Kinetic study of the reactions of chlorine atoms and Cl2.- radical anions in aqueous solutions. II. Toluene, benzoic acid, and chlorobenzene

Article abstract of DOI:10.1021/jp004630z

The reactions of chlorine atoms and Cl₂^.- radical ions with toluene, benzoic acid, and chlorobenzene were studied using laser conventional flash photolysis of Na₂S₂O₈ aqueous solutions containing Cl^- ions. The reaction rate constants was diffusion-controlled, thus showed no dependence on the σ-Hammett parameter of the substituent in the aromatic ring. The high reactivity of Cl atoms was distinct with that of Cl₂^.- radical ions, for which the rate constants for the reactions with the substituted benzenes were < 1 × 10^-/M-sec. The π-complex of benzene yielded very low bond energies (< 12 kJ), indicating that, it cannot survive to play any role in a sequence of chemical reactions unless highly stabilized by the solvent. An electron-transfer reaction pathway was not important for the substrates benzene, toluene, and benzoic acid since phenolic derivatives formed from the disproportionation of HO-CHD radicals were not observed as reaction products. The dissimilarity in the behavior of the Cl-CHD radicals in the gas phase and in organic solvents compared with that in the aqueous phase seemed to be the lack of dissociation of Cl-CHD in aqueous solutions.

Full text of DOI:10.1021/jp004630z

J. Phys. Chem. A 2001, 105, 5385-5392
5385
•
-
Kinetic Study of the Reactions of Chlorine Atoms and Cl2 Radical Anions in Aqueous
†
Solutions. II. Toluene, Benzoic Acid, and Chlorobenzene
‡
§
Daniel O. M a´ rtire,* Janina A. Rosso, Sonia Bertolotti, Galo Carrillo Le Roux,
Andr e´ M. Braun, and M o´ nica C. Gonzalez*
|
Instituto de InVestigaciones Fisicoqu ´ı micas Te o´ ricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas,
UniVersidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina
ReceiVed: December 31, 2000; In Final Form: March 14, 2001
-
Laser and conventional flash photolysis of Na
investigate the reactions of chlorine atoms and Cl
A mechanism is proposed which accounts for the faster decay of Cl
2
S
2
2
•
O
8
-
aqueous solutions containing Cl ions were employed to
radical ions with toluene, benzoic acid, and chlorobenzene.
•-
2
in aqueous solutions containing increasing
concentrations of the organic substrates. Interpretation of the experimental data is supported by kinetic computer
simulations. Chlorine atoms react with the three substituted aromatics studied here almost with diffusion-
1
0
-1 -1
controlled rate constants, k ) (1.8 ( 0.3) × 10 M s . The high reactivity observed for Cl atoms contrasts
•
-
with that of the Cl
2
radical ions, for which the rate constant for its reactions with the substituted benzenes
6
-1 -1
is e 1 × 10 M s . The organic radicals produced from these reactions, as well as the nature of the
reaction products are discussed. The observed results seem to support an addition mechanism yielding
chlorocyclohexadienyl radicals (Cl-CHD) as the most significant reaction channel following reaction of Cl
atoms and the organic compounds. In air-saturated solutions, subsequent thermal reactions of Cl-CHD radicals
lead both to chlorination and oxidation of the aromatics.
•
•-
Introduction
The reactions of Cl and Cl2 are studied by laser and
conventional flash photolysis as well as by steady-state pho-
Chloride is one of the most abundant anions in the tropo-
spheric aqueous phase. Chlorine atoms, which in the presence
2
-
tolysis of S2O8 solutions containing various concentrations
of Cl in the presence of substituted benzenes.
1
-
•
-
2,3
of chloride ions reversibly yield Cl2 radical ions, may be
formed in cloud droplets by the reaction of chloride with
strongly oxidizing radicals such as NO3 , SO4 , and HO .
Therefore, oxidation of chloride may represent an important sink
of strong oxidants in tropospheric multiphase systems, depend-
Results and Discussion
•
•-
•
SO4•- radical ions, absorbing in the wavelength range from
300 to 550 nm, were generated by photolysis of aqueous
peroxodisulfate solutions
In the presence of chloride ions at concentrations >10 M,
•
•-
9-11
ing on the final fate of the Cl atoms and Cl2 radicals.
in the pH range 3-4 (reaction 1).
•
•-
•-
-
-4
9
Cl and Cl2 radicals are strong oxidants (E(Cl2 /2Cl ) )
•
-
4,5
•-
2
.0 V and E(Cl /Cl ) ) 2.4 V vs NHE) and may react with
SO4 radicals are readily depleted due to the high efficiency
of reaction 2 yielding chlorine atoms. The reverse reaction
(reaction -2) is of no significance under our experimental
6
many organic compounds. Cl atoms react rapidly with many
species7 and can contribute to the loss of Cl2 under
-9
•-
2
-
atmospheric conditions. In a previous paper on the reactions of
conditions, where SO4 ions of very low concentration are
•
•-
9
•
-
Cl /Cl2 with benzene in water, we have shown that the high
formed only after reaction 2. Reaction of Cl with Cl ions
reversibly yields Cl2 radical ions, reactions 3 and 4.
•
-
•-
reactivity of Cl atoms toward benzene contrasts that of the Cl2
radical anions.
