Formation of radical anions on the reduction of carbonyl-containing perfluoroaromatic compounds in aqueous solution: A pulse radiolysis study

Article abstract of DOI:10.1021/jp960992o

Radical anions are formed on addition of hydrated electrons to pentafluoroacetophenone (PFA) and pentafluorobenzaldehyde (PFB) in aqueous solutions. On the other hand, addition of hydrated electrons to pentafluorobenzoic acid (PFBA) leads to rapid fluoride elimination. The spectrum of the radical anion of PFA has λ_max at 300 and 440 nm with absorption coefficient at 440 nm ε₄₄₀ = 2100 L mol^-1 cm^-1. PFA^?- decays with a rate constant of (7 ± 3.0) × 10³ s^-1. It has a pK_a = 7.5 and the spectrum of the conjugate acid has λ_max at 270 and 460 nm with ε₄₆₀ = 900 L mol^-1 cm^-1. The spectrum of the radical anion of PFB has λ_max at 285 and 430 nm with ε₄₃₀ = 800 L mol^-1 cm^-1. PFB^?- decays with a rate of (4 ± 2) × 10³ s^-1. It has a pK_a = 7.2 and the spectrum of the conjugate acid has weak absorption at 330 nm. Evidence for the formation of the radical anion was obtained from intermolecular electron transfer from the radical anions of PFA and PFB top-benzoquinone (Q), methyl viologen (MV²⁺), and 9,10-anthraquinone-2-sulfonate (AQS^-). Strong reductants derived from reduction of 2,2-bipyridine (BpyH^?) and 1,10-phenanthroline (PhenH^?) can reduce both PFA and PFB. From the kinetics of these electron transfer reactions the reduction potentials of PFA and PFB have been determined to be -0.86 ± 0.1 and -0.75 ± 0.1 V vs NHE at pH 9.4. Addition of OH^? radical to the aromatic ring of these fluorinated compounds led to rapid HF elimination and the formation of phenoxyl radicals, and addition of H^? atoms led to the formation of cyclohexadienyl radical.

Full text of DOI:10.1021/jp960992o

14022
J. Phys. Chem. 1996, 100, 14022-14027
Formation of Radical Anions on the Reduction of Carbonyl-Containing Perfluoroaromatic
Compounds in Aqueous Solution: A Pulse Radiolysis Study
Lian C. T. Shoute* and Jai P. Mittal*,†
Chemistry DiVision, Bhabha Atomic Research Centre, Bombay 400 085, India
ReceiVed: April 2, 1996; In Final Form: May 29, 1996X
Radical anions are formed on addition of hydrated electrons to pentafluoroacetophenone (PFA) and
pentafluorobenzaldehyde (PFB) in aqueous solutions. On the other hand, addition of hydrated electrons to
pentafluorobenzoic acid (PFBA) leads to rapid fluoride elimination. The spectrum of the radical anion of
PFA has λmax at 300 and 440 nm with absorption coefficient at 440 nm ꢀ440 ) 2100 L mol-1 cm-1. PFA•-
decays with a rate constant of (7 ( 3.0) × 103 s-1. It has a pKa ) 7.5 and the spectrum of the conjugate acid
has λmax at 270 and 460 nm with ꢀ460 ) 900 L mol-1 cm-1. The spectrum of the radical anion of PFB has
λmax at 285 and 430 nm with ꢀ430 ) 800 L mol-1 cm-1. PFB•- decays with a rate of (4 ( 2) × 103 s-1. It
has a pKa ) 7.2 and the spectrum of the conjugate acid has weak absorption at 330 nm. Evidence for the
formation of the radical anion was obtained from intermolecular electron transfer from the radical anions of
PFA and PFB to p-benzoquinone (Q), methyl viologen (MV2+), and 9,10-anthraquinone-2-sulfonate (AQS-).
Strong reductants derived from reduction of 2,2-bipyridine (BpyH•) and 1,10-phenanthroline (PhenH•) can
reduce both PFA and PFB. From the kinetics of these electron transfer reactions the reduction potentials of
PFA and PFB have been determined to be -0.86 ( 0.1 and -0.75 ( 0.1 V vs NHE at pH 9.4. Addition of
OH• radical to the aromatic ring of these fluorinated compounds led to rapid HF elimination and the formation
of phenoxyl radicals, and addition of H• atoms led to the formation of cyclohexadienyl radical.
