONATING AGENT : SALICYLIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
(°C)
Compound
1,2-DNB
Eo
(mv)
kf
HKO
Eo
(mv)
kf
HKO
RKS
(s-1)
RKS
(s-1)
(cm s-1)
(cm s-1)
0
5
10
15
0
-0.820
-0.855
-0.855
-0.860
-0.815
-0.825
-0.845
-0.855
-0.630
-0.655
-0.660
-0.660
20.40
39.70
589.0
1730.0
8.70
18.60
335.0
2320.0
10.20
47.90
90.40
2560.0
1.00
-
-
-
6.00
1.00
-
-
0.0105
-0.825
-0.840
-0.860
-0.870
-0.825
-0.845
-0.845
-0.855
-0.605
-0.625
-0.650
-0.640
6.92
255.0
854.0
2880.0
4.18
55.40
125.0
516.0
2.29
15.00
-
-
-
24.00
-
-
-
24.00
1.00
-
0.1581
-
-
-
-
-
-
0.632
0.0105
0.2530
1,3-DNB
1,4-DNB
5
-
-
-
10
15
0
5
10
15
-
-
3.00
0.0316
0.2530
0.0105
-
-
-
-
-
-
20.30
136.0
743.0
-
-
-
TABLE-5
THERMODYNAMIC PARAMETERS (∆H°, ∆G°, ∆S°) CALCULATED FROM DISPROPORTIONATION
CONSTANTS IN CH2Cl2 (PROTONATING AGENT = BENZOIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
(K)
Compound
1,2-DNB
∆G°
(kJ mol-1)
11.14
11.14
11.23
10.89
12.90
13.49
12.69
12.53
7.75
∆H°
-∆S°
(JK-1 mol-1)
40.04
40.09
39.67
37.82
46.02
48.52
44.85
43.49
27.80
28.03
25.91
∆G°
(kJ mol-1)
12.78
11.78
11.31
10.89
11.56
11.02
12.23
12.15
6.62
∆H°
-∆S°
(JK-1 mol-1)
44.35
42.35
39.97
-ln KD
-ln KD
(J mol-1)
(J mol-1)
273
278
283
288
273
278
283
288
273
278
283
288
4.91
4.82
4.77
4.55
5.68
5.84
5.39
5.23
3.41
3.37
3.12
3.22
5.63
5.09
4.81
4.55
5.09
4.77
5.20
5.07
2.92
3.26
3.09
2.97
213.1
338.6
160.1
669.0
68.77
5.03
37.82
42.10
39.63
43.23
42.18
24.23
27.13
25.76
1,3-DNB
1,4-DNB
7.79
7.33
7.54
7.29
7.71
26.77
7.12
24.73
reversible at customary scan rates (50-100 mv/sec) i.e.,
Ep – Ep/2 values are greater than 100 mv and ∆Ep = Epc – Epa
values range between 100 to 200 mv for the solvent at glassy
carbon electrode. The higher values of ∆Ep,s in the case of
second reduction process at glassy carbon electrode may be
due to the instability of the dianion at glassy carbon electrode
which may be due to the catalytic decomposition of slow
electron transfer or this may be due to the large surface area of
working electrode. All the three dinitrobenzenes showed
reversible peaks corresponding to the first and second reduction

2928 Hanif et al.
Asian J. Chem.
