16
Bansal, Gratzel, Henglein, and Janata
o ~ a r o g r and ~ p Optical ~ ~ ~ Absorption Studies of Radicals Produced adiolysis of Aqueous Solutions of Ethylene Glycol . M. Bansal, M. Gratzel, A. Henglein," and E. Janata Hahn-itfeitner-lnstitut fur Kernforschung Berlin GmbH, Sektor Strahlenchemie, 1 Berlin 39, West Germany! (Received September 9, 1972) Publication costs assisted by Hahn-Meimer-Institut fur Kernforschung Berlin GmbH
Simultaneous optical absorption and polarographic current measurements were carried out to investigate the free radicals formed in the pulse irradiation of NzO saturated 10-1 M ethylene glycol solutions. In che pH range from 3 to 7, only the 1,2-dihydroxyethyl radical could be observed. It disappears with 2k = 5.7 % 108 M - 1 sec-1. Below pH 3, the dehydration of this radical to yield the formylmethyl radical was observed. The equilibrium constant of the protonization of 1,2-dihydroxyethyl and the decay constant of 1 he protonated radical were found to be 1.8 X 10-1 M (or p K = 0.74) and 8.6 X lo5 sec-l, respectively. Thk formylmethyl radical disappears with 2k = 9 X lo8 M - 1 sec-1. In alkaline solutions, a rapid dehydralion also occurs. The diffusion-controlled reaction OH- + OH-CHCH20H H2O -0CHCH20H OHOCHCHz are postulated. The short time polarograms of and the rapid decay -0CHCH20H 1,2-dihydroxyethyl and formylmethyl differ significantly. 1,2-Dihydroxyethyl is oxidized a t the mercury Jdertrode a t potentials more positive than -1.0 V ( u s saturated calomel electrode) and reduced at more negative potentials. Formylmethyl is oxidized beyond -0.1 V and reduced a t potentials more negative >,haxi -0.3 V. The 1,2-dihydroxyethyl radical therefore is a much stronger reducing agent than the rorniylmethyl radical.
-
I. Introduction The optical absorption spectra and p K values,lS2 electron spin resonance spectra,3-5 and the polarographic behavior6i7 of ,short-lived free radicals formed by the reaction of OH radicals wrth aliphatic alcohols have previously been reported by several authors. From the esr studies of the uv photolyzed aqueous solutions of ethylene glycol containing 1% HzOz, Livingston and Zeldes5 concluded that the 1,2-dihydroxyethyl radical was the primary species obtained by the reaction of the -OH radical with ethylene glycol. In the presence of M2SO4, this radical undergoes dehydration to produce the formyhethy1 radical vCH~CHO, the formation of which was explained by a proton-catalyzed dehydration of the 1,2-dihydroxyet.t1yl radical.5 The existence of this radical was also posluiated to explain.the yieids of various products formed in the photolysis and radiolysis of aqueous ethylene glycol solutioiis.8,S To understand certain aspects of the mechanism of this dehydration reaction, we investigated the pulse radiolysis of aqueous solutions of ethylene glycol. The optical absorption spectra of the intermediates and the kinetics of their decay at different pH values of the solutions were investigated to firid the value of the equilibrium constant of the protonization reaction. Furthermore, polarographic studies of the radicak a t different pH values of the solution were undertaken Previous studies on the polarographic behavior of different types of radicals, a compilation of which will be published elsewhere, clearly pointed out that this tcwbnique would provide valuable information on the id ?nttty of 1,2-dihydroxyethyl and formylmethyl radicals.
