THE
J O U R N A L
OF
ICAL CHEMISTRY
Registered in U.8.Patent Ofice
@ Copyright, 1966, by the American Chemical Society
VOLUME 69, NUMBER 12 DECEMBER 15, 1965
Electron Capture by Solutes in the Radiolysis of Methanol and Ethanol
by E. Hayon and M. Moreau S W Vde~ ChimiePhysique, C.E.A. Baclay (Seine et Oise), France
(Received August 31, 1966)
The yields of hydrogen produced on y-irradiation of air-free methanol and ethanol at lowest doses are G(EJ = 5.25 and G(H2) = 5.0, respectively. Addition of certain salts (NiC12, CoSOr) has been used to determine the “readily scavengeable” yields of solvated electrons in methanol, G(e-) = 1.05, and ethanol, G(e-) = 0.90. Addition of nitrate and chloroacetate ions, over a wide concentration range, gives rise to a decrease in hydrogen yields and an increase in yields of nitrite and chloride ions. Above a certain solute concentration, the sum of the yields of G(H2) G(N02-) or G(HJ G(Cl-) increases with further increase in [SI. This increase has been shown to be proportional to the rate constant of the solute with solvated electrons and is attributed to electron capture by solutes in the RCH20Hspurs leading to a decrease of the combination reaction RCHzOH+ 2RCH20H. The “total scavengeable” yield of electrons is G(e-) = 1.85 in methanol and G(e-) = 1.65 in ethanol.
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The action of ionizing radiations on simple aliphatic alcohols is in many ways quite similar to that on water, giving rise tO solvated electrons and hydrogen atoms. I n the radiation chemistry of water it has recently been shown,l on addition of certain oxidizing agents, e.g., H+, NOa.-, and CICHzCOOH,that the yield of total reducing species increase, above a certain solute concentration, with further increase in solute concentration finally reaching a maximum yield. This increase in radical yields was shown to be proportional to ~RSCS, where IGRB is the rate constant for reaction of the solute with H:D- and CSis the solute concentration, and was attributed to the capture by solutes of electrons produced in regions of high radical concentration, ie., spurs. Such scavenging leads to a reduction of the combination reaction HzOOH 3 HzO OH- to form water.
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It seemed of interest to demonstrate that the same phenomenon took place in the radiolysis of simple aliphatic alcohols. With this view in mind, the yirradiation of lithium nitrate and chloroacetate solutions was investigated. Experimental Section A 200-curie 6oCoy source was used, and the dose rate in pure alcohols was 2.2 X 1OI6 e.v./g. min. based on the ferrous sulfate dosimeter, taking G(Fea+) = 15.5. The ethanol was a Merck “pro analysis” product and was used without further purification since it was found to give G(H2) = 4.85 =t0.05, in good agreement with other recently published v a ; l u e ~ . ~ -A~ (1) E. Hayon, J. Phys. Chem., 68, 1242 (1964) ; Trans. Faraday Soc., 61,723 (1965). (2) E. Hayon and J. J. Weiss, J. Chem. Soc., 3262 (1961).
4053
E.HAYON AND M.MOREAU
4054
later batch of Merck ethanol was found to give G(H2) = 5.00 i: 0.05 (see below). The Merck “pro analysis” methanol was found to contain aldehydes, and various ways of purifying it were tried. The best results were obtained by refluxing overnight under a nitrogen gas atmosphere about 800 ml. of methanol containing 1 ml. of HzS04 and 3 g. of 2,4-dinitrophenylhydrazine sulfate. The column having been cleaned by the reflux, this methanol was discarded. On the following morning a fresh quantity (with added H2S04and hydrazine) with N2 bubbling through the methanol was refluxed for 3 hr. The middle fraction was then collected in a receiver kept a t 0”. All reagents used were analytical research grades supplied by Hopkin and Williams except LiW03 which was supplied by Baker and Adamson. They were all dried in a desiccator containing silica gel. All irradiations were carried out in Pyrex ampoules of ca. 5-ml. capacity; the degassing procedure and gas analysis by chromatography have already been de~cribed.~ Nitrite ions were determined by the method of ShinnJ6using E 53,200 a t 5400 8.,and chloride ions determined7as silver chloride.
