RICHARD A. HOLROYD AND TIMOTHY E. PIERCE
1392
Mercury-Photosensitized Decomposition of Propane, Isobutane, and n-Pentane at 1849 A.l
by Richard A. Holroyd and Timothy E. Pierce Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania (Received December 6 , 1963)
The relative radical yields from the reaction of excited mercury ('Pl) atoms (produced by the 1849-A. line of Hg) with propane, isobutane, and n-pentane have been determined by use of the ethyl-C'* radical scavenger. The 1849-A. line was isolated with a preirradiated (with y-rays) LiF filter. The primary dissociation of propane by Hg('P1) atoms yields 50% n-propyl and 50% isopropyl radicals. Similarly the dissociation of isobutane yields 42% isobutyl and 58% t-butyl radicals, and the dissociation of n-pentane yields 28% n-pentyl, 43% 1-methylbutyl, and 29% 1-ethylpropyl radicals. Carbon-carbon bonds are not dissociated at this wave length. The relative rates of removal of tertiary, secondary, and primary hydrogen atoms by Hg(lP1) atoms are in the ratio 12:3:1 contrasting with sensitization of hydrocarbons by Hg(3P1) atoms for which this ratio is 350:65:1. The mechanism of quenching of Hg(lP1) atoms by aliphatic hydrocarbons is formation of radicals by the direct interaction with, and cleavage of, a carbon-hydrogen bond. The ratios, D/R, of disproportionation (in which CzHa and an olefin are formed) to combination are estimated: for CzH6 f n-C3H7--, D/R = 0.06; for C2H6f i-C4He-+, D/R 0; for C& f n-C6HI1, D/R = 0.08.
-
Introduction In a study of the mercury-photosensitized decomposi tion of hydrocarbons a t 2537 A. in which radical yields were determined it was shown that tertiary and secondary hydrogen atoms are much more readily abstracted by Hg(3P1) atoms than primary hydrogen atoms.2 Further it was shown that there was a correspondence between primary radical yields and quenching cross sections for individual carbon-hydrogen bonds. In the course of that investigation it was observed that for npentane the relative yield of n-pentyl radicals was greater when the unfiltered iight (2537- and 1849-A. lines both present) from a low-pressure mercury arc was used than when the 2537-A. line was used alone. It was suggested that mercury ('PI) atoms, formed by 1849-A. excitation, are more effective than (3P1)atoms in breaking primary carbon-hydrogen bonds. Until this time the reactions of paraffins with excited singlet metal atoms has received little attention. The present study is concerned with the reaction of Hg('P1) a t o m with propane, isobutane, and n-pentane. Th,e Journal of Physical Chemistry
The 1849-A. line of mercury may be isolated with a LiF filter.3 When LiF is irradiated with y-rays this filter absorbs strongly at 2537 and allows the shorter 1849- and 1942-A. mercury linea to pass. Sensitization with mercury ('PI) atoms is of interest both for comparison with the mercury ("1) atom sensitization results and to help establish the mechanism of the sensitization with excited singlet mercury atoms. Although more energy, 6.7 e.v., is available at 1849 A., it is not expected that excited electronic levels of the hydrocarbon will be reached in the sensitization since if spin is to be conserved a singlet state would be formed which for simple hydrocarbons is a t higher energies for a vertical transition. The technique used to determine radical yields was (1) This investigation was supported, in part, by the U. S.Atomic Energy Commission: presented at the 146th National Meeting of the American Chemical Society, Denver, Colo., January. 1964. (2) R. A. Holroyd and G. W. Klein, J . P h y s . Chem., 67, 2273 (1963). (3) J. L. Weeks, S. Gordon, and G. M . A. C. Meaburn. Nature, 191, 1186 (1961).
