2394
M. H. J. WIJNEN
[CONTRIBUTION FROM
THE
1701.
80
CHEMICAL DIVISION,RESEARCH AND DEVELOPMENT LABORATORIES, CELANESE CORPORATION OF AMERICA]
Reactions of Alkoxy Radicals.
11. Photolysis of Ethyl Propionate
BY M. H. J. WIJNEN RECEIVED KOVEMBER 13, 1957 The photolysis of ethyl propionate has been investigated over the temperature range 30 to 345' at different light intensities and initial pressures. As main reaction products were observed: CO, CO?, CHa, CnHs, C Z H ~C~H.R, , C4H10, CHZO,CzHjOH, CHDCHOand C2H60C2H5, These primary steps are suggested to occur in the photolysis of ethyl propionate: C2HjCOOC2Hj hv +CnHj CO C2HjO (14 +2C&& Con (Ib) Several main reactions, involving ethoxy radicals, take place subsequently CnH5O CzHj +CzH60CzHj (4) 2CnHj0 +CpH,OH CHICHO (7) +CzH6 CHiCHO (5) C&sO +CHj CHzO (8) --+ CzH4 CzHjOH (6) C2Hj0 CzHjCOnCzHj + CzHjOH CiHgCOl (15) An activation energy difference of about 7 kcal. has been established for E.R- 231~. Activation energies of, respectively, 8.2 and 9.8 kcal., were determined for the abstraction of a hydrogen atom from ethyl propionate by methyl and ethyl radicals.
+
+
+ +
Introduction Little quantitative information on alkoxy radicals is available in the literature, although they are generally believed to play an important role in many combustion processes. I n a recent studyi at this Laboratory, inethoxy radicals were produced by photolysis of methyl acetate. Since methoxy radicals were thus obtained in a relatively simple system, i t was possible to obtain considerable information regarding their reactivity. This study has been extended, therefore, to the photolysis of ethyl propionate, where ethoxy radicals are produced by the main primary step. Experimental The apparatus has been described previously.2 A. Hanovia $500 medium pressure mercury arc was used as light source. As a rule the light of the $500 arc was not filtered. Some runs were carried out using Corning filter No. 9-54 (transmittingabove 2200 A.). No difference, other than the difference due t o increased light intensity, was observed when using filtered or non-filtered light. The light intensity was varied by inserting wire gauze screens between reaction cell and arc. The reaction products and the excess ethyl propionate mere cooled down t o - 150". The products volatile a t this temperature were pumped off by Toepler pump and their total volume was measured. This fraction was then analyzed b y mass spectrometer, and quantitative measurements were obtained for these several components: Hz, CO, COS, CHI, C2H6, C2H4 and CzHs. I n addition, some CaHl.iwas present. The residual butane and the ethanol, acetaldehyde, ethyl ether and excess ethyl propionate were determined by direct mass spectrometric analysis of the residue. No accurate analysis could be obtained for the amount of formaldehyde produced. T h e conversion v a s less than 5% in most experiments.
Results The photolysis of ethyl propionate has been carried out over the temperature range 30 to 344' at different light intensities and initial pressures. The results of these experiments are given in Table I. At each temperature, the relative light intensity for the experiments may be obtained from the rate of formation of CO and /'or CO,. (1) M. H. J. Wijnen, Am. Chem. SOC. 131st h-ational Meeting, Miami, April, 1957; J . Chenz. Phys., 27, 710 (1957). (2) K . 0. Kutschke, M . H. J. Wijnen and E. W. R. Steacie, THIS JOURNAL. 74, 714 (1952).
+ + +
+
+
+ +
Primary Steps.-Two primary steps are suggested to occur in the photolysis of ethyl propionate C&COOC?Hj
+ hv --+ C2Hj + CO + C2Hj0 2CzHj + Cot
(la) (1b)
Step l a produces carbon monoxide and ethyl and ethoxy radicals. Carbon monoxide has been observed to be the main reaction product in all experiments. The formation of ethanol and ethyl ether indicates the presence of ethoxy radicals during the reaction. Step l a has been represented here as giving directly both C2HS and CO, rather than CzH&O. It is possible that C2HbC0radicals are formed intermediately, although their presence could not be shown under any conditions of this investigation by formation of stable products such as diethyl ketone. In this respect, i t may be pointed out that the relatively high quantum yield of 0.6 to 0.7 for the production of CO in the photolysis of diethyl ketone a t 25', as reported by Kutschke, Wijnen and Steacie, indicates that the CzH,CO radical is quite unstable. Step l b is proposed to explain the formation of C 0 2 , which has been observed a t all temperatures as a reaction product. No direct evidence has been obtained in this investigation indicating any molecular rearrangement of the ester molecule in the primary process. I t should be mentioned, however, that i t is not unlikely that the step may occur to some extent CsHjCOOCzHj
+
?ZP -j. CzHjCOOH
+ CzH4
(IC)
Evidence for this type of primary step in esters has been first obtained by A ~ s l o o s . ~Additional evidence for this type of decomposition also has been found in the photolysis of n- and isopropyl propionate where step IC contributes to an extent of about 15yoof the total decomposition in the primary process.* Careful examination of the mass spectrometer data did not yield any evidence for step IC. This may, however, he explained by the fact that C,H6COOHis absorbed easily on the walls of the reaction vessel and thus was not present among the reaction products. X vague indication as to the maximum extent to which step ICpossibly (3) P. Ausloos, private communication. (4) M. H. J. Wijnen, t o be published.
