Reaction of OH radicals with benzoquinone in aqueous solutions. A pulse radiolysis study

Article abstract of DOI:10.1039/a708772a

Hydroxyl radicals have been generated by pulse radiolysis in N₂O-saturated aqueous solutions. Their addition to 1,4-benzoquinone BQ (k₃ = 6.6 × 10⁹ dm³ mol^-1 s^-1 by competition with thiocyanate) in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone concentration, and its rate constant agrees with the value (k₃) obtained by competition. At the longer wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10^-4 mol dm^-3 (k₆ = 6.9 × 10⁵ s^-1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone-OH-adduct radical 1 undergoes rapid (k₄ = 2.5 × 10⁶ s^-1) keto-enol tautomerization yielding the 2,4-dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pK_a(2) ≈ 4.9 ± 0.2] has been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using ·OH in acidic or N₃· in neutral and basic solution. Its absorption characteristics compare favourably with those observed in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k₁₅ ≥ 2 × 10⁹ dm³ mol^-1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pK_a(4) = 4.1 ± 0.1; λ_max(4) = 380 nm; λ_max(4a) = 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into benzoquinone and hydroquinone (2k₁₆ = 3.1 × 10⁸ dm³ mol^-1 s^-1). In acidic solution the rate of oxidation of 2 by benzoquinone is considerably slower (k₁₃ ≤ 2.4 × 10⁷ dm³ mol^-1 s^-1). The assignment of the final product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and pK_a value of the authentic material obtained by the two-electron electrochemical oxidation of 1,2,4-trihydroxybenzene.

Full text of DOI:10.1039/a708772a

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Reaction of OH radicals with benzoquinone in aqueous solutions.
A pulse radiolysis study
Man Nien Schuchmann,a Eberhard Bothe,a Justus von Sonntagb and
Clemens von Sonntag*,a
a Max-Planck-Institut für Strahlenchemie, Stiftstr. 34–36, PO Box 101365,
D-45413 Mülheim an der Ruhr, Germany
b Institut für Oberflächenmodifizierung, Permoserstr. 15, D-04303 Leipzig, Germany
Hydroxyl radicals have been generated by pulse radiolysis in N2O-saturated aqueous solutions.
Their addition to 1,4-benzoquinone BQ (k3 ؍ 6.6 × 109 dm3 mol؊1 s؊1 by competition with thiocyanate)
in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at
around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone
concentration, and its rate constant agrees with the value (k3) obtained by competition. At the longer
wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10؊4 mol dm؊3
(k6 ؍ 6.9 × 105 s؊1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone–
OH-adduct radical 1 undergoes rapid (k4 ؍ 2.5 × 106 s؊1) keto–enol tautomerization yielding the 2,4-
dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pKa(2) ≈ 4.9 0.2] has
ؒ
been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using OH in acidic
ؒ
or N3 in neutral and basic solution. Its absorption characteristics compare favourably with those observed
in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed
that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k15 у 2 × 109 dm3
mol؊1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pKa(4) ؍ 4.1 0.1; ëmax(4) ؍ 380 nm;
ëmax(4a) ؍ 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into
benzoquinone and hydroquinone (2k16 ؍ 3.1 × 108 dm3 mol؊1 s؊1). In acidic solution the rate of oxidation
of 2 by benzoquinone is considerably slower (k13 р 2.4 × 107 dm3 mol؊1 s؊1). The assignment of the final
product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and
pKa value of the authentic material obtained by the two-electron electrochemical oxidation of
1,2,4-trihydroxybenzene.
Quinones and phenoxyl radicals have been studied extensively
since the early days of radiation chemistry because of their
Experimental
involvement in many biologically important processes such as
photosynthesis, electron transport (see ref. 1 for a collection of
references) and in the antioxidant properties of such vitamins
as A, E and K as well as of many natural phenolic substances in
plants.2–4 Benzoquinone is employed in certain cases to increase
the shelf-life of industrial polymerizable monomers.