MERCURY-PHOTOSENSITIZED DECOMPOSITIOR' AT 1844 8.
identical with that used earlier.2 A low concentration of e t h ~ 1 e n e - Cis~ present ~ to generate the C142H5radical scavenger. Labeled hydrocarbons are produced from which the radical yields in reaction 1 may be determined. The precision and higher sensitivity of this C'14 Hg(lP1)
+ RH
-+
R,
+ H + Hg
(1)
tracer technique waR useful because the 1849-A. line in the arc employed was relatively weak. Experimental Light Source. The source of 18491-A. radiation was a conventional low pressure mercury arc mounted 3 cm. above the front window of a quartz reaction cell (220 cc.). The space between the arc and filter was flushed with Nz.A 3-mm. thickness of LiF crystal which had been irradiated with y-rays was used to isolate the 1849-A. line. This filter was found to be quite satisfactory if certain precautions are observed. Since the intensity of the 1849-8. line is -5% of the 2537-A. line, which also sensitizes the decomposition efficiently, the filter must reduce the 2537-8. iintensity a t least to 0.1%. In actual practice the LiF filter was placed in the y-source until its O.D. at 2537 8. was >4.0. The optical density was determined on a Cary Model 14 recording spectrophotometer; measurements up to an O.D. of 4 were possible by using a reference solution whose O.D. a t 2537 A. is about 2 0 . The filter wa,s checked after each photolysis. Photobleaching of the absorption band occurred but in every case the 0.1). at 2537 A. after the photolysis was greater than 3.5. Thus, the intensity of the 2537-A. line was reduced to -0.01%. This filter passed aplproximately 20% of the 1849-A. line (measurements actually made a t 1950 k ) . After eabli experiment the LiF filter was rcturned to the y-source and subjected to a dose of -0.2 Mrad before reuse. The filter gradually darkened at 1849 A. after repeated irradiations but could be cleared by heating to a higlh temperature. The efficiency of the filter in removing the 2537-A. line was demonstrated by carrying out a photolysis in ithe usual manner utilizing the irradiated LiF filter and also a Corning 7910 filter (which cuts out the 1849-w. line). No products whatsoever were formed in this blank experiment. The photolysis cell was mounted vertically so that it could be immersed in a cooling bath. In the case of propane most of the results are for 25" but in later work on isobutane and n-pentane the cell was cooled to approximately 0" so that the mercury concentration in the cell would be reduced and the effective reaction volume increased. It was noted that sensitization occurred (at a reduced rate) even a t -78" with 1849 Approximately 1hr. was allowed prior to each photolysis
1393
to ensure equilibration and to allow mixing of the C142H4 and the hydrocarbon. The temperature in the reaction volume, which is close to the front window, may have been somewhat above the temperature of the bath since the front window was not actually immersed in the bath. The hydrocarbons were Phillips research grade. The e t h ~ 1 e n e - Cwas ~ ~ obtained from Sew England Nuclear Corp. The purification of these materials and details of the analytical technique have already been described.2 Results and Discussion The results obtained for propane, isobutane, and n-pentane are summarized in Tables I, 11, and 111, respectively. The products in every case are the same as those observed in the mercury sensitization2 a t 2537 A., namely, ethane-C14, butane-C14, and from two to three labeled hydrocarbons formed by reaction of an ethyl-C14 radical with the parent radicals. I n general, substantial yields of radicals resulting from the scission of a primary carbon-hydrogen bond are observed, in contrast to sensitization at 2537 8. Fragment radicals are not observed. Kinetic Considerations. Mercury sensitization of hydrocarbons with 1849-A. light is expected to satisfy the requirements of applicability of the C14zH6radical scavenging technique. Hydrogen atoms must be available from the primary decomposition to generate C14*H5radicals. That this occurs is indicated by the results which show that labeled hydrocarbons are produced. Also high intensities prevail because of the high value of the extinction coefficient of mercury vapor a t 1849A. ; thus, radical combination reactions are favored. Therefore, it is reasonable to assume that the same secondary reactions, 2-8, follow the primary decomposition, reaction 1, as were shown to occur at 2537 8.
