R. C. BAETZOLD
3596
method of Furman and Miller.21 The area under the esr absorption curve of a standard K(aMo(CN)8 solution was obtained by double integration and compared with the area of the esr signal obtained from the sample, allowing the number of spins in the sample to be estimated. The error of the method is estimated to be f15-20%. The g values for the esr signals were determined by comparison with the g value of l,l-diphenyl-2-picrylhydrazyl, either as an internal or external standard.
All esr spectra were obtained on a Varian V-4500-10 X-band spectrometer equipped with 100-KHz field modulation.
Acknowledgment. This work was supported by the U. S. Public Health Service, Grant GM 08347, National Institute of General Medical Sciences, and by Research Career Development Award 5-1(3-CRiI-22,643 to J. T. S. (21) N. H. Furman and C. 0. Miller, Inorg. Syn., 3, 160 (1950).
Kinetics of Redox Reactions of Oxidized p-Phenylenediamine Derivatives.
I1
by R. C. Baetzold Research Laboratories, Eaatman Kodak Company, Rochester, New York 14660
(Received June 8, 1970)
The kinetics and mechanisms of the reactions of a quinonediiminederivative (QDI), produced by photolysis of p-azido-N,N-diethylaniline in water, with hydroquinone (HtQ),tin(I1) ion, and azomethine leuco dye (LD) are reported. The QDI reacts with HzQor tin(I1) producing a semiquinone species which reacts more slowly with the reducing agent to form p-phenylenediamine derivative. These reactions proceed by a mechanism involving hydrogen atom transfer. The reaction of LD with QDI does not produce any detectable SQ and may proceed by a hydride transfer mechanism. The LD ionizes with p K , 9.9; the basic form of LD is almost twenty times as reactive with QDI as the acid form.
-
I. Introduction It has recently been shown' that a quinonediimine derivative (QDI) is produced when p-azido-N, N-diethylaniline is photolyzed in water. This method of forming QDI permitted a study of the kinetics and mechanism of the Michaelis reaction2 (eq 2) between QDI and substituted p-phenylenediamine derivatives (designated PPD).
QDI
PPD "2
I
SQ
AZIDE
QDI
The product of this redox reaction is semiquinone (SQ), which is a one-electron intermediate oxidation state between PPD and QDI. A technique for photolyzing azides containing pdialkylamino groups was used to produce QDI. The reactions of the latter with a variety of reducing agents The Journal of Physical Chemistry, Vol. 74, No. 20,1070
are the major concerns of this report. First, we were interested in determining whether QDI could be reduced to P P D without going through the SQ form; Michaelis3 (1) R. C. Baetzold and L. K. J. Tong, submitted for publication in J . Amer. Chem. Soc. (2) L. Michaelis, M. P. Schubert, and 8 . Granick, ibid., 61, 1981 (1939). (3) L. Michaelis, Trans. Electrochem. Soc., 71, 107 (1937).
REACTIONS OF OXIDIZED PHENYLENED DIAMINE DERIVATIVES has stated that no simultaneous two-electron transfers occur in the P P D system. We were also interested in determining the origin of the proton which is present on SQ or P P D when QDI is reduced. The evidence reported here is that an electron and proton can be transferred simultaneously when QDI is reduced. In this report reduction of QDI by hydroquinone (HzQ), by tin(I1) ion and by leuco dye is examined. Each of these reducing agents can lose two electrons forming a stable oxidation state. Hydroquinone is the only species in this study known to have a stable oneelectron oxidation statee4 11. Experimental Section The experimental procedure and apparatus have been described.l Runs are carried out with an excess of reducing agent relative to QDI to obtain pseudofirst-order kinetics whenever possible. The initial M , of which roughly azide concentration is 2 X 25% is photolyzed by the Pyrex-filtered radiation of a 200-5 xenon flash. These runs were carried out at 25" =t 0.5" with phosphate buffers of ionic strength 0.188, The materials used in this work were Eastman Reagent Grade or highest purity available. Azide was prepared as described. Leuco dye is produced by reducing a pure sample of the dye with sodium borohydride. Borohydride is added until the solution becomes colorless; a slight molar excess is then added. There is no reaction of QDI with excess borohydride, because QDI reacts almost three orders of magnitude faster with leuco dye. The leuco dye solution is prepared just before photolysis and is kept deaerated by bubbling in nitrogen in order to prevent autoxidation of the leuco dye. The SQ and dyes that are formed as reaction products were identified by comparison of their absorption spectra with known spectra, as before.