PHOTOLYSIS OF ETHYL PROPIONATE
May 20, 1958
2395
TABLE I PHOTOLYSIS OF
Run no.
31 32 33 40 41 55 56 57 17
18 19 20 21 22 23
Reaction time, sec.
1680 5160 1620 16740 4380 5820 1560 5640 9600 1560 9180 5080 1500 1500 1500
[EPli. molec./ cc. AcetaldeX 10-1' Ethanol hyde
7.76 7.80 5.84 8.75 8.98 8.23 5.90 9.07 7.64 7.67 5.10 5.22 7.15 7.87 4.83
Rate of formation of products in molecules/(sec. cc.) X 10-1; Ethyl Carbon Carbon ether Methane Ethane Ethylene Propane Butane monoxide dioxide
Hydrogen
6.55 2.51 6.68 0.81 5.29 2.54 6.64 2.30
Temperature 30" 4.82 1.44 0.09 1.68 0.71 0.33 .03 0.47 3.96 .70 .09 1.53 0.46 .03 Trace 0.14 2 . 4 5 Notdetd. 0.03 0.92 0.50 0.14 .02 0.82 1.60 .06 .38 2.23 .ll .02 0.48 0.84
3.09 0.88 3.00 0.28 1.72 1.00 2.98 0.92
0.94 .28 .86 .07 .46 .26 .80 .26
7.77 2.43 6.34 0.83 3.58 1.82 5.48 1.67
12.92 3.94 11.29 0.97 6.64 3.54 10.47 3.68
3.44 1.04 3.11 0.28 1.98 1.01 3.17 0.99
..
1.69 8.75 1.01 2.15 8.25 8.38 5.05
Temperature 72' 0.07 0.45 .22 2.29 0.28 .07 0.56 .I5 3.29 .24 .19 2.14 .18 2 . 3 7
0.41 3.70 0.32 0.64 3.14 3.02 2.60
0.34 1.76 0.25 0.49 2.26 1.92 1.79
2.88 11.85 1.18 2.62 10.40 16.00 5.62
2.12 15.50 1.79 3.82 14.74 14.60 11.21
0.57 3.62 0.45 0.94 4.09 3.74 3.08
.. ..
Temperature 108" 3.31 0.80 .81 4.41 .60 2.56 .77 3.74 1.28 .26 0.50 .21 1.55 .48 1.03 .34 .75 4.02
3.54 3.11 3.14 3.50 1.27 0.35 1.63 1.01 3.36
3.14 3.74 2.78 3.47 1.22 0.37 1.63 1.01 3.96
5.58 8.71 6.99 6.27 2.76 1.39 6.73 3.30 10.80
15.88 18.71 13.52 16.62 5.65 2.42 7.94 4.88 16.94
3.74 4.50 3.22 4.57 1.48 0.61 2.00 1.24 4.63
..
2.56 0.37 0.91 2.36 0.81
9.82 1.24 3.66 9.29 2.52 1.04
14.85 2.11 4.48 16.52 3.88 1.29
4.29 0.61 1.25 4.08 0.99 0.50
..
...
5.22 0.50 1.16 4.47 1.11 0.65
Temperature 202' 0.95 0.97 3.39 4.18 1.72 1.85 1.32 1.52 6.03 5.72 1.56 1.77
0.38 2.84 0.71 0.56 3.06 0.59
0.30 3.55 0.76 0.53 3.69 0.37
0.58 6.02 1.94 1.21 7.88 1.46
1.90 11.95 3.86 3.56 14.90 3.23
0.53 3.47 1.13 0.82 3.88 0.74
0.47 .73 .45 .29 1.40 0.32
Temperature 239" 2.50 2.95 3.92 5.35 0.77 1.00 3.32 3.82 7.13 7.32
1.01 1.58 0.25 1.08 4.02
0.47 .87 .09 .38 2.58
1.12 1.60 0.58 1.29 4.97
4.65 7.36 1.23 5.94 14.93
1.28 2.16 0.38 1.34 4.82
0.50 .87 .21 .90 1.51
Temperature 272" 9.40 12.00 5.03 7.20 4.68 6.45 4.21 6.40
4.49 2.54 2.33 1.88
1.92 0.78 .74 .46
4.70 2.30 2.46 1.27
20.27 10.48 10.01 9.05
4.49 2.31 2.16 2.24
2.48 0.92 0.85 1.16
0.41 3.00 0.34 1.06 1.26
...
0.15
.. .. .. ..
..
1.02 0.43
.. ..