The determination of the reactivity of Cl and Cl2 radicals
toward aromatic compounds is of environmental importance,
as these reactions may both lead to the formation of undesired
chlorinated derivatives and/or promote their oxidation as
observed for benzene. Here we report a kinetic and mechanistic
study on the aqueous phase reactions of Cl2 and Cl with
toluene, benzoic acid, and chlorobenzene.
2
-
+ hν f 2SO4•-
•
•-
S2O
(R,1)
8
SO4•- + Cl f SO + Cl
-
2-
•
4
1/2 2,9
log k ) 8.43 + 1.018 × I
(R,2)
9
•
-
•
•
2-
-
•-
2.1 × 10 M s-1 7b (R-2)
8
-1
Cl + SO f Cl + SO
4
4
•
-
•-
8.5 × 10 M s-1 7
9
-1
Cl + Cl f Cl
2
(R,3)
(R,4)
*
Corresponding authors. Fax: 54 221 425 4642. E-mail: dmartire@inifta.
unlp.edu.ar; gonzalez@inifta.unlp.edu.ar.
†
Cl2•- f Cl + Cl 6.0 × 10 s
•
-
4 -1 2
Part II of series. Part I: Reaction with Benzene. M. L. Alegre, M.
Geron e´ s, J. A. Rosso, S. Bertolotti, A. M. Braun, D. O. M a´ rtire, and M. C.
Gonzalez. J. Phys. Chem. A 2000, 104, 3117.
Time-resolved experiments with [Cl ] > 10 M and [S2O82-
-
-3
]
‡
Current address: Universidad Nacional de Rio Cuarto, C o´ rdoba,
Argentina.
) 25 mM (laser experiments) or 5 mM (conventional flash
experiments) show a transient species with an absorption
maximum at λ ∼ 340 nm, whose spectrum is in agreement with
§
Current address: Escola Politecnica da USPs Departamento de
Engenharia Quimica, S a˜ o Paulo, Brasil.
|
Current address: Lehrstuhl f u¨ r Umweltmesstechnik, Universit a¨ t Karlsru-
•-
12
-
he, D-76128 Karlsruhe, Germany.
that reported for the Cl2 radical ions. In the [Cl ] range used
1
0.1021/jp004630z CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/10/2001

5386 J. Phys. Chem. A, Vol. 105, No. 22, 2001
M a´ rtire et al.
Figure 2. Linear dependence of the first-order decay component (kapp
)
•
-
-
of Cl
2
3
on [CH Ph] for experiments with [Cl ] ) 0.1 M (2), 0.2 M
(0), and 0.4 M (O).
Figure 1. Signals obtained at λobs ) 340 nm by laser flash photolysis
experiments with 25 mM K and 49.2 mM NaCl solutions
containing variable concentrations of chlorobenzene from up to down:
9
to eq 1. The calculated constant g is independent of λ but
linearly depends on [XPh], as shown in Figure 2 for toluene.
The small absorption term h(λ) [< 5% of the initial absorption
2 2 8
S O
0
2
2
, 1.3, 2.17, 2.82, and 3.47 mM. Inset: Idem for solutions containing
•
-
-
of Cl2 , f(λ)] may be associated with a longer lived species
formed after Cl2 depletion.
5 mM K
2 2 8
S O and (a) [ClBz] ) 0 M, [Cl ] ) 0.069 M; (b) [ClBz] )
•
-
-
-
.8 mM, [Cl ] ) 69 mM; and (c) [ClBz] ) 2.8 mM, [Cl ] ) 96.2
mM. The solid lines represent computer simulations (refer to text).
-
g×t
A(λ) ) f(λ) × e
+ h(λ)
(E,1)
5
-1
in the present study and taking K3,4 ) 1.4 × 10 M for the
7
•-
stability constant for the reversible reactions 3 and 4, and
The effect of XPh on the decay rate of Cl2 may be understood
if reactions 9 and 10 efficiently compete with the decay reactions
of Cl2 and Cl , reactions 5 to 8.
•
-1
-1 13
ꢀ
(Cl , 350 nm) ) 3800 M cm , any contribution of chlorine
•
-
•
atoms to the observed absorption traces can be neglected.
The transient decay follows a complex kinetics strongly
Cl2•- + XPh f Cl + organic radical
-
(R,9)
-
depending on [Cl ]. A complete description of the mechanism
can be found in refs 7 and 9. The most significant reactions
•
•
-
accounting for the observed experimental decay of Cl2 are
Cl + XPh f organic radical
(R,10)
•
-
the reversible reactions 3 and 4, the recombination of the Cl2
radical ions (reaction 5), and the complex reversible reaction
•
The total concentration of the organic radicals R formed from
the reaction of XPh with Cl and Cl2 cannot exceed the initial
concentration of sulfate radicals [SO4 ]o, therefore [R ] < 10
•
•-
•
•-
system of Cl /Cl2 with water schematized by reactions 6, -6,
•-
•
-6
7
, -7, and 8.
-
5
M (laser experiments) or < 3 × 10 M (conventional flash
Cl2•- + Cl2 f Cl + 2Cl
•-
-
experiments). Consequently, a diffusion-controlled reaction
2
•
•-
between R and Cl2 , reaction 11, may accelerate the decay
kinetics of Cl2 radical ions. However, in a first approximation,
the contribution of reaction 11 will not be considered.