Introduction
The rate of fluoride elimination from the radical anions of a
perfluorinated compound depends on several factors. If the
unpaired electron occupies a σ or a mixture of σ and π
orbitals,11,12 the rate is expected to be very fast. The rate is
expected to be relatively slow for a π radical anion as the rate
of fluoride elimination in this case can be visualized as electron
transfer from π to the orthogonal σ orbital of the C-F bond.13
The rate also depends strongly on the electron affinity of the
other substituent present in the ring, because in the presence of
such substituents, fluoride elimination can be visualized as an
intramolecular electron transfer from the substituent to the C-F
bond.13 In this study, we have investigated the formation of
the radical anion of carbonyl containing perfluoroaromatic
compounds C6F5COX (X ) COOH, CHO, COCH3) on reduc-
tion and their intermolecular electron transfer reactions in
aqueous solution.
Due to their chemical inertness, there is increasing concern
for accumulation of perfluorinated compounds in the environ-
ment.1,2 As expected for compounds containing highly elec-
tronegative fluorine atoms, a chemical reaction to which they
are most susceptible is reduction. Electrochemical technique
has been increasingly used for the preparation of perfluorinated
derivatives.3 A serious problem encountered in this method is
the ease with which fluoride is lost on reduction. Strong
reductants led to complete mineralization of fluorine.4 Using
the photopolarographic technique, Konovalov et al.5 have
estimated a rate constant of ca. 109 s-1 for fluoride elimination
from the radical anion of pentafluorobenzoate. Rapid fluoride
elimination has been reported on reduction of perfluorobenzene.6
Recently, we have reported7 that the addition of hydrated
electrons to pentafluorophenol and pentafluoraniline led to rapid
fluoride elimination and the initially formed electron adducts
could not be detected. The rate constants for fluoride elimina-
tion from these compounds were estimated to be >107 s-1. This
is in contrast to the radical anion of tetrafluoro-1,4-benzoquinone
which does not lose fluoride.8
Due to the high C-F bond strength, it was speculated that
addition of an electron to a perfluorocarbon may not lead to
heterolytic breaking of the bond to form fluoride ion. In the
gas phase9 or in the rigid organic glassy matrices at low
temperature,10,11 the radical anions of perfluorocarbons were
observed to be stable with respect to fluoride elimination. The
difference in the dynamics of the radical anion in the gas phase
and in the rigid low-temperature matrices on the one hand and
aqueous solution at room temperature on the other can be traced
to the solvation of fluoride in aqueous solution. In aqueous
solution, fluoride ion is very strongly solvated and drives the
release of fluoride from the radical anion.
Experimental Section
The kinetic spectrophotometric pulse radiolysis system con-
sists of a linear accelerator which delivers 7 MeV electrons of
50 ns, 500 ns and 2 µs pulse width and with doses in the range
of 5-100 Gy per pulse. The dose absorbed was determined in
an aerated 0.05 mol L-1 KSCN solution assuming Gꢀ ) 2.5 ×
104 L mol-1 cm-1 per 100 eV (G is radical formed per 100
eV). Details of the experimental setup are described else-
where.7,8 The bimolecular rate constants (k) reported here were
obtained from the slope of the plots of the observed rate kobs vs
[S], the substrate (S) concentration, using the equation kobs
)
k0 + k[S]. The uncertainties in the values of k are estimated to
be 10-20%.7 2,3,4,5,6-Pentafluoroacetophenone (PFA), 2,3,4,5,6-
pentafluorobenzaldehyde (PFB), and 2,3,4,5,6-pentafluoroben-
zoic acid (PFBA) obtained from PCR Research Chemicals were
used as received. All the chemicals used are either analytical
or spectroscopic grade purity. A flow cell of 1.55 cm path
length was used for spectral recording. The solutions for pulse
radiolysis were adjusted to the desired pH by using HClO4, H2-
† Also affiliated with the Jawaharlal Nehru Center for Advanced Scientific
Research, Bangalore, India.
X Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00992-6 CCC: $12.00 © 1996 American Chemical Society

Reduction of Carbonyl-Containing Perfluoroaromatic Compounds
J. Phys. Chem., Vol. 100, No. 33, 1996 14023
produced by rapid loss of fluoride (reaction 2), because such
radicals are known to have
eaq- + PFA f PFA•-
(1)
(2)
C6F5COCH3•- f •C6F4COCH3 + F-
weak absorption below 350 nm and rapidly abstract hydrogen
from alcohol.15 Since the radicals produced by fluoride
elimination from PFA are not expected to have reducing
properties, a direct evidence for the formation of the radical
anion can be obtained from intermolecular electron transfer from
PFA•- to a suitable acceptor discussed below. PFA•- decays
with a rate constants of (7 ( 3) × 103 s-1 presumably by
fluoride elimination.
