H. J. VAN DER LINDEAND 6. R. FREEMAN
20
The Influence of Temperature on the 7 Radiolysis of Isopropyl Alcohol Vapor.
Effect of Molecular Structure on the Nonchain
ecompositions of Alcohol Vapors1 by H. J. van der Linde and G. R. Freeman* Dsparkment of Chemistry, University of Alberta, Edmonton, Alberta, Canada
(Received August 7 , 1970)
Publication costs assisted by the University of Alberta
Isopropyl alcohol vapor a t a density of 0.78 g/l. was irradiated to a dose of 3.2 x eV/g a t temperatures from 125 to 400". The yields of all the measured products except pinacol were independent of temperature between 125 and 205'. The G values were Hz, 7.2; CHI, 4.8; CO, 0.52; CzH4,0.21; CgHs, 0.16; CHSCHO, 2.9; GdhoH, 0.66; 2.5; (CH&CO, 4.0; diisopropyl ether, 0.36; and pinacol, 1.0-1.5. The result.; are compared with those from the radiolyses of methanol and ethanol vapors. ils the alcohol molecule size increases the amounts of C-0 and C-C bond cleavage increase a t the expense of C-H and 0-H cleavage. The total ionization yield and the net amount of alcohol decomposition do not vary greatly from one alcohol to the next: (?(ionization) = 4.1 i: 0.2 and G(-alcohol) = 14 f 1 for each of the alcohols in the "nonchain" decomposition region of temperatures. At temperatures above about 250" chain reactioris became important in isopropyl alcohol vapor. There were four modes of radiation induced chain decomposition. The stoichiometric representation of the modes, with their G values a t 380' and activation energies (kcal/mol) were CaH7QH .-t Hz (CHa)zCO, 600 (32 f 4); C&OH ---t CH4 CHaCHO, 43 (18 It: 2 ) ; CsH7OH + HyO C)&, -1200 (40 f 10); 2C3H70H-+ HzO (C&)20, 300 (41 It 2). The activation energies were similar to those of the corresponding modes in the radiation-induced pyrolysis of ethanol under similar conditions. However, the rates of all the modes except demethanation were an order of magnitude greater in isopropyl alcohol than in ethanol. Demethanation occurred a t similar rates in the two alcohols.
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Introduction Tbe radiolvsis of ethanol vapor is relatively insensitive to temperature at temperatures below 300" , 2 , 3 but above 300" both free-radical3 and cationic4 chain reactions occur. The radiation-induced cationic chain reaction forms ether and water It also occurs in isopropyl alcohol vapors5 but it begins at a lower temperature in isopropyl alcohol (-250O) than in ethanol. The present article reports the normal radiolysis and several modes of radiation-sensitized chain decomposition of isopropyl alcohol vapor. Effects of molecular structure upon the different modes of alcohol reaction are discussed. Experimental Section Materials, Sarvlple Preparation, and Handling. The methods of purification of materials and the experimental techniques used were the same as those reported earlierj5 unless otherwise mentioned. The irradiation dose rate in is~propylalcohol was 3.5 X 1019eV/(g hr). Product Analysis. The gaseous products were separated from the irradiated sample by low-temperature distillation under vacuum. The gases were collected and measured in a McLeod-Toepler apparatus, then injected into a gas Chromatograph that had a thermal conductivity detector. The Journal of Physical Chemistry, Yol. 76,N o . 1 , 3971
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The liquid products were measured by injecting 1 pl of the irradiated sample into a gas chromatograph that had a flame ionization detector. The different products were analyzed with the following gc columns and conditions : (a) hydrogen, in. copper methane, and carbon monoxide; 9 ft X tubing packed with molecular sieve 5A9 loo', helium carrier gas a t 75 mljmin; (b) methane, ethane, ethylene, and propylene; 6 ft x 1/4 in, copper tubing packed with Porapak Q (150-200 mesh), programmed from 80 to 120°, helium carrier gas a t 75 ml/min; (e) acetone and diisopropyl ether; 12 f t X 3//16 in. glass tubing packed with 10% Carbolvax 1540 on Chromosorb WAW, 75', helium at 60 ml/min; (d) acetaldehyde and ethanol; 3 ft X 3/16 in. glass tubing packed with Porapalr QS (150-200mesh), 130') helium at 75 ml/min; (e) pinacol; 6 ft X 3/16 in, glass tubing packed with 1% FFAF on glass beads (60-80 mesh), 110', helium at 30 ml/min.
