Charge transfer complex role in the formation of chlorobenzene in the γ-irradiated carbon tetrachloride-benzene system

Article abstract of DOI:10.1002/jccs.200700204

The formation of carbon tetrachloride-benzene charge transfer complex was confirmed by UV and NMR spectrometric studies. Achange in UVspectrumof benzene is observed upon addition of carbon tetrachloride. Whereas the appearance of new bands supports the formation of charge transfer complex. NMR study shows that, chemical shift of benzene pmr signal depends on the CCl₄-C ₆H₆ molar ratio. This observation is another criterion for the formation of benzene-carbon tetrachloride charge transfer complex. Job's Continuous Variation method indicates that a 2:1 CCl₄-C ₆H₆ charge transfer complex (2:1 CTC) is formed. The association constants (K_2:1) of (2:1 CTC) was found to be 0.0197M^-2. Themaximum concentration of (2:1 CTC) was found to be in samples with 2:1CCl₄-C₆H₆ molar ratio (33% benzene mole). On the other hand the maximum yield of chlorobenzene was obtained, also, upon radiolysis of CCl₄-C₆H₆ samples at a 2:1 molar ratio (33% benzene mole). Therefore, it could be concluded that (2:1 CTC) participates in the formation of chlorobenzene upon radiolysis of the benzene-carbon tetrachloride system. This conclusion was supported by the dependence of the chlorobenzene yield of a γ-irradiated carbon tetrachloride-benzene system (2:1 molar ratio) on irradiation time according to a third order kinetic equation with a very good linearity (R ² = 0.9977). Accordingly, the rate constant for the chlorobenzene formation under this condition was found to be ≈ 5.5 × 10^-7 L².mol^-2.h^-1. We propose a radiation chemical mechanism in which the 2:1 CTC plays a role in the formation of chlorobenzene.

Full text of DOI:10.1002/jccs.200700204

Journal of the Chinese Chemical Society, 2007, 54, 1443-1450
1443
Charge Transfer Complex Role in the Formation of Chlorobenzene in the
g-Irradiated Carbon Tetrachloride-Benzene System
Kh. A. Sife-Eldeen* and S. Ebraheem
National Center for Radiation Research and Technology, P. O. Box 29, Nasr City, Cairo, Egypt
The formation of carbon tetrachloride-benzene charge transfer complex was confirmed by UV and
NMR spectrometric studies. Achange in UV spectrum of benzene is observed upon addition of carbon tet-
rachloride. Whereas the appearance of new bands supports the formation of charge transfer complex.
NMR study shows that, chemical shift of benzene pmr signal depends on the CCl4-C6H6 molar ratio. This
observation is another criterion for the formation of benzene-carbon tetrachloride charge transfer com-
plex. Job’s Continuous Variation method indicates that a 2:1 CCl4-C6H6 charge transfer complex (2:1
CTC) is formed. The association constants (K2:1) of (2:1 CTC) was found to be 0.0197 M-2 . The maximum
concentration of (2:1 CTC) was found to be in samples with 2:1CCl4-C6H6 molar ratio (33% benzene
mole). On the other hand the maximum yield of chlorobenzene was obtained, also, upon radiolysis of
CCl4-C6H6 samples at a 2:1 molar ratio (33% benzene mole). Therefore, it could be concluded that (2:1
CTC) participates in the formation of chlorobenzene upon radiolysis of the benzene-carbon tetrachloride
system. This conclusion was supported by the dependence of the chlorobenzene yield of a g-irradiated car-
bon tetrachloride-benzene system (2:1 molar ratio) on irradiation time according to a third order kinetic
equation with a very good linearity (R2 = 0.9977). Accordingly, the rate constant for the chlorobenzene
formation under this condition was found to be » 5.5 ´ 10-7 L2.mol-2.h –1. We propose a radiation chemical
mechanism in which the 2:1 CTC plays a role in the formation of chlorobenzene.
Keywords: Gamma radiation; Benzene; Carbon tetrachloride; Chlorobenzene; Charge transfer
complex; Job’s Continuous Variation; NMR; External reference method; UV; As-
sociation constant.
INTRODUCTION
Budyka et al.1 studied the role of the charge transfer
g-Radiolysis of carbon tetrachloride has been studied
in solutions of different solvents,10,11 as well as in the pure
state.12,13 Pulse radiolysis of carbon tetrachloride was also
carried out.14,15 Sife-Eldeen16 studied some radiation chem-
ical aspects of the carbon tetrachloride-benzene system,
where chlorobenzene was found to be the major product.
The aim of this work is to investigate the role of carbon tet-
rachloride-benzene CTC in the mechanism of chloroben-
zene formation via g-radiolysis of carbon tetrachloride in
the presence of benzene.
complex (CTC), formed between CBr4 and aromatic amines,
in a photochemical system. Grafting of monomers CTC in
the presence of UV or ionizing radiation was studied by
Dennis et al.2 Zhongqing et al.3 studied the complex-radical
terpolymerization of styrene, maleic anhydride, and N-vi-
nyl pyrrolidone via g-ray radiation. Recently, participation
of the CTC in the g-radiation-induced chain oxidation of ar-
omatic amines in polymer films was studied by Kolninov et
al.4
Moreover, it has been known for a long time that car-
bon tetrachloride and benzene associate in solution.5 The
formation of CTC was proposed between carbon tetrachlo-
ride and benzene with different molar ratios including 1:1
and 2:1.6-9
EXPERIMENTAL
Materials
Carbon tetrachloride was distilled in the presence of
anhydrous CaCl2, and the middle cut was taken.16 Benzene,
* Corresponding author. E-mail: sifekhdr@hotmail.com