Table I : Rediictiou of QUI by H2&
HgQ concn, J4
6.12 x 10-5 1 . 2 2 x 10-4 2 . 4 3 X 10-4 4.90 x 10-4
p H 6.78 I'seudo-first-order rate constant, Bet-1
13.2 27.7 555 113.5
Bimolecular rate constant, l./mol see
2.16 x 105 2.27 X 105 2.26 x 105 2.32 x 105 2.25 0.05-x105
Reduction of SQ by H2Q
HzQ concn, M
3.06 6.12 1.22
x x x
10-5 10-5 10-4
p H 6.78 I'seudo-first-order rate constant, sec-1
0.20 0.403 0,792 1.35
Bimolecular rate constant, l./mol sea
6.54 x 103 6.58 x 103 6.49 x 103 5.50 x 103 6.22 i.0.42 X 103
3597
The SQ extinction coefficient (E), expressed as molar absorptivity, is determined by completely oxidizing a known amount of P P D with ferricyanide to the SQ state. Optical density of SQ is measured within milliseconds of mixing in a flow machine. At 526 mp, E = 7.6 X lo3l./mol em for the N,N-diethyl derivative. 111. Hydroquinone
Hydroquinone reduces QDI to SQ and further reduces SQ to PPD. When excess HZQ is present, the formation of SQ follows first-order kinetics according to eq 3.
0
"2
I
1
Q
+
Q+
H+
(3)
0
-
Et&t
SQ
Variation of H2Q concentration shows that the reaction is also first order in H2Q (Table I). Measurement of the reaction a t various pH's (Figure 1) shows that nonionized H2Q is the reactive form of the reducing agent since HzQhas pK,, = 9.85. We write the anion radical of hydroquinone as a product of eq 3 since the hydroquinone radical ionizes5 with pK, = 5.9. Equilibrium measurements4show that the anion radical disproportionates almost completely in the pH range used in these experiments, making the back reaction in eq 3 negligible. Reduction of SQ to P P D is a first-order reaction in SQ. The data in Table I indicate that this reaction is also first order in H2Q concentration, although at the higher concentrations of H2Q there may be some deviation from this behavior. Figure 1 shows the dependence of the rate constant on pH and indicates that the monoanionic and the nonionized forms of HzQ are reactive. The rate constants for the hydroquinone monoanion and the nonionized form are determined using pK1,, as mentioned before. Any involvement of hydroquinone dianion in the reduction could not be evaluated since at the higher pH's required for its formation, SQ is unstable because of disproportionation.1 The observed reactions are written in eq 4a and 4b. (4) C. A. Bishop and L. K. J . Tong, J . A m e r . Chem. Soc., 87, 501
(1965).
(5) N. K. Bridge and G. Porter, Proc. Roy. SOC. (London), 244, 259, 276 (1958). T h e Journal of Physical Chemistry, Vol. 74,N o . 20, 1970
R. C. BAETZOLD
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OH
A ,+
Et
0-
Et
“2
I
I
6
k-78 X 10Bl/(msec)
OH
Et/T\Et
A kinetic isotope effect is observed for reaction 3 when rates are compared in normal and heavy water. The ratio I C H ~ O / ~ D ~ O = 2 8 3 j= 0.10 at pH 6.90 indicates that a bond involving hydrogen isotope is broken in the rate-determining step of this reaction. This effect is consistent with the transfer of a hydrogen atom from HzQ to QDI, as shown in transition state I.