1
/2 1/2
2,9
log k ) 8.8 + 1.6 × I /(I +1)
(R,5)
(R,6)
•-
Cl2•- + H O f (HOClH) + Cl- 1300 s
•
-1 2,7
2
Cl2•- + R f products
•
(R,11)
•
-
•-
2,9
(
HOClH) + Cl f Cl + H O
(R,-6)
(R,7)
2
2
•
The efficient removal of HO radicals by the substituted
1
4
•
•
5
-1 2,7
benzenes does not allow the attainment of equilibrium
Cl + H O f (HOClH) 2.5 × 10 s
2
conditions for reactions 6 to 8, and reactions of Cl2•- and Cl•
with water may be considered as simple first-order reactions9
with rate constants k6 and k7, respectively. Under these condi-
tions, reactions 6, 7, 9, and 10 are the main reactions depleting
•
2,9
(
HOClH) f Cl + H O
(R,-7)
2
-
H+
•
•-
-
• 2,9
•
-
• 9
-
(
HOClH) {+ H+}(HOCl) a Cl + HO
(R,8)
Cl2 which are in equilibrium with Cl . As K3,4 × [Cl ] > 1
- 9
and k7/K3,4[Cl ] , k6, the apparent first-order decay rate
•
-
constant of Cl2 , kapp, is given by eq 2.
Photolysis of air-saturated aqueous peroxodisulfate solutions
containing chloride ions in the range 0.03 M to 0.6 M and [XPh]
-
3
k10
<
2 × 10 M where XPh stands for the substituted benzenes,
kapp ) k +
+ k9 × [XPh]
(E,2)
6
-
leads to the formation of a transient species whose spectrum
agrees with that of the Cl2 radical ion.
For a given [Cl ], faster decay rates of Cl2 are observed
by addition of increasing amounts of XPh, as shown in Figure
[
K3,4 × [Cl ]
]
•
-
-
•-
In fact, plots of kapp (factor g in eq 1) vs [XPh] fit to eq 2, as
shown in Figure 2 for toluene. The intercepts of the straight
lines in Figure 2 are (939 ( 245), (976 ( 260), and (949 (
1
for chlorobenzene. The experimental absorption traces at a
-
1
-
given wavelength of analysis, A(λ), could be fitted according
243) s for [Cl ] ) 0.1, 0.2, and 0.4 M, respectively. In this

Reactions of Cl Atoms and Cl2•- Radical Anions in H2O
J. Phys. Chem. A, Vol. 105, No. 22, 2001 5387
TABLE 1: Main Products Obtained after Reaction of Cl.
Atoms with XPh after Four Minutes Irradiation
experimental
conditions
substrate
identified productsa
benzoic acid
3-chlorobenzoic acid,
[CO2HPh] ) 7.2 mM
2
-
4-chlorobenzoic acid,
[S2O8 ] ) 5 mM
-
and chlorobenzene
1-chloro-2-methylbenzene,
benzaldehyde, benzyl alcohol, [Cl ] ) 0.5 M
[Cl ] ) 0.5 M
2
-
toluene
[S2O8 ] ) 5 mM
-
benzyl chloride,
-chloro-4-methylbenzene,
and bibenzyl
chlorobenzene phenol, 2 chlorophenol,
[CH3Ph] ) 0.8 mM
1
[S2O82-] ) 5 mM
-
2
2
,4,6-trichlorophenol,
,4,5-trichlorophenol,
[Cl ] ) 0.5 M
[ClPh] ) 0.56 mM
b
and 2,3,6-trichlorophenol
a
Chromatographic peaks with mass spectra corresponding to C
4
-
C
5
fragmentation products containing carbonyl functions, but no Cl
b
atoms were detected for all the substrates. 2,3,4,6-Tetrachlorophenol,
,3,5,6-tetrachlorophenol, hydroxyquinone tetrachloro, and phenol
pentachloro were also detected after irradiation for 8 min .
2
Figure 3. Dependence of the slopes of the plots of kapp vs [XPh] on
-
-1
[
Cl ] for (b) toluene, (4) benzoic acid, and (O) chlorobenzene. Since
•
the organic radical (R ) formed from benzoic acid shows significant
absorption at 340 nm (vide infra), the observation wavelength for the
experiments done with this substrate was 360 nm.
k9 to a value e2 × 10 M s-1, the goodness of the fitting and
the values of k10 and k11 are not changed, also in agreement
with the simplified kinetic analysis. If k11 is fixed to a value of
5
-1
1
0
-1 -1
9
-1
1
s
× 10
M
s , the optimal k10 ) (8.2 ( 1.6) × 10 M
work the error bars stand for a Student’s distribution taking a
-
1
is retrieved though the agreement between simulated and
99% confidence level. Similar results for the intercept values
experimental curves is not good, thus further indicating that
reaction 11 does not significantly contribute to the Cl2• decay.
When the program considers a moderate absorption coefficient
of R (ꢀ (340 nm) e 1500 M cm ) and the decay reactions
observed in oxygen-saturated solution (vide infra), the same
values for k9 and k10 are obtained, within the error bars.
Consequently, the small absorbance of R has no effect on the
simplified kinetic analysis.
Organic Transient Detection. Information on the pathways
involved in the reaction of XPh with Cl may be gained if the
nature of the organic transients (R ) formed after reaction 10 is
established. For this purpose, time-resolved experiments were
performed with high concentrations of organic substrates ([XPh]
10 M), such that reaction 10 is so efficient that Cl atoms
are readily scavenged and the R radical products may be
observed. Due to the very low reactivity of Cl2 radical ions
toward XPh, the contribution of reaction 9 to the observed
transients is expected to be of no significance.