IT. Experimental Section A 12-12/leV,6-A linear accelerator was used for the combined optical and polarographic measurements. The pulse The Journal of Physicai Chemistry. Vol 77, No. 1, 1973
-
+
+
length usually used was 20 nsec. Additional optical measurements were carried out with a Van de Graaff generator (1.5-MeV electrons, 10-pA beam current, I-psec pulse length). The aqueous solutions, containing 0.1 M ethylene glycol (Merck Co., analytical grade), were saturated with N 2 0 (oxygen was removed by bubbling through a column of 0.2 M Cr2+ in 1 M HC104 acid) to scavenge the hydrated electrons according to reaction 1. Such a high ethylene glycol concentration was used to ensure complete scavengenq-
+
N,O
a .OR + OR-
iN*
(1)
ing of -OH radicals and .H atoms during the 1-5-psec pulse. In the polarographic measurements, a capillary with a hanging mercury drop of 0.78 mm diameter (Metrohm Co.) was placed into the optical quartz cell. The solution was slowly flowing through the cell and passing a saturated calomel electrode as reference point for the potential measurements. The light beam for the optical measurements passed through the cell in the vicinity of the hanging mercury drop. Some pertinent details of the technique have been published earlier7 and other necessary details will be published elsewhere .lo The solutions (1) K.-D. Asmus, A. Henglein, A. Wigger. and G , Beck, Ber. Bunsenges. Phys. Chem., 70, 756 (1966). (2) M . Simic, P. Neta, and E. Hayon, J. Phys. Chern., 73,3794 (1969). (3) R. Livingston and H. Zeldes, J. Chem. Phys., 44, 1245 (1966). (4) A. L. Buley, R. 0. C. Norman, and R . J. Prilchett, J. Chem. Soc. B, 849 (1966). (5) R. Livingston and H. Zeldes, J. Amer. Chem. Soc., 88, 4333 (1966). (6) J. Lilie, G. Beck, and A. Henglein, Ber. Bunsenges, Phys. Chem., 75,458 (1971). ( 7 ) M. Gratzel, A. Henglein, J. Liiie, and M. Scheffier, Ber. Bunsenges. Phys. Chem., 76,67 (1972). (8) C. E. Burchill and K. M. Perron, Can J. Chem., 49,2382 (1971). (9) C. v. Sonntag and E. Thoms, 2. Nafurforsch. €3, 25, 1405 (1970). (IO) See papers in Ber. Bunsenges. Phys. Chem., in press, by (a) M. Gratzel and A . Henglein; (b) M. Gratzei, A. Henglein, and K. M. Bansal; (c) M. Gratzel, K. M. Bansai. and A. Henglein; (d) A . Hengiein and M. Gratzei.
Pulse Radiolysis of Ethylene Glycol
17
r
I
G(H) = 6.0. Spectrum h taken after IN psec was normalized to spectrum a immediately after the pulse a t 250 nm. The decay of the optical ahsorption of the radical at 250 nm follows second-order kinetics (this result was ohtained from the straight lines that resulted in a plot of the reciprocal optical density us. time after the pulse at different dose rates). A value for 2klc = 1.1 x IO6 cm sec was 2k = fi.7 X ohtained. Using a value of t Z 5 0 610 M-' cm 108 M - 1 sec - 1 A typical oscilloscopic trace tor the decay of the optical
pH = 6.0
250 nm
pH = 10 260 nm
'
~~
c
-
1
260 nm
50 ps
Figure 1. Optical absorption as function of time at different pH values (ethylene glycol concentration. 10.' M: [NzO]. 2.5 X t o - ' M, pH established with HCIOI and NaOH. pulse dose 2000 rads).
350
300
-A[nm]m
/i'
0
-
a : p H - 6 ; Ops b:pH= 6;lOOps c:pH=lO; 10 ps
~
a
\
A-b
500-
g ..Y .'
t -
--5 .
c; l
-k w
100-
"-.,
v--*-
p ,
.._ .
*
~~
~~
mediately followed by a much slower decay. The spectrum of the transient present a t the end of the first fast decay is presented by curve c in Figure 2. The comparison with curve a of the figure makes it clear that two different intermediates exist a t pH 10. I t will he pointed nut later that spectrum c in Figure 2 is that ot the formylmethyl radical .CHzCHO. It decays by second-order kinetics with 2klr = 3.5 X IO6 cm sec-I and using a value of e300 260 M-' em-', a value of 2k = 9.1 X 108 M - 1 sec-' is ohtained. A t pH 1.4, two distinct steps in the decrease in absorption can he recognized (Figure IC). The ahsorption spectrum immediately after the pulse was found to be identical with that of 1.2-dihydroxyethyl radical (Figure 2a) and the one a t the end of the first decay was found to he identical with that of the formylmethyl radical (Figure 2cl. Curve analysis showed that the fast decay immediately after the pulse was of first order provided that the half-life of the fast decay was significantly smaller than the first half-life of the radical at pH 6. We attribute the fast decay in absorption to the dehydration of the 1.2-dihydroxyethyl radical to yield the formylmethyl radical. This subsequently decays by second-order kinetics. The rate constant of the first-order decay was found to increase with H+ concentration which can he explained by the mechanism of eq 2. In Figure 3, the half-life is plotted us. the reciprocal H i concentration, The straight line ohtained intersects the ordinate axis a t R X 10-1 sec and has a slope of 1.5 X 10-7 M sec-'. If it is assumed that the equilibrium of eq 2 is rapidly attained and no sig-
Y Y
H Y
I I C-C-H
I I
OH OH
+Ht
K #
1
1
I 1
OH OHi
I -I -"I,
.C-C-H
-11 0
It,
H
I
.C-ctI
I bH
H
H
i
H
I
C-CH
II
(2)
i,
nificant radical-radical reaction occurs during the fast dehydration, the following relationship results from the kinetic treatment of the mechanism of eq 2. r w = (InZ/kl) + ( K I n 2 / k 1 ) ( 1 / l H + l )
(31
It is also assumed that the rate constant k in eq 2 is higher than k l . Using eq 3. a value of k l = 8.6 X IO5 sec is calculated from the intersection of the straight line in Figure 3 with the ordinate axis and an equilibrium constant K = 1.8 X IO-' M is calculated from the slope of the straight line. A pK of 0.74 corresponds to this equilihrium constant. Below pH 1, the rate of dehydration is so fast that it is practically finished after a 1-5-psec pulse. The absorptirrn
~'
The Journal olPhysical Chemrstry. Vol 77, No 1. 1973
Bansal. Gratrel. Henglein. and Janata
pH = 6.0
- 0.70 V
- 0.30V 10
20
30
c
LO
60
50
70
80
1 IM-']
'/[HI
Figure 3. Plot 01 the hall-life of the first-order decay ciprocal tit concentration of the solution.