1,4-Benzoquinone (Fluka) was purified by fractional sublim-
ation under argon. Hydroquinone (Fluka, >99%), 1,2,4-tri-
hydroxybenzene (Aldrich, 99%), sodium azide (Fluka, >99.5%)
and other chemicals used were available commercially. Water
was purified by a Millipore Milli-Q system. Solutions of
benzoquinone and of 1,2,4-trihydroxybenzene were freshly
prepared by dissolving the solid substance in oxygen-free N2O-
saturated water immediately before each experiment and were
kept from exposure to either light or air during the experiment.
Pulse radiolysis was carried out using 0.4 µs electron pulses
generated by a 2.8 MeV van-de-Graaff electron accelerator at
the Max-Planck-Institut für Strahlenchemie. The pulse radioly-
sis apparatus and the set-up of the optical and AC-conductance
detection methods as well as the corresponding computer-
assisted data processing arrangement have been described else-
where.7 The experiments with the higher 1,4-benzoquinone
concentrations were carried out with the 11 MeV linear acceler-
ator (Elektronika, Torium, Moscow) at the Institut für Ober-
flächenmodifizierung using 17 Gy pulses of 5 ns duration. Thio-
cyanate dosimetry was used in the optical measurements, and
dimethyl sulfoxide in N2O-saturated solution at pH 4 was used
for dose determination in the conductance measurements.8
For the evaluation of the conductivity signals the equivalent
conductance of the anions was assumed to be equal to that of
the methyl sulfinate anion formed in the dimethyl sulfoxide
dosimeter system.
The reaction of the OH radical with 1,4-benzoquinone has
been investigated by Adams and Michael by pulse radiolysis,
and the observed transient has been attributed to the benzo-
quinone–OH-adduct radical.5 In a more recent study of
the reaction of the methyl radical with benzoquinone,
Veltwisch and Asmus have proposed that the benzoquinone
methyl adduct radical tautomerizes to the methyl semi-
quinone.6 This initial process is then followed by the reaction
of this semiquinone-type radical with a second molecule
of benzoquinone to produce 2-methyl-1,4-benzoquinone
(identified as a final product) and the unsubstituted semibenzo-
quinone radical. Due to the slowness of the reaction of the
methyl radical with benzoquinone the kinetics of the tauto-
merization and the subsequent oxidation could not be
resolved.
In the present paper it will be shown that the benzoquinone–
OH-adduct radical is very short-lived; it rapidly tautomerizes
and is further oxidized by excess benzoquinone. Since all
these reactions occur early on the µs timescale, the absorp-
tion reported by Adams and Michael is in fact already due to a
mixture of the semiquinone radical anion and 2-hydroxy-1,4-
benzoquinone.
In the absence of authentic 2-hydroxy-1,4-benzoquinone,
a putative product, which was unavailable commercially, an
J. Chem. Soc., Perkin Trans. 2, 1998
791

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N2-saturated aqueous solution of 1,2,4-trihydroxybenzene
(5 × 10Ϫ4 mol dmϪ3) in 0.1 mol dmϪ3 KCl was electrolysed in
an electrochemical set-up.9 This consists essentially of a closed
coulometric cell with two platinum electrodes and a SCE as
reference.10 The potential drop across the cell is kept constant
by means of a EG & G 273A potentiostat. During the
electrolysis small samples (typically 10Ϫ3 dmϪ3) of the reaction
solution are taken from the cell at regular intervals via
a syringe-pump arrangement into an optical cell (0.1 cm
pathlength) in a Hewlett-Packard 8452A spectrophotometer
for analysis. Each sample is then returned to the electrolysis
cell after the spectrum is recorded. The pH of the 1,2,4-
trihydroxybenzene solution was adjusted by using 5 × 10Ϫ3
mol dmϪ3 acetate, phosphate or borate buffers between pH 3
and 8.
