W. A. SEDDON AND A. 0. ALLEN
1914
and N, and the rotation is evidently governed by the interaction of the bridge atoms with the d electrons in much the same way as that of the C0(en)3~+complex. It is an interesting fact that the rotational strength of c ~ [ ( e n ) ~ ( N H ~ )is* approximately ]~+ one-third that of the C ~ ( e n ) , ~transition.’ + This is just the ratio of the number of three-way interactions of bridge atoms with the cobalt atom. From this work, which is only intended to point the way toward a better qualitative understanding of transition metal complexes, have emerged several observations on the general behavior of degenerate transitions. The qualitative behavior of a chromophore
in a dissymmetric molecule will depend on the type of dissymmetry. Confining our discussion to magnetic dipole transitions and those electric dipole transitions where the coupled oscillator effect is minimal, we may say that the rotational strengths of a degenerate transition sum to zero provided there is no dissymmetric array of atoms lying on centers or planes of symmetry of the chromophore. I n general, when this condition is not satisfied, the sum rule will not apply. It would be interesting to see if the sum rule is satisfied for a complex such as Co [(NH3)4(NH~CHCH3CH~NH~) 13+, in which a methyl group lies in one of the octants of the chromophoric coordinate system.
Radiation Chemistry of Aqueous Solutions of Ethanol’
by W.A. Seddon and A. 0. Allen Chemistry Department, Brookhaven National Laboratory, Upton, New York 11975
(Received November 89, 1966)
Hydrogen yields from y rays on neutral solutions of ethanol and oxygen were determined over wide ranges of concentration. The results indicated a simple competition between O2and EtOH for an H atom, generated with a yield GH = 0.6; the properties of this radical did not change at very low oxygen concentrations. I n acid solutions, where H is generated with G = 3.3, the same competition was found. Oxygen-free solutions containing ethanol and hydrogen peroxide show a chain reaction under y rays, the kinetics of which were found to be in agreement with the anticipated free-radical mechanism. The reaction was studied with intermittent X-rays (“rotating sector’’ method) and an absolute rate constant obtained for the reaction of ethanol radicals with H202. The results are all con0.05. These sistent with values for the primary radical yields of G,- = GOH = 2.25 values are significantly lower than those deduced from some other systems and the reasons for the discrepancies are not completely understood.
*
Solutions of alcohols have played an important role in the study of the occurrence and properties of OH and H radicals formed in water radiolysis. Here we present some new data on the reactions of radicals with ethanol and on the radiation-induced reaction between ethanol and hydrogen peroxide.
in the ratio 23.2 :1; the more intense source gave about 60 pM Fe3+/min in the Fricke dosimeter. Irradiations were all carried out in syringes a t a temperature of 23”. Each G value quoted here is obtained by exposure of samples at four or more different doses. Interrupted radiations were carried out using X-rays
Experimental Section Two W o y-ray sources were used, having intensities
(1) Research performed under the auspices of the U. 5. Atomic Energv Commission.
The Journal of Physical chemistry
RADIATION CHEMISTRY OF AQUEOUS SOLUTIONS OF ETHANOL
generated a t 1.95 Mev by a Van de Graaff electron accelerator, as described by Schwarz.2 Two intensities were used, giving Fricke dosimeter responses of 24.0 and 4.8 pLM Fe3+/min, respectively. The ratio (time off/time on) was kept constant at 3.0, while the period was varied. All oxygen-free samples were deaerated by bubbling with Ar for at least 30 min. The gas passed successively through a Dry Ice trap, through pure water, and finally through a solution of the same composition before reaching the actual solution to be irradiated. Syringes were rinsed several times with the deaerated solution before final filling. Oxygen concentrations were fixed by bubbling with 02-Nz mixtures made and analyzed by the Matheson Co., Inc. Hydrogen was determined gas chromatographically by an adaptation of the method of Swinnerton, Linnenbom, and Cheek.3 Samples of 10 ml were ejected via a hypodermic needle through a rubber cap into a bubbler containing water through which Ar carrier gas passed at 10 cc/min under 5 psig. Calibration was accomplished by injection of 0.200 ml of water saturated with hydrogen a t atmospheric pressure. Three calibrations were run a t the start of the day; thereafter a calibration run was made after each determination, though usually no change in the calibration occurred during any one day. A 6-ft column of Molecular Sieve 5A, 30-60 mesh, a t room temperature was used in a Perkin-Elmer Model 154 vapor fractometer, with th3rmistor detector. Hydrogen elution time was about 1.5 min; oxygen appeared after a further 2 min and nitrogen after a further 5 min. Peak areas were determined with a planimeter; peak heights could not be used because of the finite injection times. Acetaldehyde was determined by the method described by Hummel and Allen.4 The correct extinction coefficient to use in this method is 19,100; it was wrongly quoted in the reference. The usual iodometric method for peroxide was used.