Results I n all cases the alcohols were irradiated to low doses
(7 X 1017e.v./g.) so as to determine initial yields, and the maximum total dose given in the determination of low concentrations of chloride ions (> L(H S). This condition was not usually observed in previous studies. The radiolysis in the presence of the divalent ions Ni2+ and Co2* was there-
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The Journal of Physical Chemistry
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fore examined since Baxendale and Dixon8 have indicated that these ions react very much more slowly with H atoms than with electrons in aqueous solutions, M2+)/k(H M2+) 2 104.5r8 The such that k(eHZyields were determined as a function of solute concentration of these divalent salts, and the results are shown in Figure l(b) using NiCl2.6HZOand C0S04. 7H20. A plateau is obtained over almost a 40-fold change in the solute concentration to give G(H2) = 4.10 i 0.05, as compared to G(H2) = 5.0 f 0.05 in pure ethanol from the same batch. Addition of lithium nitrate, which is soluble in alcohols, was found to reduce the Hz yield in the radiolysis of 2-propanoLQ A similar effect was observed in the irradiation of ethanol and methanol. The decrease in H2yields and the formation of NO2- ions with increase in [LiNOz] are shown in Figure 2. There appears to be an inflection in the curves in the region M LiNO,, and its significance is not clear. of 5 X G(N02-), seems to Gtotal,that is, the sum of G(H2) reach a plateau value of 5.6 a t -1.0 M LiN03. It is interesting to note that this increase of 0.75 G units over the yield in absence of LiNOs is reflected in the NO2- yields which increase more rapidly than the corresponding decrease in the H2yields. On addition of chloroacetic acid, the same effect is observed, as shown in Figure 3. At low solute concentrations, the yields of Hzdecrease more rapidly than with LiN03, indicating a greater reactivity with the precursor of HZin this region. Similarly, the yield of 61- increases more rapidly with concentration than that of NO2-. The sum of G(Hz) G(C1-) is plotted in Figure 3, and one obtains a maximum G value of 5.65. Again, the increase in 0.80 G units due to scavenging by the chloroacetate is to be seen in a greater yield in G(Cl-) over and above the corresponding decrease in G(H2). Above 2 X 10-l M chloroacetate, the yield of Hz continues to decrease regularly, but the yield of C1- increases abruptly bearing no relation to the corresponding decrease in Hz. Such a rapid increase in C1- yields seems to indicate the presence of a chain reaction. The yields of H2 in chloroacetate are somewhat different from those previously found2 and are closer to those recently reported.
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(3) G. E. Adams and R. D. Sedgwick, Trans. Faraday SOC.,60, 865 (1964). (4) J. J. J. Myron and G . R. Freeman, Can. J . Chem., 43, 881 (1965).
(5) (6) (7) (8) (9)
E. Hayon and M. Moreau, J . chim. phys., 62, 391 (1965). M. B. Shim, Ind. Eng. Chem., Anal. Ed., 13, 33 (1941). E. Hayon and A. 0. Allen, J . Phys. Chem., 65, 2181 (1961). J. H. Baxendale and R. S. Dixon, Proc. Chem. HOC., 148 (1903). L. Gilles, private communication.
RADIOLYSIS OF ALCOHOLS
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I
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1
,I
I
'
"""'I '
'
'"",
"""I
-
I
I 1 1 . 1 . 1
4
1 1 1 . 1 3 1
I
1 1 ' * ' * 1
'
' I " *
2-
12
x
10-6
10-4
10-8 Cs, M.
10 -9
lo-'
Figure 1. (a) Radiolysis of methanol in presence of NiCl2, 0. (b) Radiolysis of ethanol in presence of NiC12, 0, and CoSOd, m.
w5
XrL
1r2
IOd
[CLCH2 COOH], 9
Figure 3. ?-Irradiation of air-free ethanolic solutions of chloroacetate. Yields of G(Hz), W, G( Cl-), A, and the sum G(H2) G(Cl-), 0, are shown.
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n1
[LiNO-,],!?
Figure 2. ?-Irradiation of air-free ethanolic solutions of LiNOa. Yields of G(Hz), m, G(N02-1, A, and the sum G(H2) G(N02-), 0, are shown.