PFA•- protonates at lower pH and the spectrum exhibits a
new absorption peaks at 270 and 460 nm (ꢀ460 ) 900 L mol-1
cm-1). The spectrum (Figure 1) was recorded at 2 µs after pulse
radiolysis of a deoxygenated solution of 7.8 × 10-4 mol L-1
PFA containing 0.5 mol L-1 t-BuOH at pH 4.8. The spectrum
is different from that of cyclohexadienyl radical obtained by
addition of H• atom to PFA,
Figure 1. Transient absorption spectra obtained on pulse radiolysis
of deoxygenated solution of (2.7-8) × 10-4 mol L-1 PFA containing
0.9 mol L-1 t-BuOH (9) at pH 9.5 (20.3 Gy), and (b) at pH 4.8 (24.6
Gy). Inset: Dependence of the absorbance at 460 nm on pH of the
solution (42 Gy).
H+ + PFA•- h PFAH•
(3)
TABLE 1: Rate Constants of Reactions of e-aq, H, and OH
with C6F5COX
indicating that the site of protonation and the localization of
the unpaired electron spin density is on the oxygen atom of the
carbonyl group. The pKa of the radical anion was determined
from the dependence of the PFA•- absorbance at 440 nm as a
function of pH in the pulse radiolysis of deoxygenated solutions
of 1.3 × 10-3 mol L-1 PFA containing 0.2 mol L-1 t-BuOH
(insert of Figure 1). The pKa ) 7.5 of PFAH• obtained is 2.5
pKa unit more acidic than that of the pKa ) 10 of acetophenone
anion,14 as expected for the effect of perfluorination.16
radical
substrate
pH
k/(L mol-1 s-1
)
e-
PFA
PFB
PFBA
PFBA
PFA
7
7
12
0.6
7
7
0.6
9
13.3
(1.9 ( 0.3) × 1010
(2.4 ( 0.4) × 1010
(9.6 ( 1.5) × 109
(5.5 ( 1.0) × 107
(1.5 ( 0.3) × 109
(2.0 ( 0.3) × 109
(1.1 ( 0.2) × 109
(1.1 ( 0.2) × 1010
(1.2 ( 0.2) × 108
aq
e-
aq
e-
aq
H•
OH•
OH•
OH•
OH•
O•-
PFB
PFBA
PFBA
PFBA
Direct evidence that the spectrum is due to the radical anion
PFA•- was obtained from the observation of intermolecular
electron transfer from PFA•- to p-benzoquinone (Q) and methyl
KPO4, Na2HPO4, Na2B2O7, and NaOH and were saturated with
high-purity N2, O2, or N2O gas prior to the introduction of
volatile compounds such as PFA, PFB, etc. Distilled water was
further purified in a Millipore Milli-Q system.
PFA•- + MV2+ f PFA + MV•+
PFA•- + Q f PFA + Q•-
(4)
(5)
viologen (MV2+) to yield the radical Q•- and MV•+, respec-
tively. The formation of MV•+ and Q•- in reactions 4 and 5
were confirmed from their absorption spectra.17,18
Results and Discussion
Reduction of Pentafluoroacetophenone. Radiolysis of
water produces eaq-, H•, and OH• as primary reactive radicals.
The reaction of PFA with hydrated electrons was studied under
condition where OH• was scavenged by 2-methyl-2-propanol
(t-BuOH), which transforms it to a relatively unreactive radical.
