π-Complexes as "Modulators" of Bromine Atom Reactivity in Solution

Full text of DOI:10.1021/jo991806o

1892
J . Org. Chem. 2000, 65, 1892-1894
π-Com p lexes a s “Mod u la tor s” of Br om in e
Atom Rea ctivity in Solu tion
Mo´nica Barra* and Kevin Smith
Department of Chemistry, University of Waterloo, Waterloo,
Ontario, Canada N2L 3G1
Received November 22, 1999
In tr od u ction
It is well-established that halogen atoms form π-mo-
lecular complexes with aromatic solvents,1 in which the
aromatic compound acts as an electron donor and the
halogen atom as an electron acceptor, and hence the
π-complex has a certain charge-transfer character. For-
mation of halogen atom/aromatic molecular complexes
has been shown to influence significantly the selectivity
and reactivity of reactions such as free-radical chlorina-
tion.2
F igu r e 1. Transient absorption spectra for halogen atom/
benzene complexes recorded upon 355-nm laser excitation of
R-bromoacetophenone (A) and R-chloro-acetophenone (B) in
benzene (O) and in 9:1 (v/v) acetonitrile:benzene (b) solutions.
Very few equilibrium constants for these complexes
have been determined.2,3 Furthermore, in contrast to the
case of chlorine atoms, and of reactions in the gas phase,
there are only a handful of reports on absolute rate
constants for hydrogen abstraction by bromine atoms in
solution.4-8 The work described herein focuses on a
quantitative analysis of bromine atom/aromatic π-com-
plex formation and its effect on the reactivity of hydrogen
abstraction by bromine atoms in solution.
Resu lts a n d Discu ssion
Bromine atoms are generated upon photochemical
dehalogenation of R-bromoacetophenone. In neat ben-
zene, bromine atom/benzene π-complex formation leads
to an absorption band centered at 550 nm (Figure 1), in
excellent agreement with literature values.9 However, as
the polarity of the medium increases, a hypsochromic
(blue) shift of the wavelength of maximum absorption
(λmax) is observed (e.g., λmax 530 and 510 nm in acetoni-
trile/benzene 82:18 v/v and 99:1 v/v, respectively). This
hypsochromic shift suggests that solvation of the π-com-
plex in the ground state would result primarily from
localized interactions with the uncharged component
species, rather than from the dipole due to the contribu-
tion from the charge-transfer structure, as indeed ob-
served in the case of several charge-transfer complexes
formed from neutral species.10 Interestingly, no signifi-
cant shift could be detected in the λmax corresponding to
F igu r e 2. Dependence of the change in relative absorbance
for bromine atom/benzene π-complex as a function of benzene
concentration. Inset: double reciprocal plot.
chlorine atom/benzene π-complexes when working under
the same conditions (Figure 1).
Formation of bromine atom/aromatic π-complexes takes
place instantaneously (i.e., within the time resolution of
our system ca. 20 ns). When the signal intensity (∆A)
corresponding to π-complexes in acetonitrile solution is
recorded as a function of the concentration of aromatic
species (such as chlorobenzene, benzene, and tert-butyl-
benzene) a nonlinear dependence is observed (Figure 2
is representative). The double reciprocal plots are indeed
linear (Figure 2, inset) in agreement with a 1:1 interac-
tion. Nonlinear fittings of ∆A values vs concentration of
aromatic species according to the well-known Benesi-
Hildebrand equation11 lead to values of equilibrium
constants for π-complex formation (Keq) of (0.8 ( 0.2), (2.2
( 0.3), and (4.6 ( 0.8) M-1 for chlorobenzene, benzene,
and tert-butylbenzene, respectively. These Keq values
agree very well with those determined from the ratio
between the intercept and the slope of the double-
(1) Bu¨hler, R. E. Rad. Res. Rev. 1972, 4, 233.
(2) Ingold, K. U.; Lusztyk, J .; Raner, K. D. Acc. Chem. Res. 1990,
23, 219.
(3) J arzeba, W. J . Mol. Liq. 1996, 68, 1.
(4) Lilie, J .; Koskikallio, J . Acta Chem. Scand., Ser. A 1984, 38, 41.
(5) Shoute, L. C. T.; Neta, P. J . Phys. Chem. 1990, 94, (a) 2447, (b)
7181.
(6) Lind, J .; J onsson, M.; Shen, X.; Eriksen, T. E.; Mere´nyi, G.;
Eberson, L. J . J . Am. Chem. Soc. 1993, 115, 3503.
(7) Scaiano, J . C.; Barra, M.; Kryzwinski, M.; Sinta, R.; Calabrese,
G. J . Am. Chem. Soc. 1993, 115, 8340.
(8) Mere´nyi, G.; Lind, J . J . Am. Chem. Soc. 1994, 116, 7872.
(9) McGimpsey, W. G.; Scaiano, J . C. Can. J . Chem. 1988, 66, 1474.