TABLE-6
THERMODYNAMIC PARAMETERS (∆H°, ∆G°, ∆S°) CALCULATED FROM DISPROPORTIONATION
CONSTANTS IN CH2Cl2 (PROTONATING AGENT = SALICYLIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
Compound
1,2-DNB
1,3-DNB
∆G°
(kJ mol-1)
10.81
10.93
10.89
11.23
12.99
12.82
13.15
13.32
7.54
∆H°
-∆S°
(JK-1 mol-1)
39.37
39.33
38.49
38.98
47.27
46.11
46.48
46.26
27.08
28.63
26.35
∆G°
(kJ mol-1)
12.23
11.86
11.52
10.85
11.81
11.35
11.13
11.64
5.95
∆H°
-∆S°
(JK-1 mol-1)
42.87
42.65
40.71
37.68
42.49
40.84
(K)
-ln KD
-ln KD
(J mol-1)
(J mol-1)
273
278
283
288
273
278
283
288
273
278
283
288
4.76
4.73
4.63
4.69
5.72
5.55
5.59
5.56
3.32
3.44
3.17
3.15
5.39
5.13
4.90
4.53
5.20
4.91
4.81
4.86
2.62
2.83
3.19
2.94
61.23
82.26
149.1
530.2
215.6
251.7
39.97
40.44
20.87
1,4-DNB
7.96
7.46
6.53
7.50
23.51
26.49
24.44
7.54
26.18
7.04
TABLE-7
ACTIVATION PARAMETERS (∆G*, ∆H*, ∆S*) CALCULATED FROM DISPROPORTIONATION
CONSTANTS IN CH2Cl2 (PROTONATING AGENT = BENZOIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
(K)
∆G*
(kJ mol-1)
50.30
48.77
48.46
48.46
54.63
48.78
46.62
43.19
44.03
42.67
41.69
40.68
∆H*
(kJ mol-1)
91.66
91.61
91.57
91.53
137.5
137.5
137.4
137.4
55.79
55.75
55.71
55.67
∆S*
(JK-1 mol-1)
151.5
154.1
152.2
168.5
303.6
319.1
320.9
327.1
43.09
47.05
49.54
52.04
∆G*
(kJ mol-1)
52.86
48.85
47.30
40.55
46.41
45.30
45.93
44.07
44.73
43.18
40.83
40.35
∆H*
(kJ mol-1)
159.0
159.0
159.0
158.9
42.99
42.94
42.90
42.86
69.64
69.60
69.56
69.52
∆S*
(JK-1 mol-1)
388.8
396.1
394.4
410.9
-12.54
-8.48
-10.70
-4.21
91.26
Compound
1,2-DNB
Ea
Ea
(kJ mol-1)
(kJ mol-1)
273
278
283
288
273
278
283
288
273
278
283
288
93.93
139.8
58.06
161.3
45.25
71.91
1,3-DNB
1,4-DNB
95.03
101.5
101.3
TABLE-8
ACTIVATION PARAMETERS (∆G*, ∆H*, ∆S*) CALCULATED FROM DISPROPORTIONATION
CONSTANTS IN CH2Cl2 (PROTONATING AGENT = SALICYLIC ACID)
Glassy carbon electrode
Hanging mercury drop electrode
Temperature
(K)
∆G*
(kJ mol-1)
49.36
43.45
43.34
41.57
51.30
50.52
44.67
40.87
50.93
48.33
47.75
46.14
∆H*
(kJ mol-1)
96.24
96.20
96.15
96.11
136.9
136.9
136.8
136.8
71.28
71.24
71.20
71.16
∆S*
(JK-1 mol-1)
171.7
189.7
186.6
189.4
313.7
310.7
325.7
333.1
74.53
82.39
82.85
86.85
∆G*
(kJ mol-1)
51.82
44.47
42.47
40.35
52.96
48.00
47.00
44.47
54.33
50.32
46.79
43.59
∆H*
(kJ mol-1)
136.2
136.2
136.1
136.1
106.9
106.8
106.8
106.7
134.9
134.8
134.8
134.8
∆S*
(JK-1 mol-1)
309.2
329.9
331.0
333.1
197.5
211.6
211.3
216.3
295.1
304.1
311.0
316.6
Compound
1,2-DNB
Ea
Ea
(kJ mol-1)
(kJ mol-1)
273
278
283
288
273
278
283
288
273
278
283
288
98.51
139.2
73.55
138.5
109.1
137.2
1,3-DNB
1,4-DNB
processes and ∆Ep = ∆Epc - ∆Epa values for first reduction
process were exactly 60 mv at hanging mercury drop electrode.
In the present study, the anion radical is generated by
reducing 1,2-DNB, 1,3-DNB and 1,4-DNB.
on the surface of the working electrodes i.e., glassy carbon
electrode and hanging mercury drop electrode, respectively.