Re sults Samples of isopropyl alcohol at a density of 0.78 g/l. (1) The authors are grateful to the National Research Council of Canada for financial assistance. (2) K. M. Bansal and G. R Freeman, S.A m e r . Chem. Soc., 90, 7183 (1968). (3) K. M. Bansal and G. R. Freeman, ibid., 90, 7190 (1968). (4) K. M. B a n d and G. R. Freeman, ibid., 92, 4173 (1970). (5) H. J. van der Linde and G. R. Freeman, ibid., 92, 4417 (1970).
THE~ F L U E N C EOF '~EMPERATUBEON THE y RADIOLYSIS OF ISOPROPYL ALCOHOLVAPOR (500 Torr a1 350") were irradiated to a dose of 3.2 X 1019 eV/g a t tenipcratures from 125 to 400". The yields oi :dl {he measured products except pinacol were irrdepcndenl 01 temperature between 125 and h 5 " , but increased at highre tmiperatures. The temperature at which thcb yield began to increase varied from about 250 to ahout :iW, depending on the product. However, t h e a: val-trcof pinacol m n t through a maximurn at ahout '38Cr", i t L.0 wt 124O, 1.5 at 205",0.7 at 271 ", 0.2 ai 301", ant1 25.0 at 333" and higher temperatures. Thp large y J t ~ l dof~ products at the high temperatures were f - ~ r n by ~ ~elriain d reactions, so it is convenient to divide the ternperalurc range into 'honchain" and "celiairi" rq$oncJ and i o present the results of each region sepax 51t ely. "Bltink"' samples were prepared and heated in the sanw way as the normal samples, but were not irradiated. 'The anaoctnts of products formed in the blanks were negligible at temperatures below 300", but a t high temper-aturw A ere c,ometimes several per cent of the amoiii ts formed in the irradiated samples. The reportoci G value i have had the blank yields subtracted from theni. No?~chainRegiunz. The 100-eV yields of the measured producls at temperatures from 125 to 205" are listed m Table I. The major products in decreasing order of Lhrir ye1dF are hydrogen, methane, acetone, aeets Ideiivde, water, propylene, and pinacol. W
~
C
Table 11 : Tdonchnin Product Yields, 125 to 205""
G
7.2 4.8 0.52 0.21 0.16 2.9 0.66 2.5 4.0 0.36 1.0-1.5 -2. gb 14"
21
r("C1 400
350
300
250
3 z2
9
-
U
r - 1
V 3
a - 4 (3
( 3 3
9
2 1
0 1.5
1.6
1.7
1,8
1.9
IO~/T(OK) Figure 1. Arrhenius plots of the major radiation-induced chain product yields in isopropyl alcohol vapor. Vapor density = 0.78 g/l. A: 0, hydrogen; A, acetone. B: 0, propylene; A, diisopropyl ether. Gobaln at temperature t is the measured value at 1 minus the norichain value listed in Table I.
Chain threshold temp, "C
-250 -250 -310 -340 -340 -250 -250 -250 -250 -250
Isopropyl alcohol density 0.78 g/l. (350 Torr at 165'); dose 3.2 x 1019 eV/g. * Sum of CsHa and (i-C3H~)~0 yields. G( -C&I+3H) = 0.33 2 G(carbon atoms in products). a
=
The approximate upper temperature limit of the nonchain region, which is also the threshold temperature of the chain region, is listed for each product in Table I. Chain 2Zegiun. The G values of the products of the chain reactions were estimated by subtracting the nonchain G values of Table I from the yields measured at
higher temperatures. Subtraction of the nonchain yields was necessary to obtain a reasonable Arrhenius plot of the chain yields a t temperatures just above the chain threshold temperature. At higher temperatures the correction was negligible. This procedure was satisfactory, though not exact. The main products formed by chain mechanisms were hydrogen, acetone, propylene, and diisopropyl ether. Arrhenius plots of the chain yields of these compounds are shown in Figure 1. Water, the conjugate product of propylene and diisopropyl ether, was also detected but was not measured quantitatively. The results for propylene were less precise than those for the other products because of the lack of reproducibility of the propylene blanks. Isopropyl alcohol was evidently catalytically dehydrated to propylene on the vessel wall, sometimes at a rate that was similar to that of the radiation-induced reaction. Minor chain mechanisms formed methane, acetaldehyde, ethanol, ethylene, ethane, and carbon monoxide. Arrhenius plots of the chain yields of these compoiinds are shown in Figure 2. The activation energies of formation of the various The Journal of Physical Chemistru, Vol. 76, AVO.1, 1971
H. J. VAN
22
t("c1 2.0
400 II
350 I(
300 I
,
E a
I 1
A
1.5 1.0
I
w 0.5 IU
2
Ia?