The experimental conditions were carefully chosen. On one
hand, high [Cl ] to [XPh] ratios guarantee a negligible contribu-
tion of the reactions of XPh with SO4 radicals competing with
reaction 2. On the other hand, low [Cl ] yield Cl lifetimes
< 300 µs (see eq 2) necessary for detecting R and obtaining
information on its decay reactions.
Photolysis of XPh was minimized using a XPh saturated
aqueous solution as filter. Blank experiments performed under
identical experimental conditions used for detecting the organic
transients but in the absence of S2O8 showed no signal,
indicating that any contribution of XPh photolysis is negligible
and that the observed transients are mainly formed after reaction
of the curves of kapp vs [XPh] are obtained for benzoic acid
-
and chlorobenzene (data not shown). The intercept values agree,
9
within the experimental error, with reported values for k6. The
•
-1
-1
slopes of the straight lines obtained by plotting kapp vs [XPh]
-
-1
linearly depend on [Cl ] as expected from eq 2 (see Figure
).
Figure 3 shows that, within the experimental error, data
3
•
obtained for the three aromatics fall in the same straight line.
From the slope of the linear function in Figure 3 obtained for
all the substrates, the bimolecular rate constant k10 ) (1.8 (
•
•
1
0
-1 -1
0
.3) × 10
M
s
is obtained. From the intercept k9 is
5
-1 -1
estimated to be (2 ( 4) × 10 M s .
To validate the simplified kinetic analysis (vide supra) and
to evaluate the contribution of reaction 11 to the concentration
-
3
>
•
•-
profiles of Cl2 , a detailed kinetic analysis was performed with
the aid of computer simulations (for details of the program see
Experimental section). For this purpose, the set of reactions
depicted in Table 1 of ref 9 along with reactions 9 to 11 and
•
-
•
•-
15
the reactions of ClPh with HO and SO4 radicals were used.
No equilibrium conditions were assumed a priori for the
reversible reactions 3 and 4 and those involving the reactions
-
•-
-
•-
•-
•
of Cl2 / Cl with water, reactions 6 to 8. The program considers
2
•
•-
the flash emission as a delta function producing SO4 radicals.
•
-
•
-
Simulated Cl2 , Cl , Cl2, and Cl3 concentration profiles were
converted into the corresponding absorption curves and then
added and compared with the experimental data to fit the
complete set of experiments with the substrate chlorobenzene
in a global analysis.
2
-
Initial [SO4•-]o and the reaction rate constants k9, k10, and
k11 were allowed to vary to fit the experimental absorption
profiles. Figure 1 shows the excellent agreement observed
between simulated and experimental profiles. The adjusted
1
0.
Similar considerations were taken in steady irradiation
•
-
[
SO4 ]o agree, within the 15% experimental error, with the
experiments of air-saturated solutions performed to identify the
reaction products. In all cases the experiments showed depletion
of XPh and the simultaneous formation of the products depicted
in Table 1. Due to the particular behavior observed for the
organic transients and reaction products generated from the
values measured in corresponding experiments in the absence
-
10
of Cl and ClPh. The rate constants k10 ) (1.7 ( 0.3) × 10
-1
-1
6
-1 -1
M
s
and k11 ) (0.003 ( 16) × 10 M
s
are obtained in
agreement with the values calculated from the simplified kinetic
•
analysis also considering a negligible rate for k11. For k9 the
reaction of Cl with the different XPh, these will be discussed
separately.
5
-1 -1
value (9 ( 3) × 10 M
s
is obtained. However, constricting

5388 J. Phys. Chem. A, Vol. 105, No. 22, 2001
M a´ rtire et al.
SCHEME 1: Reactions of Benzoic Acid
Figure 4. Absorption spectrum of the organic transients obtained 200
µs after irradiation with a conventional flash lamp, of nitrogen-saturated
of the H-CHD and HO-CHD radicals of benzoic acid, it may
be assigned to a mixture of the three possible isomers of Cl-
CHD radicals of benzoic acid. Cl-CHD radical formation is
further supported by the nature of the observed reaction
products, as shown in Scheme 1.
2 2 8
solutions containing 5 mM K S O and 0.05 M (O) or 0.1 M (b) NaCl
in the presence of 7.2 mM benzoic acid. The dashed line corresponds
to the normalized reported spectrum of the HO-CHD radicals of
1
6
benzoic acid.
Disproportionation of the 3- and 4-Cl-CHD radicals of
benzoic acid with loss of HCl leads to the formation of the
isomers 3- and 4-chlorobenzoic acid, respectively, supporting
an addition/elimination route (reactions 13 and 14). Formation
of 3- and 4-chlorobenzoic acid was not quantified, however,
assuming that their chloroform/water partition coefficients and
GC detector sensitivities are similar to those of benzoic acid,
the total yield of the chlorobenzoic acids accounts for a 30%
the amount of benzoic acid depleted.
Benzoic Acid. Preliminary experiments showed that the
transient species formed after reaction of Cl with benzoic acid
HCO2Ph) presents an absorption maximum close to that of
•
(
•
-
-
Cl2 . Thus, for the optimal experimental conditions [Cl ] )
2-
0
.05 or 0.1 M, [S2O8 ] ) 5 mM and [HCO2Ph] ) 7.2 mM,
•
-
Cl2 lifetime is e 120 µs, and consequently, the transient
spectra were taken 200 µs after the pulse of light in order to
•
-
minimize any contribution of Cl2
.
