2253
REACTIONS OF RECOIL CARBOX ATOMS
Reactions of Recoil Carbon Atoms with Oxygen-Containing Molecules. 111. Reaction Mechanisms in Methanolla by R. L. WilliamsIband A. F. Voigt* Institute for Atomic Research and Department of Chemistry, Iowa State Uniaersity, A m e s , Iowa (Received December 81, 1970)
60010
Publication costs borne completely by The Journal of Physical Chemistry
Carbon-11 was produced by the reaction l*C(y,n)"C in liquid methanol, and the yields of labeled products were determined by radiogas chromatography. Reaction alternatives for the intermediate resulting from carbon atom insertion into a C-H bond of methanol are discussed. The dependence of the yields of carbon monoxide and acetaldehyde on the duration of the irradiation period is attributed to reactions of the solvated electron; kinetic expressions are developed.
The behavior of energetic carbon atoms produced in denser placed between the heated counting tube and the nuclear reactions has been studied rather e x t e n s i ~ e l y , ~ - ~ crystal wall. With this configuration, the effluent tube could safely be heated to 160'. but only a few of the studies of organic systems have Yield and Dose Measuyements. Yields were calcuinvolved compounds containing alcoholic or ether lated as previously describedg and are based on the total groups. Voigt, et a1.,6 have recently discussed the produced. As before this activity was found to be structural dependence of carbon monoxide yields from alcohols and ethers. Deoxygenation and other reacdirectly proportional to the integrated dose as measured by cobalt glass d o ~ i m e t r y which ~ , ~ ~ in turn was tions of vapor-deposited carbon atoms with oxygencontaining molecules have been reported by Skell, calibrated against Fricke dosimetry. The integrated dose received by the sample can be et a1.6-s Palino and Voigtg have presented a rather complete calculated from eq 1 study of the methanol system. Additional studies of 9.09( h0.88) X 1021Mtbext"' carbon atom reactions with methanol are reported in dose (rads) = (1) (1 - e - ' t b 1P o this paper in an attempt to clarify and evaluate some of the reaction schemes. Carbon-11 was produced by the in which the constant is determined from the dosimetry reaction '*C(y,n)"C and the yields were determined by experiments,1'r'2 po is the carbon atom density (atoms/ radiogas chromatography. The strong dose depencm9, and tb and t, are the irradiation time and time bedence of the yields of carbon monoxide, acetaldehyde, tween the irradiation and monitor count M . The and glycolaldehyde is discussed, and a brief kinetic dose rate was taken as this dose divided by the irradiatreatment involving reactions of the methanolated election time; thus the assumption was made that the tron is presented. synchrotron intensity was constant during the irradia-
Experimental Section The experimental system has been described re~ e n t l y . Recoil ~ atoms of llC are produced in situ by the nuclear reaction "C(y,n) llC using the bremsstrahlung beam from a General Electric Model AI electron synchrotron operating a t a maximum energy of 70 MeV. The products were separated by gas chromatography using the columns reported previously. The purification of methanol with 2,4-dinitrophenylhydrazine and sulfuric acid was also reported. The detection system for monitoring the total activity and for the radioactive effluent consisted of a 7.5 X 7.5 cm SaI(T1) crystal (Isotopes, Inc.) with a 1.6-cm transverse hole through which the effluent was carried in an 8-mm quartz tube wound with iron resistance wire and heated. The crystal, which is very sensitive to thermal shock, was protected by a silvered air con-
( l j (a) Work was performed at the Ames Laboratory of the U. S. Atomic Energy Commission, Contribution No. 2787; (b) based on part of the Ph.D. Thesis submitted by R. L. Williams to Iowa State Universit,y. (2) A. P.Wolf, Advan. P h y s . Org. Chem., 2, 201 (1964) (3) R. Wolfgang, Progr. React. Kinet., 3, 97 (1965). (4) R . Wolfgang, Science, 148, 899 (1965). (5) A. F. Voigt, G. F. Palino, and R. L. Williams, J . P h y s . Chem., 75, 2248 (1971). (6) P. S. Skell, J. H. Plonka, and R. R. Engel, J . A m e r . Chem. SOC., 89, 1748 (1967). (7) P. S. Skell and R. F. Harris, ibid., 91, 4440 (1969). (8) P. S. Skell and J. H . Plonka, ibid.,92, 836 (1970). (9) G. F. Palino and A. F. Voigt, ibid., 91, 242 (1969). (10) N. J. Kreidl and G. E. Blair, Nucleonics, 14 (3), 82 (1956); 17 (lo), 58 (1959); G. E. Blair, J . A m e r . Ceram. Soc., 43, 426 (1960). (11) G. F. Palino, Ph.D. Thesis, Iowa State University, Ames, Iowa, 1967. (12) R. L. Williams, Ph.D. Thesis, Iowa State University, Ames, Iowa, 1970. The Journal of Physical Chemistry, Vol. 7'6, N o . 16, 1971
R. L. WILLIAMS AND A. F. VOICT
2254 Table I : Yields (%) of Selected Products from Methanol Systems Product
Methyl iodoacetate Methyl acetate Acetaldehyde 1,2-Propanediol 1,3-Propanediol 2,2-Dimethoxyethane 2,2-Dimethoxyiodoethane 2-Iodoethanol Carbon monoxide Glycolaldehyde
------Unscavenged-----a
(ooncn i n mol %)-0 , 1 Br8
---Soavenged 0 . 5 Iz
b
3.5
3.6 2.4 5.7 1.1
3.0 0.8 8.5 1.3
...