When the protonating agent is added, the cathodic peak
shifted anodically and the anodic peak completely disappeared.
The pseudo first order rate constant is calculated from the
DNB +
DNB−•
(13)
e

Vol. 27, No. 8 (2015)
Electrochemical Studies of Protonation Reaction of Anion Radicals of Some Dinitroaromatics 2929
voltammograms recorded after the addition of protonating
agent.
reactive towards the protonating agent than 1,3-DNB as the
later species is more stable. However, it should be noted that
the rate constant for 1,2-DNB anion radical is comparable to
1,3-DNB in CH2Cl2 at 5 °C. It may be due to the closely placed
nitro groups which hinder the attack of bulky protonating agent
(benzoic acid). For 1,4-DNB the nitro groups are farthest apart
and hence the rate constant is higher.
In this case it is observed that on addition of protonating
agent the first peak increased in height at the expense of the
second peak. It is thought initially that the increase in the peak
height is due to the presence of water as impurity in the solvent.
But addition of more water produced no effect on the observed
peak height. The possibility of catalytic and kinetic reactions
is also ruled out as no cathodic shift in the peak position is
Disproportionation for the dinitrobenzenes system may
be depicted as follows:
2DNB–
DNB + DNB2–
(14)
kf
Disproportionation constant (KD) values for dinitro-
benzenes are given in Tables 5 and 6. In the solvent CH2Cl2
disproportionation constant for the anion radical of 1,4-DNB
at both electrodes is greater than 1,2-DNB–• and 1,3-DNB–• at
all the temperatures selected for the present study. When the
protonating agent is salicylic acid the similar trend is observed
(Figs. 2 and 3).
observed and its broadening with increase in
ratio occurred
a
when protonating agent is added. From the shift in the peak
potential (Ep-E1/2) the pseudo first order rate constant kf is
obtained from Nicholson and Shain equation. The electron
transfer reaction between these anion radicals and the proto-
nating agent could be ruled out on the basis of the shape of the
cyclic voltammogram. The homogeneous reaction of dinitro-
benzenes and a protonating agent involves electron transfer
reaction. The second order rate constant (k2) is obtained by
dividing the kf by the concentration of the protonating agent
(benzoic acid/salicylic acid).
1,2-DNB:
–
–
–
–
O
–
–
O
O
O
O
O
O
O
+
N
+
N
+
N
+
N
–
O
O
O
O
+
N
+
N
+
N
+
N
+
O
O
–
O
–
O
–
+
Minor contributor
1,3-DNB:
In solvent CH2Cl2 at 0 °C, the k2 (1,4) (2.14 × 104 L mol-1 s-1)
at the glassy carbon electrode and (1.57 × 104 L mol-1 s-1) at
hanging mercury drop electrode are higher than k2 (1,2) and
k2 (1,3). At 5 °C temperature the k2 (1,2) (3.96 × 102 L mol-1 s-1)
and k2 (1,3) (3.95 × 102 L mol-1 s-1) observed for glassy carbon
electrode are comparable. The k2 (1,4) values (5.56 × 102 L
mol-1 s-1) and (4.45 × 102 L mol-1 s-1) observed for the glassy
carbon electrode and hanging mercury drop electrode respec-
tively were higher than for 1,2-DNB and 1,3-DNB. The bimole-
cular rate constant for the protonation of the anion radicals of
dinitroberzenes follow the following pattern.
–
–
–
–
–
–
O
O
O
O
O
O
O
O
+
N
+
N
+
N
N
+
–
–
–
O
O
O
O
+
N
N
N
N
+
O
O
O
O
–
1,4-DNB:
–
–
–
–
–
–
O
O
O
O
O
O
O
O
+
+
N
+
N
+
N
N
+
N
+
N
+
N
+
N
+
O
–
O
O
O
–
O
O
–
O
O
–
Minor contributor
Fig. 1. Resonance structures of the dinitrobenzenes
k2 (1,4) > k2 (1,2) > k2 (1,3)
This behaviour can be explained if we consider the
resonance structures of the dinitrobenzenes as shown in Fig. 1.
