Radiation-induced reactions of benzoyl chloride and acrylates in solution. A pulse radiolysis study

Article abstract of DOI:10.1039/a907162h

Using electron pulse radiolysis with optical detection, the radiation- induced reactions of benzoyl chloride and acrylates were studied in tetrahydrofuran and acetonitrile solution at room temperature. In both solvents electron transfer leads to the formation of transient radical anions of acrylates (k ? 2-3 x 10¹⁰ dm³ mol^-1 s^-1) and of benzoyl chloride (k ? 3 x 10¹⁰ dm³ mol^-1 s^-1). The latter dissociates into chloride ion and the benzoyl radical (k = 3 x 10⁶ s^-1), whereas the acrylate anion transforms by protonation into a ketyl-type radical. In mixed solutions a fast electron transfer from acrylate anions to benzoyl chloride is found (k ? 1 x 10¹⁰ dm³ mol^-1 s^-1). The benzoyl chloride anion reacts with the monomer (k = 2.8 x 10⁹ dm³ mol^-1 s^-1) with simultaneous release of Cl^- , forming species which may initiate polymerization. In the presence of oxygen the formation of benzoylperoxy radicals (k = 1.6 x 10⁹ dm³ mol^-1 s^-1), showing a strong absorption band in the near-UV (λ(max) = 400 nm), is observed.

Full text of DOI:10.1039/a907162h

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Radiation-induced reactions of benzoyl chloride and acrylates in
solution. A pulse radiolysis study
Wolfgang Knolle,*a Uwe Mullerb and Reiner Mehnerta
a Institut fur OberÑachenmodiÐzierung, Permoserstrae 15, D-04303 L eipzig, Germany.
E-mail: knolle=rz.uni-leipzig.de
b Martin-L uther-Universitat Halle-W ittenberg, Institut fur Organische Chemie, Geusaer Strae,
D-06217 Merseburg, Germany
Received 6th September 1999, Accepted 31st January 2000
Using electron pulse radiolysis with optical detection, the radiation-induced reactions of benzoyl chloride and
acrylates were studied in tetrahydrofuran and acetonitrile solution at room temperature. In both solvents
electron transfer leads to the formation of transient radical anions of acrylates (k B 2È3 ] 1010 dm3 mol~1
s~1) and of benzoyl chloride (k B 3 ] 1010 dm3 mol~1 s~1). The latter dissociates into chloride ion and the
benzoyl radical (k \ 3 ] 106 s~1), whereas the acrylate anion transforms by protonation into a ketyl-type
radical. In mixed solutions a fast electron transfer from acrylate anions to benzoyl chloride is found
(k B 1 ] 1010 dm3 mol~1 s~1). The benzoyl chloride anion reacts with the monomer (k \ 2.8 ] 109 dm3
mol~1 s~1) with simultaneous release of Cl~, forming species which may initiate polymerization. In the
presence of oxygen the formation of benzoylperoxy radicals (k \ 1.6 ] 109 dm3 mol~1 s~1), showing a strong
absorption band in the near-UV (j \ 400 nm), is observed.
max
grating monochromator (ARC, SP275), 1P28 photomultipliers
Introduction
and a fast transient recorder (Tektronix, TDS620). Linac oper-
ation and data acquisition were done in the computer-
controlled mode.
Radical reactions induced by benzoyl radicals play an impor-
tant role in photocuring of acrylates and unsaturated poly-
ester resins. Reactions of the benzoyl radicals have been
extensively studied using photoinitiators as a source of
benzoyl radicals. The primary reaction of these photoinitia-
tors is known to be photochemical a-cleavage [eqn. (1)]
leading to the formation of the benzoyl radical.1
All experiments were performed at room temperature.
Tetrahydrofuran (THF, Merck, spectrograde), acetonitrile
(ACN, Merck, spectrograde), benzoyl chloride (PhCOCl,
Merck, [99%), and butanediol diacrylate (BDDA, Merck,
[99.5%) were used as obtained; butyl acrylate (BA, Merck,
[99%) was puriÐed by distillation. In some experiments a
small amount of water (2 vol.%, Millipore Q) was added to
THF to extend the lifetime of solvated electrons. It was con-
Ðrmed that for solute concentrations [10~2 mol dm~3 the
addition of water did not inÑuence either the transient signals
measured or the bimolecular rate constants derived.