The observed transient absorbs in the wavelength region from
80 to 460 nm with maximum at around 330 nm, as depicted
in Figure 4. In N2-saturated solutions a second-order decay
•
A further reaction pathway for the reaction of Cl with benzoic
2
acid is H-abstraction from the carboxylic group yielding
benzoylperoxyl radicals. The latter radical shows an absorption
maximum at λ > 800 nm and is known to decarboxylate in
organic solvents yielding phenyl radicals and carbon dioxide
with a first-order oxygen-independent rate constant of the order
kinetics is observed with a bimolecular decay rate of 2k/ꢀmax-
5
-1
(
330 nm) of 1 × 10 s cm. Experiments performed under
identical conditions but in the presence of molecular oxygen
show the same transient spectrum, although with lower absor-
bance.
6
-1 21,22
of 6 × 10 s .
Assuming that benzoylperoxyl radicals
decarboxylate with similar rates in aqueous solutions, their
lifetime should be of fractions of microseconds. The phenyl
The spectrum and kinetic behavior of the organic transient
is typical of cyclohexadyenyl radicals (CHD). In fact, the
hydroxycyclohexadienyl radical (HO-CHD) of benzoic acid,
also shown in Figure 4, shows a single absorption maximum at
radicals thus formed efficiently react with molecular oxygen
3b
(
phenyl lifetime < 3 µs in air saturated solutions)2 yielding
-
1
-1
arylperoxyl radicals which absorb in the wavelength range from
400 to 500 nm and decay by bimolecular recombination with
3
50 nm (ꢀ ) 3600-3800 M cm ) and a bimolecular decay
5
-1
16
rate 2k/ꢀmax(350 nm) of 1 × 10 s cm. The cyclohexadienyl
7
-1 -1 23,24
second-order rate constants < 10 M s .
The observed
radical of benzoic acid (H-CHD) shows an absorption maxi-
mum at around 320-330 nm.1
6,17
transient does not show absorption in this wavelength range,
and we may conclude that the H-abstraction route is of no
significance. This is also supported by the fact that germinate
products resulting from cage recombination of the phenyl/
benzoyloxyl radical pairs (vide infra) were not formed.25 For
the same reasons, an H-abstraction pathway from the aromatic
ring yielding phenyl radicals and HCl should also be of no
significance.
Moreover, HO-CHD radicals
of various substituted benzenes18 and the chlorocyclohexadienyl
radical (Cl-CHD) of benzene11 are known to rapidly and
reversibly react with molecular oxygen, yielding peroxyl radicals
(
reaction 12). Peroxyl radicals (PR) typically exhibit broad
19,20
absorption only at λ < 280 nm.
Consequently, after a few
microseconds the equilibrium condition for reaction 12 is
established and the main effect of oxygen on the CHD spectrum
taken 200 µs after the pulse of light is that of lowering the
absorption.
Chlorobenzene also observed as a reaction product though
in very small yields (<0.4%), may be formed from the reaction
•
-
•
of phenyl radicals with Cl2 /Cl . However, under our experi-
•
-
3
HO-CHD + O a PR
(R,12)
mental conditions, [O2]/[Cl2 ] > 10 , the reaction with O2 is
expected to be an important reaction depleting phenyl radicals.
Since benzoyloxyl radical formation is not supported by our
time-resolved experiments (vide supra), we suggest chloroben-
2
•
Addition of Cl to the aromatic ring of benzoic acid yielding
chlorocyclohexadienyl radicals (ClCHD), reaction 13 in Scheme
9
•
1
, in a reaction similar to that observed for benzene is a possible
zene may be formed after an ipso addition of Cl to benzoic
reaction pathway. On the basis of comparison of the kinetic
and spectroscopic patterns of the observed transient with those
acid yielding an ipso Cl-CHD radical, reaction 15. Subsequent
bimolecular reaction with the other isomers of Cl-CHD radicals

Reactions of Cl Atoms and Cl2•- Radical Anions in H2O
J. Phys. Chem. A, Vol. 105, No. 22, 2001 5389
in air- and O2-saturated solutions and the band at 315 nm under
N2 saturation present similar patterns to that of the HO-CHD
2
6-28
radical of toluene
(see Figure 5). The shoulder observed
at λ > 290 nm in N2-saturated solutions may be due to the
contribution of benzyl radicals which were reported to present
absorption in the 290-250 nm range in aqueous solutions.27
Benzyl radicals irreversibly react with molecular oxygen with
9
-1 -1
a rate constant of 2.8 × 10 M
s
to yield peroxyl radicals
absorbing at λ < 280 nm and thus should not be observed in
air- and O2-saturated solutions. Note that benzyl radicals in
MTHF matrixes show an absorption maximum at 308 and 319
nm. If such absorption is also present in aqueous solutions,
its contribution to the visible absorption band observed in N2-
saturated solutions cannot be discarded.
2
9
Similar absorption spectra were observed for the Cl-CHD
9
and HO-CHD radicals of benzene and also for those of
benzoic acid (vide supra), consequently, the observed transient
in the presence of oxygen may be assigned to Cl-CHD radicals
of toluene. The lower absorbance observed with increasing [O2]
may be explained by a reversible reaction with O2 as observed
Figure 5. Absorption spectrum of the organic transients obtained 250
µs after irradiation with a conventional flash lamp for N
2
(2), air (b)
and O (O)-saturated 5 mM K solutions containing 0.5 M NaCl
2
2 2 8
S O
9
for the Cl-CHD radicals of benzene and benzoic acid (vide
in the presence of 5.6 mM toluene. The solid line corresponds to the
normalized published spectrum of the HO-CHD radicals of toluene
from refs 27 (upper spectrum) and 26, 28] (lower spectrum).