Results and discussion
Fig. 1 Transient spectrum (᭹) obtained 8 µs after electron pulse
Radical-generating system
Hydroxyl radicals have been generated in the radiolysis of water
[reaction (1)]. The radiation chemical yields (G values) of these
irradiation (ca. 6 Gy/pulse) of N2O-saturated solution of BQ (4 × 10Ϫ4
mol dmϪ3) at pH 6.2. Inset A: absorbance build-up at 330 nm; inset B:
absorbance build-up at 440 nm. For comparison the spectrum of the
semibenzoquinone radical anion (᭝) obtained 8 µs after electron pulse
irradiation (ca. 5 Gy/pulse) of an N2O-saturated solution of NaN3
(5 × 10Ϫ2 mol dmϪ3) and hydroquinone (10Ϫ3 mol dmϪ3) at pH 7.6 is
also shown.
ionizing
Ϫ
ϩ
ؒ
ؒ
(1)
H2O
eaq , OH, H , H , H2O2, H2
radiation
Ϫ
Ϫ7
ؒ
primary water radicals are G(eaq ) ≈ G( OH) = 2.9 × 10 mol
JϪ1 and G(H ) = 0.6 × 10Ϫ7 mol JϪ1. N2O is used to convert the
solvated electrons into OH [reaction (2)].
ؒ
11
ؒ
Ϫ
Ϫ
ؒ
eaq ϩ N2O ϩ H2O → OH ϩ OH ϩ N2
(2)
Rate constant of the reaction of the OH radical with
benzoquinone
Pulse radiolysis of an N2O-saturated aqueous solution of
benzoquinone (BQ) at pH 6.2 (natural pH) produces a transient
spectrum (represented by ᭹ in Fig. 1) which agrees with the
one reported by Adams and Michael.5 For the determination
of the rate constant of the addition of OH radicals to BQ
[reaction (3)] they monitored the absorbance build-up at 450
O
O
H
•OH
OH
H
(3)
•
Fig. 2 Dependence of the observed first-order rate constants of
absorbance build-up at 330 (᭝) and at 440 nm (᭡) on BQ concentration
in N2O-saturated solution at pH 6.2 (conditions as in Fig. 1). Inset: pH
dependence of kobs of absorbance build-up at 440 nm in 10Ϫ3 mol dmϪ3
BQ solution.
O
O
1
nm from which a rate constant of k3 = 1.2 × 109 dm3 molϪ1 sϪ1
was calculated.5 At the time, the mechanistic complexities (see
below) were not realized. We have now redetermined this rate
constant by competition kinetics with thiocyanate and obtained
tored at this wavelength has been plotted against the BQ
concentration (᭝ in Fig. 2). Indeed, according to this linear
plot, k3 = 5.0 × 109 dm3 molϪ1 sϪ1 is obtained. This is in good
agreement with the value of 6.6 × 109 dm3 molϪ1 sϪ1 obtained
independently from competition kinetics.
k3 = 6.6 × 109 dm3 molϪ1 Ϫ1. Corrections have been made for
s
the absorbance of the transient species resulting from reaction
(3) at 480 nm where (SCN)2 has its maximum. Further data
presented below support the new (higher) value.
Ϫ
᝽
Unlike the fast build-up at 330 nm, the slower absorbance
build-up at 440 nm represents a much more complicated pro-
cess. Its first-order rate constant (᭡ in Fig. 2) increases with the
BQ concentration only up to about 4 × 10Ϫ4 mol dm3, at higher
The most conspicuous feature of the transient spectrum
observed in the BQ solution (represented by ᭹ in Fig. 1) is
that the rate of absorbance build-up is several times higher
when monitored at around 330 nm (inset A in Fig. 1,
BQ concentration kobs reaches a plateau value of 6.9 × 105 sϪ1
,
.
kobs = 2.1 × 106
s
Ϫ1) than at 400 nm and above (cf. inset B
independent of the BQ concentration up to 2 × 10Ϫ3 mol dmϪ3
in Fig. 1, kobs = 6.9 × 105
s
Ϫ1). These two distinct kinetic
Pulse radiolysis experiments carried out at the Institut für
Oberflächenmodifizierung have shown that kobs remains con-
stant up to a BQ concentration of 6 × 10Ϫ3 mol dm3 (data not
shown). Furthermore, when the pH of the BQ solution is varied
(at a fixed BQ concentration of 10Ϫ3 mol dmϪ3) this plateau
value of kobs decreases with increasing acidity of the solution
(from 6.9 × 105 at pH 6.2 to 2.4 × 104 sϪ1 at pH 2.5). This pH
dependence is a sigmoidal curve with an inflection point at pH
4.2 (inset in Fig. 2).
processes represent the formation of two different species.