Quantum chemical calculations were performed using the
semi-empirical method PM3 which is included in the program
HYPERCHEM( 5.01. The geometries of the ground states
were optimized using the PolacÈRibiere conjugate gradient
method in unrestricted HartreeÈFock (UHF) approximation.
The Direct Inversion in the Iterative Subspace (DIIS) method
was used for better convergence of self-consistent Ðeld (SCF)
calculations of the electronic structure. All excitation energies
were obtained for the restricted open-shell HartreeÈFock
(ROHF) approximation at the single excitation (CIS) level of a
conÐguration interaction wave function and with the orbital
criterion.
The major problem in investigation of radical reactions fol-
lowing the photochemical a-cleavage is the superposition of
the transient spectra. Mostly, both radicals of the pair formed
by photolysis absorb in the same wavelength region. Conse-
quently, primary reactions of the benzoyl radical with scav-
engers such as acrylates can hardly be distinguished from
reactions of the counter radical.
Electron attachment to benzoyl chloride is expected to o†er
a simple way for generating benzoyl radicals with unreactive
chloride ions as counterparts. Under these conditions spectro-
scopic properties and kinetics of benzoyl radicals could be
studied independently of the counter radicals. Here we report
the results of a pulse radiolysis study on tetrahydrofuran
(THF) and acetonitrile (ACN) solutions containing either
benzoyl chloride or acrylate, as well as both at di†erent con-
centration ratios.
Experimental
The pulse radiolysis experiments were carried out using 5 or
15 ns electron pulses from the 11 MeV linear accelerator of
the Institut fur OberÑachenmodiÐzierung. Corresponding to
the pulse width the dose per pulse was in the range 20È35
or 100È120 Gy, respectively (all spectra normalized to the
dose). The optical detection system consisted of a pulsed 450
W xenon lamp, Suprasil cell (light path 1 cm), a high intensity
Results and discussion
The experiments were performed in two di†erent solvents to
check for possible solvent e†ects. In THF electron-pulse irra-
diation leads to the formation of solvated electrons as the
reducing species,2 whereas in ACN a monomeric and/or a
DOI: 10.1039/a907162H
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1425

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dimeric form of an anion is observed within a few ns giving
of observing a short-lived benzoyl chloride radical anion on
pulse radiolysis in solution likely, too.
reducing conditions.3 It is widely accepted that CH CN~~ can
3
be used to represent both reducing species under rapid equi-
C H COCl ] e~ ] [C H COCl]
(2a)
C H COCl ] CH CN ] [C H COCl] ] CH CN (2b)
librium.3,4 It should be noted that at high concentration the
solute reacts directly with the primary solvated electrons.
6
5
solv
6 5
6
5
3
6
5
3
Both solvated electrons and CH CN~~ show broad absorp-
3
tion bands from the UV into the near-IR and exhibit lifetimes
[C H COCl] ] C H CO ] Cl~
(3)
(4)
6
5
6 5
up to several hundred nanoseconds. Solvent radicals and non-
reactive cations formed2,5 contribute to an unspeciÐc long-
lived absorption at j \ 350 nm.
C H CO ] product
6
5
With increasing PhCOCl concentration new intermediates
appear in the spectrum. A short-lived component with a
shoulder (peak) around 330 nm (cf. right inset in Fig. 2, spike
of the 330 nm trace) showing roughly a Ðrst-order decay
[k/106 s~1 \ 3.5 ^ 0.2 in THF (7.0 ^ 0.2 in ACN)] indepen-
dent of the solute concentration, and a long-lived component
with a maximum at 370 nm (Fig. 2, inset for 370 nm) can be
distinguished. The spectrum remaining 80 ls after the pulse
and the Ñat part of the 330 nm trace are due to solvent rad-
icals.
We assign the spectrum of the short-lived component [Fig.