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Radiolysis of MeEhanolic Xolutions. Considerable attention was given to the purification of methanol. Making the assumption that the highest yields of hydrogen obtainable are an indication of the absence of impurities capable of reacting with the solvated electrons and H atoms produced on irradiation, and hence of a higher degree of purity, we endeavored to vary our purification procediire with this end in mind. The method of purificatjon found to give the highest G(HJ yields has been described above. A mean of 12 ir-
radiations gave G(H2) = 5.26 f 0.1, on y-irradiation of deaerated methanol. This yield is to be compared with earlier values of 4.1,101114.8,12" 4.99,12b and the high values of 5.39I2 and 5.414 obtained after careful purification of the methanol. As was done for ethanol, the yield of readily scavengeable solvated electrons was determined by measuring the yield of hydrogen produced in the presence of NiCI2. A plateau value of G(H2) = 4.2 f 0.05 was obtained, Figure l(a), indicating G(e-) = 1.05 f 0.05. On addition of LiNO, the yields of G(H2) and G(N02-) were irreproducible, particularly in moderately dilute solutions. In concentrated solutions the results were better, e.g., in 1.0 M LiN03, G(H2) = 1.82 and G(N02-) = 4.18, giving GT = 6.0. The results in this region of concentration independence correspond to the maximum yield obtained by Baxendale and Mellows14 on addition of H f ions, G(H2) = 6.05, and indicates that the total scavengeable yield of electrons produced in methanol is a(€!-),= 1.85 f 0.1.
Discussion Chemical kinetic e v i d e n ~ e , ~ *as~well t ~ ~ Jas~spectral16 (10) E. Hayon and J. J. Weiss, J. Chem. Soc., 3970 (1961). (11) G. E. Adams and J. H. Baxendale, J . Am. Chem. Soc., 8 0 , 4215 (1958). (12) (a) hl. Imamura, S. U. Choi, and N. N. Liohtin, ibid., 85, 3565 (1963); (b) L. M. Theard and M. Burton, J . Phus. Chem., 67, 59 (1963). (13) G. Meshitsuka and M. Burton, Radiation Res., 8, 285 (1958). (14) J. H. Baxendale and F. W. Mellows, S. Am. Chem. Soc., 83, 4720 (1961).
Volume 89, Number 13 December 1986
E. HAYON AND M. MOREAU
4056
and e.s.r.l6 evidence, has recently established the formation of solvated electrons in the radiolysis of simple aliphatic alcohols. The curves showing the variation of G(H2) with the concentrations of various solutes (ref. 2, 3, 10, 14 and Figures 1-3) show two regions which are sometimes separated by a range of solute concentration over which G(H2) remains constant. These observations have been interpreted to indicate the presence of two precursors of hydrogen-the solvated electron and H atoms-each of which is capable of being scavenged by solutes, although with significantly different rate constants. From known reactivities of various solutes, the decrease of G(H2)down to the plateau region has been attributed to the reaction of the solute with solvated electrons, and the decrease of G(Hz) at higher solute concentration to a competition between the solute and alcohol for H atoms. Reactions 1-3 are considered to be the main processes occurring on y-irradiation.
+ RCH20HRCHOH + H
2RCH2OH --+ RCH20H+ RCHzOH
-j-
+ H2
(3)
+ Hf RCH20H- +RCHzO- + H H + RCHzOH +H2 + RCHOH ---f
RCHOH
On using solutes known to be good reactants for electrons, such as nitrate ions and chloroacetic acid, one can explain the results obtained up to -5 X lop8 M solutions when Gtotal remains constant (Figures 2 and 3) as due to reactions 4 to 8 RCH20H-
+ NOa- + NO2
+ RCH20- + OH-
+ ClCH2COOH C1- + CH2COOH + RCHzOH H + NOa- *NO2 + OHH + ClCHzCOOH -+- Hz + CICHCOOH
RCH2OH-
H
+ ClCHzCOOH
(4)
--j-
-j-
Hf
+ CI- + CH2COOH
(5)
(6)
(7) (8)
The NO2formed in reactions 4 and 6 reacts, as has been The Journal of Physical Chemistrg
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"""'
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I
' ' 1 " " '
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' ' ~ " " " '
"'""
'"'-7
GT 5.0 0
Ht ( A.and S.)