0 I I
I
I
I
8
Table 111 : Radiolytic Decomposition of Methanol, Ethanol, and Isopropyl Alcohol Vapors
u
0
1.5
0
9
LINDEAND G. R. FREEMAX
amount of C-H and O-H cleavage is given by the value of G(H2). Similarly, the amounts of C-0 and C-C cleavage are indicated by the G values of the resulting products. It is interesting to coinpare the extents of the different modes of decomposition of methanol,6 ethanol,2 and isopropyl alcohol during y radiolysis in the vapor phase. As one proceeds through the alcohols in order of increasing molecular size, the amounts of C-0 and C-C cleavage increase a t the expense of C-H and O-H cleavage (Table 111).
25( ,
DER
1.0
G ( H ~ ~ G(C-0 cleavage) G(C-C cleavage)
0.5
G(C0)
0
1.6
1.8
1.7
1.9
Figure 2. Arrhenius plots of the minor radiation-induced chain product yields in isopropyl alcohol vapor. Vapor density = 0.78 g/1. A: 0, acetaldehyde; 0, carbon monoxide; A, ethanol; n, (CHaCHO CZH~OH CO). B: 0, methane; 0, ethylene; A, ethane; 0, (CH4 - 0). @chain a t temperature t is the measured value at t minus the nonchain value listed in Table I.
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Table I1 : Activation Energies of Formation and G Values of Chain Products from Isopropyl Alcohol at 380'" Product
E% kcel/mol
32 f 4 32 f 4 40 f 10 41 f 2 20 rt 2 13 f 2 15 f 2 58 f 3 54 i 3 45 f 3 a Isopropyl alcohol density 3.2 X 1019 eV/g.
=
Gohain
600 600 w1200 300 46 27 6 10
6 3
0.78 g/l. (530 Torr); dose =
products are listed in Table 11. To give an idea of the relative extents of the different reactions, G values estimated for 380" are also given in the table.
Discussion Nonchain Region (t < 2 ~ 7 0 ~ )Alcohol . molecules can decompose in several different ways, by cleavage of a C-H, O-H, G O ,or C-C bond. A measure of the total T h e Journal of Physical Chemistry, Vol. 76, N o . 1, 2071
10.4
9.9 1.8
0.3
4.1f0.2 13
i-CsH70HC
3.3 0.7 4.1f0.2 15
7.2 2.9 5.3 0.5
4.1f0.2 14
Reference 6, t = 21'. Reference 2, t = 150'. Present work, t = 125-205'. G(H2) is a measure of the total of C-H and O-H cleavage. References 10 and 11.
~o~/T(oK)
+
CsHaOHb
0.8
G(ion pairs)E G(-alcohol) 1.5
CHsOHa
The large increase in G(C-0 cleavage) on going from methanol to ethanol is related to the fact that methanol cannot be dehydrated to form an olefin, whereas ethanol can. Dehydration to form an olefin is much less endothermic than simple C-0 bond ~ l e a v a g e . ~ CHaOH
---f
CzHsOH ---+
+ OH -92 kcal/mol CZH4+ HzO -I1 kcal/mol
CHa
(1) (2)
The further increase on going from ethanol to isopropyl alcohol correlates with the fact that secondary alcohols are more readily dehydrated than are primary alcohols by acid c a t a l y s i ~ . ~ The ~ ~net reactions 2 and 3 are nearly equally endothermic7
i-CsHTOH +CaHa
+ HzO -12
kcal/mol
(3)
In methanol there is no C-C bond t o cleave, but in both ethanol and isopropyl alcohol G(C-C cleavage)/ G(C-0 cleavage) = 1.8 (Table 111). The ratio (no. of C-C bonds)/(no. of C-0 bonds) for isopropyl alcohol is double for ethanol, so the constancy of the cleavage ratio is somewhat surprising. Carbon monoxide must result from the decomposition of highly excited species, because all four appendages of the 3 C-O- group in the alcohol must be removed. The (6) J. H. Baxendale and R. D. Sedgwick, T r a n s . Faraday SOC., 57, 2157 (1961). (7) S. W.Benson, J . Chem. Educ., 42, 502 (1965). (8) C. R. Noller, "Chemistry of Organic Compounds," 3rd e d , W. B. Saunders Co., London, 1965,p 148ff. (9) J. L. Beauchamp, J. Amer. Chem. SOC.,91, 5925 (1969).