2
0
supra) and for most HO-CHD.
An addition reaction yielding Cl-CHD, reaction 17, and a
subsequent loss of HCl catalyzed by acids, reaction 19, may
also explain the formation of benzyl radicals. The latter reaction
SCHEME 2: Reactions of Toluene
•
pathway resembles the addition of HO radicals to toluene,
where benzyl radicals are formed from the acid-catalyzed
30,31
elimination of water from the HO-CHD radical.
Therefore,
formation of benzyl radicals does not unambiguously support
an H-abstraction reaction from the CH3 group. In fact, kinetic
isotope studies of the reactions of toluene with Cl atoms in CCl4
shows that the isotope effect on the rates of the reaction of Cl
atoms with CH2DC6H5 is very small, thus further supporting
3
2
an addition mechanism.
Disproportionation of the Cl-CHD radicals of toluene yields
toluene and chlorotoluene, reaction 20. Mainly 2- and 4-chlo-
rotoluene were detected as reaction products, in agreement with
3
3
free radical substitutions at an aromatic carbon. Reaction of
Cl-CHD radicals with O2 yields PR which upon subsequent
addition of O2, reaction 21, leads to fragmentation products as
9
,20
reported for most CHD radicals.
Formation of bibenzyl can be explained from the recombina-
34
tion of benzyl radicals, reaction 22. Reaction of benzyl radicals
3
5
with O2 yields peroxyl radicals which undergo subsequent
disproportionation yielding benzyl alcohol and benzaldehyde,
reactions 23 and 24. Reaction of benzyl radicals with Cl2• leads
to the formation of benzyl chloride and chloride ions, reaction
-
yields either chlorobenzene upon elimination of formic acid,
as shown in reaction 16, or benzoic acid when HCl is eliminated
2
5.
(not shown).
Photochlorination of toluene by chlorine in organic solvents
Reaction of Cl-CHD radicals with molecular oxygen leads
to oxidized fragmentation products, as will be discussed for the
other substrates (vide infra).
at low temperatures and high chlorine concentrations is also
•
reported to undergo an attack of Cl , both, on the ring and on
the side-chain C-H bonds, with the former being highly
Toluene. The absorption spectra of the transient obtained at
3
6
reversible.
2
50 µs after the flash of light in N2-, air-, and O2-saturated
2-
-
solutions containing 7.3 mM S2O8 , 0.5 M Cl , and 5.6 mM
CH3Ph is shown in Figure 5. In N2-saturated solutions the
spectrum shows a sharp peak at about 310-315 nm and a
shoulder below 290 nm. In the presence of molecular oxygen
but otherwise identical experimental conditions, the transient
spectrum shows lower absorbance and a less structured band
with a maximum at around 305-315 nm.
Chlorobenzene. The absorption traces of the organic inter-
mediate of chlorobenzene were obtained in experiments with
2
-
-
[S2O8 ] ) 0.01 M, [Cl ] ) 0.05 M, and [ClPh] ) 4.46 ×
-
3
10 M. The transient spectrum taken 150 and 500 µs after the
flash pulse shows an absorption maximum around 310-320 nm,
both for N2- and air-saturated solutions, as shown in Figure 6.
In N2-saturated solutions a second-order decay kinetics is
observed with a bimolecular decay rate of 2k/ꢀmax(320 nm) of
•
The reaction of Cl with toluene may proceed either by
5
-1
addition to the aromatic ring yielding Cl-CHD radicals, reaction
4 × 10
s
cm. Experiments performed under identical
1
7 in Scheme 2, or by H-abstraction from the CH3 group
conditions but in the presence of molecular oxygen show the
same transient spectrum although with lower absorbance and
yielding benzyl radicals, reaction 18. The observed spectrum

5390 J. Phys. Chem. A, Vol. 105, No. 22, 2001
M a´ rtire et al.
SCHEME 4: Reactions Leading to the Formation of
Trichlorophenol
experimental conditions for product detection, Cl-CHD radicals
readily decay by reactions other than disproportionation.
Cl-CHD radicals may lose HCl to yield an unstable
chlorobenzene radical cation which efficiently adds water to
yield HO-CHD radicals (reactions 29 and 28). The latter
radicals undergo subsequent disproportionation to chlorophenol
(an observed minor reaction product) and chlorohydroxycyclo-
hexadienes, which may either eliminate HCl yielding phenol
or eliminate water leading to chlorobenzene (reactions 31 and
Figure 6. Absorption spectrum of the organic transients obtained 150
µs and 500 µs (black and white symbols, respectively) after irradiation
with a conventional flash lamp for N
saturated 5 mM K and 0.05 M NaCl solutions in the presence of
.5 mM chlorobenzene. The dashed line corresponds to the normalized
2
- (squares) and air- (triangles)
2 2 8
S O
4
reported spectrum of the HO-CHD radicals of chlorobenzene. Inset:
Absorption traces obtained at 320 nm in (a) N
solutions.