Hydrated electrons react with PFA with a rate constant of k1 )
(1.9 ( 0.4) × 1010 L mol-1 s-1 at pH 7, as determined from
Figure 2 presents time-resolved spectra obtained on pulse
radiolysis of deoxygenated solution of 8.3 × 10-4 mol L-1 PFA
containing 2.1 × 10-5 mol L-1 MV2+ and 0.8 mol L-1 t-BuOH
at pH 9.4. In this solution 91% of the hydrated electrons
produced on radiolysis react with PFA and the remaining 9%
react with MV2+. The spectrum recorded at 1 µs after the pulse
shows absorption bands at 440 nm due to PFA•- and an intense
band at 605 nm due to MV•+ formed directly by reaction of
MV2+ with hydrated electrons. The decay of PFA•- absorption
at 440 nm is accompanied by concurrent increase in the MV•+
absorption at 605 nm due to electron transfer from PFA•- to
MV2+. At 15 µs after the pulse, the spectrum obtained is
entirely due to the radical cation MV•+ absorption. This shows
that the decay of 440 nm absorption led to the formation of
MV•+. The rate constant k4 ) (2.5 ( 0.4) × 109 M-1 s-1 was
determined by monitoring the rate of formation of MV•+ at 605
nm as a function of MV2+ concentration in the pulse radiolysis
of deoxygenated solutions containing 1.4 × 10-3 mol L-1 PFA
and 1 mol L-1 t-BuOH at pH 9.6. The rate constant k5 ) (3.3
( 0.5) × 109 L mol-1 s-1 was determined by monitoring the
rate of formation of benzosemiquinone absorption at 430 nm
-
the dependence of the rate of eaq decay monitored at 700 nm
as a function of PFA concentration in the pulse radiolysis of
deoxygenated solutions containing 1 mol L-1 t-BuOH. The rate
constants of the reactions of PFA with the primary radicals of
water radiolysis are summarized in Table 1.
The transient absorption spectrum recorded at 5 µs after pulse
radiolysis of deoxygenated solution of 3 × 10-4 mol L-1 PFA
containing 0.9 mol L-1 t-BuOH at pH 9.5 has absorption
maxima at 300 and 440 nm (Figure 1) with ꢀ440 ) 2100 L mol-1
cm-1. The spectrum is similar to the spectrum of the radical
anion of acetophenone.14 At low concentration of PFA (10-5
mol L-1), the rate of eaq decay at 700 nm was concomitant
-
with the formation of PFA•- absorption at 300 nm. This
indicates that the spectrum can be assigned to the radical anion
PFA•-. The spectrum cannot be due to the phenyl radicals

14024 J. Phys. Chem., Vol. 100, No. 33, 1996
Shoute and Mittal
TABLE 3: Comparison of the Rate Constants of Electron
Transfer Reactions from the Radical Anion of PFA and Ac
to Different Acceptors
donor
acceptor
product
PFA•-
Ac•-
MV2+
AQS-
Q
MV•+
AQS•2-
Q•-
(2.5 ( 0.4) × 109a
(2.5 ( 0.4) × 109a
(3.3 ( 0.5) × 109a
(7.5 ( 1.1) × 109b
(3.0 ( 0.4) × 109b
(3.1 ( 0.6) × 109c
a Rate constant in L mol-1 s-1 determined at pH 9.4-9.6. b Rate
constant determined at pH 12. c Rate constant determined at pH 9.4.
and III.
∆E ) 0.0591 log K
(II)
and
∆E ) E(PFA/PFA•-) - E(Phen/PhenH•)
(III)
Bipyridinium radical (pKa ) 14)19 produced by reduction of
2,2′-bipyridine (Bpy) also reduced PFA.
BpyH• + PFA f Bpy + PFA•- + H+
(8)
Figure 2. Time-resolved absorption spectra recorded at (b) 1, (2) 5,
and ([) 15 µs after pulse radiolysis of deoxygenated solution of 8.3 ×
10-4 mol L-1 PFA and 2.1 × 10-5 mol L-1 MV2+ containing 0.8 mol
L-1 t-BuOH at pH 9.4. Dose ) 15.2 Gy.
An increase in the rate constant k8 ) (1.1 ( 0.2) × 107 L mol-1
s-1 reflects the higher E(Bpy/BpyH•) ) -0.97 V of Bpy. The
rate was determined by monitoring the decay of BpyH• at 365
nm in deoxygenated solutions of 1.2 × 10-3 mol L-1 Bpy
containing 1 mol L-1 t-BuOH at pH 9.5.