1444 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007
Sife-Eldeen and Ebraheem
(a) UV spectrophotometric study
analar grade, from BDH was used as received.
UV spectra of samples, containing different CCl4-
C6H6 molar ratios in cyclohexane, are shown in Fig. 1. CCl4
mole % increases, in the direction of the arrow, in the order
0, 10, 20, 30, 40, 50, 60,70, 80 and 90. In this figure, it is
clear that new shoulders are observed at 248 and 254 nm in
the presence of CCl4. These shoulders are not related to ei-
ther CCl4 or C6H6 UV spectra. The appearance of these
shoulders is a criterion for the formation of a charge trans-
fer complex.20,21
Irradiation
Samples were prepared by introducing 2 mL of
freshly prepared reaction mixture into ampoules. The am-
poules were then deareated by freeze-evacuated-thaw cy-
cles. The ampoules were then sealed off. The sealed sam-
ples were irradiated at ambient temperature by a Co-60
gamma irradiator with a cylindrical irradiation chamber
(dose rate = 8.02 kGy/h).16
The stoichiometry of this charge transfer complex
could be determined according to Job’s continuous varia-
tion method.22 The absorbance values at 248 and 254 nm
are plotted as a function of benzene mole % in CCl4-C6H6
samples diluted with cyclohexane (Figs. 2 and 3, respec-
tively). The total number of CCl4 and C6H6 moles remained
constant (1.35 ´ 10-2 mol/L).
UV spectroscopic studies
A matched pair of silica cuvettes (path length, 1 cm)
was used in order to cancel out cyclohexane absorbance
and to correct for variations in dielectric constant of the so-
lutions. UV spectra were recorded for CCl4-C6H6 samples
with different molar ratios diluted with cyclohexane(spec-
troscopic grade) against blank cyclohexane on a Jasco
spectrophotometer (model V560, made in Japan) at room
temperature.
NMR measurements
1H-NMR spectra were measured with a BRUKER
300 MHz NMR spectrometer according to the external ref-
erence method.17-19 The chemical shifts of benzene pmr
signal for different molar ratio unirradiated CCl4-C6H6 so-
lutions should be referred to a common external reference
outside of each sample solution. Deuterated dimethyl
sulfoxide (DMSO) was used as an (external reference).
DMSO, sealed in a cylindrical capillary tube, was inserted
into the sample tube.
Fig. 1. UV spectra of samples containing different
CCl4-C6H6 molar ratios in cyclohexane (CCl4
% increases in the direction of the arrow in the
order 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90).
GC Analysis of chlorobenzene
The GC analytical method used for quantitative de-
termination of chlorobenzene has been previously de-
scribed in detail.16
RESULTS AND DISCUSSION
Formation of the charge transfer complex
Studies on the formation of 1:1 and 2:1CCl4-C6H6
CTC were carried out previously by several authors.6-9 In
the current study the formation of CTC between carbon tet-
rachloride (CCl4) and benzene (C6H6) is confirmed by new
evidence.
Fig. 2. Dependence of the UV absorbance at 248 nm
on the benzene mole % in CCl4-C6H6 samples
diluted with cyclohexane.