H -I- C142H4+C'4&, H
-t R H +HZ
+
+ Ri
CL42H5 Ri +Ri-C142H5
+
+
C142Hs R i +C142He Ri(-H)
+
(2)
(3) (4)
(5)
C142Hs-t R i +C142H4 RH
(6)
C'42H5 ---+Cl44H1o
(7)
+
2C142H5 +C142H4 C142Ha
(8) If even higher light intensities are employed, the reac(4) This was expected since t h e vapor p r e s p r e of mercury
at -78' is sufficient t o absorb completely the 1849-A. line in a few centimeters: cf. "The Photochemistry of Gases" by W. A. Noyes. Jr., and P. A. Leighton, Reinhold Publishing Co., New York, N. Y., 1941, p . 218.
Volume 68, A'umber 6
June, 1964
RICHARD A. HOLROYD AND TIMOTHY E. PIERCE
1394
Table I : Propane CaHa, mmoles
C'42H4, amoles
CzHe
1.02 1.08 1.05 1.08 1.04 1.11
8.43 3.11 1.91 1.38 0.87 0.78
9.17 7.18 11.0 7.18 2.96 4.46
1.05
1.40
Here reaction 5 is CzHj
1.89
+ n-C,H,
kdkP
108
Temp. 19.0 10.7 9.96 6.56 2.22 2.68
27.0 25.4 29.4 25.9 11.9 19.6
24.5 17.9 18.9 14.8 7.0 10.4
0.05 0.03 0.19 0.06 0.03 0
1.54 1.51 1.46 1.41 1.58 1.63
5.9
3.9
0.09
...
=
=
24"
-78"
2.36
+ C2H6
+ C&,
kdkl x
n-Cs&
Temp.
5
-
Rate, picomoles/sec. n-C~Hlo i-CaHij
k5/kd for the reaction of C2H5 with i-CaHi is taken as 0.21.
Table 11: Isobutane (0")
a
Rate, picomoles/sec. 2,%Dimethyln-C~Hla butane
i-CIHl0, mmoles
C142H4; amoles
CsHa
1.13 1.09 1.12 1.12 1.02
13.1 5.53 3.39 2.75 1.84
6.63 5.35 4.09 3.04
Here reaction 5 is CZH5
...
+ isobutyl
+
6.10 2.77 2.03 0.98 0.57
13.0 11.5 10.0 7.76 5.67
7
ka/kz X
Isohexane
kK/k4"
108
12.2 9.41 7.62 5.60 3.97
... 0.05 0.00 0.00 0.02
... 8.6 7.0 8.4 7.9
+
CZHB i-CaHs.
~
~~
Table I11 : Pentane (0')
a
n-CsHit, mmoles
C142H4, pmoles
CzHs
0.9 0.9 0.9 0.9 0.9 0.9
14.4 6.42 2.93 2.00 1.44 1.28
6.12 6.36 5.56 6.25 3.68 4.67
Here reaction 5 is CzHs
+ n-CbHll
+
CzHs
n-CaHlo
Rate, picomoles/sec. 3-Methylhexane
3-Ethylpentane
9.77 8.86 5.65 4.51 2.45 3.00
10.4 12.3 11.6 12.5 8.22 9.65
5.93 7.35 6.71 7.09 4.72 5.54
+
+R>
f - '$RJ-C~%HK(~
+
+R~-cLI~H~(~
k6/k4 &/k4
+ kdk4)+ ks/k4)
where k6/k4 and k ~ / k 4are the appropriate disproporThe Journal of Physical Chemistry
7 29 8.00 6.63 6.81 4.06 5.23
kK/kP
101
0.12 0.05 0.04 0.12 0.04 0.10
2.5 2.3 2.2 2.4 2.2 1.9
+ i-CbH1,.