2, left inset (…)] to the benzoyl chloride radical anion. This
assignment is supported by the following arguments: (i) the
reaction of PhCOCl with reducing species in THF and ACN
is di†usion-controlled, which is expected for an electron trans-
fer reaction, (ii) the short-lived transient [which is only slightly
In both solvents under de-aerated conditions the same
solute transients are formed showing similar time proÐles and
concentration dependences. However, in air-saturated solu-
tions a di†erent decay behavior of the transients formed was
observed. Whereas in THF the benzoylperoxy radicals
decayed without formation of another observable transient,
the formation of a very broad unspeciÐc absorption band
(from 300 to 500 nm) was observed in ACN, inÑuencing the
time proÐles. This might be related to the di†erent hydrogen
donating capabilities of the solvents: the formation of
(unobservable) hydroperoxides is likely to proceed in THF,
but it is not expected in ACN. The investigation of the di†er-
ent decay behavior was beyond the scope of this work.
For better identiÐcation of the spectra of aerated solutions,
results are given for THF solutions. In the other cases mea-
surements done in ACN solutions will be discussed, as higher
G-values for the formation of reducing species lead to better
signal-to-noise ratios.
De-aerated solutions of benzoyl chloride
On the timescale of our experiments, the time proÐles of the
unspeciÐc long-lived absorption at j \ 350 nm due to solvent
radicals and non-reactive cations formed after electron irra-
diation did not change with increasing benzoyl chloride
(PhCOCl) concentration. However, increasing PhCOCl con-
centration leads to a faster decay of the broad absorption
bands due to solvated electrons (in THF) and CH CN~~ (in
3
ACN, cf. Fig. 1), respectively.
From the rate of the pseudo-Ðrst-order decay at increasing
benzoyl chloride concentration bimolecular rate constants of
k
\ (3.2 ^ 0.1) ] 1010 dm3 mol~1 s~1 and k
\ (3.5
ACN
THF
^ 0.1) ] 1010 dm3 mol~1 s~1 near the di†usion-controlled
limit are deduced for the electron transfer reactions (rate con-
stants are summarized in Table 1). The electron attachment
process [cf. eqn. (2)] for organohalides is often accompanied
by a fast (immediate) dissociation; therefore, the radical
formed by release of the halide ion [cf. eqn. (3)] is observable.
However, in low-temperature optical absorption measure-
ments on c-irradiated benzoyl halides in methyl-
tetrahydrofuran matrix the radical anion has been observed
with benzoyl chloride, while the benzoyl radical was found
with benzoyl bromide and iodide.6 This makes the possibility
Fig. 1 Time proÐles of the absorbance of the solvated electron
absorption measured in ACN at 500 nm for di†erent concentrations
of PhCOCl [in 10~4 mol dm~3]: 0.87 (A), 1.75 (B), 3.5 (C) and 7.0
(D). Inset: Plot of decay rate against PhCOCl concentration. Dose
per pulse: 100 Gy.
Table 1 Rate constants and characteristic wavelengths of transients
k/
j
of product/
max
nm
Reaction
Solvent
dm3 mol~1 s~1
e~ ] BA
THF
ACN
ACN
THF
ACN
THF
ACN
ACN
THF
ACN
THF
3.0 ] 1010
2.4 ] 1010
2.5 ] 1010
3.5 ] 1010
3.2 ] 1010
9.5 ] 109
1.2 ] 1010
2.0 ] 109
1.6 ] 109
290, (BA)~~
e~ ] BDDA
e~ ] PhCOCl
290, (BDDA)~~
330, (PhCOCl)~~
Fig. 2 Transient optical absorption spectra observed after electron
(BA)~~ ] PhCOCl
(BDDA)~~ ] PhCOCl
(PhCOCl)~~ ] BDDA
PhC~O ] O
(PhCOCl)~~ decay
330, (PhCOCl)~~
pulse irradiation (100 Gy per pulse) of N -saturated 0.125 mol dm~3
2
PhCOClÈACN solutions (time after the pulse as indicated). Left inset:
(…) di†erence between spectra measured at 20 and 400 ns (PhCOCl
radical anion), (L) di†erence between spectra measured at 5 and 80 ls
(benzoyl radical). Right inset: time proÐles of the absorbance at di†er-
ent wavelengths.
