PULSE RADIOLYSIS OF ALIPHATIC ACIDSIN AQUEOUS SOLUTIONS of studies that the introduction of bulky organic groups aroitnd the metal ion will frequently increase the rate of substitution reactions.1s
4207
Acknowledgment. This work was supported, in part, by a grant from the Ohio University Research Committee.
Pulse Radiolysis of Aliphatic Acids in Aqueous Solutions. I.
Simple Monocarboxylic Acids by P. Neta,’ M. Simic,’ and E. Hayon Pioneering Research Laboratory, U.9. Army Natick Laboratories, Natick, Maesachusetts 01 760 (Received May 1 , 1969)
Transient optical absorption spectra due to carboxyalkyl radicals have been observed on pulse radiolysis of aqueous solutions of some aliphatic acids (formic, acetic, propionic, n- and isobutyric, and trirnethyl acetic acids). These radicals were produced as a result of dehydrogenation by H atoms and OH radicals at the CY and/or /3 positions. The assignment of these radicals was supported by the results obtained from the reaction of eaQdwith the corresponding monochloroaliphatic acids when dechlorination takes place. The a-carboxyalkyl radicals (Cz-C4) studied have absorption maxima in the region 290-350 nm, while the /3 radicals (Ca-C,) have maxima below 250 nm. The observed change with pH in the transient absorption spectra of these radicals was attributed to the dissociation of the carboxyl group. The pK values of these radicals were determined spectrophotometrically and were found to correspond in most cases to the pK values of the parent aliphatic acids. The extinction coefficients and decay rates were also found to be dependent upon pH. The reactivity of CH20H, CHsCHOH, .CH2C(CH&OH, oCH~COO-,and -COz- radicals with .COz- and eCH2COO- was investigated.
Introduction The radiation chemistry of a number of simple aliphatic carboxylic acids in aqueous solution has been examined and many of the products formed have been determined (for general review, see Allen2). These products resulted from the reactions of the acids with the reactive species formed in the radiolysis of water. H20 -w+
eaq-, OH, H, Hz, H202
The main radiolytic products from formic acid were Hz, COZ and/or oxalic acid depending on the pH of the solution. The main products from acetic acid were hydrogen and succinic acid. Carbon dioxide, methane and biacetyl were also found, the yields of which become significant only a t high concentrations ( > 1 M ) . The yields of Hz and C 0 2from the radiolysis of higher acids were also studied.a From the product analysis it was concluded that the main reactions taking place in these systems are hydrogen abstraction by OH radicals and H atoms
+ OH +R2&OOH + H 2 0 RzCHCOOH + H R2C’COOH+ Hz
R2CHCOOH
--t
(1) (2)
At higher concentrations (>0.1 M ) these acids were
suggested4 to react with hydrated electrons to produce carbonyl compounds and CO.
CHaCOOH
/*CH3c0
+ eaq-
I
CHaCO-
+ OH-
+ OH
(34
(3b)
A transient absorption spectrum assigned to C02has been obtained on pulse radiolysis of formic acid solutions,6 but none of the higher aliphatic acids has been investigated. This paper presents a pulse-radiolysis study of aqueous solutions of formic, acetic, propionic, n- and isobutyric, and trimethylacetic acids. The observed transients were produced mainly by reaction with OH radicals and H atoms. The eaq- was (1) National Academy of Sciences National Research Council R e search Associate a t Natick. (2) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” D. Van Nostrand, Company, Inc., Princeton, N. J., 1961. (8) H.Fricke, E. J. Hart, and H. P. Smith, J. Chem. Phys., 6 , 229 (1938). (4) E. Hayon and J. Weiss, J . Chem. Soc., 5091 (1960). (5) J. P. Keene, Y. Raef, and A. J. Swallow, in “Pulse Radiolysis,” M. Ebert, 5. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press, New York, N. Y., 1966,p 99.
Volume 78, Number 13 December 1969
4208
P. NETA,M. SIMIC,AND E.HAYON
converted to OH radicals or H atoms by reaction with N20or H30+.