CTC Role in Formation of ClPh in g-Irradiated CCl4-C6H6 System
J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 1445
In both Fig. 2 and Fig. 3 there is a break point (x). This
break point (x) corresponds to about 2:1CCl4-C6H6 molar
ratio (33% benzene mole). Therefore, according to this re-
sult it could be concluded that 2:1CCl4-C6H6 charge trans-
fer complex (2:1CTC) is formed between benzene and car-
bon tetrachloride.
association in a CTC.
In the current study 1H NMR spectra for different mo-
lar ratio samples of CCl4-C6H6 were investigated. It is well
known that the 1H NMR chemical shift of benzene refer-
ring to DMSO as an internal reference appears at 7.37
ppm.27 But in the current work the 1H NMR chemical shift
of benzene referrs to DMSO as an external reference (see
experimental part). Therefore, 1H NMR chemical shifts of
100% and 33% in benzene samples, under these conditions,
were observed at 6.6205 and 6.838 ppm, respectively, (Fig.
4), whereas a down field shift is observed in the presence of
CCl4 in the sample.
1
(b) H NMR Study
Charge transfer complexes have been studied by dif-
ferent methods including nuclear magnetic resonance
(NMR) spectroscopy.23-26 NMR is particularly sensitive to
small changes in the electronic environment of a magnetic
nucleus that can occur upon intimate contact or short-range
The dependence of the benzene 1H NMR chemical
shifts of unirradiated CCl4-C6H6 samples on benzene mole
% is illustrated in Fig. 5. A down field shift increases with
the increase of carbon tetrachloride mole %, i.e. decrease of
the benzene mole %.
This dependence gives evidence for the formation of
CTC, whereas the change in the NMR chemical shift for a
signal according to donor-acceptor mole %, is considered
as an indication for the formation of the CTC.28-34 A break
point (x) is observed in Fig. 5 corresponding to about 2:1
CCl4-C6H6 molar ratio (33% benzene mole). This observa-
tion also indicates that the stoichiometry of the CCl4-C6H6
charge transfer complex is 2:1.
Fig. 3. Dependence of the UV absorbance at 254 nm
on benzene mole % in CCl4-C6H6 samples di-
luted with cyclohexane.
Fig. 4. 1H NMR spectra of the benzene-carbon tetrachloride samples measured under the external reference method. (A)
100% benzene (benzene pmr signal appears at 6.6205 ppm). (B) 33% benzene (benzene pmr signal appears at 6.8381
PPM).

1446 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007
Sife-Eldeen and Ebraheem
Determination of 2:1 CTC association constants (K2:1)
It is apparent from the current stoichiometric studies
that 2:1 CTC is formed in the CCl4-C6H6 system according
to the following equation:
of 1H NMR measurements of benzene in CCl4-C6H6 sam-
ples at different CCl4 concentrations (mol/L) according to
eq. (2).35
1
1
1
=
+
(2)
Ddobs
K2:1[CCl4]2Dd0 Dd0
K2:1
Ddobs represents the difference between the frequency at
which the 100% benzene pmr signal appears and that ob-
served in the presence of different concentrations of carbon
tetrachloride.
2CCl4 + C6H6 € [2CCl4-C6H6]
(1)
2:1 CTC
The association constant K2:1 was determined from a series
Dd0 represents the difference between the frequency
at which the 100% benzene pmr signal appears and that of
100% 2:1 CTC.
According to eq. (2),35 plotting of 1/Ddobs versus
1/[CCl4]2 leads to a linear relationship with a slope of
1/k2:1Dd0 and a y-intercept of 1/Dd0 (Fig. 6). The associa-
tion constant (K2:1) can be readily determined from the ra-
tio of the intercept and the slope. Accordingly, in our case
k
2:1 was found to be 0.0197 M-2.
This linear dependence is also a criterion for the 2:1
stoichiometry of the CCl4-C6H6 complex.36
Dependence of the concentration of 2:1 CTC on ben-
zene mole percentage in the CCl4-C6H6 system
According to equation (1) the equilibrium constant
K2:1 could be defined in mole fraction terms.37,38
1
Fig. 5. Dependence of the H NMR chemical shift of
benzene in carbon tetrachloride-benzene sam-
ples on benzene mole %.
*shift in benzene nmr signal (Hz) = signal fre-
quency in Hz of C6H6 proton in the presence of
CCl4 - signal frequency in Hz of C6H6 proton in
the absence of CCl4.
xctc
(xCCl )2(xC6H
where K2:1=
(3)
)
6
4
knowing that xctc, xCCl and xC6H6 are the mole fractions of
4
Fig. 6. Graph of 1/Dd0 vs. 1/[CCl4]2 according to eq. (2) for the association of CCl4-C6H6.