3 4 5 6 7 8 9 10
2460 1860 1800 1800 3900 11520 4080 5280 1620
7.87 8.76 5.37 8.64 4.61 6.95 7.01 6.91 7.00
7.02 6.40 5.90 6.54 2.48 0.82 3.64 1.65 6.92
2.22 1.16 3.47 1.87 0.61 0.40 1.16 0.52 4.04
11 12 13 14 15 16
1680 10260 4800 1680 4500 8100
6.82 6.47 6.40 6.25 4.57 4.12
3.48 0.44 0.89 3.37 0.79 0.28
0.86 0.13 0.20 1.11 0.26
0.53 1.82 0.31 0.91 3.04 0.96
..
..
..
1.21 1.71 0.38 2.68 3.92
.. .. .. ..
12.18 5.67 6.40 4.35
..
2
ETHYL PROPIONATE"
27 28 29 30 36 37
8040 1500 4980 7560 1560 7380
4.90 3.75 5.23 5.16 6.02 5.55
0.12 .99 .31 .19 .68 .32
45 46 47 49
5.17 5.17 5.63 5.58 5.67
.. ..
50
4320 2820 7380 4260 1200
35 42 43 44
1080 1800 7200 2340
4.90 4.82 4.88 5.17
0.08 0.40
...
.. ..
.. .. 0.08 .08 .29
.. ..
..
0.21
..
..
.. ..
.. ..
..
.. ..
.. ..
.. ..
..
Temperature 152' 2.77 5.86 0.67 0.89 1.04 1.41 2.22 3.96 1.13 0.75 0.51 1.46
.. ..
.. .. ..
.. I
.
.
I
.. .. ..
..
..
..
..
.. .. ..
..
..
.. .. ..
.. ..
Temperature 344" 6 . 5 0 66.00 11.20 4.10 .. 18.60 46.20 38.20 2.44 720 4.82 . . 52.70 52 9.16 60.00 11.68 4.92 .. 54.70 2 . 2 4 60.20 14.86 47.20 240 4.82 .. 53 14.63 66.20 1Z.ll 5.37 . . 62.70 .. 16.40 51.60 57.20 2.42 240 4 . 9 2 54 Open spaces indicate that under the given conditions this compound was not formed or formed in too small amounts to allow detection by mass spectrometer analysis.
M. H. J. WIJNEN.
2396
VOI.
so
may occur may be obtained by assuming that in the low temperature runs all ethylene, not produced by disproportionation of the ethyl radicals, is due to step IC. Such calculations indicate that step IC may be responsible for about 10% of the decomposition occurring in the primary steps. Steps l a and l b are similar to the primary steps in the photolysis of CH3COOCH32 and CHICOOCDZ,~ where in the case of CH3COOCD3 the following steps could be shown to occur
produces formaldehyde and methyl radicals. The results show clearly an increase in methyl radicals with increasing temperature as might be expected by the thermal decomposition of ethoxy radicals according to reaction 8. Not included in the reaction mechanism are the reactions of methyl radicals with ethoxy radicals to form methyl ethyl ether and/or methane and acetaldehyde. The main reason for this is that, in the low temperature region where such reactions preferably occur, the amount of decomposition of the ethoxy radical is CHaCOOCDa $- hr +CHaCO CDaO too small to allow these radical-radical reactions --f CHI CDI COz The ratio Rco/Rco, was found to be 3.8 =t0.3 in to occur to any marked extent. It may be menthe temperature range 30 to 157". This indicates tioned that methyl ethyl ether never was observed that step l a occurs about 4 times as frequently as among the reaction products. Reactions 13, 14, step l b . At higher temperatures this ratio of 15 and 16 represent the hydrogen abstraction reacI ~ c o / R c oincreases ~ somewhat. It seems likely tions from ethyl propionate by methyl, ethyl and that this is caused by secondary reactions such as ethoxy radicals and by H atoms produced by redecomposition of the CH$2HCOOCzH~and Cz- action 9. The radicals formed by these abstracH5COOCHCH3 radicals rather than by a tem- tion reactions are given as C4H9COz,rather than experature effect on the relative occurrence of steps pressing an opinion whether the abstraction would occur a t the propionyl or ethoxy group of the ester l a and l b . Reaction Mechanism.-Accepting reactions la molecule. I n the suggested reaction mechanism, methane is and l b as primary steps, these main reactions are formed by reactions 12 and 13. According to Ausproposed to explain the observed results 100s and Steacie,12 RCH,(IP) = O.O~RC,H,, where 2CzHs --t CIHIO (2) RcH,(~ indicates ~) the rate of production of methane *CZHI CZH5 (3) Equation I is therefore by reaction 12 only. CBHBO +C ~ H I O C ~ H L (4) CZH5 obtained +CzHa + CHICHO (5)
+
+
+
+
+
CzH4 4- CzHsOH (6) CzHsOH CHaCHO (7) +CH3 CHtO (8) -.--f CH3CHO H (9) C2H5 (10) 2CH3 -+ CsHs (11) CHI f C2H5 CH4 f CZHI (12) CH, C2H5COOCzHs +CHI C~HBCOI (13) C&sCOOCsHs +C ~ H B C'HoC02 (14) CZH5 C2H60 CZH~COOCIH~ +CIH~OH4-C4H9COz (15) H CzHsCOOCzHs +Hi 4-CIHDCOZ (16) ----f
2CzHsO CZH~O
----f
*
+
+ + +
*
+ + +
+ +
Reactions 2, 3, 10, 11 and 12 are generally accepted recombination and disproportionation reactions of ethyl and methyl radicals and do not warrant any discussion. Reaction 4 explains the formation of ethyl ether, especially observed a t low temperatures. Reactions 5 and 6 are possible disproportionation reactions between ethyl and ethoxy radicals. Disproportionation reactions of alkoxy radicals, such as reaction 7, have been suggested frequentlya-* to explain alcohol and aldehyde formation. Not included in the reaction mechanism is the recombination reaction of two ethoxy radicals to form diethyl peroxide. The mass spectrum of diethyl peroxide shows considerable peaks a t mle 90 and m/e 62. Total absence of these peaks proved the absence of diethyl peroxide as a reaction product. Reactions 8 and 9 have been shown to occur by several i n v e s t i g a t i ~ n s . ~ - ~Reaction l 8 (5) bl. H. J. Wijnen, J. Chcm. Phys., to be published. (R) J, B. Levy, THISJ O U R N A 75, L , 1801 (1953). (7) J. H.Raley, L. M. Porter, F. F. Rust and U'. E. Vaughan, ibid.,
73, 15 (1951).