Evidently only the build-up at 330 nm is sufficiently fast to
reflect reaction (3), since k3 is known from competition kinetics
(see above).
Fig. 2 shows some aspects of the kinetic behaviour of the
system in near neutral solution. To show that the above
observed fast absorbance build-up at 330 nm represents the
initial addition of OH radicals to BQ [reaction (3)], kobs moni-
792
J. Chem. Soc., Perkin Trans. 2, 1998

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Fig. 3 Pulse radiolysis (5 ns pulses of ca. 17 Gy/pulse) of N2O-
saturated BQ solution (5 × 10Ϫ3 mol dmϪ3) at pH 2. Spectra recorded
80 (᭿) and 750 ns (᭝) after the pulse. Inset: the absorption build-up at
330 nm.
Fig. 4 Pulse radiolysis of 1,2,4-trihydroxybenzene (10Ϫ3 mol dmϪ3) in
N2O-saturated solution, ca. 3 Gy/pulse. Experiments for pH >7 were
carried out in the presence of NaN3 (5 × 10Ϫ2 mol dmϪ3). The spectrum
of the 2,4-dihydroxyphenoxyl radical 2 at pH 3.3 (᭹) was obtained 30
µs after the pulse. The spectrum of the 2,4-dihydroxyphenoxyl radical
anion 2a at pH 7.6 (᭝) was obtained 16 µs after the pulse. Inset A: pH
dependence of the absorbance at 430 nm (᭡) and of G(Hϩ) (᭺)
obtained by conductance measurements. Inset B: Spectrum of 2a
obtained 16 µs after the pulse at pH 10 in the presence of 10Ϫ2 mol dmϪ3
borate buffer.
To obtain further information concerning the OH-adduct
radical 1 and its subsequent transformation the ns pulse radi-
olysis facilities were used to extend our kinetic measurements
into the very short time of <1 µs after the pulse.
Enolization of the benzoquinone–OH-adduct radical
deprotonation/protonation processes in C᎐H-acid systems are
Fig. 3 shows the transient absorption spectra in 5 × 10Ϫ3 mol
dmϪ3 BQ solution at pH 2 (spectra recorded in neutral solution
are distorted considerably by the semiquinone radical anion
which is produced at this high BQ concentration by direct scav-
enging of about 40% of solvated electrons by BQ and are not
shown here). Under these conditions the reactions of OH rad-
icals, H atoms and about one-fifth of solvated electrons with
BQ are completed within 50 ns after the pulse. The spectrum
recorded 80 ns after the pulse (᭿ in Fig. 3) is thus essentially the
spectrum of the OH-adduct radical 1 (with minor contributions
from the H-adduct and the semiquinone radical). As shown in
Fig. 3, radical 1 undergoes a transformation reaction to pro-
duce a new transient species, the spectrum of which (recorded
750 ns after the pulse, ᭝ in Fig. 3) has a maximum at 400 nm.
The inset in Fig. 3 shows the absorption build-up at 330 nm
comparatively slow.16
2,4-Dihydroxyphenoxyl radical
To support the above reaction mechanism we have taken a
closer look at the 2,4-dihydroxyphenoxyl radical 2 which can
be readily generated directly from 1,2,4-trihydroxybenzene
independent of the BQ system. This can be achieved either by
addition of the hydroxyl radical [reaction (7)] followed by water
elimination [reaction (8)] of the tetrahydroxycyclohexadienyl
radical in acidic solution (spectrum ᭹ in Fig. 4), similar to the
case of hydroquinone and other polyhydroxycyclohexadienyl
radicals,5,17 or by the reaction of the azide radical [reaction (9)]
O•
OH
OH
OH
with kobs = 2.5 × 106 sϪ1
.