CTC Role in Formation of ClPh in g-Irradiated CCl4-C6H6 System
J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 1447
2:1 CTC, CCl4 and C6H6 respectively.
Equation (3) could be written in the following con-
centration terms:
plex (2:1 CTC*) could be formed upon gamma radiolysis of
ground state 2:1 CTC:
(2CCl4 -C6H6)
2:1 CTC
(2CCl4-C6H6)*
2:1 CTC*
(5)
w®
2
N2:1 CTC(N0 6 + N0 - 2 N2:1 CTC
)
C H
6
CCl
4
K2:1
=
(4)
(N0
- N2:1 CTC) (N0
- 2N2:1 CTC)
2
C H
CCl
4
6
6
where the N2:1 CTC, N0 6, and N0 4 are the number of 2:1
whereas, excitation of 2:1 CTC (eq. 5) could lead to com-
plete electron transfer from the donor to the acceptor.41
Therefore 2:1 CTC* could dissociate in a manner similar to
that proposed by Budyka et al.1 and Kolninov et al.4 for the
dissociation of the CTC formed between CBr4 and aro-
matic amines. Accordingly, the following equation is pro-
posed:
C H
6
CCl
CTC moles, the initial number of benzene moles, and the
initial number carbon tetrachloride moles, respectively. As
the value of K2:1 , the initial concentrations of benzene and
carbon tetrachloride are known; the concentration of 2:1
CTC could be determined at different benzene mole % by
solving equation (4) mathematically.
The dependence of the 2:1 CTC concentration on
benzene mole % is shown in Fig. 7. The maximum concen-
tration of 2:1 CTC is obtained at 2:1 carbon tetrachlo-
ride-benzene molar ratio (33% mole of benzene).
k1
*
(2CCl4-C6H6) ® (C6 H6 , CCl3 , Cl , CCl4) (6)
+
.
–
×
The following ion recombination reaction was pro-
posed before:16
Dependence of the chlorobenzene concentration on
benzene mole percent in gamma irradiated CCl4-
C6H6 samples
k2
+
C6 H6 + Cl ® C6H6Cl
-
×
×
(7)
Different molar ratios of carbon tetrachloride-ben-
zene samples were gamma irradiated with the same ab-
sorbed dose (44.1 kGy), and the amount of chlorobenzene
formed was determined by GC analysis. It is apparent from
Fig. 8. that the maximum chlorobenzene yield (expressed
in mmol/L) was obtained also at a 2:1 carbon tetrachlo-
ride-benzene molar ratio (33% mole of benzene).
As the maximum concentration of 2:1 CTC was
found also at the 2:1 carbon tetrachloride-benzene molar
ratio Fig. 7, therefore it could be concluded that 2:1 CTC
participates in the mechanism of chlorobenzene formation.
Therefore, excited 2CCl4-C6H6 charge transfer com-
.
It is well known that CCl3 tends to abstract a hydro-
gen atom.39 Therefore, it could be supposed that CCl3
.
formed in reaction (6) abstracts a hydrogen atom from
C6H6Cl., formed according to reaction (7), as follows:
k3
×
C6H6Cl + CCl3 ® C6H5Cl + HCCl3
×
(8)
Accordingly, the process through which 2:1 CTC par-
ticipates in the mechanism of C6H5Cl formation could be
summarized as:
Fig. 8. Dependence of the chlorobenzene concentra-
tion (mmol/L) on benzene mole percent in g-ir-
radiated CCl4-C6H6 samples.
Fig. 7. Dependence of 2:1 CTC concentration on ben-
zene mole % in CCl4-C6H6 system.