(&Hi
total
- 0.041icla,)/(li'/rC~H~(lo) [EPI)= k i a / k ~ o ' / ~
(1)
Rc,H,(lo)-the rate of ethane production by reaction 10-may be calculated from data obtained by Ausloos and Steacie12and by Wijnen,13which indicate that Rc,H~(~o) = O.~~R*QR~/RC Values ~H~~. calculated via equation I for k13/k1O1/' are given in Table 11. By plotting log kl3/k10'/' against 1/T (Fig. 4), an activation energy of about 8.2 kcal. is obtained for reaction 10. TABLE I1 RATIOOF RATECOXSTANTS 1076
kii/kia"a,
10" kii/kz'/z,
.. 97 -50 7 .. 45;37(V) 108 13 3 9.2(V) 152 46 8 3.1 -0.2 202 I15 22 2.1 251 43 ... 239 277 397 105 344 817 289 ... a Values given for k l j / k a marked by ( V ) were obtained via equation V, all other data for k15/k8 were obtained from equation IV. 30 72
..
.
I
.
According to the given reaction mechanism, ethane is produced by reactions 3, 5, 10 and 14. R c ~ H ~may ( ~ O )be calculated as indicated above, and previous investigations have shown that R c ~ H ~=( ~ ) O . ~ ~ R C , H , ,The . ~ *results ~ ~ in Table I indicate that
(8) Y.Takezaki and C. Takeuchi, J . Chem. P h y s . , 22, 1527 (1954). (12) P. Ausloos and E. W. R. Steacie, Can. J . Chcm., 33 1062 (9) R. E.Rebbert and K. J. Laidler, ibid., 20, 574 (1952). (IO) I?, F. Rust, F. H. Seubold and W. E. Vaughan, THIS JOURNAL, (1955). 73 338 (1950). (13) M.H.J. Wijnen. J. Chenr. Phys., 22, 1631 (1954). (11) F. H. Pollard, H. S. B. Marshall and A. E. Pedler, Trans. (14) P. Ausloos and E. W. Steacie, BdI. mc. chim. Bclgcr, 63, 87 Fnrndnv S n r . . 52. 59 (1956). (1954).
May 20, 1958
PHOTOLYSIS OF ETHYLPROPIONATE
2397
from 239' upward in most experiments no ethanol could be detected. This proves that reaction 6 and therefore in all probability also reaction 5 do not occur a t these temperatures. It may be added here that the formation of acetaldehyde may be explained easily by other reactions as will be pointed out later. At 239' and higher, the ethane formation by abstraction of a hydrogen atom from ethyl propionate may therefore be given by the equation (Rc~H total ~
- 0.12Rc4ai0 - R c ~ H ~ ( I o ) ) / ( R ~ / [EPI) ~ c ~ H I= o kic/ki'/2 (11)
Values for k14/kz1'', thus calculated, are given in Table I1 for the temperatures 239 to 344'. At temperatures below 239 ', reaction 5, producing ethane via disproportionation of ethyl and ethoxy radicals, may become more important. Therefore, equation (111) is derived, which includes possible CIHs production via reaction 5.
- RC~HI(IO) -
E/ 3
-
I
1.o
0.5
10 -6 [ E P ] /Re&.