...
16.3 3.8
10.2 1.3
2.7
...
... ...
, . . I
.
Otlierd
K.D.e 7.3 1.2
...
.
... 3.0 9.4 1.3 24.8
N.D. 9.4 1.3 N.D.
N.D.
24.7
, . .
a Dose = 0.009 eV molecule-1. Dose = 0.036 eV molecule-'. c Yields of corresponding bromo products. glycolaldehyde carrier present during irradiation. ' Yield not determined under these conditions.
tion period. Dose rate and irradiation time are variables in the kinetic treatment. Bromine Scavenger Studies. Bromine was substituted for iodine as the scavenger in several experiments to aid in product identification and to verify the free-radical nature of particular reactions. The product 2-iodoethanol is unstable toward thermal decomposition and has a long retention time on the columns used in these studies, bul 2-bromoethanol can be handled more easily. Identification of dimethoxybromoethane in the bromine-scavenged system led to laboratory preparations of the analogous iodo compound. A synthesis method was suggested by the formation mechanism proposed for the recoil system, and its success lends considerable support to that proposal. RIethyl vinyl ether, the methoxy analog of a proposed enol intermediate, [H2C=CHOH], and iodine were found to react in methanol a t 0" yielding dimethoxyiodoethane. Alternatively dimethoxyiodoethane was prepared by direct exchange of iodide for bromide by equilibration of a solution of S a 1 and BrCH,CH(OCH& in acetone. I n either preparation aliquots of the reaction mixture were injected onto the chromatograph column with an irradiated sample, and the unknown activity peak was identified by retention time without purification of the preparation product. Acetal Formation. The formation of an acetal from the corresponding aldehyde and alcohol under the conditions of these experiments was discussed previously.9 It appears that this reaction, which is usually considered to be an acid-catalyzed, high-temperature process, can also be catalyzed by elemental iodine, possibly through a halohydrin intermediate.
Results Product Yields. Table I presents selected product yields observed in dose and scavenger studies. Several of these yields show significant dose dependence in contrast to most products from methanolg I n the presence of iodine or bromine the yields of methylaceThe Journal of Physical Chemistry, Vol. 76, A-0. 16,1971
Acetaldehyde or
tate, acetaldehyde, glycolaldehyde, 1,2-propanediol, and 1,3-propanediol are reduced to below detection limits. These products are replaced quantitatively by the new products methyl iodoacetate, dimethoxyethane, 2,2-dimethoxyiodoethane, 2-iodoethanol, and by the increased yield of carbon monoxide. The yields of acetaldehyde, carbon monoxide, and 1,2-propanediol are also reported for samples to which carrier quantities of acetaldehyde or glycolaldehyde were added before irradiation. Dose and Dose Rate Eflects. The yield of "CO from methanol is dependent on both dose and dose rate. The results are listed in Table I1 over the ranges of these two variables studied. Fewer data were obtained for acetaldehyde, and the dose dependence is reported for samples irradiated over a range of dose rates.