TABLE 2: Rate Constants of Electron Transfer Reactions
Involving PFA
radical
substrate
pH
k/(L mol-1 s-1
)
The protonated radical PFAH• is a weaker reductant. The
rate constant
BpyH•
PhenH•
PFA•-
PFA•-
PFA•-
PFA•-
PFAH•
PFAH•
PFAH•
PFA
PFA
Phen
Q
9.5
9.4
9.4
9.5
9.6
9.4
4.7
4.8
4.7
(1.1 ( 0.2) × 107
(3.3 ( 2.0) × 106
(4.2 ( 2.0) × 106
(3.3 ( 0.5) × 109
(2.5 ( 0.4) × 109
(2.5 ( 0.4) × 109
(8.0 ( 2.0) × 108
(2.9 ( 0.4) × 109
(5.0 ( 0.8) × 109
PFAH• + MV2+ f PFA + MV•+ + H+
(9)
MV2+
AQS-
MV2+
AQS-
Q
k9 ) (8.0 ( 2.0) × 108 L mol-1 s-1 was determined from the
buildup of MV•+ at 605 nm as a function of MV2+ concentration
in deoxygenated solutions of 1.1 × 10-3 mol L-1 PFA
containing 0.7 mol L-1 t-BuOH at pH 4.7. The rate constant
k10 ) (5.0 ( 0.8) × 109 L mol-1 s-1 for reduction of Q was
as a function of Q concentration in the pulse radiolysis of
deoxygenated solutions of 4 × 10-3 mol L-1 PFA containing
0.5 mol L-1 t-BuOH at pH 9.5. The rate constants of electron
transfer reactions are summarized in Table 2.
PFAH• + Q f PFA + QH•
(10)
obtained from the dependence of the rate of build-up of Q•-/
QH• (pKa ) 4.2)18 absorption at 430 nm in deoxygenated
solutions of 1.1 × 10-3 mol L-1 PFA containing 0.7 mol L-1
t-BuOH at pH 4.7.
With the reducing radical (PhenH•), derived from reduction
of 1,10-phenanthroline (Phen) in reaction 6
eaq- + Phen/Bpy f PhenH•/BpyH• + OH-
PhenH• + PFA h Phen + PFA•- + H+
(6)
Electron transfer from the radical anion of a perfluorinated
compound to an acceptor is a topic of considerable interest
because the rate has been reported to be much less than that
would normally be expected from their reduction potentials.21
This is because of the drastic change in the geometry with the
formation of the radical anion. In order to see whether the
radical anions of pentafluoroarmatic compounds in aqueous
solution exhibit this property, we compare the rate constants
for electron transfer from PFA•- to several acceptors with that
of the corresponding rates of the radical anion of acetophenone
(Ac•-). The rate constants for electron transfer reactions from
Ac•- to different acceptors were determined by monitoring the
formation of the product absorption as a function of acceptor
concentration in the pulse radiolysis of deoxygenated 3 × 10-3
mol L-1 Ac and 0.7 mol L-1 t-BuOH solution at pH 12. The
rate constants listed in Table 3 indicate that there is no
significant difference in their rates. This indicates that both
PFA•- and AC•- have similar geometry, and this conclusion is
in agreement with the theoretical calculation which shows that
the radical anions of perfluoroaromatic compounds which have
electron affinic substituent such as CO group are normal π
radicals.12
(7)
the reduction potential of the PFA/PFA•- couple was determined
from the kinetics of the electron transfer reaction between PFA•-
and Phen with phenanthroline (Phen) as reference couple
(E(Phen/PhenH•) ) -0.85 V vs NHE).19 The rate constants
of the forward (k7) and the backward (k-7) reactions were
determined from the dependence of the observed rate, kobs
,
monitored at 350 nm, on PFA concentration in the pulse
radiolysis of deoxygenated solutions of 1.3 × 10-3 mol L-1
Phen containing 1 mol L-1 t-BuOH at pH 9.4. From the linear
plot of kobs/[PFA] vs [Phen]/[PFA] as in eq I,20 k7 ) (3.3 (
2.0) × 106 L mol-1 s-1
kobs ) k7[PFA] + k-7[Phen]
(I)
and k-7 ) (4.2 ( 2.0) × 106 L mol-1 s-1 were obtained from
the intercept and the slope, respectively. A reduction potential