400, PhCOOO~
380, PhCO~
2
3.5 ] 106 s~1
7.0 ] 106 s~1
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Table 2 Comparison of optical transitions measured and calculated
for the PhCOCl radical anion and the benzoyl radical (in parenth-
eses: oscillator strength; s \ shoulder, b \ broad band)
formed for a structure ““in vacuoÏÏ and that the position of the
absorption band of the anion is difficult to measure accu-
rately, because of the soluteÏs self-absorption. Additionally, the
observed red-shift of approximately 50 nm for the dissociation
reaction agrees well with the calculated shift of about 60 nm
(the calculations usually reproduce tendencies better than
absolute values).
The spectrum of the benzoyl radical measured in this work
compares well with the spectrum found (absorption peak at
370 nm) in laser Ñash photolysis experiments.1
j
(exp)/
j(calc)/
max
nm
Transient
nm
B300 (s)
B370 (b)
290(0.60)
370(0.13)
313(0.11)
353(0.34)
Air-saturated solutions of benzoyl chloride
In air-saturated electron-irradiated neat solutions oxygen
reacts with the primary anionic species [k(e
(2.5 ^ 0.2) ] 1010 dm3 mol~1 s~1, this work: decay of solvat-
ed electrons in neat THF followed at 700 nm with increasing
] O ) \
solv, THF
2
oxygen-sensitive (see later)] points to an ionic species, and (iii)
it shows a Ðrst-order decay with a rate constant independent
of the solute concentration, which is expected for the disso-
ciation reaction [cf. eqn. (3)]. The spectrum of the long-lived
component (Fig. 2, spectrum at 5 ls), which appears after the
decay of the radical anion, is attributed to benzoyl radicals
produced according to eqn. (3).
To support our assignment, semi-empirical quantum chemi-
cal calculations were performed (see Table 2). The calculations
give the strongest transition at 290 nm for the benzoyl chlo-
ride radical anion, and at 353 nm for the benzoyl radical.
Although these values di†er slightly from the experimental
values (330 and 380 nm), the result is satisfactory taking into
account that the quantum-chemical calculations are per-
O
concentration; k(CH CN~~ ] O )7 \ 1 ] 1011 dm3
2
3
2
mol~1 s~1] as well as with the solvent radicals. Long-lived
absorption (lifetime [0.5 ls) due to solvent peroxy radicals is
observed in air-saturated neat solutions only below 350 nm as
shown in Fig. 3 (L).
In solutions containing PhCOCl, the grow-in of a new tran-
sient with a pronounced peak around 400 nm (and a shoulder
below 330 nm) is observed (Fig. 3, left inset). Its formation rate
is a function of the oxygen concentration (Fig. 3, inset at 380
nm) and the corresponding rate constant was estimated from
the superimposed time proÐles at 380 nm as follows: (i) com-
paring di†erent spectral regions the underlying short-lived
absorbance observed in nitrogen-saturated solutions was
found to be una†ected by oxygen (cf. Fig. 3, inset at 310 nm)
and therefore its decay rate in nitrogen-saturated solutions
was used in the Ðtting procedure, (ii) the decay of the new
band proved to be independent of oxygen concentration and a
corresponding decay rate of 4 ] 105 s~1 was derived in
oxygen-saturated solutions, (iii) Ðnally, the formation rate at
each concentration was determined by Ðtting the sum of three
exponential functions with two Ðxed time constants to the
experimental curves. Taking an oxygen concentration of
2.1 ] 10~3 mol dm~3 for air-saturated THF,8 a bimolecular
rate constant of (1.6 ^ 0.3) ] 109 dm3 mol~1 s~1 was esti-
mated for this reaction. The new band is assigned to benzoyl-
peroxy radicals formed according to eqn. (5).
Fig. 3 Transient optical absorption spectra observed 0.5 ls after
electron pulse irradiation (100 Gy per pulse) of air-saturated THF
solutions. (L) Neat THF; (@) 0.1 mol dm~3 PhCOCl and (…, left
inset) spectrum of benzoylperoxy radical corrected for solvent peroxy
radicals. Right insets: time proÐles of the absorbance at 310 and 380
Carbon-centered peroxy radicals usually show unspeciÐc UV-
absorption, but absorption bands in the near-UV or even in
the visible region have been found to be characteristic of
vinylperoxy and phenylperoxy radicals.9 The origin of these
absorption bands is discussed in terms of possible overlap of
the odd electron with the p-system of the vinyl double bond
or of the phenyl ring. In the present case such an overlap is
possible with the p-system either of the C2O double bond or
of the phenyl ring.
nm for
a
0.5 mol dm~3 PhCOClÈTHF solution [bottom, N -
2
saturated; top, air-saturated; middle, N Èair 1 ] 1 (only for 380 nm)].