Table I1 : Dependence of Carbon Monoxide and Acetaldehyde Yields on Dose and Dose Rate Dose rate, eV molecule-1 min-1
x
103
------Dose, 0.5
eV molecule-' X 102-----0.9 1.5
3.0
Carbon Monoxide Yield 1.5 1.9 2.2 2.7 3.2
19.3 19.4 19.2 18.5 17.8
1.9-2.6
4.0
15.8 16.1 16.3 14.8 14.2
13.3 13.7 14.0 12.5 11.6
11.2 10.8 10.4 10.2 9.1
1.2
0.9
Acetaldehyde Yield 2.4
Discussion Reactions of the Insertion Intermediate. A primary concern is the behavior of the C-H insertion intermediate which is the precursor of several products. The proposed reaction scheme is shown in Figure 1.
2255
REACTIONS OF RECOIL CARBON ATOMS
-+[HCCOH]
[CH2=C=O]
-+ CHaC
\
OCH,
Iz CHaOH
CH3 OH
+ "C
+ [H-11C-CH20H
ICH2C !//o
I*-
\
OH
/
OCH3 a-, H +
'
OH OH
Iz 2CHaOH
I
ICH2CH(OCHa)2 Figure I .
Proposed reaction sequence.
The insertion intermediate [H-"C-CHZOH]* can result from the interaction of methanol with carbon atoms over a wide range of energy. It is suggested that enough energy is deposited locally in some of the insertion processes to result in the loss of both hydrogen atoms from the methanol carbon atom producing an alkynol intermediate which rapidly collapses to ketene. I n the unscavenged system the ketene is efficiently trapped by methanol to form the ester, methyl acetate. I n the scavenged system iodine reacts with the ketene, and the product and solvent further react to produce methyl iodoacetate. The quantitative replacement of the ester by the iodo ester is shown in Table I. If the insertion intermediate is produced with less energy, it can stabilize by molecular collision and intramolecular hydrogen transfer to the enol form of acetaldehyde which in the unscavenged system rapidly tautomerizes to the aldehyde. If iodine is present, the enol isomer is trapped and reacts with solvent to produce dimethoxyiodoethane, the dimethylacetal of iodoacetaldehyde. The acetaldehyde yield (Table 11) is strongly dose dependent, an effect which Palino" attributed to reaction with the solvated electrons in the system to produce 1,2-propanediol. The addition of carrier quantities of acetaldehyde or glycolaldehyde to the sample before irradiation protects the trace amounts of labeled acetaldehyde and increases its yield to 7.375, which we consider to be its undisturbed production value. Radical Reactions. The yield of 1,2-propanediol was reduced from its dose dependent values of from 5.7 to 8.570 in the unscavenged system to zero in the presence of scavenger and to 1.2% with aldehyde carrier present. Several mechanisms apparently operate for its formation. Part of its yield is attributed to the free-radical mechanism shown in eq 2 (Figure 2 in ref 9) This is considered the mechanism responsible for the
CHgOH CH~CHOH+CH3CHOHCHZOH '
I2
(2 )
2CHaOH
I
The Journal of Physical Chemistry, Vol. 76, Xo. 16,1971
2266
R. L. WILLIAMS AND A. F. VOIGT
ently account for systematic yield changes, the interference by radical reactions becoming more significant with increasing dose. Closer examination of the mathematics involved indicates that this explanation is not the most direct approach. The yield of any product is the fraction of the total activity of the sample measured after the irradiation. Carbon-11 is produced a t a constant production rate dependent on the synchrotron beam intensity (dose rate), but decays by positron emission a t a rate expressed by its decay constant X. Thus the carbon activity does not increase linearly with irradiation time but instead follows the usual isotope production curve, A = P(l - e-htb). Since the carbon-labeled products have the same decay constant, the yield of most products is not time dependent. However, if radical reactions remove some of the product molecules the yield must decrease, but the nature of the change is not obvious. During the beam burst the radicals are present a t a steady-state concentration dependent on the reaction medium and the intensity of the radiation beam. The radical concentration is much higher than the concentration of “C-labeled products. Radical reactions would then be expected to remove a constant fraction, a, per unit time of the “C-labeled product present a t any time. Therefore the activity attributed to the product would increase during the irradiation by the modified production formula, A = P ( l - e-(xta)tb) and the yield, given by eq 3 would
show a nonlinear decrease with increasing irradiation time. Evidence for such nonlinearity will be presented. Solvated Electron Reactions. The yields of carbon monoxide, glycolaldehyde, and acetaldehyde in the unscavenged methanol system show unusually strong dependence on dose and dose rate (Table 11). Other products in the methanol systemg show less pronounced dependences which are nearly linear with dose. Decreases in yields for unsaturated hydrocarbon products have been attributed1’ t o reactions of hydrogen atoms resulting in increased yields of more-saturated products with increased time. The yields of some products, such as ethanediol, 1,3-propanediol, and ethanol, are attributed a t least in part to combination of labeled radicals with the predominant radiation species in methanol, l 3 .CH,OH and .H, and show increases in yield with irradiation time. These minor dependences can be considered normal behavior for labeled products in methanol, and similar effects have been reported for other systems. l4 A carbon monoxide production site model5 was based on results of samples scavenged with iodine in which the !