OH
OH
•OH
(7)
–H2O
(8)
HO•
We suggest that the OH-adduct radical 1 undergoes a very
rapid, possibly water-assisted keto–enol tautomerization to
give the 2,4-dihydroxyphenoxyl radical 2 [equilibrium (4),
OH
OH
OH
2
O•
N3– + •OH
OH
N3• + OH–
(9)
O
H
OH
(H2O)
(H2O)
(4)
O•
OH
•
(–4)
OH
N
OH
H
3•/–N3–,–2H+
(10)
O
OH
1
2
O•
O–
OH
OH
(5)
(–5)
(6)
2a
+H+
(–6)
in basic solution [reaction (10)] [spectra at pH 7.6 (᭝) and at
pH 10 (inset B) in Fig. 4].
O–
A comparison of the authentic spectrum of radical 2 (᭹ in
Fig. 4) with the spectrum obtained above in 5 × 10Ϫ3 mol dmϪ3
BQ solution (᭝ in Fig. 3) confirms our assignment of the latter
to that of radical 2.
Fig. 4 also shows the shift of the absorption maximum of
radical 2 at 400 nm in acidic solution to 430 nm at pH 7.6. In
the inset A of Fig. 4 the absorbance at 430 nm as well as the
value of G(Hϩ) obtained by conductometric measurements are
plotted as functions of pH. From this sigmoidal curve the first
pKa(2) = 4.9 0.2 is obtained [equilibrium (6)]. With this pKa
2a
k4 = 2.5 × 106 sϪ1]. Not surprisingly, reaction (4) is much faster
than the enolization of non-radical enone species.12,13 The
spectrum obtained in Fig. 3 (᭝) is attributed to radical 2. This
is likely to have its first pKa value in the pH region of 4–5 as
found for other hydroxyphenoxyl radicals of similar type.14,15 In
neutral solution it dissociates into the 2,4-dihydroxyphenoxyl
radical anion 2a [equilibrium (6)]. The formation of 2a
from 1 directly [reaction (5)] is considered insignificant as
J. Chem. Soc., Perkin Trans. 2, 1998
793

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O•
OH
OH
O•
N3• / –HN3
(11)
(12)
+ H+
(–12)
OH
O–
3a
3
anion 3a (᭝ in Fig. 1) [pKa(3) = 4.0, equilibrium (12)]5 pro-
duced by the reaction of the azide radical with hydroquinone
[reactions (9) and (11)], the close similarity of these two spectra
in the wavelength region 300–450 nm is obvious.
The divergence of the two spectra in Fig. 1 in the wavelength
region above 450 nm suggests the presence of an additional
species besides the semibenzoquinone radical anion 3a. Indeed,
whereas the absorbance < 320 nm disappears (bimolecular
rate constant, 2k16 = 3.1 × 108 dm3 molϪ1 Ϫ1), the absorbance
s
between 320–500 nm decays only partly, and that above 500 nm
does not change at all, even at 0.1 s after the pulse, i.e. the
absorption at 500 nm can be ascribed to a permanent product
with an absorption maximum at 480 nm at pH 6.2 as shown in
Fig. 6.
The explanation of these phenomena is as follows. At pH 6.2,
the 2,4-dihydroxyphenoxyl radical 2 is deprotonated to the
corresponding semiquinone radical anion 2a [equilibrium (6)].
This is rapidly oxidized by BQ [reaction (15)] to produce the
semiquinone radical anion 3a and the 2-hydroxy-1,4-benzo-
quinone anion 4a. The former subsequently decays bimolec-
Fig. 5 Pulse radiolysis of 1,4-benzoquinone (10Ϫ3 mol dmϪ3) in N2O-
saturated solution at pH 3.1, ca. 3 Gy/pulse. Rapid conversion of
radical 1 into the 2,4-dihydroxyphenoxyl radical 2. Spectra of the tran-
sients taken 1 (᭹) and 2 µs (᭝) after the pulse. Inset A: absorption
build-up at 370 nm. Inset B: absorption build-up at 470 nm. For
comparison, the spectrum taken 30 µs after the pulse in the radiolysis of
1,2,4-trihydroxybenzene (10Ϫ3 mol dmϪ3) in N2O-saturated solution at
pH 3.3 is also shown (main graph, dotted line).
value the acid dissociation [reaction (6)] of radical 2 has a
rate constant of k6 = 6.3 × 105 sϪ1 if one assumes kϪ6 = 5 × 1010
dm3 molϪ1 sϪ1
.