I
I
I
Experimental Section This work was carried out using a Febetron 705 pulsed radiation source, and the experimental conditions have been describeda in detail. Briefly, this source produces an electron beam of 2.3 MeV energy and single pulses of -30 mec duration. The monitoring light source was an Osram XBO 450W xenon lamp, and the light output from the lamp was increased by a factor of 25-30 times by increasing the current to the lamp with pulses of -1.2 msec duration. Two highintensity Bausch and Lomb monochromators were used in series to reduce scattered light. Solutions were prepared using water purified by triple distillation, radiolysis, and photolysis. Analytical reagent grade and spectrograde chemicals were employed as supplied by Baker and Adamson, Mallinckrodt, and Eastman. Solutions were buffered using perchloric acid, potassium hydroxide, sodium tetraborate (1-5 mM) and potassium phosphate (1-5 mM). Carbon dioxide was supplied by Matheson and was further purified on a vacuum line. Dosimetry was carried outa using a 0.1 M KCNS solution and the (CNS)2radical formed was observed at 500 nm using €600 = 7600 M-l cm-l. :Doses of 8-36 krad/pulse were used and were derived on the basis of g(esq-) = g(0H) = 2.8. Due to the high dose per pulse, the choice of solute concentration was made on the basis of known radicalradical and radical-solute rate constants' at the various p H values studied. This was also checked by varying the concentration of the acids. Full scavenging of the radicals was also ensured in the determination of the OD vs.pH curves. Results and Discussion Pulse radiolysis of aqueous solutions of aliphatic acids was carried out in NaO-saturated (1 atm) solutions to convert practically all (-98%) the esq- into OH radicals eaq-
+ N20
---t
N2
+ OH- + OH
(4)
Solutions at lower p H were studied under conditions such that all the esq- would react with H30+ to form hydrogen atoms eaq-
+ H30+ +HzO + H
(5)
Both H atoms and OH radicals are considered to dehydrogenate carboxylic acids and their ions in a similar way, reactions 1 and 2. Since the reactivity of carboxylic acids with H atoms is relatively low, high concentrations (up to 1 M ) of these acids were used to assure full scavenging of both hydrogen atoms and hydroxyl radicals. The nature of the transient species produced in the pulse radiolysis of these monocarboxylic acids was T h e Journal of Physical Chemistry
d 0
0.I
0
A,nm Figure 1. Absorption spectrum of COZ- in irradiated aqueous solutions (8 krads/pulse). E is given in M - l om-l units. 0, 0.03 M HCOONa, NzO (1 atrn), pH 9; 0, 0.03 M HCOONa, NzO (1 atm), pH 13; 0, 0.03 M HCOONa, COZ (1 atrn), pH 3.1; €3, 0.1 M EtOH, COZ(1 atm), pH 5 ; 0, 0.2 M MeOH, GOz (1 atm), pH 5.
established in a number of cases by irradiating the corresponding CY- or P-chloroaliphatic acids. These are known8 to react with eaq- to form halogen ions RzCClCOOH
+ eW-
RzCClCH2COOH
+ e,-+
---t
+ C1-
(sa)
+ C1-
(6b)
RbCOOH
Rz6CH2COOH
To eliminate the reaction of OH radicals with the chloroaliphatic acids, t-butanol was used to scavenge the OH radicals. Tertiary butanol was chosen for this purpose since it was showna to form a transient with low absorption above 250 nm and a spectrum independent of pH. I n all the cases presented in this paper, the absorption due to the t-butanol radical was subtracted from the total absorption of the transient species observed. Furthermore, the CH&(CH&OH radical was found to be unreactive with the chloroaliphatic acids. This was shown by irradiating solutions of t-BuOH N20 ClCH2COO- or CHsCHClCOO- under conditions such that RCHCICOO- would not react with either eaq- or OH. Only .CH2C(CH3)20H radicals were observed with no absorption in the region where the a-carboxyalkyl radicals absorb. Transient absorption spectra of aliphatic acid radicals, with the exception of COZ-, are in acidic solutions invariably different from those in alkaline solution. By following the change in absorption of the transient
+
+
(6) M. Simio, P. Neta, and E. Hayon, J. Phys. Chem., 73, 3794, 1969. (7) M. Anbar and P. Neta, I n t . J. Appl. Radiat. Isotopes, 18, 493 (1967). (8) E. Hayon and J. Weiss, Proc. 2nd Intern.'Conf. Peaseful Uses A t . Energy, Geneva, 39, 80 (1968); E. Hayon and A. 0. Allen, J. Phys. Chem., 65,2181 (1961).