Results Measured hydrogen yields, G(HZ), from various solutions are shown in Table I. The molecular yield from water, GH,, is seen to be slightly lower in acid than in neutral solutions, as previously reported by Hayon.6 The additional HP arising from ethanol is a measure of the independent yield of H atoms, which are formed in water and react with alcohol by eq 1 H
+ CzH6OH = Hz + *C*HdOH
1915
Table I : Some Measured Hydrogen Yields Solution
GWd
Neutral KBr (1.38 mM), air free Neutral KBr (1.5 mM), air saturated Neutral KBr (1.38 mM), with NzO (0.32 mM)" HC104 (pH 1.69), KBr (1.53 mM), air free HClO, (pH 1.69), KBr (1.53 mM), air saturated Ethanol (10.5 mM), with NzO (0.32 mM)" Ethanol (105 mM), with NzO (0.32 mM)" Ethanol (10 mM), NaHzP04 (0.1 M , pH 4.5)b
0.4593I0.017 0.4313I0.010 0.429 f:0.013 0.4193I0.010 0.3663I0.010 1.051 f 0.02 1.156f0.02 3.50 f O . 0 7
a Saturated with Ar containing 1.26% NzO. Fifteen separate determinations, with several points taken in each determination.
Simic;6 the increase presumably results from removal from the spurs at high alcohol concentrations of OH radicals, which normally react with H atoms or their precursors. I n the phosphate solution, the solvated electrons react with the acid phosphate anions to form additional H atoms and the observed yield is the sum of GH~, GH,and Geaq-. When low concentrations of 0 2 are present, reaction 2 competes with reaction 1. A simple competition should lead to a linear relation between the oxygenH
+
0 2 =
HOz
(2)
alcohol ratio and the reciprocal of the yield of H2 in excess of the molecular
+
+
G(Hz) = GH, G H [ ~ K(Od/(alc)I-' (A) where K = k2/kl, the ratio of rate constants for reactions 2 and 1. I n this work G H ~a t the different oxygen concentrations is obtained by interpolation from the values of Table I, assuming that G H is ~ linear in the cube root of the oxygen concentration. The yield of hydrogen varies with total dose because consumption of the reactants alters their ratio. Integration of eq A, under the condition z > (H202), so that all H and OH react with the alcohol, the mechanism pre-
2
0
(4)
dicts 6 8 IO (o,)/(c,H,oH)~
12
14
16
18
G(-H202)
103
+
and the yield a t each dose was corrected using eq B with G(-02) = 2.8, GH = 0.62 in neutral or 3.4 in acid solution, and K = 380 or 400. The corrections in G amounted to only 1 4 % . The results in neutral solution are shown in Figure 1. From the reciprocal of the intercept we find GH = 0.62 and from the ratio of slope to intercept K = k H + O 2 / k H + a l o = 390 f 60, in good agreement with Willson and Scholes.’ The reasonably good fit to a straight line shows that the entity being competed for does not appreciably change its properties over the range of concentrations studied and refutes Hayon’s contentions that it acts like an H atom below about 60 p M (02) but like an electron at higher O2concentrations. In acid solution, all ea,- are converted to H atoms, SO that a much higher G(H2) is found. Again we find simple competition with oxygen (Figure 2). GH = 3.3, in good agreement with the value expectedg a t the pH used; IS = 447 66, in agreement with the value found in neutral solution. Hydrogen peroxide, in the absence of oxygen, also competes with alcohol for hydrogen atoms, as shown in Figure 3. Here the molecular yield G H was ~ estimated as a function of peroxide concentration by interpolation from the d:tta of Ghormley and Hochanade1,’O again assuming linearity of G H ~with the cube root of the scavenger concentration. The ratio k H + H z o 2 / k H + c 2 R s o n is 2.2 A 1.0. Thus k H + 0 2 / k H + H 2 0 2 is about 200; Thomas, in three different papers,ll finds 175, 210, and 300; Hochanadel12finds 455. During these irradiations, peroxide disappeared with a very high yield, evidently by a chain reaction with the alcohol. Such a chain would be expected from the reactions ( R = . C2H40H)
The Journal of Physical Chemistry
(3)
+
where GR = GOH G,,,G H and I is the radiation intensity or “dose rate” in units which may be described most conveniently as (1/15.5)(d(Fe8+)/dt in Fricke dosimeter) (Mlsec). Thus the yield of peroxide disappearance should rise linearly with the peroxide concentration and the reciprocal square root of the intensity and should be independent of the alcohol concentration. Figure 4 shows that these laws are accurately followed. The quantity plotted here is G(-H202) G H ~ Owhere ~ , we take GH~o, as 0.7.g
+
7
2*81
2.4
22.0 -W I
-
1.6
I
1.2
0.8
*
H, OH, H2, H202
=
[G~”21~”Zk~/(2k~)”2](H~O~) (C)
Figure 1. Hydrogen yields in neutral solutions containing ethanol and oxygen: e, 02 = 42.2 p M ; 0,65.8 phf; [I, 13.1 phf; A, 270 pM.