-
-
Fig. l.-PlOtS Of lo6(RCzHatotd RCZH~(IO)0.1ZRCrHio)/ (RCHaR'/'CrHio) ucfs1(s lo-' [EP]/RCHa at 108, 152 and 202".
decomposition of ethoxy radicals into acetaldehyde and hydrogen atoms. However, if this were the case, then i t might be expected that, especially I n equation 111, R C Hrepresents ~ the rate of production of CHs radicals. I n the proposed reaction a t high temperatures, the hydrogen production mechanism CH3 radicals are produced only by ther- would closely parallel the acetaldehyde production. mal decomposition of C1H50 radicals. If this is This is not the case. The possibility that acetalcorrect, RCH: will be a measure of the concentration dehyde may be produced by some other reaction(s) of ethoxy radicals. To calculate R c H ~i t, has been should therefore be considered. The C4H9COI assumed that CH3 radicals will only react to form radical, produced by abstraction of a hydrogen ethane (reaction lo), propane (reaction 11) and atom from ethyl propionate, may well be it source methane (reactions 12 and 13), in which case R C H ~for acetaldehyde production. Acceptiiig that mainly a secondary hydrogen atom is abstracted is given by R C H ~= ~ R C (10) ~ Hf~R C H f ~ Some CHz radicals undoubtedly may have dis- from ethyl propionate, these two C4H9CO2 appeared undetected, possibly by addition to C4- radicals may be formed: CH3CHCOOCzH5and HgCO2 radicals. As will be pointed out later, the C2H5COOCHCH3. Little speculation seems inextent to which CHI radicals disappear by addition volved in suggesting that the CH3CHCOZCzH5 or other, not given, reactions, seems to be small radical may thermally decompose into C&, CO and the evidence indicates, therefore, that R c is~a and CzH60, into CZH4, COz and C2H5, or possibly good approximation of the C2H50 radical concen- even into CH3CHCO and CzH50, although methyl ketene was not observed among the reaction prodtration. Equation TI1 is plotted in Fig. 1 for the tempera- ucts. The CzH6COzCHCH3radical on the other tures 108, 152 and 202". Values thus obtained for hand may decompose into C2H6, Con and C2H4or k14/k2'/' are given in Table 11. Figure 1 shows into CHICHO, CO and CZ&. Either radical, in clearly that the results are less accurate a t lower decomposing, produces a CzHs or C4HS0 radical temperatures. Considering that R c ~ H is ~ (cal~ ~ )and thus starts a chain reaction. Indicalion for reaction is obtained by comparing the culated from RCzHs tots‘ - R c ~ H ~-( ~Oo. )~ ~ R C ~such H ~a~chain , this is not surprising. With increasing tempera- product yields a t 344' with those obtained a t lower ~ > >> ture, R c ~ H ~approaches (I~) Rc~H total, , and errors in the temperatures. The fact that R c ~ Htotal ( O . ~ ~ R Cf, H 0 .~0 ~ 4 & ~ ~ )is added proof for the dedetermination of R C ~ H ~and ( I O0.)~ ~ R c ~become, H,,, composition of the C4H9COZradical. In addition, therefore, negligible compared to RCzHa total. Log k14/kl'/' is plotted against 1/T in Fig. 4, and acetone was photolyzed in presencc of ethyl proan activation energy of 9.8 kcal. is obtained for E14, pionate a t wave lengths above 3000 A., where ethyl the energy required to abstract a hydrogen atom propionate does not absorb. Both ethylene and acetaldehyde were formed a t temperature:; above from ethyl propionate by ethyl radicals. I n plotting equation 111, small intercepts were 152', proving that a t these temperatures ethylene observed a t 108 and 152' which, if real, indicate and acetaldehyde are formed by decomposition of that reaction 5 occurs. The results are, however, the C4H9C02radicals. No appreciable ethylene and acetaldehyde pronot accurate enough to give reliable data for k5/ duction was observed when acetone was photolyzed kgkz'/'. The results show a considerable acetaldehyde pro- in presence of ethyl propionate a t 152'. I t may duction a t temperatures above 239". Since a t therefore be assumed that a t this and lower temthese temperatures no ethanol is observed, i t is peratures, the C4H9COzradical will not decompose. obvious that acetaldehyde cannot be produced by Equations (IV) and (V) may be derived under this disproportionation of ethoxy radicals. A possible condition explanation for this increased acetaldehyde production might be given by reaction +the thermal (Rose total
~ . ~ ~ R ~ , O ) / ( R C H . ~ * /= 'C~I~)
+
R u / ~ z ' / ~ [ E P I / R c H Iks/kskz'/r
(111)
M. H. J. WIJNEN
2398
VOl. 80
and
I n equation IV, R c % H ~is( the ~ ) rate of ethylene production by reaction 6 only and is given by R C * H ~ ( ~ ) = Rc~H tot~ al - O . ~ ~ R C , H-, , 0.04Rcl=, disre.* garding possible ethylene production by primary 72'C step IC. The maximum extent to which primary step IC may take part is 10% of the total decomposition occurring in the primary steps. Taking this ethylene production by step IC into account, 2 4 data obtained for kl5/ks via equation IV will inlo-" [EP] /R1/2~4~10. crease by not more than 15% a t 30 and a t 72". At 152" the value of 10lg k15/k8 is 3.1, disregarding Fig. 3.- Plots of lo6 (RQH~OH - RCHaCHO)/(R'"C~HiuRCH8) primary step ICand 5.4 if i t is accepted that step IC uersus lo-" [EP]/R*/'QHIO at 72 and 108'. occurs to an extent of 10% of the total decomposition in the primary steps. Equation IV is plotted Equations IV and V are not applicable to the in Fig. 2 for the temperatures 30, 72 and 152'. results obtained a t 202' and higher, since a t those temperatures additional acetaldehyde and ethylene are produced by the thermal decomposition of the C4H9COe radicals a t 202", k16/k8 therefore was calculated via equation VI. o.2 0.1
"
%
t r
4
2
50
6
100
10"[EP] /RcH~. Fig. 2.- Plots
Of
10'' (RCZHSOH- RCzH4(a,)/R2CH3Versus [ E P ] / R c H at ~ 30, 72 and 152'.