4209
PULSE RADIOLYSIS OF ALIPHATIC ACIDSIN AQUEOUS SOLUTIONS
Table I : Absorption Maxima, Extinction Coefficients, Decay Rate Constants, and Dissociation Constants for the Carboxyalkyl Radicals Produced in Pulse Radiolysis of Aqueous Solutions of Some Aliphatic Acids PH
Acid
Formic
Amax; nm
3.1 9
L,
M - 1 pm-1"
2k; M - 1
1.5 x 1.5 x 1.7 x 1.8 x
235 235
3000 3000
320 350 300 335 -280 335 -295 325 260 nm are in agreement with the results of Keene, et aL6 However, we find the absorption maximum at 235 instead of 250 nm, with 8236 = 3000 M-I cm-l. This difference is probably due to the relatively high correction factor for scattered light used in the earlier experiments.6 The equivalence of the products from reactions 7 and 8 could also be demonstrated in 7-irradiated aqueous solutions of 0.5 M HC02Na N2O and 0.5 M HC02Na 0.5 M NaHC03 (HC03- e C02 OH-) at pH values between 6 and 9. The yield of oxalic acid, produced by dimerization of COZ-, was determined by precipitation as calcium oxalate. Identical yields were foundgin both systems, with G(oxa1ic) = 3.4 and 3.2 i 0.2, respectively. Aqueous solutions of MeOH C02 and EtOH C02 were also irradiated (Figure 1). After correcting for the absorption due to the alcohol transient, which originates from the reaction of OH radicals with the alcohol, the spectrum was found to be identical with that in formic acid solutions, supporting further the above view. The transient absorption in the alcohol C02 solutions was about one-half that in HC02N2O solutions, in agreement with the relative yields of cap- and ea,OH H. -3
+
+
+
+
+
+
+
+
+
PKn of
PKn of radicalb
Radical
8ec-1°
acid
C
3.77
4.5
4.76
4.9
4.88
4.8
4.82
5.8
4.85
4.8
5.02
Deviation ~t0.2. ' See Discussion.
I n acid solution (pH 3 a decrease of the C02 yield3 and an increase in the oxalic acid yielde was observed, suggesting preferential dimerization of the radicals. Since no changes in the absorption spectra and decay rates of the carboxyl radical were observed down to p H 3 (this work) and pH 2.4,6it is possible that .C02H and eC02- have identical spectra and decay rates. The pK value for the dissociation of this radical might also be below 2.4, and an explanation for the change in the nature of the products produced would have to be found. A transient absorption down to 260 nm, similar to that of .C02-, was observed on pulse radiolysis of aqueous solution of oxalate ions saturated with N20 a t p H 9. Complete scavenging could not be achieved due to the low reactivity of OH radicals with oxalate ions. This spectrum could be assigned either to .CO2-CO2- or to .C02-. However, using the extinction coefficient of * COZ-, the decay rate of the oxalate transient was found to be identical with that of C02-. Acetic Acid. The transient absorption spectrum produced on pulse radiolysis of aqueous solutions of acetate ions saturated with N 2 0 showed a broad absorption band with a maximum a t 350 nm (Figure 2). This transient species is considered to be due to the .CH2COO- radical, and support for this assignment was obtained from the pulse radiolysis of chloroacetate ions. The spectra obtained from solutions of CH8COOClCHzCOO- and of t-BuOH C1CH2COO- were identical with those obtained from CH3COON2O solution. The absorption in the t-BuOH ClCH2COO- system was about one-half that in the other two systems, in accord with the relative radical yields. On pulse radiolysis of acetic acid or chloroacetic acid
+
+
+
+
(9) M. Simio and G. Scholes, unpublished results.