THEINFLUENCE OF TEMPERATURE ON THE y RADIOLYBIS OF ISOPROPYL ALCOHOLVAPOR
23
Table r!i : Moder, of Di:composition during Radiation-Induced Pyrolysis of Alcohols at 380"
Mode
+ + + +
(7) alcohol -+ B 2 carbonyl (8) -+ GH4 aldehyde (9) -.v I F I 2 0 olefin (10) -+HZO ether a
100 64
30 f 2 20 f 2
45
27 f 2 43 f 4
21 2: = 230
600
32 f 4 18 rir 2 40 & 10
43 -1200
300 Z = 2140
41 1 2
Values taken from Figures 8 and 9 of ref 3 and Figure 2 of ref 4. Ethanol pressure = 580 Torr. Dose rate = 4 X 1018 eV/(g hr). Isopropyl alcohol pressure = 530 Torr. Dose rate = 3.5 x 1019 eV/(g hr).
' Present work.
decrease in carbon monoxide yield as one proceeds from methanol to isopropyl alcohol (Table 111) reflects the increasing number of possible modes of decomposition of the alcohol molecules, and other related properties. The total ionization yield and the net amount of alcohai decomposition do not vary greatly from one alcohol to the next: G(ionization) = 4.1 f 0.210t11 and G(-alcohol) -- 14 1 for each of the three compounds listed in Table 111. Chain Region (1 > 26zsOo). Pinacol was formed by reaction 4. The pinacol yield declined at temperatures
*
2(CH3)2bOH
=+ -
[(CH&COH JZ
(4)
above about 250°, in the same region where the chain mechanisms began to operate. The decline in the pinacol yield is attributed to the decomposition of the alcohol. radical. (CHJ&%H
-+ (CH8)zCO
+H
(5 )
Reaction 5 is alt;o one of the chain propagation reactions for aeetonc! formation, the other one being
R
+ (CH,)2CEEOH--+RH + (CH3)&OH
(6)
where R is any free radical in the system. Four main modes of chain decomposition occur during the radiation-induced pyrolysis of isopropyl alcohol. The modes are analogous to those that were found in the ~ , ~it is inradiatron-induced pyrolysis of e t h a n ~ l ,and structive to compare the two systems. The four rn0dr.s are represented by the following stoichiometric equatiions.
+ carbonyl --+ CH4 + aldehyde
(8)
+H 2 0 4-olefin
(9)
+ ether
(10)
alcohol +Hz
+HzO
the diflerent modes are dehydrogenation (7) > demethanation (8) > dehydration to olefin (9) > dehydration to ether (lo), whereas in isopropyl alcohol under similar conditions the relative rates are dehydration to olefin (9) > dehydrogenation (7) > dehydration to ether (10) > demethanation (8). The activation energy of the dehydrogenation chain reaction in isopropyl alcohol vapor is 32 f 4 kcal/mol, obtained from an Arrhenius plot of the hydrogen and acetone yields. The acetone yield was corrected slightly for the amount produced by the secondary reaction 11. CHyCHO
CJ&OH
+ (CHa)ZCO
(11)
By analogy with the thermal reaction 12 that occurs CHzO
+ CzH50H
---?t
CHyOH
+ CHZCHO
(12)
readily in the vapor phase a t temperatures above 250°,4 it is suggested that the ethanol in the present system is produced by (11). The rate of the demethanation chain reaction 8 in isopropyl alcohol is not represented simply by the rate of formation of methane or of acetaldehyde, because the latter two are not equal (see Table I1 or Figure 2). The aldehyde undergoes secondary reactions including (11) and decomposition to methane and carbon monoxide CHsCHO +CH,
+ 60
(13)
The yield of (8) can be estimated from either eq i or ii.
GS = G(CH3CHO) 3. G(C2H,0H) + G(C0) (i)
(7)
The rates of dehydrogenation (7) and dehydration (9 and 10) a,re roughly an order of magnitude greater in isopropyl alcohol than in ethanol, while the rates of demethanation (8) are similar in the two systems under the same conditions ((TableIV). I n ethanol at 380" and 530 Torr the relative rates of
+ i-CsH,OH *
Gs = G(CH4)
- G(CQ)
(ii)
An Arrhenius plot of the right-hand side of eq i gave an activation energy Es = 16 kcal/mol (Figure 2A), and that of the right-hand side of eq ii gave E8 = 19 kcal/mol (Figure 2B). It is concluded that Es = 18 f 2 kcal/mol. The activation energies of the dehydration reactions (10) P. Adler and H. K. Bothe, 67. Naturforseh. A , 20, 1700 (1965). (11) R. M. Lebleno and J. A. Herman, J . (>him.Phus., 6 3 , 1055
(1966).
The Journal of Phgsical Chemistru, Vol. 76,No. 1, 1071
NEDE. BIBLER
24
9 and 10 are obtained from Arrhenius plots of the olefin and ether yields, respectively. In isopropyl alcohol we obtained E 9 = 40 rt 10 kcal/mol and El0 = 41 f 2 kcal/rnol.