Except for the results a t l52", no intercepts were observed, indicating that k7 ' k g 2 is small compared to the units in which the ordinate of Fig. 2 is expressed. Values of k15/ks, obtained from Fig. 2, are given in Table 11. KO accurate results were obtained a t 108", although there was a general agreement of the data with equation IV. Equation V is plotted in Fig. 3 for the temperatures 72 and 108". The intercepts are not pronounced enough to allow an accurate determination of (ks - kb) /(k8k2'/2). Values of k16/k8 obtained via equation V are given in Table 11. No plot was possible for equation V a t 30 and a t 152". I n this respect, it should be mentioned that acetaldehyde is probably the compound most subject to analytical error. An explanation for the failure to be able to plot equation V a t 152" may in addition be given by the possibility that a t this temperature some acetaldehyde is produced by reaction 9-the thermal decomposition of ethoxy radicals producing CH3CHO and H atoms.
Equation VI is valid only if reactions 6 and 7 do not occur a t 202" which is difficult to prove. Some justifications for equation VI may be found in the following considerations. Plots of equation IV a t different temperatures gave no or small intercepts, indicating that k7/ks2 is small compared to kla/ks X [EP]/RcH,. Secondly the fact that no ethyl ether is found a t high temperatures proves that no recombination-and therefore probably also no disproportionation-reactions occur between ethyl and ethoxy radicals a t these temperatures. Table I1 gives the value obtained for k15/k8 a t 202" by equation VI. No calculations have been carried out to determine k15/k8 a t 239" since the amount of ethanol produced a t this temperature is too small to allow its accurate determination. I n Fig. 4, log k15'kB is plotted against 1/T. From this plot an activation energy difference of Es - E15 f:7 kcal. is obtained. Since disproportionation and recombination reactions usually have a low activation energy, they will be most important in the lower temperature region. As an example of this may be given the production of ethyl ether, formed by recombination of ethyl and ethoxy radicals, which decreases rapidly with increasing temperature. By extrapolation of the data obtained for k14/k2'/2, i t may be shown that a t 30" the amount of ethane, produced by reaction 14, is negligible compared to the total ethane production. At this temperature R c ~ H ~therefore (~) is given by Rc2aa(s)= Rc2a6tots]
-O.~~RC~ RC%He(lO) H~~
Equation (VII) is thus obtained (RCH~CHO - R c ~ /R'cH~ H ~ = k7/ka2
0'11)
It is obvious from the derivatives made to obtain R c ~ H ~that ( ~ ) large errors are almost unavoidable. The results nevertheless indicate that a t 30" k7/ks2 = 5 f 2 molec.-'sec. cc. Material Balance.-Since CO and CzH60 are both produced by the primary step l a only, it is evident
May 20, 1958
PHOTOLYSIS OF ETHYL PROPIONATE
that RCO/RC~H~O should be equal to unity. I n the above expression R C ~ Hindicates ~O the rate of production of CzH60radicals and is given by R C ~ H=~ O RC~H~f OC RC ~ H ~~C-k HO R C ~ H ~ O HRcH~. These calculations have been carried out, although they are not reported in order to save space. Within experimental error R c o / R ~ ~was H ~found o to be equal to unity-largest observed deviations were - 0.2 and f0.3. The fact that experimentally Reo/ R C ~ Hequals ~ O unity indicates also that R c H ~as , suggested earlier, is a good approximation for the rate of disappearance of ethoxy radicals by thermal decomposition into CH3 and CHzO. Combining the reaction products of steps l a and lb, i t may also be shown that 2(Rco RcoJ/ o ) equal unity, which has, (RczHa R c ~ H ~should within experimental error, been observed to be true.
2399
I\
+
+
+
A
\
t I
/\
I
A
\.