(10) Foster, R. Organic charge-transfer complexes; Blomquist, A. T.,
Ed.; Academic Press: New York, 1969; Chapter 3.
(11) Benesi, H. A.; Hildebrand, J . H. J . Am. Chem. Soc. 1949, 71,
2703.
10.1021/jo991806o CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/01/2000

Notes
Ta ble 1. Secon d -Or d er Ra te Con sta n ts (kH) for
J . Org. Chem., Vol. 65, No. 6, 2000 1893
Sch em e 1
Hyd r ogen Abstr a ction by Br om in e Atom s in Va r iou s
Solven ts
a
kH (106 M-1 s-1
)
CH3CN/
C6H6
9:1 v/v
CH3CN/
t-BuC6H5
9:1 v/v
c
c
CH3CNb
C6H6
c
H-donor
methanol
1.6 ( 0.8 0.49 ( 0.02 0.44 ( 0.02
d
(1.0)
24 ( 1
(0.14)
32 ( 3
(0.05)
70 ( 6
(0.025)
(1.6)
6.4 ( 0.2
(0.45)
9.0 ( 0.3
(0.5)
(1.6)
5.6 ( 0.4
(0.16)
8.5 ( 0.2
(0.25)
diethyl ether
2-propanol
toluene
0.46 ( 0.03
(0.90)
In the presence of 10% (v/v) benzene or tert-butylben-
zene in acetonitrile solution, the second-order rate con-
stant kH for H-abstraction can be related to the equilib-
rium constant for π-complex formation (Scheme 1)
according to eq 1.
0.54 ( 0.04
(1.2)
19.5 ( 0.6
(0.055)
17.4 ( 0.5 0.64 ( 0.03
(0.06) (0.85)
a
Maximum molar concentration of H-donor used given in
b
•-
d
parentheses. Br2 as probe. c π-Complex as probe. Reaction
k1
k2Keq[XC6H5]
kH
)
+
(1)
was too slow to determine an accurate value.
1 + Keq[XC6H5] 1 + Keq[XC6H5]
reciprocal plots. The changes observed in Keq values as a
function of the type of aromatic π-donor are interpreted
in terms of electronic effects (i.e., Keq increases in the
presence of an electron-donating group such as t-Bu,
whereas a decrease is observed in the presence of an
electron-withdrawing group such as Cl). Not surprisingly,
Keq values are solvent dependent, and solvent effects can
be interpreted in terms of a balance between the ionizing
power and the complexation ability of the solvent. For
benzene π-complex formation, Keq in cyclohexane (a less
ionizing solvent than acetonitrile) is found to be (1.4 (
0.2) M-1, whereas in dichloromethane (complexing sol-
vent)12 the plot (not shown) of ∆A vs benzene concentra-
tion ([benzene] e 1 M) is essentially linear, indicating
The values of kH determined in acetonitrile solution
using Br2 as probe correspond indeed to k1, and are in
•-
excellent agreement with data in the literature.7 The
values of kH determined in benzene solution, on the other
hand, do not necessarily correspond to the reactivity of
the 1:1 π-complex, since in neat benzene complexes of
higher stoichiometry may most likely exist.15 The de-
crease in reactivity in the presence of aromatic solvents
clearly indicates that k2 is significantly lower than k1.
Assuming the second term of eq 1 to be negligible for 9:1
(v/v) acetonitrile/benzene and acetonitrile/tert-butylben-
zene solutions, one obtains kH/k1 ) (1 + Keq[C6H6])-1
.
From the ratio kH/k1, Keq values of (2.3 ( 0.4) M-1 and
(4.6 ( 0.6) M-1 are calculated for benzene and tert-
butylbenzene, respectively, in excellent agreement with
Keq values determined from absorption data (vide supra).
These results support the assumption that the second
term of eq 1 is negligible and clearly show that aromatics
act as modulators of bromine atom reactivity by control-
ling bromine atom concentration via complexation.