O
O•
O•
O
The assignment of the second pKa value of radical 2 is more
difficult. From neutral to basic pH (cf. inset B in Fig. 4) the
main absorption of radical anion 2a appears unchanged at 430
nm (with a shoulder at 410 nm). Between pH 8 and 9 there is a
sharp reduction of the absorbance at 360 nm. This is accom-
panied by a very weak increase at the longer wavelength region
650–700 nm. However, since these changes appear not to follow
a smooth sigmoidal curve pattern, other possible factors such
as a base-catalysed tautomerization of 2a to a dienone radical
anion (cf. ref. 17) might bring about these changes rather than
the deprotonation of the radical anion 2a to the corresponding
dianion.
OH
OH
O
+
(13)
O
OH
OH
4
2
3
(–6) (6)
(–12) (12)
+H+ –H+
(–14) (14)
+H+ –H+
+H+ –H+
O
O
O•
O
O•
/–H+
OH
O–
Radiolysis in acidic solution
Further confirmation of the above reaction mechanism was
obtained in µs pulse radiolysis of low concentration of BQ in
acidic solution. As shown in Fig. 5 the situation here is similar
to that observed in the ns timescale for a high concentration of
BQ (cf. Fig. 3). The initial spectrum taken 1 µs after the pulse
(᭹ in Fig. 5, attributed to 1), is transformed rapidly into spec-
trum ᭝ which resembles closely that of the authentic 2,4-
dihydroxyphenoxyl radical 2 (dotted line in Fig. 5 is taken from
the spectrum in Fig. 4). This transformation is best observed in
the wavelength region >430 nm where radical 1 shows stronger
absorption than radical 2. In the inset B of Fig. 5 the initial fast
build-up immediately after the pulse [reaction (3)] is followed
by an absorption decay as 1 tautomerizes into 2 [reaction (4)].
This absorption decay is too fast here for kinetic evaluation,
but its rate is comparable to the rate of reaction (4) determined
above.
+
(15)
O–
O–
O
4a
2a
3a
OH
O
2 x
2 H+
+
(16)
OH
O
ularly producing hydroquinone and BQ [reaction (16)]. The
final product observed 0.1 s after the pulse with an absorption
maximum at 480 nm is therefore attributed to 4a (Fig. 6).
Reaction (15) is similar to the rapid oxidation of the methyl-
semiquinone radical by BQ reported previously.6 This results in
the quantitative formation of methylbenzoquinone.
The second (slower) absorption build-up observed after 2 µs
(cf. insets in Fig. 5) represents the subsequent reaction of radical
2 which is much slower in acidic solution than in neutral solu-
tion as already mentioned above (cf. inset in Fig. 2).
Returning to Fig. 2, one observes that at BQ concentrations
<4 × 10Ϫ4 mol dmϪ3, kobs at 440 nm (᭡) increases with BQ con-
centration. Reaction (15) is responsible for this build-up. From
Oxidation of the 2,4-dihydroxyphenoxyl radical by benzoquinone
At the end of a build-up process with kobs = 6.9 × 105 sϪ1 the
spectrum recorded 8 µs after the pulse (᭹ in Fig. 1) shows that
the transient here is neither the radical 2 nor its anion 2a
(cf. Fig. 4). On the other hand, if this spectrum is compared
with the spectrum of the well-known semibenzoquinone radical
the slope of this plot, k15 ≈ 2 × 109 dm3 molϪ1 sϪ1 can be esti-
Ϫ3
mated. At BQ concentrations >4 × 10Ϫ4 mol dm
k
obs
reaches
the limit at 6.9 × 105 sϪ1 which is the same as k6 calculated
for the acid dissociation reaction of radical 2. In other words,
under these conditions the rate of formation of the products in
reaction (15) is limited by k6.