Discussion An activation energy of 8.2 kcal. was obtained for E13, the energy required for the abstraction of a hydrogen atom from ethyl propionate by methyl radicals. No other data are reported in the literature for E13. An activation energy of 10 kcaL4 has been reported for the reaction CHa
+ CHaCOOCH3 +CH, + CHsCOOCH3
These values seem to be in good agreement since in methyl acetate a primary-and in ethyl propionate a secondary-hydrogen is abstracted. A similar difference in activation energy for the abstractions of primary and secondary hydrogen has been observed in acetone and methyl ethyl ketone. CH, CH:,
+ CHICOCHI +CH, + CHzCOCH3 (A) + CzHsCOCHa +CHI + CzHrCOCHa (B)
The activation energies have been reported as 9.7 kcal.15for reaction A, and 7.4 kcal.'" for reaction B. As activation energy for the hydrogen abstraction from ethyl propionate a value of E14 = 9.8 kcal. was obtained. Since the GH5-H bond is somewhat weaker than the CH3-H bond, i t might be expected that the hydrogen abstraction by ethyl radicals needs a higher activation energy than the similar reaction by methyl radicals. The ratios of the steric factors P13/P101/z and P14/Pz'/2 is in the order of as generally observed in abstraction reactions by methyl and ethyl radicals. Information obtained from this work indicates that with increasing temperature the ethoxy radical decomposes rather than forming ethanol by hydrogen abstraction or by disproportionation. It is interesting to compare these data with those obtained by Rebbert and Laidler,7 who studied the thermal decomposition of ethyl peroxide in a flow system in presence of a large excess of toluene. As main reaction products were observed ethane and formaldehyde and in much smaller amounts, methane and dibenzyl. They suggested the following reaction mechanism to explain their data CZH~OOC~HS --f 2CzH50 CzHs0 --f CH3 CHzO 2cH1 +CzHs CH, CsH5CHs ----f CHI CBHSCHZ 2CsHsCHz --f (C6H5CHz)z
+
+
(16)
(8) (10)
(17) (18)
(15) A. F. Trotman-Dickenson and E. W. R . Steacie, J. Chem.
Phys., IS, 1097 (1950).
I 2.0
2.5
3.0
103 I / T . Fig. 4.-Plots of 13 log k13/k:/b ( 0 ) ; 13 log k l t / k ' p (0); and 19 log kls/ks (A)versus lo3 X l/T.
+
+
+
Because of the lack of any appreciable peak a t 17 m/e in the mass spectrometric analysis of the reaction products, they concluded that reaction 8 would be much faster than reaction 19. CZHSO$ CBH6CH3 --3 CzH50H
+ CeHICHz (19)
These results were obtained in the temperature range 200 to 245". Our data show that small amounts of ethanol were observed a t 200°, but within experimental error no ethanol was found in four of the five runs a t 239". Qualitatively speaking, these data are in excellent agreement and indicate, as pointed out by Rebbert and Laidler, that a t these temperatures the thermal decomposition of the ethoxy radical proceeds much more rapidly than the hydrogen abstraction reaction b y ethoxy radicals. Evidence for the occurrence of a disproportionation reaction between ethoxy radicals to form ethanol and acetaldehyde has been obtained a t 30 and a t 152'. Ethanol formation by disproportionation of ethoxy radicals seems to be mainly important in the low temperature region. A similar observation was made regarding methoxy radica1s.l~~ As difference in activation energy between reactions 8 and 15 was obtained E8 - E15 = 7 kcal. The heat of dissociation of ethoxy radicals into CH3 and CHzO has been reported as 10.4 kcal. by Rebbert and Laidler' and as 12.8 kcal. by Gray.'B I n his calculations Gray accepted A€€ = - 9.0 kcal. as enthalpy of formation for the GHbO radical. Recent investigations by Pollard, Marshall and Pedlere place AH = -6.7 kcal., in which case the heat of dissociation of the CzH50 radical into CH3 and CHzO would become 10.5 kcal. as reported by Rebbert and Laidler. Accepting therefore AH (C2H60 --t CH3 CH20) as 10.5 kcal., i t is obvious that E8 3 10.5 kcal.; from this a minimum value of 3.5 kcal. is obtained for El5.
+
(16) P. Gray, Proc. Roy. SOC.( L o n d o n ) , A221, 462 (1954)
E. G. KING
2400
The author of this investigation feels that E8 and El5 are close to the minimum values given above and bases his opinion on the following considerations. Granting that the values reported for kt/ks2 are not very accurate, since they were mainly determined from intercepts, they nevertheless indicate that E8 is small. I n an investigation of methyl acetate,' an activation energy of about 4.5 kcal. was obtained for reaction 20. CHsO
+ CHICOOCHZ+
CHjOH
+ CHzCOOCH3
(20)
Although the RO-H bond has been reported" as
THE
so
100 kcal. for CHaO-H and as 99 kcal. for C2HsO-H, i t seems logical that E15should not exceed Ezoto any great extent, if a t all. This is even more so if we consider that a primary hydrogen is abstracted in reaction 20 while a secondary hydrogen is abstracted from ethyl propionate. Acknowledgments.-The author acknowledges many helpful discussions with Drs. S. D. Cooley and H. D. Medley during the progress of this work. Special thanks are due to Messrs. R. LfT. Jarrett and R. 11.Guedin for calculation and interpretation of the mass spectrometric data. CLARKWOOD, TEXAS
(17) P. Gray, Trans. Faraday SOL, 62, 344 (1950).