T h e Journal of Physical Chemistry, Vol. 7 6 , N o . 15, 1971
carbon monoxide yield is independent of irradiation time for all the compounds studied. I n all cases the carbon monoxide yield is somewhat higher if iodine is present during the irradiation, but for unscavenged methanol the carbon monoxide yield falls from near the scavenged yield for short irradiation times to only onethird of this value for irradiation times of 20-30 min. The yields of acetaldehyde and glycolaldehyde show similar time dependencies in the same unscavenged system. The solvated electron, an important radiation product, appears to be a common reactant for all three of these species. The rate constants of reactions of eaq- and other solvated electrons with a large number of compounds have been measured using the technique of pulse radiolysis and have been conveniently tabulated. l5 An intensive review of the current understanding of the solvated electron and its reactions has recently been prepared16 and is recommended as an indepth summary of the subject. The study of reactions of eaq- with more than 300 compounds suggests that the reactivity of different chemical species toward eaq- is a function of the availability of a vacant orbital on the substrate as well as of the change in free energy on incorporation of an additional electron. The compounds carbon monoxide and acetaldehyde are among the more reactive toward eaq- with bimolecular rate constants 1 X lo9and 3.5 X lo9 M - l sec-I. The rate constant for glycolaldehyde has apparently not been determined although all the simple aldehydes arid ketones show similar reactivities, with the exception that formaldehyde is about two orders of magnitude less reactive. The low reactivity of formaldehyde and its precursor radical .CH20H in the radiolysis of methanol, coupled with the low reactivity of methanol itself toward esolv- ( k < lo4 M-l sec-’) makes methanol an ideal solvent for the study of the solvated electron. The anomalous behavior of formaldehyde provides the reason why reactions of the solvated electron have been observed with recoil products in methanol but not in other systems. I n the radiation chemistry of ethanol, for example, large quantities of acetaldehyde are produced, G = 3.14.13 Acetaldehyde effectively removes the solvated electron, thus eliminating its effects on yields in recoil studies. I n methanol radiolysis the only product which can significantly reduce the concentration of solvated electrons is carbon monoxide, G = 0.26, which due to its low concentration would only be important in extended irradiations. The rates of eaQ-reactions have been used in the pre(13) J. W. T. Spinks and R. J. Woods, “An Introduction to Radiation Chemistry,” Wiley, New York, N . Y . , 1964. (14) D. E. Clark and A. F. Voigt, J . A m e r . Cham. Soc., 87, 5558 (1965). (15) M . Anbar and P. Neta, Int. J . A p p l . Radiat. Isotopes, 16, 227 (1965). (16) (a) L. M. Dorfman, A d a m . Chem. Ser., No. 50, 36 (1965) ; (b) M. Anbar, ibid., 50, 45 (1965).
REACTIONS OF RECOIL CARBON ATONS
2257
ceding discussion since these values have been determined for a wide variety of compounds. Very similar rate constants for the solvated electron in methanol and ethanol have been reported for some of the same compounds. For example, the rate constants for 0 2 with the various solvated electrons are 2.0 X 1O1O for ivater and 1.9 X 1O1O for both methanol and ethanol.16a Although such close agreement cannot be assumed for all compounds, no significant differences have been reported. The subsequent reactions of the molecular ions formedin the esolv-reaction have not been as thoroughly studied. The work of Weiss" is of particular interest and is the basis for the reaction proposals t o be made here. He reported the formation of glycolic acid by the combination of the ' ( 3 0 2 - radical ion with the methanol radical . CHzOHin the radiolysis of solutions of methanol in water with a G value of 2.7. Similarly, .COH combines with .CH20Hto produce glycolaldehyde. The radical .COH could be formed by the reaction of CO with esolv- followed by neutralization, or by the combination of CO with 'H. The hydrogen atom reaction is apparently unimportant in methanol since similar concentrations of .H are present during the irradiation of the other alcohols in which the carbon monoxide yields show little time dependence. Kinetic Analysis. Acetaldehyde. The reaction of the methanolated electron with acetaldehyde accounts for the dose dependence of its yield by a simple kinetic analysis. The following reactions must be considered.