794
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The value estimated above for reaction (15) is not unusual for
rate constants of electron transfer for semiquinones to benzo-
quinones2 since the difference in one-electron reduction poten-
tials (the driving force) of such a reaction, ∆E ≈ 210 mV [the
provided by the measurement of conductance changes. In an
N2O-saturated solution of BQ at pH 6.2 (natural) irradiated
with an electron pulse, following the initial jump caused by the
formation and subsequent neutralization of Hϩ within the
pulse [reaction (1)] a transient conductance build-up equivalent
to G(Hϩ) = 11.8 × 10Ϫ7 mol JϪ1 is observed several µs after
the pulse (Fig. 7A). This conductance increase subsequently
decays via second-order kinetics to a lower and permanent
level (monitored up to 0.1 s after the pulse) equivalent to
G(Hϩ) = 5.5 × 10Ϫ7 mol JϪ1 (Fig. 7B).
reduction potential of benzoquinone2 E(Q/Q Ϫ) = 99mV at pH
᝽
7 and that of the 2,4-dihydroxyphenoxyl radical18 2 (or anion
Ϫ
ؒ
2a) E(ArO /ArO ) = Ϫ 110 mV at pH 13.5], is sufficiently
high. The rate constant for the analogous reaction by methyl-
semiquinone has been estimated to be ca. 4 × 108 dm3 molϪ1 sϪ1
on the basis of ∆E = 76 mV.6
In acidic solution the equilibrium (6)/(Ϫ6) lies increasingly to
the side of radical 2. The latter is oxidized by BQ [reaction (13)]
with more difficulty than its anion.18 Under these conditions
reaction (13) becomes the rate-determining step. Since kobs (440
nm) is reduced to 2.4 × 104 sϪ1 at pH 2.5 (cf. inset of Fig. 2),
At neutral pH, radical 3 is fully deprotonated [equilibrium
(12)/(Ϫ12); pKa(3) = 4.0]. The G value of proton formation
Ϫ7
ؒ
observed at twice the value of G(2) = G( OH) = 5.8 × 10 mol
JϪ1 6 µs after the pulse (Fig. 7A), indicates that product 4 is
also fully deprotonated at pH 6. After the disappearance of
the semibenzoquinone radical anion by bimolecular decay
[reaction (16)], the remainder of the conductance increase is
solely due to the final product, the anion of 2-hydroxy-1,4-
benzoquinone, 4a.
In acidic solution, the pattern of conductance change in
the N2O-saturated solution of BQ is similar to that observed
at pH 6.2, except that the magnitude of the conductance
increase [i.e. G(Hϩ)] becomes smaller in more acidic solution
(Fig. 8). Both curves in Fig. 8, measured 20 µs and 0.1 s after
the pulse, show their inflection points at pH ca. 4.3. The curve
measured at 20 µs reflects the sum of the semibenzoquinone
radical 3 and 2-hydroxy-1,4-benzoquinone 4, whereas the
value measured at 0.1 s, i.e. after all radicals have disappeared
by bimolecular decay, must reflect pKa(4) = 4.3 [equilibrium
(14)].
it follows that k13 р 2.4 × 107 dm3 molϪ1 sϪ1
.
From the above reaction scheme, where the subsequent
reactions of radical 2 depend on its state of dissociation
[equilibrium (6)/(Ϫ6)], it can be readily shown that at the
inflection point of the kinetic-pK plot shown in the inset of
Fig. 2, [Hϩ] = (k6 ϩ k15 × [BQ])/kϪ6. Under these conditions
k15 × [BQ] = 2 × 109 × 10Ϫ3 = 3 k6. Hence, [Hϩ] = 4 k6/kϪ6
,
pH = Ϫ0.6 ϩ pKa = Ϫ0.6 ϩ 4.9 = 4.3. The actual inflection
point found at pH 4.2 neatly confirms the above reaction
scheme.