H 2 0 = e,,-,
+GH~O~
- G,,,-
0.4
0
2
4
6
8
IO
(o,)/(c,H,oH)
12
14
16
18
20
xio3
Figure 2. Hydrogen yields in solutions of ethanol and oxygen containing 0.0225 M HC104, pH 1.69. Meaning of symbols same as in Figure 1.
(7) G. Scholes, Discussions Faraday SOC.,36, 311 (1963). E.Hayon, Trans. Faraday SOC.,60, 1059 (1964). (9) A. 0.Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” D. Van Nostrand Co., Inc., Princeton, N. J., 1961. (10) J. A. Ghormley and C. J. Hochanadel, Radiation Res., 3 , 227 (1955). (11) (a) J. K.Thomas, J . Phys. Chem., 67, 2593 (1963); (b) J. P. Sweet and J. K. Thomas, ibid., 68, 1363 (1964); (c) H.Fricke and J. K. Thomas, Radiatwn Res., S u p p l . , 4, 35 (1964). (12) C. J. Hochanadel, Radiation Res., 17, 286 (1962). (8)
RADIATION CHEMISTRY OF AQUEOUS SOLUTIONS OF ETHANOL
I
I
1
G( -HzOz)
1917
+ G H ~ -o ~G
e. 9 -
=
h‘ks (HzOz) (D) Go - 2kdc~(CzHsOH)I
I.o
0.1
0
0.3
0.2
where Go is the expression on the right-hand side of eq C. Figure 5 gives data on yields a t two different peroxide and three different alcohol concentrations, which vary with the concentrations according to eq D, although the intercepts (for infinite (CzHsOH)) are a little low. From the slopes of the two lines, we find ks/lc6 = 0.060 and 0.058, respectively, but the uncertainty in the intercepts reduces one’s confidence in the significance of the slopes. It is possible that a small quantity of oxygen is formed a t the higher (H202)/ (GHsOH) ratios, which tends to interrupt the chains. If we accept the above ratio, since k7 = 4.5 X we find kS = 7.6 X lo8,not very different from Adams’ value13 of about 10.5 X los and Thomas’ value14 of 7.2 X lo8. Measured aldehyde yields are shown in Table 11. The missing compound in the material balance is RZ (2,3-butanediol), presence of which was shown qualitatively by vapor-phase chromatography, using an F & M Model 300 with flame ionization detector. We also showed that isomeric diols are absent, or are a t most 10% of the major isomer. If the product of chain-breaking reaction 6 were entirely diol, a yield of G R / ~or about 2.6 would be expected. We conclude that disproportionation to aldehyde and alcohol occurs in about 30% of the reactions. Taub and Dorfman15 estimate about 20%.
0.4
MEAN(H,O,)/(C,H,OH)
Figure 3. Hydrogen yields in neutral solutions containing hydrogen peroxide and ethanol but no oxygen : 0, 1.75mM C,HbOH; 0,3.5mM; A, 10.5mM. I
I
I
I
1s 16
14
0 12
k
0
+
6
2
0
t
4 0 200 300 600 100
400 500 ~ H z O z N 1 8 / If’: p M
Table 11: Material Balance in the Reaction of Ethanol (3.5 mM) with Hydrogen Peroxide
100
Figure 4. Peroxide destruction yields in solutions containing hydrogen peroxide and ethanol (3.5 mM except 0.875 and 7.0 mM where indicated): 0,high-intensity source; 0 , low-intensity source; abscissa, peroxide concentration in p M multiplied by the square root of the ratio of the standard intensity (that of the high-intensity source on an arbitrary date) to the actual intensity.
The intercept should then be equal to Geaq-; the value found is 2.3. From the slope, we find k7/(2ks)”’ = 3.30. At lower ratios of CzHsOH to HzOZ,reaction 9 must be taken into account. Inclusion of reactions 9 and 10 in the scheme leads to a complicated expression which,
+ = HzO + HOz HOz + R = CHsCHO + HzOz OH
H202
if (C2H50H)> (HzOz),can be approximated by
(9) (10)
Source intensity
(HzOz),
Low High High
99 99
p M
G(dio1) = G(-HsOz) G(Hd G(a1d)
+
G( -HzOa) G(a1dehyde) G(Hz)
160
14.3
13.5
1.0
5.05 7.2
4.4
1.0
6.7
1.0
1.8 1.65 1.5
To obtain absolute values of reaction rates of the alcohol radical R, we turned to the interrupted beam or “rotating sector” technique. The theoretical curve for this case, in which chains are terminated by a reac(13) G. E. Adams, J. W. Boag, and B. D. Michael, Trans. Faraday Soc., 61, 1417 (1965).