When toluene is used as H-donor in acetonitrile solu-
tion, at [toluene] > 0.01 M the corresponding π-complex
can be detected immediately after laser excitation. In fact,
bromine atom/toluene π-complex can be detected even in
neat toluene (λmax ) 535 nm, spectrum not shown) with
a lifetime of 172 ns. The latter is in very good agreement
with the lifetime estimated from the second-order rate
constant measured in neat benzene, i.e., 1/(6.4 × 105 M-1
s-1 × 9.39 M) ) 166 ns. Alternatively, if an intramolecu-
lar H-abstraction were to take place within toluene
π-complex, the second-order rate constants shown in
Table 1 for acetonitrile and benzene solutions would
represent the product between the corresponding equi-
librium constant for bromine atom/toluene π-complex
formation (assumed to be similar to that for tert-butyl-
benzene π-complex) and the first-order rate constant for
intramolecular H-abstraction. A first-order rate constant
K
eq e 0.1 M-1. This observation is in good agreement with
the value reported for bromine atom/benzene π-complex
formation in carbon tetrachloride (i.e., 0.2 M-1).3
Observed bromine atom reactivity (kobs) was deter-
mined by means of laser-flash photolysis, using the probe
technique13 based on the formation of detectable com-
plexes between bromine atoms and (i) aromatic com-
pounds (π-complex) or (ii) bromide anions (Br2•-). The
latter are characterized by having a strong absorption
band centered at 360 nm.14 In the presence of 10% (v/v)
benzene or tert-butylbenzene in acetonitrile, no change
could be detected in the second-order rate constant for
formation of Br2•- (i.e., (2.1 ( 0.3) × 1010 M-1 s-1), which
indicates that the reactivity of “free” and complexed
bromine atoms toward bromide anions in acetonitrile
solution are rather similar (i.e., diffusion controlled). In
neat benzene, the second-order rate constant for forma-
•-
tion of Br2 is (3.4 ( 0.2) × 109 M-1 s-1
.
Rates of hydrogen abstraction by bromine atoms were
determined by monitoring either the buildup at 390 nm
(Br2•- as probe) or the decay signal at 510 nm (π-complex
as probe) as a function of H-donor concentration ([RH]).
In all cases, second-order rate constants (kH) were
obtained from the slopes corresponding to the linear plots
(not shown) of kobs vs [RH]. Resulting values are sum-
marized in Table 1. Not surprisingly, the reactivity of
bromine atoms decreases in the presence of aromatic
compounds.
of 1.5 × 107 s-1 results from data in acetonitrile (Keq
)
4.6 M-1), whereas a value lower than 6.4 × 105 s-1 (and,
consequently, a lifetime of the order of microseconds)
would result from data in neat benzene. Since hydrogen
abstraction rates for bromine atoms seem to be largely
unaffected by the polarity of the solvent,8 the (at least)
23-fold change in first-order rate constant contradicts an
intramolecular process. However, the decrease in reactiv-
(12) Chateauneuf, J . E. J . Org. Chem. 1999, 64, 1054.
(13) Paul, H.; Small, R. D., J r.; Scaiano, J . C. J . Am. Chem. Soc.
1978, 4520.
(14) Hug, G. L. Optical Spectra on Nonmetallic Inorganic Transient
Species in Aqueous Solution; National Bureau of Standards: Wash-
ington, 1981; NSRDS-NBS 69, pp 160.
(15) J arzeba, W.; Thakur, K.; Ho¨rmann, A.; Barbara, P. F. J . Phys.
Chem. 1995, 99, 2016.

1894 J . Org. Chem., Vol. 65, No. 6, 2000
Notes
from National Instruments. This computer is interfaced (GPIB)
to a Tektronix TDS 620 A digitizer used for data acquisition.
Other aspects of this laser system have previously been de-
scribed.16,17 Solutions were contained in quartz cells constructed
of 7 × 7 mm2 Suprasil tubing. All measurements were performed
at 21 °C using air-equilibrated samples.
ity observed when the solvent is changed from acetoni-
trile to benzene agrees very well with a change in reactive
species, i.e., from “free” (reactive) to complexed (less
reactive) bromine atoms. Thus, it is inferred that in the
case of toluene H-abstraction does not take place within
the corresponding π-complex, but rather toluene acts both
as a reactant and, as in the case of the other aromatics
employed in this study, as a modulator of bromine atom
reactivity via complexation.
Ackn owledgm en t. Financial support from the Natu-
ral Sciences and Engineering Research Council (NSERC)
of Canada is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Plots of signal in-
tensity (∆A) for bromine atom/aromatic π-complexes as a
function of the concentration of aromatic species; observed
bromine atom reactivity as a function of H-donor concentra-
tion; transient spectra recorded in neat toluene. This material
is available free of charge via the Internet at http://pubs.acs.org.
Exp er im en ta l Section
Acetonitrile, benzene, methanol, 2-propanol, and toluene
(BDH Omnisolv), tert-butylbenzene (Aldrich), and tetrabutyl-
ammonium bromide (Across) were used as received. Diethyl
ether (BDH ACS) was distilled before use. R-Bromoacethophe-
none (Across) and R-chloroacethophenone (BDH) were recrystal-
lized from aqueous ethanol and hexane, respectively, before use.
Laser experiments were carried out using a Q-switched Nd:
YAG laser (Continuum, Surelite I) operated at 355-nm (4-6 ns
pulses, <30 mJ /pulse) for excitation. The system is controlled
by a Power Macintosh 7100/80 computer running LabVIEW 3.1.1
J O991806O
(16) Barra, M.; Agha, K. A. J . Photochem. Photobiol., A: Chem. 1997,
109, 293.
(17) Sanchez, A. M.; Barra, M.; de Rossi, R. H. J . Org. Chem. 1999,
64, 1604.