Further support of the proposed mechanism by conductance
measurements
Additional evidence in favour of the foregoing mechanism is
Fig. 6 Pulse radiolysis of N2O-saturated solution of BQ (10Ϫ3 mol
dmϪ3; ca. 15 Gy/pulse). Spectra of the final products taken 0.1 s after
the pulse at pH 2.5 (᭿, 2-hydroxy-1,4-benzoquinone) and at pH 6.2
(᭝, anion of 2-hydroxy-1,4-benzoquinone).
Fig. 8 Dependence of proton formation monitored as conductance
increase 20 µs (᭿) and 0.1 s (᭝) after the pulse (6–8 Gy/pulse) in N2O-
saturated solutions of BQ (10Ϫ3 mol dmϪ3
)
Fig. 7 Pulse radiolysis of N2O-saturated solution of BQ (10Ϫ3 mol dmϪ3) at pH 6.2. Conductance changes in the µs (A), and ms (B) time ranges.
The initial spike appearing during the first 3 µs after the pulse arises from interference of the pulse with the detection devices and does not
reflect the actual conductance change.
J. Chem. Soc., Perkin Trans. 2, 1998
795

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pKa(4) = 4.0 is obtained, which is in good agreement with the
value determined by the conductometry method.
The assignment of the final product as 2-hydroxy-1,4-
benzoquinone 4 receives further support from the results of an
electrochemical oxidation of 1,2,4-trihydroxybenzene to give
the product 4.19 The cyclic voltammograms of 1,2,4-trihydroxy-
benzene exhibit irreversible waves at 0.2–0.4 V vs. SCE, depend-
ing on pH. The electrolysis was carried out at 0.5 V. In Fig. 10
the spectral changes during the anodic oxidation process are
shown at pH 8.2 (upper) and at pH 2.0 (lower). After about
180 s the current dropped to background level, and two elec-
trons per molecule of the starting material were transferred.
The final product 2-hydroxy-1,4-benzoquinone 4 thus obtained
(spectra shown in bold) has an absorption maximum at 482 nm
(ε = 2200 dm3 molϪ1 cmϪ1) at pH 8.2 while in acid solution its
maximum is shifted to 380 nm (ε = 1400 dm3 molϪ1 cmϪ1).
The absorption characteristics of product 4 are thus identical
to those of the final product obtained by pulse radiolysis (Fig.
6). The pH dependence of the absorption of this product moni-
tored at 482 nm (inset in Fig. 10) shows an inflection point at
pH 4.1 which is attributed to pKa(4). This value is in good
agreement with that determined by pulse radiolysis (conduct-
ance and optical measurements, see above).
Fig. 9 Pulse radiolysis of N2O-saturated solutions of BQ (10Ϫ3 mol
dmϪ3). pH dependence of the absorbance at 380 nm (᭝) and at 480 nm
(᭹) of the final product (2-hydroxy-1,4-benzoquinone 4) monitored
0.1 s after the pulse.
Acknowledgements
This work was initiated and partially financed by Bundesminis-
terium für Bildung, Wissenschaft, Forschung and Technologie
Project 02WT9583.
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Fig. 10 Time-dependent spectra taken during the two-electron oxid-
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Upper: experiment carried out at pH 8.2. Lower: experiment carried
out at pH 2.0. Inset: pKa plot of the absorbance monitored at 482 nm.
pKa value of product 4 determined by optical and spectro-
electrochemical measurements
It has been shown in Fig. 6 that the absorption characteristics
of the final product assigned to 2-hydroxy-1,4-benzoquinone 4
change from acidic pH to neutral solution. The absorption
maximum at 480 nm (ε = 1800 dm3 molϪ1 cmϪ1 assuming a G
value of 5.6 × 10Ϫ7 mol JϪ1 for 4a) in neutral solution is
replaced by a maximum at 380 nm at pH 2.5 (ε = 1080 dm3
molϪ1 cmϪ1, for 4). The changes in absorbance of the BQ solu-
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curves shown in Fig. 9. From the intercept of these two curves,
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Paper 7/08772A
Received 5th December 1997
Accepted 8th January 1998
796
J. Chem. Soc., Perkin Trans. 2, 1998