Volume 78, Number 18 December 1960
4210
P. NETA,M. SIMIC,AND E. HAYON
03
0.D. O
I
t
0.2
T
I
I
300
350
0.I 4 0O
450
100
X,nm
Figure 2. Absorption spectra and OD vs. pH curve of CHzCOOH and CHZCOO-radicals in irradiated aqueous solutions (30 krads/pulse). 0 , 1 M CHaCOONa,NzO (1 atm), pH 9; a, 1 M CH3COONa,0.02 M CICHzCOONa,pH 9'; 0, 0.1 M CICHzCOONa, 1 M t-BuOH, pH 9; 0, 0.1 M CICHzCOOH, 1 M t-BuOH, pH 3; pH curve: 0.1 M ClCHzCOOH, 1 M t-BuOH.
-
0
0.2
0.D. 0.I
solutions at p H 3, the peak of the transient absorption is shifted to 320 nm, and the extinction coefficient is found to be about 20% lower than that at pH 9 (Figure 2). This absorption is assigned to the CHzCOOH radical, since its formation was found to be dependent on the dissociation of the acid (see below). It is evident from these results that the site of reaction of the OH radical with the dissociated or undissociated acid is primarily at the C-H bond (under the experimental conditions used). An attack on the carboxyl group, if taking place, has only a minor contribution. This is in agreement with previously reached conclusions from product analysis, kinetic measurements,1° and esr studies.l1JZ The p K value of the .CH2COOH radical has been derived by monitoring the transient absorption a t 380 nm in irradiated solutions of t-BuOH CICHZCOOH (inset in Figure 2). It was found to be pK = 4.5, in agreement with the pK value for acetic acid itself (Table I). The second-order decay rate constants of the acetic acid transients were determined in NzO-saturated solutions a t p H 3 and pH 10. The rate was found to be about twice faster for the undissociated radical compared to the dissociated one (Table I), in general agreement with the relative rates of reaction of neutral and singly charged species. Propionic Acid. The transient absorption spectrum obtained on irradiation of aqueous solutions of propionate ions in the presence of N20 (1 atm) at pH 10-13 had a broad maximum at 330 nm and a second absorption with a peak below 230 nm (Figure 3). To demonstrate that the composite structure of this spectrum arises from two species, possibly the a- and p-carboxyalkyl radicals, these radicals were produced independently from the pulse radiolysis of the a- and p-chloropropionic acids. On irradiation a t pH 9, the 01 radical formed shows a broad peak at 330 nm, whereas the p radical has a maximum below 240 nm (Figure 3).
+
The Journal of Physical Chemistry
0 250
300
350
400
450
X ,nm Figure 3. Absorption spectra and OD vs. DH curve of transients produced on pulse radiolysis of aqueous solutions of propionic and chloropropionic acids (36 krads/pulse). a. 0,0.25 M GH~CHZCOON~, NzO (1 atm), pH 9 and NzO (1 atm), pH 3 and pH 13.5; €3, 0.5 M CH~CHZCOOH, pH 1. b. 0, 0.05 M CH&HClCOONa, 1 M t-BuOH, pH 9; €3, 0.05 M CH&HClCOONa, 1 M t-BuOH, pH 3.1; 0,0.1 M CICHzCHzCOONa, 1 M 2-BuOH, pH 9; El, 0.1 M ClCHzCHzCOOH, 1 M t-BuOH, pH 4 ; pH curve: 0.05 M CHsCHClCOONa, 1 M t-BuOH.