2
The decay of benzoylperoxy radicals in THF is nearly
pseudo-Ðrst-order (k \ 4 ] 105 s~1) and independent of
benzoyl chloride concentration and dose; therefore, we
assume hydroperoxide formation via hydrogen abstraction
from the solvent.
Similar results were found in time-resolved IR experiments,
following the formation and decay of benzoylperoxy radicals
after laser Ñash photolysis of 2-hydroxy-2-methyl-1-
phenylpropan-1-one (Darocure 1173)Èhexane solution (cf. Fig.
4).10 The decay of the benzoylperoxy radicals (cf. Fig. 4) shows
a half-life of approximately 2 ls, i.e. the rate of 3.5 ] 105 s~1
compares well with our measurements (k \ 4 ] 105 s~1).
Owing to the limited time resolution (approximately 50 ns, for
experimental details see e.g. ref. 11) the formation rate can
Fig. 4 Decay of benzoylperoxy radicals followed at 1820 cm~1 after
laser Ñash photolysis (355 nm, 30 mJ per pulse, 7 ns) of air-saturated
Darocure-1173Èhexane solution.10 The inset shows the formation of
the benzoylperoxy radical band and the concomitant decay of the
benzoyl radical band at 1830 cm~1.
Phys. Chem. Chem. Phys., 2000, 2, 1425È1430
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only roughly be estimated, but the half-life of O200 ns is in
good agreement with our value of 150 ns deduced in THF.
In air-saturated solutions containing PhCOCl at concentra-
tions [10~1 mol dm~3, again the short-lived transient
(radical anion) of PhCOCl is observed with the same decay
rate as found in de-aerated solutions, showing that the reac-
tion of oxygen with the radical anion cannot compete with the
fast dissociation reaction.
Solutions of acrylates
If acrylates are added to the neat solvents, the primary anionic
species react with the acrylate. Rate constants of approx-
imately 3 ] 1010 dm3 mol~1 s~1 (see Table 1) near the
di†usion-controlled limit were determined. Two new interme-
diates in the transient spectra can be distinguished: a short-
Fig. 5 Transient optical absorption spectra observed after electron
pulse irradiation (100 Gy per pulse) of a N -saturated 0.1 mol dm~3
2
lived one (Fig. 5, spectrum at 0.01 ls, inset for 420 nm, N )
BAÈTHF solution: (…) 0.01 ls and (@) 4 ls after the pulse. For com-
2
parison spectra of acrylate radical anion (K) and ketyl-type radical
(|É ) measured in aqueous solutions15 are given. Insets: time proÐle of
with a Ðrst-order decay rate of 5 ] 106 s~1 which does not
depend on the acrylate concentration, and a long-lived one
the absorbance for N - and air-saturated THF solutions.
(Fig. 5, spectrum at 4 ls, inset for 340 nm, N ).
2
2
The assignment of the transients can be made according to
pulse radiolysis measurements of aqueous solutions of acry-
lates.12,13 There, it was found that electron attachment leads
to a radical anion [cf. eqn. (6)] which subsequently protonates
forming either a ketyl-type radical [eqn. (7)] or an a-
carboxyalkyl radical [eqn. (7a)]. Comparing the spectra mea-
sured in this work with the spectra measured in aqueous
solution (see Fig. 5), it is evident that in ACN and THF the
electron attachment leads to the acrylate radical anion as well
as subsequent formation of the ketyl-type radical. [The
absorption band of a-carboxyalkyl radicals has a clear
maximum at 320 nm,13 which di†ers signiÐcantly from the
spectrum observed at 4 ls (Fig. 5).] The decay of the acrylate
radical anion does not depend on the solute concentration but
on the dose applied. Therefore we assume that protonation
mainly takes place via the associated protons [denoted as H`
in eqn. (7)] of the solvent cations THF(H`) and ACN(H`)
formed during the irradiation by deprotonation of the
primary radical cations.2,5 Quantum chemical calculations
show that these proton transfer reactions are favourable as the
energy balance is exothermic by approximately 35È40 J
mol~1.