o LiNO3 ,f=1/7 ClCH2COOH,f=l
10 Normalized C,
Figure 4. Plot of Gtokl as a function of solute concentration normalized with respect to [H+]: [H+], e, data from ref. 3; [LiNOa], 0, f = 1/7; [ClCH&OOH], 0, f = 1. (fis the normalization factor.)
shown in aqueous solutions,l7 with RCHOH radicals to yield NOz- ions and the corresponding aldehyde RCHOH
(2)
is followed by RCHzOHf
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5.5
(1)
where RCH20H- represents the solvated electron. Reaction 3 which could account for the formation of unscavengeable (molecular) hydrogen RCHSOH +RCHO
1
+ NO2
---f
NO2-
+ RCHO + H +
(9)
In this way one reducing species leads to the formation of one NOz- ion. At solute concentrations above 5 X loy3M one finds, however, that the sum of the H2 and NOz- (or Cl-) yields does not remain constant but increases with further increase in [SI finally reaching a plateau value. This increase is reflected in the yields of NOz- and C1ions which are greater than the corresponding decreases in the H2 yield. These results are interpreted to indicate a scavenging of electrons in the spurs by the solutes present in relatively high concentration, resulting in a decrease of the back reaction RCHzOH+
+ RCH20H- +2RCHzOH
(10)
As was done in the radiation chemistry of water,l?17 the increase in total yields with increase in solute concentration was normalized with respect to [H+] since the rate constant k(eH+) is known16: in ethanol k(eH+) = 2 X 1Olo M-l sec.-l and in methanol k(eH+) = 4 X 1O1O M-1 sec.-1. Figure 4 shows the results normalized for LiN08 and ClCHzCOOH solutions against [H+], using the results of Adams and Sedgwick3 of G(H2) us. [Hf]. The normalization coincidence of the curves is not so good as that ob-
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(16) I. A. Taub, M. C. Sauer, and L. M. Dorfman, Discussions Faraday Sac., 36, 206 (1963). (16) C. Chachaty and E. Hayon, Nature, 200, 69 (1963); J . chim. phya., 61, 1116 (1964). (17) E. Hayon, Trans. Faraday Soc., 61, 734 (1965).
RADIOLYSIS OF ALCOHOLS
tained in aqueous systems. However, from the normalization factors, f, obtained one can calculate rate constants to better than 140%. Thus, in ethanol, NOa-) = 5L9 i 0.9 X lo9 M-l sec.-l and k k(e(eC1CHzC00€3) = 2.0 =k 0.7 X 1O1O M-l sec.-l. Such rate constant values are in support of the capture by these solutes of electrons since the rate constants for reaction with IH atoms are much smaller (see, e.g., ref. 5). It is seen, therefore, that in the radiolysis of ethanol the yield of readily scavengeable solvated electrons derived using Ni2+ or Go2+ salts is G(e-) = 0.90 f 0.10, and the yield. of electrons scavengeable at relatively high concentrations of LiN03 or ClCH2COOH in the spurs is G(e-) =: 0.75 f 0.10. This total G(e-)T = 1.65 f 0.20 obtained on irradiation of liquid ethanol at room temperature is to be compared with the yield of electrons G(e-) :=: 2.3 f 0.7 as determinedls by e.8.r. in the radiolysis oE glassy ethanol at 77°K. On yirradiation of methanol, the yield of readily scavenge-
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able electrons is 1.05 0.05, and the maximum yield of electrons is G(e-) = 1.85 h 0.1. The corresponding yield obtainedla by e.s.r. on irradiation of glassy methanol at 77°K is G(e-) = 2.2 f 0.7. The yields of readily scavengeable electrons given above can be compared with G(e-) = 0.95a and 0.g4for ethanol and G(e-) = 1.314for methanol. In conclusion, it would appear to be a general phenomenon in the radiation chemistry of liquid systems that some of the electrons, which are produced on ionization of the medium and which normally return to the parent positive ions or react with other positive ions or radicals formed on irradiation, can be captured in the tracks or spurs by certain solutes present in relaS is tively high concentrations provided the ~ R S Cvalue sufficiently high (where T~RS is the rate constant for reaction of the solute with electrons and CSis the solute concentration). Such electron capture by solutes in the spurs leads to a higher yield of radical production and hence a higher net decomposition of the liquid.
Volum 60,Number 13 December 1066