The peak of the /3 radical could not be observed due to high absorption of p chloropropionate ions below 240 nm. By comparing the absorption obtained in irradiated solution of CH3CH2COONzO with those of the a and p radicals produced from chloropropionic acids (Figure 3) it can be estimated that the extent of hydrogen abstraction by OH radicals a t the a position is about 55%, in fair agreement with a previous estimate.*O In acid solutions of propionic and a- and p-chloropropionic acids, the absorption spectra are shifted toward lower wavelengths (Figure 3). The maximum of the a radical was shifted to 300 nm and its extinction coefficient is reduced by -25%. These changes are attributed to the formation of the undissociated radicals CH&HCOOH and .CH2CH2COOH. Comparing the N2O with spectrum obtained from CH&HzCOOH
+
+
(10) M. Anbar D. Meyerstein, and P. Neta, J . Chem. SOC.,B , 742 (1966). (11) W.T. Dixon, R. 0. C. Norman, and A. L. Buley, J . Chem. SOC., 701 (1964). (12) H. Taniguohi, K.Fukui, S. Ohnishi, H. Hatano, H. Hasegawa, and T. Maryuaura, J . Phys. Chem., 72,1926 (1968).
421 1
PULSE RADIOLYSIS OF ALIPHATIC ACIDSIN AQUEOUSSOLUTIONS I
I
t
I
I
I
I
I
I
0.4
O.?
+
the rate found in solution of propionate N20. I n all cases a value of (1.2 f 0.4) X lo9 M-l sec-l was obtained for the dissociated radicals. The decay rate for the undissociated radicals was higher -2.2 X l o 9 M-l sec-' (Table I). n-Butyric Acid. The transient absorption spectra from irradiated solutions of n-butyric acid and a-chloron-butyric acid are presented in Figure 4. A broad ab-335 nm was observed in solution sorption with A,, of t-BuOH CH3CHzCHClCOO- at pH 9, and was assigned to the a radical, CH3CHzcHCOO-. I n solutions of CH3CHzCHzCOO- NzO a t the same p H a peak at 335 nm was also observed, in addition to a strong absorption below 280 nm. Attempts were made to unambiguously identify this increasing absorption below 280 nm as an absorption due to the p and y radicals. However, exact measurements of these spectra were not possible due to impurities present in the commercially available p- and y-chlorobutyric acids, which could not be eliminated. The main impurity seemed to be crotonic acid and when aqueous solutions of crotonic acid were irradiated a very strong absorption with Amax 275 nm and E -10,000 M-' cm-l was obtained. This absorption was also found in the impure p-chlorobutyric acid. Nevertheless, it was possible to conclude that both the p and y radicals < exhibit strong absorption below 270 nm with A,, 250 nm. A comparison of the transient absorption obtained in irradiated solutions of n-butyrate NzO and a-chloron-butyrate t-butanol (Figure 4) shows that the reaction of OH radicals with n-butyrate results in 25% hydrogen abstraction in the a position. This is in good agreement with a previous estimate (30%).1° As it was found for propionic acid, the fraction of attack on the a position appears to be smaller for the undissociated acid. A value of 12% was suggested from the esr experiments a t p H 2.12 The pK of the CH3CH2cHCOOHradical was monitored at 350 nm in irradiated solutions of n-butyric acid NzO (Figure 4) and found to be 4 3 , identical with that of n-butyric acid itself. The rate constants for the a radical interaction were derived from the decay of the transient absorption in solutions of n-butyric acid NzO at pH 3 and pH 9 and are of the order of -109 M-1 sec-l. Isobutyric Acid. The results from irradiated soluNZO (Figure 5) are similar tions of isobutyric acid to those of n-butyric acid. The peak at 325 nm was assigned to the a radical and the increase in absorption below 280 nm to the p radical. The a- and P-chloro derivatives were not available for confirmation of these assignments. I n acid solution a shift of the spectrum was observed for both the a and p radicals. The pK of the a radical was derived by monitoring the transient a t 335 nm, and a value of 5.8 was obtained, which is higher than pK = 4.85 for the acid itself.
+
d
+
0
0.2
0.I
0
300
250
350
40t
A, nm Figure 4. Absorption spectra and OD us. pH curve of transients produced on pulse radiolysis of aqueous solutions of n-butyric and a-chloro-n-butyric acids (36 krads/pulse). 0 , O . l M CH3CH2CH2COONa, NzO (1 atm), pH 9; 0, 0.1 M CH3CHzCH2COOH,NzO (1 atm), pH 3.15; 8, 0.05 M CHaCHzCHClCOONa,1 M t-BuOH, pH 9.