tion H Ri + occurs, but here it is assumed to be unimportant. In general, reaction 1 will produce several radicals R1,Rz, etc.; ai is the fraction of dissociations which gives radical, Ri. It has been shown2 that the ratio of radical yields is obtained from the yields of the corresponding labeled hydrocarbons from the relationship '#R,
ka/ki X
;-Heptane
tionation ratios for the radicals Ri and Rj. Further, the mechanism predicts that eq. I should
be obeyed, where B is a constant. Plots of Ri-C142H6/ C144Hlo us. (RH)/(C14zH4) are linear (Fig. 1-4) and the values of ai were evaluated from t,he values of the intercepts, as obtained by the least-squares method, and the
MERCURY-PHOTOSENSITIZED DECOMI?OSITION AT 1849 A.
'I A
PROPANE
A // /
Y
8
/
I
OO
I
500
(C,H,)/
1395
20
JSOBUTANE
-
A /
/
/A
/
?-.
/"
/
I
1000 (CFH4)
I
1500
Figure 1. Propane (25'). The lower two solid lines connect observed values of the ratios i~opentane-C~"/n-C'~4H10> 0; and n-pentane-C14/n-C144Hl~, a. The triangular points are values of the quantity [Z;R-C14~Hd 1 k&ca)]/l.l2 Cl44H1o a t 1849 A., A; and a t 2537 A., A (flagged points are data from ref. 2).
+
appropriate disproportionation to combination ratios, the determination of which will be discussed below. Values of k3/k2 were calculated from plots of eq. 11:
which is obtained by summing the above equation for all radicals, Ri. 'The values of k3/kZ obtained in this study from plots of eq. I1 in Fig. 1,2, and 3 for propane, fsobutane, and n-pentane were 0.0015, 0.0081, and 0.0022, respectively, in good agreement with the values obtained by Holroyd and Klein.2 Since eq. I1 does not contain any a's, it can be used to test if the same mechanism is applicable at both 2537 and 1849 A. by plotting the quantity on the left-hand side us. (RH)/(C142H4). It is found that the data a t 2537 8. (Fig. 1-3, filled triangles) lie on the same line as determined by the 1849-11. data (Fig. 1-3, open triangles). Thus, although the primary dissociation is
Figure 2. Isobutane. The lower solid lines connect observed values of the ratios: 2,2-dimethylb~tane-C~~/n-C~~,H~~, 0; and i s ~ h e x a n e - C l ~ / n - C ~8~.~ H The ~ ~triangular , points are values of the quantity [ZR-Ci4&( 1 k6/k4)]/1.12C144H10 a t 1849 A. and O", A; and at 2537 d. and 25", A (ref. 2 ) .
+
shown to be wave length dependent, the secondary reactions are essentially the same a t the two wave lengths. It can also be seen from the figures that no significant fraction of the hydrogen atoms produced are "hot" since if they were formed with excess kinetic energy and abstracted hydrogen forming H,, the intercepts of the plots of eq. I1 would be much larger than 2. Further, it is observed that in a 1% solution of Ci42134in propane the CI42H4will scavenge 83Oj, of the hydrogen atoms generated a t 1849 8. Propane. The observed products from propane are n-pentane-C14 and isopentane-C14. Thus, only npropyl and isopropyl radicals are produced ; but in this case the yield of n-pentane-C14 is nearly equal to that of i ~ 0 p e n t a n e - Ca~t ~high eth~1ene-C'~ concentration. A least-squares analysis of the data plotted according to eq. I (lower two lines in Fig. 1) gave intercepts for n-~entane-Cl~/butane-C'~ and isopentane-C14/ butane-C14 of 1.01 and 0.83, respectively. Thus, the ratio of the relative yields of radicals froin reaction 1 is aYt-pr/CY,.pr
=
O A 3x 1.01
0.21 + 0.19 (-11 ++ 0.06 + 0.08 Volume 68, Number 6
=
1.01
June, 1964
RICHARD A. HOLROYD AND TIMOTHY E. PIERCE
1396
n- PE NTA NE /
T I 100 (CsHe
I
200
+ i-C~Hio)/(C*%aHd
I
300.