2
- and (b) air-saturated
3
2, respectively). Otherwise, the reaction of HO-CHD with
O2 leads to the formation of oxidized open chain products (vide
supra). HO-CHD radical disproportionation reactions in O2-
containing solutions were reported not to contribute significantly
to product formation, in agreement with the relatively small
peaks of chlorophenol observed.
SCHEME 3: Reactions of Chlorobenzene
3
8
As HO-CHD radicals may be formed both, from the addition
and electron tranfer pathways (vide supra), the contribution of
the electron-transfer route cannot be discarded for this substrate.
Formation of tri- and tetrachlorophenols may occur from the
subsequent diffusion-controlled H-abstraction reactions by Cl
atoms from the HO group of chlorophenol yielding chlorophe-
noxyl radicals which upon subsequent Cl addition leads to an
39,40
increased chlorination of the phenol.
The latter reaction path
seems unlikely since dichlorophenol, an intermediate product
in the chlorination pathway to trichlorophenol, is not formed
in detectable yields and high ratios of trichlorophenols to
chlorophenol are already observed at short irradiation times.
These observations suggest that trichlorophenols are most
probably formed during the primary steps of the reaction. A
possible reaction pathway, shown in Scheme 4 for the para Cl-
CHD isomer, involves a reversible reaction of Cl-CHD radicals
with molecular oxygen yielding endoperoxides, which are
stabilized by subsequent rearrangement, intramolecular H-
faster decay kinetics (shown in Figure 6 inset), as expected from
a reversible reaction with O2 yielding peroxyl radicals (vide
infra).1
8,19
The transient absorption spectrum, its bimolecular decay and
the reversible reaction with O2, are characteristic of the CHD
radicals and resemble those of the HO-CHD radical of
chlorobenzene,37 except for the higher absorption observed at
shorter wavelengths, as shown in Figure 6. The polychlorinated
phenols detected as reaction products (depicted in Table 1)
indicate a complex reaction system where both, the Cl-CHD
and HO-CHD radicals of chlorobenzene are involved. Con-
sequently, the observed transient may be assigned to the
contribution of all the isomeric forms of both CHD radicals.
•
•-
abstraction and reaction with Cl /Cl2 to yield trichlorophenols,
reaction 35. Otherwise, subsequent addition of O2 to the
endoperoxides leads to the rupture of the cycle yielding oxidized
3
8
fragmentation products, reaction 36.
Conclusions
The rate constants k10 determined here in aqueous solution
The CHD radicals may be formed from two different reaction
for XPh: toluene, benzoic acid, and chlorobenzene and that of
•
channels: addition of Cl to the aromatic ring of ClPh yields a
9
benzene, as well as those reported in the literature in CH2Cl2
Cl-CHD, reaction 26 in Scheme 3, and a charge transfer from
for phenol, aniline, dimethylaniline, dimethoxybenzene, and
•
the aromatic ring to Cl yielding an unstable radical cation which
39
hexamethylbenzene all fall within the experimental error in
subsequently hydrolyses to a HO-CHD radical, as shown in
reactions 27 and 28.
10
10
-1 -1
the range from 1.2 × 10 to 2.5 × 10
M
s . The reaction
rate constants being diffusion-controlled, show no dependence
Dichlorobenzene formed from the disproportionation of the
Cl-CHD radicals of ClPh is not observed. Thus, under our
on the σ-Hammett parameter of the substituent in the aromatic
ring. The high reactivity of Cl atoms contrasts with that of Cl2
38
•-

Reactions of Cl Atoms and Cl2•- Radical Anions in H2O
radical ions, for which the rate constants for the reactions with
J. Phys. Chem. A, Vol. 105, No. 22, 2001 5391
H-abstraction from OH groups and/or to undergo electron
transfer with the anions of aliphatic alcohols, rather than
6
-1 -1
the substituted benzenes are < 1 × 10
M
s . This
8
observation is in agreement with studies involving aliphatic
abstracting hydrogen from C-H bonds.
6
12
organic compounds reported by Gilbert and Buxton. Com-
•
parison of the reactivity of Cl with some reported rate constants
Experimental Section
•
-
for the reactions of Cl2 indicates that, in many cases, these
rate constants are largely accounted for by the fraction of Cl
Materials. Potassium peroxodisulfate (Merck), sodium chlo-
ride (Merck p.a.), chlorobenzene (Merck p.a.), toluene (Merck
p.a.), and benzoic acid (Merck p.a.) were used as received.
•
-
atoms present in equilibrium with Cl2 , as already discussed
in refs 7 and 12.
No evidence for the formation of a π-complex,4
-
1
Distilled water (>18Ω cm , < 20 ppb of organic carbon) was
obtained from a Millipore system.
1-43
absorbing
at 490 nm with a lifetime of 1 to 10 µs was found. We may
therefore conclude that a π-complex between the aromatics and
Cl atoms is either not formed or decomposes in aqueous
solutions in less than 50 ns to yield Cl-CHD radicals. The same
Time-Resolved Experiments. Flash photolysis experiments
were carried out using a conventional equipment (Xenon Co.
model 720C) with modified optics and electronics (optical path
length 11 cm). Laser experiments were performed with a
Spectron SL400 Nd:YAG system generating 266 nm pulses (∼8
ns pulse width, optical path length 1 cm).
Preparative Experiments. A cylindrical low-pressure Hg
lamp (Heraeus, MNNI 35/20, Germany) emitting at 254 nm
was used. The photochemical reactor was of annular geometry
volume: 400 mL, annular optical path of 1 cm). The whole
reactor was immersed in a thermostat controlling the temperature
at 25 ( 1 °C.