+
those from CHzCHCICOOH t-BuOH and CICHZt-BuOH it is found that H abstraction CHzCOOH from propionic acid takes place -35% a t the a position. This result is in agreement with that obtained in esr study12at pH 2. The different reactivity of the a C-H bonds in CH3CHzCOOH and CH3CHzCOOtoward OH radicals is probably due to a difference in the extent of the inductive effect of the -COOH and -COO- groups. The pK of the CH3cHCOOH radical was determined a t 350 nm in irradiated solutions of a chloropropionic acid t-BuOH, and was found to be 4.9, identical with the pK of propionic acid itself (Table I). The second-order decay rate constants of the a and p radicals were measured in solutions of CH3CHzCOONzO, where they are both formed in approximately the same yield. Following the decay in the 330- and the 250-nm region it was found that both radicals decay a t the same rate. Otherwise, the a and p radicals could be formed independently from the chloropropionic acids in the presence of t-BuOH. The effect of the t-BuOH radicals on the decay of the acid radicals was found to be small for acetic acid (see below) and is probably even smaller for propionic acid. Indeed, the decay rates of both the a and the p radicals mixed with t-BuOH radicals were found to be very close to
+
+
+
+
+
+
+
+
Volume 73, Number 1.9 December 1969
4212
?:
P. NETA,M. SIMIC,AND E. HAYON
0.4
0.4
-
I500
d 0.2
0.3 0.3
d
d
0
0
IO00
0 0.2
3
0.2
5
7
E
9
PH 5 00
0. I 0.1
D
0 0
A,n m
X,nm Figure 5 . Absorption spectra and OD us. pH curve of transients produced on pulse radiolysis of aqueous solutions of isobutyric acid (36 krads/pulse); 0.1 M (CH&CHCOOH, NzO (1 atm); 0, PH 9; @, PH 3.
Figure 6. Absorption spectra and OD us. pH curve of the transient produced on pulse radiolysis of aqueous solutions of trimethylacetic acid (19 krads/pulse); E is given in M-l cm-l units; 0.2 M (CH3)&COOH, NzO (1 atm) at pH 9, 0; pH 3.1, @.
The decay rate constant was estimated to be about lo9M-l sec-'. This could not be calculated accurately because both a and p radicals are formed from isobutyric acid in appreciable amounts.12 These radicals could not be formed separately from the chloro derivatives, and therefore only an approximate value for the extinction coefficient could be derived. Trimethylacetic Acid. This acid has no a C-H bonds and a reaction with OH radicals is expected therefore to lead to the exclusive formation of a p radical. The transient absorption spectra obtained in the presence of N20 are shown in Figure 6. No absorption was found in the 300-400-nm region. At pH 9 a maximum at 240 nm was obtained and is suggested to be due to the .CHzC(CH3)zCOO- radical. I n acid solutions at p H 3.1 the absorption maximum appears to be shifted toward lower wavelengths. The peak in acid solution could not be observed due to the increase in absorption of the acid at lower wavelengths. However, the pK of the radical could be derived a t 250 nm (Figure 6) and was found to be 4.8, compared to 5.0 for the acid itself. The second-order decay rate constants a t the dissociated and undissociated radicals are 9.3 X lo8 and 1.3 X lo9 M-' sec-', respectively. Reactivity of COZ- and .CH2COO- Radicals. Reaction rates of .C02- and aCH2COO- with each other and with various alcohol radicals have been followed.