Looking at the resonance structures it may be seen that when
an electron reacts with dinitrobenzene it would react with
greater ease with 1,3-DNB as the positive charge is delocalized
over the entire ring giving it greater stability than 1,2-DNB
and 1,4-DNB. For the later two isomers, the two resonance
structures are minor contributors. On adding the protonating
agent it is expected that 1,2-DNB and 1,4-DNB would be more
In order to seek information regarding the stability of the
electrochemical system and its kinetic behaviour, thermody-
namic and activation parameters are calculated. In solvent
CH2Cl2, the entropy and enthalpy of activation of 1,3-DNB−•
is found higher than that of 1,2-DNB−• It seems that the lower
∆S* value of 1,2-DNB−• is due to the activated complex of
1,2-DNB−• being as symmetrical and planer and more ordered.
The observed decrease in ∆H* values is expected as the two
nitro groups are far apart.
0.5
0.4
0.3
0.2
0.1
0
0.5
0.5
0.4
0.3
0.2
0.1
0
5 °C
5 °C
5 °C
0.4
0.3
0.2
0.1
0
-0.3
-0.6
-0.3
-0.6
-0.9
Ep
(i)
-0.9
Ep
(ii)
-1.2
-1.5
-1.2
-1.5
-0.3
-0.6
-0.9
Ep
(iii)
-1.2
-1.5
Fig. 2. (i) Simulated cyclic voltammogram of 1,2-DNB in dichloromethane at hanging mercury drop electrode (protonating agent: Benzoic acid); (ii)
Simulated cyclic voltammogram of 1,2-DNB in dichloromethane at glassy carbon electrode (protonating agent: benzoic acid); (iii) Simulated cyclic
voltammogram of 1,3-DNB in dichloromethane at glassy carbon electrode (protonating agent: benzoic acid)

2930 Hanif et al.
0.5
Asian J. Chem.
0.5
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
5 °C
5 °C
-0.5
-0.8
-1.1
Ep
-1.4
-1.7
-0.5
-0.8
-1.1
Ep
-1.4
-1.7
(i)
(ii)
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
5 °C
15 °C
-0.3
-0.6
-0.3
-0.6
-0.9
Ep
-1.2
-1.5
-0.9
Ep
-1.2
-1.5
(iii)
0.5
0.5
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
5 °C
15 °C
-0.3
-0.6
-0.9
Ep
-1.5
-1.2
-0.2
-0.5
-0.8
Ep
-1.1
-1.4
(iv)
(v)
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
5 °C
15 °C
-0.3
-0.6
-0.9
Ep
-1.2
-1.5
(vi)
-0.2
-0.5
-0.8
Ep
-1.1
-1.4
Fig. 3. (i) Simulated cyclic voltammogram of 1,2-DNB in dichloromethane at glassy carbon electrode (protonating agent: Salicylic acid); (ii) Simulated
cyclic voltammogram of 1,2-DNB in dichloromethane at hanging mercury drop electrode (protonating agent: Salicylic acid); (iii) Simulated cyclic
voltammogram of 1,3-DNB in dichloromethane at glassy carbon electrode (protonating agent: Salicylic acid); (iv) Simulated cyclic voltammogram
of 1,3-DNB in dichloromethane at hanging mercury drop electrode (protonating agent: Salicylic acid); (v) Simulated cyclic voltammogram of 1,4-
DNB in dichloromethane at glassy carbon electrode (protonating agent: Salicylic acid); (vi) Simulated cyclic voltammogram of 1,4-DNB in
dichloromethane at hanging mercury drop electrode (protonating agent: Salicylic acid)

Vol. 27, No. 8 (2015)
Electrochemical Studies of Protonation Reaction of Anion Radicals of Some Dinitroaromatics 2931
When salicylic acid is used as protonating agent in the
CH2Cl2 the entropy and enthalpy of activation for the homo-
geneous chemical reaction of 1,2-DNB–• is higher than 1,3-
DNB–• and 1,4-DNB–•. The higher values of 1,2-DNB–• show
that the two nitro groups are present close to each other and
hence it is difficult for them to be in one plane. There is more
disorderness in the activated complex of 1,2-DNB–• than that
of 1,3-DNB–•.