+ y -% "C products CH30H + "C --%CHaWHO CHglCHO + esolv- -% CHz1'CHCH3 CH30H
I
1
prescribed rate equation. The last term must be ind [CHsCHO] dt
Rz
-
KB[CH~CHO] - A[CH,CHO] (7)
cluded since the loss by decay is significant relative to loss by the electron reaction. The solution of eq 7 is
[CH3"CHO] =
R2
+ K3[l -
___
X
and
The addition of carrier acetaldehyde eliminates the loss of labeled acetaldehyde by the electron reaction permitting direct evaluation of the ratio R2/RI from the yield in the protected system, 7.37,. The value of K 3 in the unscavenged system can be determined from the data as shown in Figure 2. The best value is estimated as 0.75 f 0.10 min-'. Values for the bimolecular rate constant k3 are dependent on estimates of [esolv-l.
-
5.0
(4)
(5)
-
4.0
(6)
OH OH Equation 4 represents the total production of products by the synchrotron radiation. The rate of production denoted R1 = kl [y J [CH,OH]is independent of time and depends only on the beam intensity [ y ] , i.e., dose rate. The rate of production of acetaldehyde, Rz = k, ["C] [CH,OH],will be a similar function of dose rate since "C is directly proportional to the beam intensity. The rate of loss of acetaldehyde by the electron reaction (cf. Figure 1) may be represented as R3 = k3 [esolv-l [CH,CHOI = K3[CH3CHO], since [esOlv-]is constant with time a t a fixed dose rate. The other reactants necessary t o convert acetaldehyde into propanediol, H+, and .CH20H will also be present in constant amounts a t a fixed dose rate. However, the fate of the product of the reaction between CH3CHO and esolv- does not affect the kinetic treatment as long as this reaction is not reversible. The expected acetaldehyde yield as a function of dose at a fixed dose rate can be determined by solving the
ae 0 J
30
-
20
-
1.0
-
w
>
IRRADIATION TIME ( M I N I
Figure 2 . Acetaldehyde yield and calculated curves.
Carbon Monoxide. A similar treatment for the carbon monoxide yield using eq 10 and the reactions (17) J. J. W-eiss, Radiat. Res., Suppl., 4, 141 (1964).
The JOtkTnal of Physical Chemistry,Vol. 76,N o . 16, 1971
R. L. WILLIAMSAND A. F. VOIGT
2258 proposed earlier provides a similar yield equation
25
yield("C0) =
where the value 0.25 has been determined from the 25% yield of "CO in iodine-scavenged samples. I n Figure 3 the smooth curves result from substituting selected values of K , into eq 11 and the points are the carbon monoxide yields a t a dose rate of 1.9 X eV molecule-' min-l. Although the experimental results show considerable scatter, a relatively unique value of Kq can be chosen by inspection to be 0.17 k 0.03. Repeating the analysis of the "CO yields for different dose rates results in a series of K 4 values; 0.15 a t 1.5 X eV molecule-' min-l, 0.20 a t 2.2 X and 0.30 a t 2.7 X Since K , = k4[esolv-][CO],this series reflects the relative concentration of solvated electrons with increasing dose rate. The relative reaction rates of the methanolated electron with acetaldehyde and carbon monoxide determined by this method a t 2.3 X eV molecule-' min-' are 0.75/0.25. Reactions of the methanolated electron significantly alter these recoil product yields, and the observed rates substantiate the similarity of this species with the hydrated electron, for which the ratio of rate constants is 3.5.
The Journal of Physical Chemistry, Val. 76, No. 16, 1971
0
5
IO
IRRADIATION TIME ( M I N I
15
Figure 3. Carbon monoxide yield and calculated curves.
Acknowledgments. The authors are very grateful to the members of the Iowa State University electron synchrotron staff, especially to Dr. Alfred Bureau and Mr. James Sayre.