IV. Tin(I1) Ion Tin(I1) ion is described as a two-electron reducing agent in standard textbook^.^ When it reacts with QDI, however, one-electron paths are favored, probably because a stable oxidation state (SQ) can be formed. SQ is formed by first-order kinetics when azide is photolyzed in the presence of excess tin(I1) ion. The T h e Journal of Physical Chemistry, Vol. 74, No. 80, 1070
8
Figure 1. Dependence on pH of the rate of SQ formation (curve I ) and decay (curve 11)at 8.90 X 10-6 M hydroquinone.
pseudo-first-order rate consta,nt of this reaction is proportional to tin(I1) ion concentration (Table 11). Since the rate-determining step of the reaction is first order in QDI and Sn2+,we believe that a trivalent oxidation state of tin is produced in the reaction. Other kinetic studiessJ’ indicate that tin(II1) can be formed, although it is unstable. Table 11: Reduction of QDI to SQ by Tin(I1) Ion
5.02 1.26 4.39 1.16 2.32 2.90
Recently it was shown that hydroquinone can transfer a hydrogen atom to R h O H Z +upon reduction of the latter.e Since QDI is also capable of accepting a hydrogen atom, we expect that the two mechanisms are similar. The activation energy, measured by an Arrheniustype plot, is 6.4 kcal/mol for reaction 3. This value is low and nearly the same as the activation energy’ for reduction of QDI by ferrocyanide. This reaction was also believed to proceed by a hydrogen atom transfer.
7
PH
x x x x x x
4370 3720 4380 5430 4750 4980
10-5
10-4 10-4 10-3 10-3
Tin(I1) ion is present as the trihydrated ionlo a t the nearly neutral pH and low chloride ion concentration of these experiments. The trihydrated species ionizes with pK, = 9.55 according to the equilibrium K*
Sn(HzO)s2+= H+
+ Sn(Hz0)20H+
(5)
This result is correlated with the pH dependence of the SQ formation rate shown in Figure 2. The pH-independent reaction is attributed to (6) G. Davies and K. Kustin, Trans. Faradag Soc., 65, 1630 (1969). (7) E. S. Gould, “Inorganic Reactions and Structure,” 1st ed, Holt, New York, N. Y.,1962. ( 8 ) T. R. Ball, W. Wulflcuehler, and R. E. Wingard, J . A m e r . Chem. Soc., 57, 1729 (1935). (9) J. Weiss, J . Chem. Soc., 309 (1944). (10) Stability Constants, Special Publication No. 17, The Chemical Society, London, 1964,p 68.
REACTIONS OF OXIDIZEDPHENYLENED DIAMINE DERIVATIVES
+
Sn(H20)32+ QDI
-% SQ + Sn(H20)20H2+
(6)
The increase in rate with pH is attributed to
+
Sn(H20)20H+ QDI
-% SQ
+ Sn(H2O) (OH)2+
(7)
When all of the hydrated complex is ionized, the rate should become pH independent. From the data in Figure 2 and pK, = 9.55, we can evaluate k~ = 5.0 X lo3 l./mol sec and k2 = 1.9 X lo6 l./mol sec. Equation 8, involving the simultaneous transfer of two electrons from tin(I1) to QDI, can be ruled out as accounting for SQ formation.