Solvent is known to play an important role in determining
the rate of electron transfer in solution20,21. The well known
Marcus-Hush outer sphere reorganization energy arises from
dielectric polarization effects in response to the transferring
electrons and can lead to decrease in the rate of electron transfer
in highly polar solvents. The solvent dynamics has pronounced
effect on the kinetics of the electron transfer reactions22. On
the other hand, substantial changes of electron transfer rate
constants are solely observed for the dinitrobenzenes due to
solvent polarity.
The dipole moment (µ) of a molecule is the vector sum of
bond moments and is a function of charge separation and
geometry of the molecule. Because of the geometry factor
and possibility of the bond moments, dipole moment is pro-
bably a less useful measure of the ability of a solvent to promote
dissociation of an ionic solute than is the dielectric constant
"ε". Solvents with substantially higher dielectric constants
should be used preferably in electrochemical work in order to
minimize the solution resistance. This will also minimize
ohmic losses and diminish the problem of potential control
error23. For solvents of dielectric constants much below 15,
substantial ion association begins to take place.
inspite of that energy λi is not neglected. The λi 's are calculated
by applying SCF-UHF method for bond orders24. The contribu-
tion of λi to λis about 8 % when the bulk dielectric constant is
taken and 10 % when the effective dielectric constant is taken.
The radius of the nitro groups is taken as 2.3 Å i.e., a1 = a2
= 2.3 Å while the value of a2 = 3.5 Å. The charge densities are
calculated from SCF-UHF and MNDO calculations. The
multisphere model is adopted since a single sphere model had
been shown to be inadequate24.
The standard theoretical free energy is calculated by using
the equation
[λ + wp − wr ]2
∆Goth = wr +
(16)
4λ
where wr and wp are work terms for the reactant and product,
respectively.
For the first reduction process wr is zero, so the above
equation becomes
[λ + wp ]2
∆Goth
=
4λ
The dielectric constant for an interfacial phase, however,
is entirely different from the bulk phase and it is the interfacial
dielectric constant which may be relevant in the present study
because we are working in the vicinity of the electrode. As far
as heterogeneous rate constant is concerned, the most important
thing is the local environment created by the solvent molecules
around the initial and transition state. So instead of taking
the bulk dielectric constant, it is more appropriate to take an
effective dielectric constant (for the evaluation of λo)25. The
expression for the variation of dielectric constant as a function
of distance from the centers of the charged ion is given by Booth
and it has been adopted for the present study26,27. In order to
study the electrode reactions, Booth's expression is used after
some modifications28,29. The charge of the ion (Z) is replaced by
the effective charge of the electrode. This effective charge
assumed to be 7 reproduced the dielectric constant of 4.1 at a
distance of 6 Å (which is the thickness of the inner layer IHP
adjacent to the electrode)28,29. Thus the value of ε for CH2Cl2
which have been considered as an effective dielectric constant
in contrast to the values of 64.40 for the bulk dielectric constant.
The rate constants calculations are carried out by using these
values of effective dielectric constants. The theoretical
The Z-parameter is a measure of the ability of the medium
to stabilize an ion-pair to a less polar electronically excited
state produced by charge transfer20.A high Z-value corresponds
to high solvent polarity.
The solvent reorganization energy λo measures the energy
required to reorganize the atoms and molecules of the environ-
ments of the reactants from their position at equilibrium to the
position around the product species. This reorganization energy
comprises of two parts: internal reorganization energy λi and
solvent reorganization energy, λo. The internal reorganization
energy λi measures the work expended in changing the bond
length and bond angles within the molecules whereas λo measures
the work expanded in changing the external environment. The
solvent reorganization energy is calculated using a multisphere
model. Charges on the sphere (that of nitro groups and benzene
ring) is estimated using SCF-UHF and MNDO methods and
equation is
kth
heterogeneous rate constant
is calculated from the equation
s,h
∆Goth
th
s,h
k
= Zel exp(−
)
(17)
kT
kT
1/2
)
Zel = (
where collision frequency
q12 q22 2q1q2
2πm
1
1
1
λo =
(
−
)(
+
+
)
Digital simulation method have played an important role
in the analysis of the electrochemical data30-32. In the present
study heterogeneous electron transfer rate constant ks,h is
calculated by simulation method. The results of these ks,h are
collected in Tables 9 and 10.