400
Figure 4. Propane and isobutane (1:1)0". The lines connect observed values of the following ratios : isopentane0 ; n-pentane-C14/n-C144Hl~, 0 ; isohexaneC1*/n-C144Hl~, 0. C14/n-C144H10,A; 2,2-dimethylb~tane-C~~/n-C~~4Hlo, Abscissa is (CaHs i-C4Hl0)/(C~~2H4).
+
It would be valuable to know quantum yields a t 1849 A. No attempt was made to measure the light flux accurately; however, the quantum yields may be estimated if the ratio of intensities of the 1849- and 2537-A. lines are known. From an experiment with propane the absorbed light intensity of the 2537-A. line I I I I I I I was calculated to be -5 nanoeinsteins/sec. assuming 2 00 400 600 8 = 0.9. The rate of formation of C142H6, the sum (CIHIZ) /(C149H4) of the rates of all labeled hydrocarbons, is 0.10 nanoFigure 3. n-Pentane (0'). The lower three solid lines connect mole/sec. in a typical run a t 1849 8. (top row of observed values of the ratios: 3-rnethylhe~ane-C~~/n-C~~4H10, Table I). The rate of formation of hydrogen atoms, 0 ; 3-ethylpentane-C1*/n-C414Hlo, V; and n-heptane-C14/ RR, is given by R ~ i r , ~ , [ l ks(C3Hs)/kz(Cl442H4)1, n-Cl44Hto, 0 . The triangular points are values of the quantity which is equal to 0.12 nanomole/sec. Since (PH cannot [ZR-C142Ha(l + /~~/k4)]/1.12C1~4Hlo a t 1849 k.,A ; and at 2537 A... A. be larger than unity this places a minimum on the intensity of the 1849-A. line of 0.60 nanoeinstein/sec., (the filter absorbed 80% a t 1849 A,). Thus, for unThe disproportionation ratios used are: for R = isofiltered light from the lamp, the intensity of the 1849propyl, k6/k4 = 0.212and k6/k4 = 0.1g5; and for R = 8. Une reaching the cell is >7% of the intensity of the n-propyl, k6/k4 = 0.06 (see Table IV) and k6/k4 = 2537-A. line. If the relative intensity of the 1849-A. 0.08.6 line is not much larger than 7% the quantum yield of The primary decomposition a t 1849 A. is hydrogen atoms is close to unity. Isobutane. The labeled products from isobutane, besides CzH6 and C4H10,are 2,2-dimethylbutane and isohexane. From the plots of eq. I (Fig. 2) the interThus, the secondary hydrogen atoms are three times as and isocepts for 2,2-dimethylb~tane-C~~/C~~4Hlo reactive as the primary hydrogen atoms. These rehexane-C14/C144Hloare 0.90 and 1.13, respectively. sults contrast sharply with the 2537-A. results, obtained by this same technique,2 which showed that the (5) J. A. G.Dominguez, J. A. Kerr, and A. F. Trotman-Dickenson, J . Chem. Soc., 3357 (1962). primary decomposition is7
+
+ H + n-C3H7 (10%) Hg + H + i-C3H7 (90%)
Hg(*Pi) f CaHs +Hg +
Ths Journal of Phusical Chemistry
(8) Predicted from eq. 111, ref. 2. (7) There is, however, some controversy regarding the relative
yields of n-propyl and isopropyl radicals from the primary decomposition of propane a t 2537 A.; cf. ref. 2 for details.
MERCURY-PHOTOSENSITIZED DECOMPOSITION AT 1849 A.