The activation energies of the four decomposition modes in the radiation-induced pyrolysis of isopropyl alcohol are similar to those of the corresponding modes in ethanol (Table IV).
The Radiolysis of Carbon Tetrachloride. Radical Yields and the Formation of Tetrachloroethylene as an Initial Product1 by Ned E. Bibler Savannah River Laboratory, E. I . d u Pont de Nemours and Company, A i k e n , South Carolina 89801 (Received August 11, 1970) Publication costs assisted by the U. S. Atomic Energg Commission
The radiolysis of liquid, air-free carbon tetrachloride containing a variety of solutes at 25' was investigated in detail. With Rrz and H I as radical scavengers, the measured initial 1OO-eV yield of CC13 radicals was equal to 7.0 f 0.2. The organic products resulting from spur reactions in these solutions were CzC14 and C2C:16 (G = 0.08 and 0.47 molecule/100 eV, respectively). The total yield of scavengeable chlorine was 8.3 atoms/100 eV. With these two scavengers, evidence for the existence of intermediates other than C1 and CC13 was obtained. When 1 2 was the scavenger, IC1 was the most abundant product. The other major product, CC&I,was very susceptible to radical attack, and eventually all the initial iodine atoms appeared as ICI. I n solutions containing IC1 as a scavenger, G(-ICl), G(Iz), and G(CCl3I) were all equal to zero. With each of the above scavengers, tetrachloroethylene was an initial product; in the absence of scavengers, tetrachloroethylene presumably is removed by reaction with C1 or Clz. To explain CzC14 formation in the presence C2H4, CH8C1, of ICl, an IC12 intermediate is proposed. Other scavengers suitable for C1 atoms included "8, CHZCl2,and CHC13; CC13Br was not an efficient scavenger as suggested by the absence of C2C14 as a product. In solutions containing "3, a precipitate containing Cl- ions was formed by the radiolysis. Also, in the presence of the chloromethanes, CH2ClZ and CHCL, the radical yield in carbon tetrachloride was lowered considerably, apparently by an energy- or charge-transfer mechanism.
Introduction Various authors have estimated2-' widely differing 100-eV yields of radicals in the 6oCo y radiolysis of liquid air-free carbon tetrachloride with a variety of scavengers (Table I). These differences may be partly attributed to differences in the reactivity of the scavengers toward electronically excited or ionic precursors of the radicals and also the reactivity of the scavengers toward the radicals themselves.* However, there is no apparent reason for the differences obtained when the halogens Brz or 1 2 are used as the scavengers. Both halogens haxe been shown to scavenge cCl3 radicals ~ have ionization in the radiolysis of c h l o r ~ f o r m ;both potentials lower than that for carbon tetrach1oride;'O a n d both can undergo exothermic reactions with CC13 radicals or C1 atoms. Accordingly, we have investigated the radiolysis of liquid carbon tetrachloride in detail using a variety. of scavengers including Brz, 1 2 , 1C1, and HI. In the presence of these and other solutes (including NHJ, CH2C12, and CzHJ capable of T h e Journal
of
Physical Chemistry, Vol. 76, No. 1, 1971
reacting with C1 atoms, tetrachloroethylene was an initial product of the radiolysis. This compound is not produced in the radiolysis of pure carbon tetrachloride2s3 or of carbon tetrachloride containing ~ x y g e n . ~ (1) The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S.Atomic Energy Commission. This work was sponsored by Division of Peaceful Nuclear Explosives. (2) J. W. Schulte, J . A m e r . Chem. SOC., 79, 4643 (1957). (3) F. P. Abramson, B. M. Buckhold, and R. F. Firestone, ibid., 84, 2285 (1962). (4) T. H. Chen, K. Y. Wong, and F. J. Johnston, J. Phgs. Chem., 64, 1023 (1960). (5) S. Ciborowski, N. Colebourne, E. Collinson, and F. S. Dainton, Trans. Faraday Soc., 57, 1123 (1961). (6) E. Collinson, F. S.Dainton, and H. Gillis, J . Phys. Chem., 65, 695 (1961). (7) A. Chapiro, ibid., 63, 801 (1959). (8) R. A. Holroyd in "Fundamental Processes in Radiation Chemistry," P. Ansloos, Ed., Interscience, New York, N. Y., 1968, p 457. (9) H. R. Werner and R. F. Fireiitone, J . Phys. (?hem., 69, 840 (1965). (IO) K. Watanabe, J. Chem. Phys., 26, 542 (195?).