We assume that an electron transfer to benzoyl chloride [cf.
eqn. (9)] occurs. Rate constants of B1010 dm3 mol~1 s~1
were found for this bimolecular reaction (Table 1). As the
absorption coefficient of the acrylate radical anion is much
larger than that of the benzoyl chloride radical anion in the
region of interest, the formation of the latter cannot be
directly observed.
If a low concentration of acrylate is added to a solution
containing 1 ] 10~1 mol dm~3 PhCOCl, the following
changes in the transient spectra can be distinguished. (i) The
transient assigned to the benzoyl chloride anion decays faster
with increasing acrylate concentration [cf. Fig. 7(a) at 330 nm]
and a bimolecular rate constant of (2 ^ 0.4) ] 109 dm3 mol~1
s~1 can be derived [Fig. 7(a), inset]. (ii) At the same time, the
intensity of the 370 nm transient assigned to the benzoyl
radical decreases [cf. Fig. 7(b)]. This behavior is expected if
the precursor (the radical anion) of the observed transient is
quenched by the acrylate. It can easily be shown that in this
case the reciprocal of the initial absorbance (A ) of the
0
benzoyl radical should be proportional to the acrylate concen-
tration and to the ratio of the decay rate of the benzoyl chlo-
In air-saturated solutions oxygen leads to a faster decay of
the radical anion, and the long-lived absorption due to ketyl-
type radicals is completely removed (see Fig. 5, insets). After
the decay of the anion ([0.5 ls after the pulse) only solvent
peroxy radicals can be observed.
Solutions containing both benzoyl chloride and acrylates
In solutions containing PhCOCl and acrylate, both solutes
compete for the reducing species produced by irradiation. To
distinguish possible reaction pathways, experiments were per-
formed with an excess of one of the two solutes. In solutions
containing an excess of acrylate, the addition of PhCOCl
leads to a faster decay of the acrylate anion formed by the
electron attachment (Fig. 6).
Fig. 6 Time proÐles of the absorbance in a de-aerated 0.1 mol dm~3
BAÈTHF solution measured at 340 nm (BA radical anion) for di†er-
ent PhCOCl concentrations (dose \ 100 Gy per pulse). Inset: Plot of
the decay rate against PhCOCl concentration.
1428
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Fig. 9 Time proÐles of the absorbance measured at 380 nm in elec-
tron irradiated (100 Gy per pulse) air-saturated THF solutions con-
taining 0.1 mol dm~3 of both PhCOCl and BA (a) and only PhCOCl
(b).
An electron transfer reaction between the benzoyl chloride
radical anion and the monomer can be excluded for the fol-
lowing reasons: (i) the high rate constant of 9.5 ] 109 dm3
mol~1 s~1 for the electron transfer reaction from the acrylate
anion to PhCOCl makes the back-transfer reaction very
unlikely, and (ii) the formation of the acrylate anion in such a
reaction would be observable in the spectrum because the
acrylate anion has a much higher absorption coefficient. The
investigation of the possible reaction pathways by means of
quantum-chemical calculations leads one to the conclusion
that the benzoyl chloride anion adds to the acrylate double
bond with the simultaneous release of Cl~ [eqn. (11)], leading
to a radical species which can initiate the polymerization.
Fig. 7 Time proÐles of the absorbance in 0.1 mol dm~3 PhCOClÈ
ACN solution measured (a) at 330 nm (PhCOCl radical anion) and (b)
at 370 nm (PhCO radical) for di†erent concentrations of BDDA [in
10~3 mol dm~3]: 0 (A), 1.7 (B), 5.2 (C) and 8.7 (D). Insets: (a) decay
rate and (b) reciprocal of initial absorbance (corrected for solvent
absorption) as a function of BDDA concentration.
ride radical anion [k
, cf. eqn. (3)] and the ““quenchingÏÏ
decay
rate (k ):
q
1
A
k ] [acrylate]
B
q
P 1 ]
(10)
A
k
0
decay
It is seen in Fig. 7(b) (time proÐles at 370 nm) that only the
absorption intensity of benzoyl radicals decreases with
increasing acrylate concentration, and that the decay rate is
nearly constant (B2 ] 105 s~1). Although the reaction of
benzoyl radicals with acrylates [cf. eqn. (12)] is known from
photolysis experiments,14 the rate constant for addition to the
acrylate double bond (B5 ] 105 dm3 mol~1 s~1) is too small
to inÑuence the decay observed in our pulse radiolysis experi-
ments.