As above the ratio
CYt-butyl/%obutyl
is given similarly by
+ 0.51 + 0.31 = 1.4 1.13 i+ 0.02 + 0.05
0.90
1
The disproportionation ratios used are: for R = tbutyl, k6/k4 = 0.51 (see below) and k6/k4 = 0.315; and for R = isobutyl, k6/k4 = 0.02 (Table IV) and k6/k4 = 0.05.6 The primary reaction in the 1849-k. sensitized decomposition of isobutane is Hg(lP1)
+ i-64H10 --+ Hg + H + isobutyl (42%) .+ Hg + H $- &butyl (58%)
The tertiary hydrogen atom is thus 12 times as reactive as the primary hydrogen atoms. Again these results contrast sharply with the 2537-A. results obtained by this same technique which showed that only 3% of the radicals in the initial dissociation are isobutyl. n-Pentane. The results in Table 111 show that three radicals: R1 = n-pentyl, RB = 1-methylbutyl, and R3 = 1-ethylpropyl, are formed from n-pentane. The values of CY as determined from the intercepts of the = ~0.69 : C Y(1~ tlower three lines in Fig. 3 are: C Y ~ : C Y 0.08 0.09) ~0.91(1 0.25 0.11) r0.54 (1 0.25 $0.29) E 1: 1.5: 1. The initial dissociation of n-pentane a t 1849 A.is therefore
+
Hg('Pd
+
+
+
+ n-CsH12
+ H + .CH&HzCHzCH&H3 (28%) +Hg + H + CH~CIHCHZCH~CH~ (43%) +I-lg + I€ + CHaCH&IICH&H3 (29%) +Hg
The disproportionation ratios used are: for RI (npentyl), k5/k4 = 0.08 (Table IV) and ks/k4 = 0.Og6; for RZ (1-methylbutyl), k6/k4 = 0.252 and k6/k4 = 0.1l6; forR3 (1-ethylpropyl), kg/kq = 0.2P and ks/k4 = O.2ge6 These resultg confirm the earlier observation that when the unfiltered light of a low pressure mercury arc is used, inordinately high yields of n-pentyl radicals are observed as a result of photosensitization by the 1849-A. line of Hg. Only 2% of the radicals formed with 2537 A. are n-pentyl radicals. Propane-Isobutane ( I : I ) . The sensitization of an equimolar mixture of propane and isobutane was studied to compare directly the relative reactivity of these two compounds toward Hg(IPl) atoms and to give some measure of the relative quenching cross sections of the two substances a t 1849 8. Further, this study provides an additional measurement of tlhe primary radicaX yields for the individual hydrocarbons. I n the mixture thermal hydrogen atoms preferentially attack isobutane; thus, the ratio of the yields of isopentane-C" and n-pentane-CI4 is largely a measure of the ratio of
1397
the primary radical yields from propane. The average value of this ratio for the four points in Fig. 4 is 0.86, in good agreement with the extrapolated ratio of the intercepts, 0.83, in Fig. 1. The primary radical yields, determined from the intercepts in Fig. 4, are 18% isopropyl, 21% n-propyl, 39% t-butyl, and 22% isobutyl. The ratio of the total yield of butyl radicals to the total yield of propyl radicals, which is 1.6, is the over-all quenching efficiency of jsobutane, relative to that of propane. The effective quenching cross section of isobutane is obtained from this ratio by taking into account the difference in molecular weights and is found to be 1.8 times that for propane. It is assumed in arriving a t this value that the quantum yield of radicals, R, is the same for both propane and isobutane. Unfortunately absolute values of quenching cross sections for Hg(lP1) atoms are not available for comparison. From theoretical considerations it is expected that quenching cross sections at 1849 A.should be greater than a t 2537 Relative Reactivity of Carbon-Hydrogen Bonds. The propane and pentane results indicate that the average rate of removal of a secondary hydrogen atom by a Hg('P1) atom is 2.8 times the rate of removal of a primary hydrogen atom. Similarly, the isobutane data show that the rate of removal of the tertiary hydrogen atom is 12 times that of a primary hydrogen atom.1° These values contrast with sensitization