of E(PFA/PFA•-) ) -0.86 ( 0.1 V vs NHE at pH 9.4 was
obtained from the equilibrium constant K ) 0.8, using eqs II

Reduction of Carbonyl-Containing Perfluoroaromatic Compounds
J. Phys. Chem., Vol. 100, No. 33, 1996 14025
TABLE 4: Rate Constants for Electron Transfer Reactions
Involving PFB
radical
substrate
pH
k/L mol-1 s-1
(CH3)2C•OH
PFB•-
PFB
9
(5.9 ( 0.9) × 108
(3.2 ( 0.5) × 109
(5.4 ( 0.8) × 109
(3.5 ( 0.5) × 105
(9.0 ( 1.4) × 108
(2.5 ( 0.4) × 109
(2.7 ( 0.4) × 109
(1.9 ( 0.3) × 109
(2.2 ( 0.3) × 109
(3.6 ( 0.5) × 107
Q
9.5
9.2
4.8
4.8
9.4
4.8
9.3
9.3
9.3
PFB•-
MV2+
PFB
MV•+
PFBH•
PFB•-
MV2+
AQS-
AQS-
PFB
PFBH•
BpyH•
PhenH•
PFB•-
PFB
Phen
PFB•- + MV2+ f PFB + MV•+
(13)
formation rate of MV•+ monitored at 605 nm as a function of
MV2+ concentration in deoxygenated solution of 1.1 × 10-3
mol L-1 PFB containing 0.5 mol L-1 t-BuOH at pH 9.2. The
rate constant of electron transfer from PFB•- to Q of k14
)
(3.2 ( 0.5) × 109 L mol-1 s-1 was obtained by monitoring the
formation of benzosemiquinone Q•-
Figure 3. Transient absorption spectra obtained on pulse radiolysis
of deoxygenated solution of 9 × 10-4 mol L-1 PFB containing 0.5
mol L-1 t-BuOH (b) in solution at pH 9.4 recorded at 2 µs and (9) in
solution at pH 4.7 recorded at 4 µs after the pulse (25 Gy). Inset:
Dependence of the transient absorption at 420 nm on pH of the solution
(123 Gy).
PFB•- + Q f PFB + Q•-
(14)
absorption at 430 nm as a function of Q concentration in
deoxygenated solutions of 4 × 10-3 mol L-1 PFB containing
0.5 mol L-1 t-BuOH at pH 9.5.
A number of radiolytically generated reducing radicals also
reduce PFB to the radical anion. 2-Hydroxyl-2-propyl radical
derived from 2-propanol reduces PFB with a rate constant k15
) (5.9 ( 0.9) × 108 L mol-1 s-1 at pH 9.
Reduction of Pentafluorobenzaldehyde. Hydrated electrons
react with PFB with a rate constant of k11 ) (2.4 ( 0.4) × 1010
L mol-1 s-1 at pH 7. It was determined by monitoring the decay
-
rate of eaq at 700 nm as a function of PFB concentration in
the pulse radiolysis of deoxygenated solutions of 1 mol L-1
(CH3)2C•OH + PFB f (CH3)2CO + PFB•- + H+ (15)
t-BuOH at pH 7.
The rate constant was determined by monitoring the formation
of PFB•- at 420 nm as a function of PFB concentration in the
pulse radiolysis of N2O saturated solutions of 0.5 mol L-1
eaq- + PFB f PFB•-
(11)
-
The spectrum of the radical anion PFB•- formed on addition
of eaq- recorded at 2 µs after pulse radiolysis of deoxygenated
solution of 9 × 10-4 mol L-1 PFB containing 0.5 mol L-1
t-BuOH at pH 9.4, has bands at 285 and 430 nm (Figure 3)
with ꢀ430 ) 800 L mol-1 cm-1. The spectrum is similar to that
of the benzaldehyde radical anion (λmax 300 and 435 nm).22
2-propanol at pH 9. In these solutions eaq produced on
radiolysis reacts with N2O to form OH• which subsequently
abstracts hydrogen from 2-propanol to form 2-hydroxyl-2-propyl
radical. The rate constant k16 ) (1.9 ( 0.3)109 L mol-1 s-1
for reduction of PFB by bipyridyl radical (BpyH•) was
determined by monitoring the decay of BpyH• at 365 nm as a
function of Bpy
PFB•- decays with a rate constant of (4 ( 2) × 103 s-1
,
presumably by fluoride elimination. The radical anion has a
pKa ) 7.2, which is 1.2 pKa unit more acidic than that of the
radical anion of benzaldehyde (pKa ) 8.4).22 The pKa was
determined from the dependence of
BpyH• + PFB f Bpy + PFB•- + H+
(16)
concentration in deoxygenated solutions of 7.2 × 10-4 mol L-1
Bpy containing 0.5 mol L-1 t-BuOH at pH 9.3.