(14) J. K. Thomas, ibid., 61, 702 (1965). (15) I. A. Taub and L. M. Dorfman, J. A m . Chem. Soc., 84, 4053 (1962).
Volume 71, Number 6 May 1967
W. A. SEDDON AND A. 0. ALLEN
1918
I
‘Y
1 .o
It-
BEAM ON TIME,CORR. A ~ I / I ~ ~ ‘ ~ , s ~ c
---& 12
0
I 1000
I 2000
I I 3000 4000 I/(C*H,OH),
I 5000
M
1 I 6000 7000
Figure 5. Effect of reduced alcohol concentration on the yield of peroxide destruction: upper line (H2Oz) = 197 pM; lower line, 122 p M ; arrows indicate intercepts calculated from the line of Figure 4; ratio of slopes, 4.35; ratio of (H202)a,4.2.
tion second order in the carriers, is given by Burnett.Is With the beam on onefourth of the time, the yield of the chain a t very short on times should be twice that for steady irradiation. I n Figure 6, the chain yield AG = G(-HzOz) - G,G H ~ divided o~ by its steady-state value is plotted against the on time (divided by 1’”). The theoretical curve shown is fitted to the data by a single parameter, the value of which fixes 2ks at 2.0 0.6 X lo9 M-’ sec-l. The value of Dorfman and Taub,l’ 1.4 f 0.4 X lo9, is not really in disagreement, but the curve corresponding to 1.4 X 109, shown dotted in the figure, fits the data somewhat less well. From the ratio found in Figure 4,we find k7 = 1.5 X lo5, which is rather smaller than might be expected.
+
*
Discussion The data on the ethanol-peroxide chain reaction point toward a value of 2.3 for the yield of solvated electrons. This is lower than the value 2.85 of Czapski and Allenls and 2.6 of Hochanadel and Casey,lS but agrees better with estimates by Hayon20 (2.3) and by Head and Walker21(2.45). The yield of peroxide in oxygenated ethanol solutions,2 3.2, should be equal to GH,o, ‘/z(GoH GH Ge-). Since G H ~ o=~0.7,12 GH = 0.6 (present work), and GOH= 2.2 (both by the difference in G(HPOZ)in the presence and absence of alcohol and by G(a1dehyde) in the ethanol-oxygen solution), this gives G,,,- = 2.2.
+
The Journal of Physical Chemistry
+ +
Figure 6. Pulsed-beam irradiation of ethanol-peroxide solutions: ordinate, ratio of the chain yield AG of peroxide destruction to that expected for steady-state irradiation under the same conditions of concentrations and radiation intensity (from Figure 4); abscissa, beam-on time A, sec, multiplied by the square root of the ratio of intensity to standard intensity; 0 , intensities near 2.6 X lo-* M/sec/unit G value; 0,near 0.50 X lo*, in the same units. Curve is drawn with theoretical shape and may be fit to the data only by sliding horizontally. Solid curve shows the best fit; the dotted curve is expected from ref 17.
If OH, H, e,,-, H2, and H202 are the only important oxidized and reduced species formed in radiolysis, then material balance requires that the above yield should be equal also to G H ~ GH Gesq-. This we measured as G(HJ in ethanol plus 0.1 M H2P04- solutions and found it to be 3.5, 10% higher than the G (HYOz) in oxygen. The difference, though outside experimental error, does not necessarily mean that other species are involved; it may be simply due to scavenging of radicals from the spur by the rather concentrated phosphate. It would be interesting to determine the effect of different phosphate concentrations on G(H202) in oxygenated solutions. The apparent inconsistencies in the measurement of radical yields, which have led to various proposals for the existence of additional transient species, are not yet all resolved. It is becoming clear, however, that radical yields may be increased by the presence of rather low concentrations of scavenger to a greater extent than was formerly realized.
+
+
(16) G. M. Burnett in “Tvfechanisms of Polymer Reactions,” Vol. 111, Interscience Publishers, Inc., New York, N. Y.,1954. (17) L. M. Dorfman and I. A. Taub, J. Am. Chem. SOC.,85, 2370 (1963). (18) G. Czapski and A. 0. Allen, J . Phys. Chem., 66, 262 (1962). (19) C.J. Hochanadel and R. Casey, Radiation Res., 25, 198 (1965). (20) E.Hayon, Trans. Faraday SOC.,61,723 (1965). (21) D.Head and D. C. Walker, Nature, 207,517 (1965).