of hydrocarbons by Hg(3P1)atoms for which the ratio of reactivities of tertiary, secondary, and primary hydrogen atoms was found to be 360:65:1. The excited singlet mercury atom is less selective in its reactions with hydrocarbons than the triplet, an effect which probably results from the increase in exothermicity of the quenching reaction. Mechanism of Quenchzng. The results obtained by 1849-A. sensitization contrast with the results of the direct photolysis of hydrocarbons a t 1470 A. In the vacuum ultraviolet photolysis of propane" and nbutane12 at 1470 A. molecular detachment of Hz and CH, are primary processes of major importance. This contrast is to be expected since as was pointed out i%*v9
(8) J. L. Magee, J . Chem. Phus., 8, 687 (1940). (9) K. J. Laidler, ibid., 15, 712 (1947). (10) It is interesting t o note t h a t the 1849-A. results on radical yields are fiimilar t o radiolysis data. For example, in the radiolysis of saturated hydrocarbon liquids, parent radicals are formed, and further a secondary hydrogen atom is two t o three times and a tertiary hydrogen is -16 times (R. A. Holroyd and G. W. Klein, unpublished results) more likely t o be removed than a primary hydrogen atom. However, in contrast, radiolysis leads t o fragment radicals and molecular products and the "primary processes" in radiation chemistry are entirely different. (11) H. Okabe and J. R. McNesby, J . Chem. Phus., 37, 1340 (1962). (12) M. C. Sauer, Jr., and L. M. Dorfman, ibid., 35, 497 (1961).
Volume 68, Number 6 June, 1964
RICHARD A. HOLROYD AND TIMOTHY E. PIERCE
1398
earlier the optically allowed excited electronic state of the hydrocarbon may not be formed by sensitization with Hg('P1) atoms. These results suggest that the quenching of Hg(lP1) atoms by saturated hydrocarbons leads to radicals by interaction with and scission of a carbon-hydrogen bond as mas predicted by L a i d l e ~ from - ~ ~ ~theore ~ tical considerations several years ago. Disproportionation to Combination Ratios. Since normal type radicals are formed in substantial yields at 1849 A., the present data may be used to estimate disproportionation (in which Cl4?Heand an olefin are formed) to combination ratios, k5/kq, for reactions of C1*2H6 with these normal radicals. The results are shown in Table IV. For this calculation the amount of ethane-CI4formed in reaction 8 is assumed to be 0.12 as much as the butane-C14 yield; and, in the case of propane, the ethane-C14 formed in the disproportionation of Cl4*H5and isopropyl radicals is taken as 21% of the isopentane-CI4 yield.* For R = n-propyl, the value obtained here for lcslk, is 0.06 f 0.04, which is somewhat lower than the value of 0.14 reported by Thynne,14 based on a single experiment. I n a similar fashion k 5 / k 4 for R = isobutyl is estimated to be 0.02 f 0.02. I n this case the amount of
The Journal of Physical Chemistry
Table IV : Disproportionation to Combination Ratio, kj/ka, for Normal Radicals Lit. Obsd.
+
CzHs n-CaH, CzHr + i-CdH8 CzHb n-CrH11
+
a
Ref. 2, eq. 111.
0.06 f 0.04 0.02 f 0.02 0.08 f 0.04
ref.
0.14b
... , . .
Calcd.a
0.14 0.10 0.14
See ref. 14.
ethane-CI4formed in the disproportionation reaction of C142H5and t-butyl radicals is taken as 0.51 as much as the 2,2-dimethylbutane-C14yield, a value which is the average of the two values reported: 0.54 (ref. 2) and 0.48 (ref. 6). The value of k 6 / k 4 obtained where R is npentyl is 0.08 f 0.04. Apparently, for reactions of ethyl radicals with normal type radicals, combination predominates as is the case for reactions of methyl and various normal-type radicals.l 5 (13) K. J. Laidler, J . Chem. Phya., 10, 43 (1942). (14) J. C. J. Thynne, Proc. Chem. SOC.,68 (1961). (15) M . H. J. Wijnen, J. Am. Chem. SOC.,83, 3752 (1961).