The reduction potential of the PFB/PFB•- couple was
determined from the kinetics of the electron transfer reaction
using Phen/PhenH• as reference couple. The overall electron
transfer rate kobs for reaction 17 was
C6F5CHO•- + H+ h C6F5C•HOH
(12)
the PFB•- absorbance at 430 nm on the pH of the solution in
the pulse radiolysis of deoxygenated solutions of 1.7 × 10-3
mol L-1 PFB containing 0.2 mol L-1 t-BuOH. (insert of Figure
3). The spectrum of the conjugate acid (PFBH•) recorded at 4
µs after pulse radiolysis of deoxygenated 9 × 10-4 mol L-1
PFB solution containing 0.5 mol L-1 t-BuOH at pH 4.7 showed
a weak absorption band at 330 nm (Figure 3) as in the case of
benzaldehyde ketyl radical.22
PhenH• + PFB h Phen + PFB•- + H+
(17)
determined from the dependence of PhenH• decay at 350 nm
as a function of PFB concentration in deoxygenated solutions
of 6.2 × 10-4 mol L-1 Phen containing 0.5 mol L-1 t-BuOH at
pH 9.3. From the linear plot of kobs/[PFB] vs [Phen]/[PFB] and
using eq I, the rate constants k17 ) (2.2 ( 0.3) × 109 L mol-1
s-1, k-17 ) (3.6 ( 0.5) × 107 L mol-1 s-1, and the equilibrium
constant K ) 60 were obtained. This yields a reduction potential
E(PFB/PFB•-) ) -0.75 ( 0.1 V vs NHE at pH 9.3 using eqs
II and III.
The radical anion PFB•- was observed to transfer electron
to a number of compounds to form their radical anions (Table
4). These reactions are direct evidence that the species formed
on addition of eaq- to PFB is a radical anion. Reaction of PFB•-
with MV2+ led to electron transfer and formation of MV•+. The
rate constant k13 ) (5.4 ( 0.8) × 109 L mol-1 s-1 was obtained
from the dependence of the
Reduction of Pentafluorobenzoate. Hydrated electrons
react with pentafluorobenzoate (PFBA) with a rate constant of

14026 J. Phys. Chem., Vol. 100, No. 33, 1996
Shoute and Mittal
Figure 5. Transient absorption spectra of cyclohexadienyl radicals
obtained on pulse radiolysis of deoxygenated 0.3 mol L-1 t-BuOH
solution at pH 7 containing (9) 1.8 × 10-3 mol L-1 PFBA, (b) 1.8 ×
10-3 mol-1 PFA, and (2) 2 × 10-3 mol L-1 PFB (dose ) 40 Gy).
Figure 4. Transient absorption spectra of phenoxyl radicals obtained
on pulse radiolysis of N2O-saturated solutions of (9) 2.8 × 10-3 mol
L-1 PFBA (24 Gy), (2) 1 × 10-3 mol L-1 PFA (27 Gy), and (b) 1 ×
10-3 mol L-1 PFB (27 Gy).
TABLE 5: Rate Constants for Oxidation of Ascorbate by
Substituted Tetrafluorophenoxyl radicals
(9.6 ( 1.5) × 109 L mol-1 s-1 at pH 12 (Table 1). This reaction
does not lead to the formation of absorbing species which can
be attributed to the formation of radical anion and was not
observed to transfer electron to Q, in contrast to the radical
anions of PFA and PFB. In low-temperature glassy 2-meth-
yltetrahydrofuran matrix,10 the radical anion formed on addition
of electron to pentafluorobenzoic acid has absorption peaks at
330 and 540 nm. The radical anion of benzoic acid (C6H5C•
(OH)O-) has absorption peaks at 310 and 435 nm at pH 9.1.23
This indicates that in aqueous solution, the radical anion PFBA•-
formed rapidly loses fluoride.5 This is in agreement with
fluoride yield G(F-) ) 3.6 measured in the steady state
γ-radiolysis of deoxygenated 1 × 10-3 mol L-1 PFBA contain-
ing 2 × 10-3 mol L-1 carbonate and 1 mol L-1 t-BuOH solution
at pH 11. In N2O-saturated carbonate solution of 1 × 10-3
PFBA at pH 11, a fluoride yield of G(F-) ) 14 was obtained.