+ QDI -% PPD + tin(1V) P P D + QDI + H + 3 2 S Q
tin(I1)
that the ratio k H 2 0 / k D z 0 should be greater a t high pH than at low p H if hydrogen atom transfer were taking place. The activation energy of this reaction in the pH-independent region is measured by plotting log koba us. reciprocal temperature. The observed activation energy a t pH 6.10 is 16.0 kcal/mol. This value is considerably higher than that observed for reduction of QDI by H2&, PPD' or ferr0cyanide.l A high activation energy is expected when an unstable product (such as tin(II1)) is formed.11 Semiquinone that is formed by reactions 6 and 7 is not stable. The rate of decay is increased by increasing pH or concentration of tin(I1) (Table 111). The decay
(8s) (8b)
Table 111: Decay of SQ in Tin(I1) System
This mechanism predicts the observed dependence of rate on tin(I1) concentration only if step Sa is rate limiting. The rate data of reference 1, however, indicate that SQ could not be formed fast enough by path 8b to account for the observed rates (Table 11). The rate of SQ formation decreases when D2O is substituted for H2O as the reaction medium. The rates are
kH20/kDz0 =
1.98
Tin(I1)O oonon, M
4.81 X 1.45 x 2.41 x 3.61 x 4.81 x
0.06 at pH 8.15
=t
9.65 X 2.85 x 4.95 x 7.90 x 1.02 x
10-4 10-4 10-4 10-4
lo8 104 104 104 106
kbimoleculsr 1 . ~ ~ ~ 1
6.34 x 4.95 x 3.30 x 2.32 x 6.24 X
6.55 x 8.24 X 1.23 x 10-4 2.46 x 10-4 4 . 1 2 x 10-4
(9)
This effect is consistent with breakage of an oxygenhydrogen bond required for the transfer of a hydrogen atom from hydrated tin(I1) to QDI in the rate-determining step of the reaction a t low pH. The effect of D2O at high pH would be to increase the pK, for ionization of the complex (eq 5). This effect alone predicts D ~1.O It is much more likely that electhat I C H ~ O / ~> tron transfer, not hydrogen atom transfer, is involved in this reaction at high pH. The low-pH isotope effect, together with the effectof heavy water on pK,, suggests
kbimoleoular, l./mol sac
PPD oonon,M b
0.20 at pH 7.00
~ H ~ o / ~= D ~ 2.28 o =t
3599
M.
pH 8.17, P P D concn = 8.24 X concn = 2.41 X M.
104 104 104 104 lo8
pH 7.86, Tin(I1)
does not follow first-order kinetics, but when P P D is added, SQ decays by second-order kinetics. The bimolecular rate constant is determined from the product of SQ extinction coefficient and the slope of Figure 3; it varies inversely with P P D concentration according to Figure 4. These observations are consistent with
lo i I
I
I
I
5
10
15
20
25
Time (sed I
7
I 8
1
J
9
10
PH Figure 2. Dependence on pH of the bimolecular rate of SQ formation by reaction of QDI with tin(I1).
Figure 3. Plot of reciprocal SQ optical density us. time for decay of SQ a t pH 7.86, PPD = 2.46 X 10-4 M , M. stannous ion = 2.41 X (11) J. Halpern, Can. J.Chem., 37,148 (1959).
The Journal of Physical Chemistry, Val. 74, No. 20,19YO
R. C. BAETZOLD
3600 I
I
1
Reciprocal PPD Concentration
Figure 4. Plot of bimolecular rate constant us. reciprocal PPD concentration for SQ decay; p H 7.86, stannous ion concentration = 2.41 X M.
for PPD protonation ( K B = 1.0 X lo8) to calculate ka/k2 = 3.0 X lo8 from this slope when k3(R)(H+)>> ,%,(A). The published12 value k3/kz = 2.9 X lo8 supports this mechanism. We conclude that the reduction of QDI by hydrated tin(I1) ion proceeds by the formation of the unstable oxidation state, tin(II1). This results in a high activation energy; the reaction may proceed by a hydrogen atom transfer. A concerted mechanism whereby the proton comes to QDI from water while the electron comes from tin(I1) ion, however, cannot be ruled out. Tin(I1) ion does not react directly with SQ because mechanism 10 results in a more rapid removal of SQ from the system. We can say that at pH 6 and Sn2+ at M , the possible direct reaction of SQ with tin(11) ion must have k lo2l./mol sec since the observed SQ decay required several seconds.