The .COZ- radical was produced either by reaction 7 or 8 at pH 5, and the initial decay was followed a t 255 nm where the absorption of the other radicalsa present contribute less than 15%. The sCH2COO- radical was produced by the reaction of chloroacetate with eas- at pH 9, and the decay was followed at 350 nm where the contribution of other radicals was negligible. The alcohol radicals were formed by the reaction of OH with alcohols. Decay rates were estimated under initial conditions (first half-life), where the concentration of the two radicals was approximately the same, and represent a composite value of k(R1 R2)and k(R1 RI). I n all cases, the decay rates of .C02- and * CH2COOradicals were found to be higher in the presence of an alcohol radical ( * CHZOH, CH&HOH, and CH2C(CH3)ZOH) by a factor of 3-5, indicating a substantial interaction between these radicals. The reactivity of .CH2- and -CH2COO- radicals was found to be the lowest with the t-BuOH radicals. Although very little is known about the kinetics of interaction of two different radicals in pulse radiolysis, the existence of such reactions has been deduced from product analysis in radiation chemistry. I n y radiolysis of aqueous solutions of alcohols in the presence of C02, formation of various a-hydroxycarboxylic acids was reported.l3-16 For instance, glycolic acid with G = 2.2 was formed in COz-saturated solutions of
~
The Journal of Physical Chemistry
+
+
0
PULSE RADIOLYSIS OF ALIPHATIC ACIDSIN AQUEOUS SOLUTIONS methanol,14J5 by the reaction of vCH20H with .COz-. Lactic acid was similarly formed from CH&HOH, and malonic acid from CHzCOO-.13 Our results show that the carboxylation reaction, that is combination of COz- with organic radicals, is not sufficiently fast with some radicals to prevent partial recombination of like radicals. However, in some cases, substantial yields can be expected. It can also be seen that the * CHzCOO- radical reacts with different alcohol radicals, and p-hydroxycarboxylic acids are expected to be formed.
Conclusions The absorption spectra of the a- and p-carboxyalkyl radicals have been found to be easily distinguishable. The dissociated a radicals R2CCOO- have absorption maxima in the region 325-350 nm, relatively independent of the number of carbon atoms in the alkyl group, with extinction coefficients of -800-950 M-I cm-l. The undissociated radicals RzCCOOH show similar absorption spectra with maxima shifted toward lower wavelengths by about 30 nm and extinction coefficients 20-30% lower. The p-carboxyalkyl radicals have absorption maxima 5 240 nm with extinction coefficients considerably higher than those of the a radicals. The spectra of the undissociated form are in all cases shifted toward lower wavelengths. It appears that y radicals absorb in the same region. I n contrast to the carboxyalkyl radicals, .COzshows an absorption maximum at 235 nm with high extinction coefficient, 3000 M-l cm-l, independent of pH between p H 3 and 13. The second-order decay rate constants for the carboxyalkyl radicals are of the order of lo9 M-' sec-l. I n all cases the dissociated form reacts about twice
4213
slower, in general agreement with the predicted rates for reactions of singly charged and neutral species according to Debye's equation. These changes in the rate constants are significantly greater than the changes which could result from variations in the ionic strength of the solutions. It appears that both a and radicals decay at the same rate. The pK values for the dissociation of the carboxyalkyl radicals (both a and p ) were found to be identical with those of the parent acids, except that for the carboxyl radical .C02- which remains unknown, and the isobutyric acid radical which appears to be higher. This increase could be explained as due to the hyperconjugation of the unpaired electron with the CHa groups, which would induce a partial negative charge on the a carbon atom. This is supported by the coupling constants of this unpaired electron as measured in esr studies.11~12 The attack of OH radicals on aliphatic carboxylic acids is directed primarily to the C-H bonds a t different positions, the distribution being dependent on the relative reactivities. The fraction of the a radicals produced was found to be considerably lower for the undissociated a,cids. This is probably due to the difference in the inductive effect of -COOH and -COOgroups on the a position and the electrophilic nature of the OH radical.
Acknowledgment. Partial financial support received from the U. S. Army Research OEce, Durham, is gratefully acknowledged. (13) G. Scholes, M. Simic, and J. J. Weiss, Nature, 188, 1019 (1960). (14) A. Appleby, J. Holian, G. Scholes, M. Simic, and J. J. Weiss, Proceedings of the 2nd International Congress of Radiation Research, (1962). (16) F. GU tlbauer and N. Getoff, Int. J. Appl. Radiat. Isotopes, 16,873 (1965).
Volume 78, Number 18 December 1969