Reactions of C6F5COX with OH• Radicals. Reaction of
OH• radical was carried out in N2O-saturated solution where
radical
substrate
pH
k/(L mol-1 s-1
)
•OC6F4COCH3
•OC6F4CHO
•OC6F4COO-
AH-
AH-
AH-
9.3
9.3
9.4
(1.5 ( 0.3) × 109
(7.6 ( 1.0) × 108
(5.0 ( 0.8) × 108
18 and 19 was recorded at 1 µs after pulse radiolysis of N2O-
saturated 3 × 10-3 mol L-1 PFBA solution at pH 6.5. It has
λmax at 285 and 405 nm. Figure 4 also shows the spectrum of
the phenoxyl radical obtained on OH• addition to PFA (λmax
310 and 400 nm) recorded at 3 µs after pulse radiolysis of N2O-
saturated 1 × 10-3 mol L-1 PFA solution at pH 7. The
phenoxyl radical produced on addition of OH• to PFB recorded
at 2 µs after pulse radiolysis of N2O-saturated 1 × 10-3 mol
L-1 PFB solution at pH 7 has λmax at 275 and 395 nm. The
absorption spectra of tetrafluorophenoxyl radicals obtained are
similar to that of the pentafluorophenoxyl6,7 and other phenoxyl
radicals.26
The rate constants of OH• attack on C6F5COX (reaction 18)
were determined from competition kinetics using OH• + SCN-
reaction as reference by monitoring the change in the absorbance
(A) of (SCN)2•- as a function of C6F5COX concentration in the
pulse radiolysis of N2O-saturated 4 × 10-4 mol L-1 KSCN
solutions at pH 7. The rate constants obtained from the slope
of the plot of A0/A vs [C6F5COX]/[SCN-]27 are summarized in
Table 1.
-
eaq was converted into OH by reaction with N2O. Addition
of OH• to pentafluoroaromatic ring would lead to the formation
of isomers of hydroxycyclohexadienyl radical24 with the pre-
dominant addition at the ortho and the para positions. The
formation of HO bond to a carbon where F is attached is
unstable25 and it rapidly undergoes HF elimination to yield
phenoxyl radical.6,7
OH• + C6F5COX f HO•C6F5COX
(18)
(19)
As in the case of other phenoxyl radicals,7,28 substituted
tetrafluorophenoxyl radicals oxidized ascorbate (AH-) to ascor-
bate radical.
HO•C6F5COX f •OC6F4COX + H+ + F-
Addition of OH• to the ipso position may lead to the elimination
of carbonyl group and formation of pentafluorophenoxyl radical.
•OC6F4COX + AH- f -OC6F4COX + AH• (22)
OH• + C6F5COX f •C6F5(OH)COX
•C6F5(OH)COX f •C6F5O• + XCHO
(20)
(21)
The rate constants for reaction 22 were determined by monitor-
ing the formation of ascorbate radical at 360 nm as a function
of ascorbate concentration in the pulse radiolysis of N2O-
saturated solutions of ca. 10-3 mol L-1 substrate at pH 9.3.
The rate constants obtained are summarized in Table 5.
The spectrum of the phenoxyl radical (Figure 4) formed on
addition of OH• to pentafluorobenzoic acid (PFBA) by reactions

Reduction of Carbonyl-Containing Perfluoroaromatic Compounds
J. Phys. Chem., Vol. 100, No. 33, 1996 14027
Reactions C6F5COX with H• Radical. H• atoms normally
add to the benzene ring of aromatic compounds to form adduct,
cyclohexadienyl radical, which have absorption in the 300-
360 nm region.24 The adduct formed on H• addition to PFBA
has λmax at 310 nm. The spectrum (Figure 5) was recorded at
10 µs after pulse radiolysis of 1.8 × 10-3 mol L-1 PFBA
containing 0.3 mol L-1 t-BuOH at pH 2.2. The spectra of H•
atom adducts of PFA and PFB have λmax at 340 and 310 nm,
respectively. The rate constant (Table 1) for reaction of H•
atoms with PFBA was determined by monitoring the adduct
absorption at 310 nm as a function of PFBA concentration in
deaerated 0.3 mol L-1 t-BuOH solutions at pH 2.0.
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Conclusion
Addition of hydrated electrons to hexafluorobenzene,6
pentafluorophenol,7a and pentafluoroaniline7b in aqueous solution
led to rapid fluoride elimination because the unpaired electron
in these radical anions has significant σ character. Increasing
the electron affinity of the carbonyl substituents in going from
-COO- to -CHO and -COCH3 led to increase stability of
the radical anion of perfluorinated compounds due to the
delocalization of the unpaired electron into the π orbital of the
CdO moiety which reduces the spin density on the C-F bond
and hence the rate of fluoride elimination. This leads to
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