YS.
-.0.70V
lhe re-
shortly after the pulse is now due to the formylmethyl radical. The absorption decays by second order after the pulse. A rate constant 2k = 9 x 10s M - 1 sec-' was calculated from this decay which corresponds to the one oh. tained in the alkaline solutions. The fast formation of the CHOH
I
CH,OH
+OH-
-I -11,o
CH-0CHPH
-I
+OH- (4)
formylmethyl radical a t pH 10 (see Figure 2h) can he explained by an OH- catalyzed dehydration of the 1.2-dihydroxyethyl radical (eq 4). The pK values of simple aliphatic alcohol radicals are higher than 10.7.' It can hardly he postulated that the pK of the 1.2-dihydroxyethyl radical is significantly lower. However, even a t pH 8.5, the decay of the 1.2-dihydroxyethyl radical is very fast. This indicates that no equilihrium on the left-hand side of eq 4 is established between the acid and base form of 1.2-dihydroxyethyl. Apparently, the O H - ion reacts very fast with the 1.2-dihydroxyethyl radical to yield HOHKCHOThis radical anion has a much higher rate of OH loss to form the formylmethyl radical than of its reaction with H20 to establish the equilihrium with its acid form. Furthermore, the rate of O H - loss must he higher than that of the reaction between HOHzCCHOH and O H - , the rate of the latter being therefore the determining rate of the overall process. This rate could not be measured accurately; however, it can he concluded from the results obtained that the rate of the reaction of OH- with the 1.2-dihydroxyethyl radical is that of a diffusion-controlled process. III. 2. Polarographic Measurements. The polarographic measurements are to give additional information ahout the identity of the radicals. We have shown that "shorttime polarograms" of short-lived free radicals can be ohtained by recording the polarographic current a t the mercuN drop electrode as a function of time after formation of the radicals by irradiation of the solution with a short electron pulse.6.7 Such current us. time curves are taken a t different potentials of the mercury drop. The plot of the current a t a given time after the pulse us. the electrode potential constitutes a polarogram. Polarograms have been determined for a large number of free radicals. They are typical for certain types of radicals and can therefore be used now for the identification of new species found in pulse experiments. A compilation of such polarograms will be published elsewhere. ~
The J ~ ~ u r n a l ~ I P h y ~ i c a l C h e m i sVol. l r y .77. NO. 1. 1973
-0.9ov
250 nm
*
CH=O CHz
p H = 2,6
100 ps Figure 4. Polarographic current vs. time curves at different pH values and potentials (vs. saturated calomel electrode) (curves a-e). Curve f shows 2500-A absorption vs. time at pH 2.6. The current us. time curves are determined (a) by the rate of radical disappearance, (h) by the formation of a concentration gradient a t the mercury drop, and (c) by the kinetics of the transfer reaction through the double layer. These curves therefore are not exactly parallel to the curves showing the optical ahsorption as function of time for the radicals in the hulk of the solution. It is not the purpose of the present paper to describe the details of the current us. time curves, i.e., the kinetics of the electrode processes involved. A description of these phenomena will he given elsewhere.10 Only those features of the current us. time curves will be discussed which are important for the identification of the radicals. Figures 4a-e show the oscilloscope traces of the polarographic currents as function of time a t different pH values and potentials a t the mercury electrode in the pulse-irradiated aqueous NzO saturated ethylene glycol solutions. On the left side, the zero line before the pulse can always he recognized. Negative signals represent oxidation. and positive signals the reduction of a radical. Figure 4f gives the optical ahsorption as function of time to complete the polarographic current us. time curves c-e a t pH 2.6. At pH 6.0, oxidation of the radical present after the pulse occurs a t -0.70 V. The negative signals decays to zero after about 200 psec (Figure 4a). In Figure 5, the maximum signal after the pulse is plotted us. the potential of the mercury electrode for pH 5.9, 4.9. and 3.9. It can he seen that a constant negative signal is obtained at potentials between -0.7 and -0.2 V. A t more positive potentials weaker negative signals are observed. This effect is due to an adsorption of the sulfate ions a t the mercury electrode by which the transfer reaction is hindered. This effect has been observed with other simple aliphatic alcohols and will be described in detail elsewhere.I0c.d At potentials more negative than -0.7 V, the negative signal he-
Pulse Radiolysis of Ethylene Glycol
19
ln
c
C: 7
L.