[CONTRIBUTION FROM
VOl.
MINERALSTHERMODYNAMICS EXPERIXENT STATION, REGION11, BUREAUOF MINES, UNITED STATES DEPARTMENT O F THE INTERIOR]
Low Temperature Heat Capacities and Entropies at 298.15 OK. of Lead Sesquioxide and Red and Yellow Lead Monoxide BY E, G. KING' RECEIVED JANUARY 10, 1958 The heat capacities of lead sesquioxide and the red and yellow forms of lead monoxide were measured over the temperaAll three substances behaved in a regular manner. Entropies a t 298.15°1C. were obtained, as shown ture range 51-298'K. (cal./deg. mole) : sesquioxide, 36.3 2~ 0.7; red monoxide, 15.6 i.0.2; and yellow monoxide, 16.1 f 0.2.
This paper reports heat capacity measurements over the temperature range 51-29S°K.', together with entropy evaluations a t 298.15'K., for lead sesquioxide, red lead monoxide (tetragonal) and yellow lead monoxide (rhombic). No previous similar data have been published for these compounds except the few results of Nernst and Schwers2for the yellow monoxide in the temperature range 2 1-93'K. Data for lead dioxide and minium (Pbs04) have been reported by Millar. Materials.-Lead sesquioxide was prepared by the following procedure. A solution of reagent grade lead nitrate was treated with a slight excess of ammonium carbonate solution. The precipitated lead carbonate was washed free of ammonium salts and dried a t 140'. It then was converted to sesquioxide by heating in air, 40 hr. a t 290°, 64 hr. a t 310" and 10 days a t 320". Analysis of the product gave 89.64% lead, as compared with the theoretical 817.62%. The X-ray diffraction pattern agreed with the ASTM catalog. Red lead monoxide was prepared by heating electrolytic lead dioxide in vacuo a t 430-480' for about 8 weeks. (The long heating period was required to eliminate yellow monoxide which is very slow to convert t o red.) The product gave no test for higher oxides when treated with strong sodium hydroxide solution. I t analyzed 92.69y0 lead (theory, 92.83'). The X-ray diffraction pattern indicated the presence of only a small amount of the yellow variety. Yellow lead monoxide was made by heating lead carbonate (obtained by the method mentioned above). The first heating, 80 hr. a t 56C-580°, was insufficient to decompose the carbonate completely or to prevent the formation of some red monoxide. The substance finally was split into small portions, heated for 10 hr. a t 725' and quenched to room temperature. No reconversion to the red form occurred during this treatment. .4nalysis of the product gave 92.847, lead (theory, 92.83%). The X-ray diffraction pattern agreed with the ASTM catalog. ( 1 ) Bureau of Mines, U. S. Department of the Interior, Berkeley, California. (2) W. Nernst and F. S c h w a s , Sitzbcr. prsuss. Acud. Wiss., 355 (1914).
( 3 ) R.W.Millar, THISJOURNAL, 61, 207 (1928).
TABLE I HEATCAPACITIES (CALJDEG. MOLE) T , OK.
53.36 58.06 62.66 67.03 71.50 75.95 81.00 85.48 94.96 104.75
CP
T , OK.
Cp
T,OK.
Pb2O3(mol. wt., 462.42) 8.891 114.88 15.90 216.45 9.524 124.78 16.81 226.13 10.12 136.53 17.82 235.97 10.70 145.90 18.58 245.91 11.24 155.87 19.33 256.50 11.77 166.02 20.02 266.45 12.38 175.85 20.62 276.38 12.87 186.19 21.25 287.09 13.91 196.05 21.78 296.56 14.92 206.42 22.33 298.15 (red, mol. 114.78 124.89 135.91 145.79 155.84 165.83 176.34 186.35 195.99 206.38
CP
22.80 23.25 23.64 24.04 24.44 24.81 25.09 25.40 25.70 (25.74)
53.40 68.07 02. 71 67.26 71.67 76.13 81.30 85.97 94.64 104.71
PbO 4.095 4.354 4.608 4.851 5.088 5.311 5.569 5.777 6.179 6.627
u t , 223.21) 7.039 216.27 7.440 226.09 7.841 236.73 8.183 245.91 8.494 256.39 8.773 266.62 9.039 276.58 9.283 286.79 296.27 9.479 9.702 298.15
9.878 10.05 10.21 10.32 10.47 10.60 10.71 10.82 10.94 (10.95)
54.13 58.39 62.59 66.82 71.12 75.57 81.03 85.21 94.91 105.01
PbO (yellow, mol. wt., 223.21) 4.353 114.74 7.184 216.48 4.590 124.80 7.568 226.22 4.806 135.95 7.958 236.04 5,041 145.71 8.292 245.94 5.260 155.95 8.599 256.39 5.472 165.87 8.868 266.26 5.734 176.28 9.118 276.22 5.924 185.83 9.337 286.42 6.363 196.21 9.547 296.11 6.793 206.23 9.747 298.15
9.915 10.08 I O . 24 10.37 10.50 10.61 10.72 10.82 10.94 (10.95)