1448 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007
Sife-Eldeen and Ebraheem
(2CCl4-C6H6)
2:1 CTC
(2CCl4-C6H6)* ®
C6H5Cl formation could be third order. Fig. 9 shows the de-
pendence of the chlorobenzene concentration (determined
by gas chromatographic analysis), formed by g-irradiation
of 2:1 CCl4-C6H6 molar ratio samples, on irradiation time
according to third order kinetic equation:40
w®
2:1 CTC*
–
+
(C6H6 , CCl3 , Cl , CCl4) ® C6H5Cl +…… (9)
×
×
It should be mentioned that chlorobenzene is the main
product formed upon radiolysis of the CCl4-C6H6 system.
Other products (minor products) formed during radiolysis
of this system were hexachloroethane, (trichloromethyl)
benzene and biphenyl.16 Another mechanism for the forma-
tion of chlorobenzene upon g-radiolysis of benzene-carbon
tetrachloride system has been discussed, previously, by the
first author.16 In that mechanism, it was mentioned that
t = 1/k(2b - a)2 [(2b - a) 2x/a (a - 2x)
+ ln b (a - 2x)/a(b - x)]
where a, b are the initial concentrations of CCl4 and C6H6
respectively (mol/L).
×
CCl3 could be formed also by dissociative electron cap-
x is the concentration (mol/L) of C6H5Cl (main prod-
uct) at elapsed time t
ture, eq. (10), and decomposition of excited neutral carbon
tetrachloride, eq. (11):
A straight line plot, according to a third order kinetic
equation with a very good linearity (R2 = 0.9977), is shown
in Fig. 9. This observation supports the concept of 2:1 CTC
participation in the mechanism of chlorobenzene formation
upon g-radiolysis of this system. The reaction rate constant
(K) for the formation of chlorobenzene could be calculated
from the slope of the straight line according to the third or-
der kinetic equation: K = 5.5 ´ 10-7 L2 ×mol-2×h-1.
-
e + CCl4 ® CCl3 + Cl
×
CCl4* ® CCl3 + Cl
-
×
(10)
(11)
In the current mechanism, according to equation (8):
d[C6H5Cl]
(12)
×
= k3 [C6H6Cl ][CCl3 ]
×
dt
×
Suppose that CCl3 formed in reaction (6), will mainly
C = 1/(2b – a)2 [(2b – a) 2x/a (a - 2x)
+ ln b (a - 2x)/a(b - x)]
×
abstract a hydrogen atom from C6H6Cl (reaction 8). There-
fore, according to the steady state approximation concept
×
the overall rate of formation-disappearance of CCl3 could
be written as follows:
×
d[CCl3 ]
*
×
×
= k1[(2CCl4-C6H6) ] – k3[C6H6Cl ][CCl3 ]
dt
= 0
(13)
(14)
k1[(2CCl4×C6H6)*]
×
\ [CCl3 ] =
×
k3[C6H6Cl ]
Fig. 9. Dependence of chlorobenzene yield in g-irradi-
ated 2:1 CCl4-C6H6 samples on irradiation time
according to a third order kinetic equation (C ´
106).
By substitution with [CCl3×], in equation (12), by its
value in (14) to produce the final kinetic equation (15):
d[C6H5Cl]
×
= k3[C6H6Cl ] [CCl3 ]
×
\
dt
= k1[(2CCl4×C6H6)*]
(15)
CONCLUSION
Reaction (15) indicates that the rate of C6H5Cl forma-
tion depends on (2CCl4×C6H6)* concentration. As (2CCl4×
C6 H6)* is considered as a physical aggregation of three
molecules (two CCl4 and one C6H6), the overall reaction of
1- UV and NMR spectroscopic studies confirmed the
formation of carbon tetrachloride-benzene charge transfer
complex.
2- The stoichiometry of carbon tetrachloride-benzene

CTC Role in Formation of ClPh in g-Irradiated CCl4-C6H6 System
J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 1449
1967, 23A, 2749.
charge transfer complex was found to be 2:1 according to
Job’s continuous variation method. The linear dependence
of 1/Dd0 (for benzene proton pmr signal) on 1/[CCl4]2 is
also a criterion for the 2:1 stoichiometry of the CCl4-C6H6
complex. The association constants (K2:1) of this 2:1 charge
transfer complex (2:1 CTC) were found to be 0.0197 M-2.
3- The maximum concentration of 2:1 CTC was
found at 2:1CCl4-C6H6 molar ratio (33% benzene mole per-
cent). At this molar ratio, also, the maximum yield of chlo-
robenzene was obtained upon gamma radiolysis of differ-
ent benzene % samples. This observation indicates that 2:1
CTC participates in the mechanism of chlorobenzene for-
mation during g-irradiation of this system.
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ACKNOWLEDGMENT
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The author would like to express his highest appreci-
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