In the inset of Fig. 7(b) the reciprocal initial intensity is
plotted against the acrylate concentration. From the slope of
the straight line a value of 180 ^ 30 is obtained for the ratio of
rate constants of eqn. (10). Using the decay rate measured in
this work, a quenching rate of approximately (1 ^ 0.3) ] 109
dm3 mol~1 s~1 is calculated, which is of the same magnitude
as the rate estimated directly from the change of absorption at
330 nm [cf. Fig. 7(a)] for the decay of the benzoyl chloride
anion [k \ (2 ^ 0.4) ] 109 dm3 mol~1 s~1].
On a much larger timescale (up to 30 ls) the formation of a
new absorption band around 330 nm was observed showing a
formation rate of 3.8 ] 106 dm3 mol~1 s~1 (Fig. 8).
Such absorption bands around 310È330 nm are expected
for the ““propagatingÏÏ acrylate radical as well as for simple
radicals formed by addition to the acrylic double bond.15,16
The relatively high formation rate observed excludes the
possibility that a kind of oligomerization proceeds, as the
addition rate for the second (and following) acrylate monomer
units is lower by two orders of magnitude (103È104 dm3
mol~1 s~1).14,17 Additionally, the polymerization process is
hard to observe directly because the absorption characteristics
of the propagating radical do not change signiÐcantly with
increasing number of monomer units. Addition of benzoyl
radicals to the acrylate leading to the formation of this band is
also ruled out, as its precursor (the benzoyl chloride anion) is
scavenged by the acrylate [cf. eqn. (11)]. The most likely
explanation for the formation of this band is the addition
Fig. 8 Time proÐles of the absorbance in 0.1 mol dm~3 PhCOClÈ
ACN solution measured after electron irradiation (100 Gy per pulse)
at 330 nm for di†erent concentrations of BDDA [in 10~3 mol dm~3]:
25.8 (A), 17.2 (B), 1.7 (C) and 0.3 (D). Inset: Observed formation rate
at di†erent BDDA concentrations.
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reaction of radiolytically formed solvent radicals to the acrylic
double bond.
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the benzoylperoxy radical is produced. In both cases the
peroxy radicals decay in the same way by hydrogen
abstraction from the solvent; any possible reaction with acry-
lates cannot compete with this fast decay at the acrylate con-
centrations (up to 1 ] 10~1 mol dm~3) used.
2
3
F. Y. Jou and L. M. Dorfman, J. Phys. Chem., 1973, 58, 4715.
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7
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Conclusion
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Pulse radiolysis experiments have shown that solvated elec-
trons react with benzoyl chloride forming a radical anion
which transforms by release of chloride to the benzoyl radical.
In the presence of oxygen, benzoylperoxy radicals are formed
10 S. Jokusch and U. Muller, unpublished work.
11 G. W. Slugget, C. Turro, M. W. George, I. V. Koptyug and N. J.
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showing a characteristic absorption band with j \ 400 nm.
max
In acrylate solutions electron transfer reactions lead to acry-
late radical anions which protonate forming ketyl-type rad-
icals. In solutions containing both solutes, electron transfer
from the acrylate radical anion to benzoyl chloride has been
observed as well as direct addition of the benzoyl chloride
radical anion to the acrylic double bond with simultaneous
release of chloride ion.
16 E. Takacs and L. Wojnarovits, Nucl. Instrum. Methods B, 1995,
105, 282.
17 L. Wojnarovits, E. Takacs and A
Š
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Appl. Chem., 1995, A23, 443.
Acknowledgement
The authors thank Dr S. Naumov for quantum chemical cal-
culations.
Paper a907162h
1430
Phys. Chem. Chem. Phys., 2000, 2, 1425È1430