A more complete description of the experimental setup for
time-resolved and preparative experiments can be found in refs
9
behavior was observed for the substrate benzene. Very low
bond energies (<12 kJ) were calculated for the π-complex of
benzene,44 thus indicating that, unless highly stabilized by the
solvent, it cannot survive to play any role in a sequence of
chemical reactions. Further evidence supporting the formation
of adducts with Cl atoms other than a loosely bound π-complex
(
4
6
are the formation of a three-electron complex, Me2S∴ Cl and
Cl-CHD radicals of pyridine and azarenes.4
,48
Chlorine atom addition to the aromatic ring of the XPh studied
9
here and to benzene yielding a Cl-CHD radical, seems to be
an important reaction channel for reaction 10. However, neither
the contribution of an H-abstraction reaction pathway from the
CH3 group of toluene nor an electron transfer from the aromatic
ring of ClPh to Cl atoms can be unambiguously neglected. An
electron-transfer reaction pathway is of no significance for the
9
and 50.
2-
Methods. Unless otherwise indicated, typical S2O8 con-
-3
centrations were 5.0 × 10 M for conventional flash photolysis
-2
and 2.5 × 10 M for laser experiments. The pH of the samples
was controlled with a Methrom-Herisau pH meter model E512.
The pH of the peroxodisulfate solutions was approximately 4
due to the acid content incorporated with the K2S2O8, containing
water and acid as impurities. The ionic strength of the solutions
was within the range of 0.005 to 0.2 M.
9
substrates benzene, toluene and benzoic acid, since phenolic
derivatives formed from the disproportionation of HO-CHD
radicals were not observed as reaction products. An electron-
transfer mechanism was also proposed for N,N,N′,N′-tetram-
ethyl-p-phenylenediamine and dimethoxy benzene in dichlo-
Solutions of the aromatics were prepared by dilution of their
saturated aqueous solution at 25 °C. For experiments performed
in the absence of molecular oxygen, the saturated solutions of
the organic substrates and the water used for dilution were
bubbled with N2.
Organic products were extracted from the aqueous solutions
with a fixed volume of chloroform and the extracts stored in
glass vials with PTFE/silicone septum-lined screw caps and
minimized headspace. Analysis of the extracts was performed
by gas chromatography with a HP 6890 chromatograph equipped
with a fused silica HP5-MS GC capillary column and coupled
to an HP 5973 mass selective detector. The analysis was
performed by using a temperature program starting at 80 °C
and ending at 200 °C at a rate of 10 °C/min and held at 200 °C
for 5 min. Helium was used as carrier gas with a flow rate of
3
9
romethane.
Cl-CHD adduct formation in the photochemical chlorination
of benzene was already postulated in the fifties supported by
45
trapping experiments with maleic anhydride. Cl-CHD forma-
•
tion is also a contributing pathway in the reaction of Cl with
benzene in the gas phase though its main fate is decomposition
to reform the reactants.44
Formation of Cl-CHD radicals was proposed to be the main
species responsible for the increased selectivity in the abstraction
of tertiary with respect to primary hydrogen observed in the
chlorination of alkanes in aromatic solvents.4
5-47
Moreover, the
apparent reactivity of such Cl-adducts toward 2,3-dimethylbu-
tane, as observed in time-resolved experiments, was reported
•
49
to reflect the extent of their reversible dissociation to free Cl .
3
The difference in the behavior of the Cl-CHD radicals in
the gas phase and in organic solvents compared with that in
the aqueous phase seems to be the lack of dissociation of Cl-
CHD in aqueous solutions. A possible explanation for the
apparent mechanistic differences is that Cl-CHD is always
formed in each phase but it does not significantly dissociate on
the time scale of our observations (see ref 9). This indicates
that the role of the aqueous medium is that of stabilizing the
Cl-CHD radicals.
29 cm /s. Injection volumes were of 20 µL.
Kinetic Computer Simulations. The model is based on
component balances and equilibrium equations that are formu-
lated in terms of a differential algebraic equations system (DAE).
The DAE system is solved by a modified version of the LSODI
51
52
routine based on Gear’s Stiff method. The version imple-
mented performs the solution of the DAE system concomitantly
to the evaluation of the parametric sensitivities based on the
53
decoupled direct method. In consequence, the simulation of
The contribution of H-atom abstraction from the aromatic
ring yielding reactive phenyl radicals is not supported by our
studies, since aryl peroxyl radical formation from the reaction
of phenyl with O2 (vide supra), neither its reaction products
were observed. The fact that H-abstraction from the aromatic
ring is not an important reaction channel is in line with the
reported behavior of Cl atoms showing a strong preference for
the system is robust, such that it can be carried out with success
for very different parameter values, while the sensitivities
obtained are accurate and obtained without much computational
effort. The estimation criterion used to solve this problem is
the least squares. The algorithm for minimization of the criterion
5
4
is based on the rotational discrimination and provides ad-
ditional features for assessing the identifiability of the problem.55

5392 J. Phys. Chem. A, Vol. 105, No. 22, 2001
M a´ rtire et al.
Acknowledgment. This research was supported by Funda-
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Tecnol o´ gica, Argentina (ANPCyT), Consejo Nacional de In-
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and the Deutsche Akademische Austauschdienst, Germany
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