2;
c
e0 1
ri -1
0
-2
0 0
c
C a,
2-3 3
u
-L
-5
0
- 0.5
-1,o
-1
Volt ---c Figure 5. Maximum sigrial of the polarographic current after the pulse as function of the potential (vs. saturated calomel electrode) at different pH values. The polarograms at p H 5.9, 4.9, and 3.9 are altributed to the 1,2-dihydroxyethyl radical, the polarogram at p t i 9.0 to the formylmethyl radical.
comes rapidly weaker and a t about -1.0 V, a positive signal after the pulse can be observed. This signal rises with increasing the negatlive potential. The polarogram of Figure 5 corresponds to similar polarograms of a alcohol radicals.7 Oxidation at the mercury electrode occurs below about --1.0 V independent of the pH in acid solutions. Reduction of the radical occurs at more negative potentials, whereby the polarographic “wave” is shifted to less negative potentials with increasing H + concentration. The polarograms 01 Figuie 5 at pH 5.9, 4.9, and 3.9 are attributed to the 1,2-dihyclroxyethyl radical. This radical differs from the ethanol radical (CH3-CII-OH) in that one H atom of the methyl group is replaced by OH. Comparing the polarogram of both radicals7 one realizes that the oxidation wave and especially the reduction wave of the
CHzOH-CHOH radical are at more positive potentials than the corresponding waves of CH3-C€IOH. At p H 9.0, a radical is present after the pulse that is reduced at the potential of -0.70 V, as can be recognized from the positive current signal of Figure 4b. The maximum signal after the pulse is plotted in Figure 5 us. the potential for a solution of pH 9.0. It can be seen that oxidation of the radical occurs only if the potential is more positive than -0.1 V. Reduction becomes noticeable a t potentials more negative than -0.30 V. The full height of the polarographic “wave” is reached at -0.75 V. The POlarographic behavior of the radical present in alkaline solution is significantly different from that present in neutral or slightly acid solutions. The polarogram at pH 9.0 in Figure 5 is attributed to the formylmethyl radical. This polarogram is similar to that of the .CHzCOO- radical7 and to the radical CH&HCOCH3 formed in the reaction of OH with ethyl methyl ketone. Apparently, the formylmethyl radical is a much weaker reducing species than its precursor, the 1,2-dihydroxyethyl radical. The curves c-e in Figure 4 show a complex behavior. The solution is now acidic enough to allow the observation of the,dehydration according to eq 2 . The optical absorption shows the typical fast decay within the first 50 psec after the pulse (Figure 4f). At both -0.30 and -0,iO V (curves c and d), a negative signal immediately after the pulse is observed which indicates the presence ot the 1,2dihydroxyethyl radical. After about 80 psec, the signal is slightly positive since the formylmethyl radical is now present. The low height of this positive signal is due to the fact that after 80 wsec an extended diffusion layer has already been established before the mercury drop and that part of the radicals have already disappeared by mutual interaction. The weakly positive signal finally decays to zero at longer times. At -0.90 V (curve e), still a weak negative signal is seen immediately afcer the pulse since the 1,2-dihydroxyethyl radical is still oxidized at this potential (Figure 5). However, the signal now rapidly switches to a strong positive signal which finally decays to zero. It has always to be kept in mind that the observed current signal is composed of two components: the oxidation (or negative) signal of the 1,2-dihydroxyethyl radical and the reduction (or positive) signal of the formylethyl radical. During the first 100 psec after the pulse, both radicals are always present although with a changing ratio in concentrations. If the signal of 1,2. dihydroxyethyl (at -0.90 V) is weak, a small amount of fcrmylmethyl formed is sufficient to switch the signal to a strong positive value (since formylmethyl has already maximum current at -0.90 V according to Figure 5).
The Journal of Physical Chemistry. Val. 77. No. 1, 1973