(15)
2 εo
ε
a1 a2
R12
where εo and ε are optical and effective dielectric constants,
respectively q1 and q2 are charges on the nitro group and benzene
ring in radical a1 and a2 are radii of nitro group and benzene
ring, respectively.
The internal reorganization energy λi measures the work
expended in changing the bond length and bond angles within
the molecules. However, the contribution of these energies is
small as compared to the solvent reorganization energy as λo
>> λi the major contribution to λarises from λO i.e., λ = λi + λo
Conclusion
Kinetic study of protonating reactions of reactive inter-
mediates by Nicholson and Shain polarographic method is
simple and results are in good agreement with homogeneous
electron transfer rate constant, which were previously reported

2932 Hanif et al.
Asian J. Chem.
TABLE-9
THEORETICAL RESULTS FOR ELECTRODE REACTIONS INVOLVING ELECTRON
TRANSFER IN CH2Cl2 at 25 °C (PROTONATING AGENT = BENZOIC ACID)
Zhet (cm sec-1)
λ1SCF-UHF
λο (e.v)
∆G°th (e.v)
Compound
q1
q2
w
kths,h (cm sec-1)
1,2-DNB
ε= 5.46
εο = 8.93
1,3-DNB
ε= 5.46
εο = 8.93
1,4-DNB
ε = 5.46
εo = 8.93
-0.371
-0.371
-0.258
-0.258
0.482
0.593
0.205
0.232
1.599
0.566
0.076
0.119
4843
-0.312
-0.312
-0.376
-0.376
0.492
0.606
0.134
0.162
26.34
08.74
0.042
0.062
0.000
0.129
4843
4843
-0.312
-0.312
-0.338
-0.338
0.488
0.601
0.210
0.236
1.350
0.477
TABLE-10
THEORETICAL RESULTS FOR ELECTRODE REACTIONS INVOLVING ELECTRON
TRANSFER IN CH2CL2 AT 25 °C (PROTONATING AGENT = SALICYLIC ACID)
Zhet (cm sec-1)
λ1SCF-UHF
λο (e.v)
∆G°th (e.v)
Compound
q1
q2
w
kths,h (cm sec-1)
1,2-DNB
ε= 5.46
εο = 8.93
1,3-DNB
ε= 5.46
εο = 8.93
1,4-DNB
ε = 5.46
εo = 8.93
-0.371
-0.371
0.076
-0.258
-0.258
0.119
0.485
0.596
0.206
0.233
1.550
0.544
4843
0.042
0.062
-0.370
-0.370
-0.260
-0.260
0.000
0.129
4843
4843
0.482
0.592
0.131
0.159
29.09
09.87
-0.390
-0.390
-0.221
-0.221
0.481
0.591
0.208
0.234
1.440
0.514
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in the literature. The digital simulation method based upon
cyclic voltammograms is used for the evaluation of hetero-
geneous rate constant ks,h. This method is found useful in the
determination of heterogeneous rate constant in the present
study since measurements are made at one scan rate only.
Theoretical heterogeneous rate constant ks,h is also calculated
using the Marcus theory with some modifications. Calculations
of the theoretical heterogeneous rate constant requires the
reorganization energy, λo, which in turn requires dielectric
constant of the solvent. However, it is found that the use of λo
calculated from effective dielectric constant (i.e. the dielectric
constant in the interfacial region) produces theoretical values
in good agreement with the experimental values obtained
through simulations. This strengthens our assumption that the
electron transfer takes place in the interfacial region. Thus the
validity of the use of modified Marcus theory is established
by agreement between theoretical and experimental ks,h. The
advantage of the theoretical calculations is that heterogeneous
rate constant can be calculated for those compounds for which
no experimental data is available.
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