Feb., 1962
REACTIVITY OF HYDROGEN ATOMSIN
THE LIQUID
PHASE
291
computed by Evans, Munson and Wagmans from the dissociation h i t of BrF, the assumption of these authors that the dissociation leaves fluorine in an excited state apparently is correct. The free energies of formation, AFOf., of BrF, BrF3 and BrF5 a t 25' are calculated as -18.0, -55.2 and -84.1 kcal./mole, respectively, from the present heats of formation and tables of ent r ~ p y . ~ The J ~ entropy of BrF3 is taken as 69.905 cal./degree mole, even though this is derived with certain assumptions about the degree of dimerization of BrF3I0 and subsequently may be revised. Brz + BrFs 3BrF (5) The free energies a t elevated temperatures indicate a t 25'. If ASo is obtained from tables of en- that BrFs dissociates into BrF3 and Fz above 550' tr~py~,'~ and that BrF3 dissociates into BrF and Fz above 800' (Fz also dissociates into atoms in the latter APO + T A P = 11.8 kcal. (6) which is in very good agreement with their value of region). The dissociation of BrF5has been verified AHo derived from the temperature coefficient of by approximate PVT measurements. Above the equilibrium constant. If, then is possible to 1000°, BrF is expected to be the stable species over obtain A H o f . B r p from AH0f.BrP3 by the reverse of a wide range of composition, in equilibrium with atomic bromine or fluorine. the calculation which they macle Acknowledgment.--The author wishes to thank A H o r B ~ F= ' / 3 ( A f f 0 -k Affaf.BrF8) (7) Dr. D. W. Osborne for advice regarding the calori' / 8 ( 1 1 . 8 - 64.8) metric techniques arid both Mr. J. L. Tague and = - 17.7 kcal./mole a t 25'' Mr. B. T. Cope, Jr., for operating the shield a t Since this agrees within 0.7 kcal./mole with AH?.B~F various times during the experiments.
mixing of the liquids and ideality of the vapors. At present, no correction is made for partial dimerization of BrF3, since the proportion of dimer in the vapor at 25' is not accurately known. The equilibrium constant for dimerization has been estimated'O from approximate vapor densities at 75 and looo, but the extrapolation to lower temperatures probably is subject to very large error. Steunenberg, Vogel and Fisher' found AFO = 1.2 kcal. and ANo = 11.9 kcal. for the gas phase reaction
THE REACTIVZTU OF HYDROGEN ATOMS IN THE LIQUID PHASE. HI, TIRE REACTIONS WlTH OLEFINS BY 1'.J. HARDWICK Gulf Reseamh ck Development Company, Pittsburgh SO, Pennsylvania Received September 7, 1961
The rates of reaction of hydrogen atoms with a geries of olefins in n-hexane solution have been measured a t 23'. Both addition arid abstraction of hydrogen occur. For olefins within a given structural type, e.g., RCH=CHR, the rates of reaction are the same, although these rates vary from one structural type to anqther. Hydrogen abstraction appears to take place only from the olefinic hydrogens, although the individual factors affecting the rate have not been determined. The reactivity of hydrogen atoms is similar to that of alkyl radicals, but different from that of oxygen atoms.
Introduction The reactivity of hydrogen atoms with olefins in the gas phase has been studied extensively. Until ten years ago, however, experiments were confined to a study of ethylene, propylene and the butenes; such work has been critically reviewed by f3teacie.l Addition of hydrogen was the main observed reaction; evidence for hydrogen abstraction was found in some cases. The most comprehensive work has been carried out by Melville, Robb and their co-workers. 2--8 The rate constmts for hydrogen atom reaction wit'h ten olefins were det'ermined at 18'. Their meas'ure(1) See "Atomic and Free Radical Rea,ctions," E. W. R. Steacie, Reinhold Publ. C,orp., New York, N. Y.. 1954, Chap. V. (2) H. W. Melville and J. C . Robb, Proc. Roy. Soc. (London), 8196, 445 (1949). (3) H. W. Melville and J. C. Robb, i b i d . , 8196, 466 (1949). (4) €1. W. Melville and J. C. Robb, ibid., 8196, 479 (1949). (5) H. W. Melville and J. C . Robb, ibid., 8196, 494 (1949). (6) H. W. Melville and J. C . Robb, ibid., A202, 181 (1950). (7) P. E. M. Allen, H. 'AT. Melville and J. C. Robb, ibid., 6218, 311 (1953). ( 8 ) J. N. Bradley, H. W. Melville and J . C. Robb, i b i d . , 8 2 3 6 , 454 (1956).
ments represent the sum of the addition and abstraction reactions, but in a later paper8they found addition to be the most probable reaction in the case of ethylene and propylene. Absolute constants were measured, varying in value from 1.8-8.0 X 10l1cc. mole-1 sec. --I. Toby and Schiff found the rates of H and D addition to ethylene to be the same, confirming earlier work by Melville. lo Some very interesting studies have been made of the reaction of hydrogen atoms with propylene in the solid state.l1--I4 Propylene molecules form part of a solid matrix (maintained near liquid nitrogen temperatures). Hydrogen or deuterium atoms, formed by dissociation of the corresponding molecules on a hot mire, diffuse into the matrix and form radicals by addition. An activation energy of 1.5 kcal. was found for the formation of isopropyl radicals. In contrast to propylene, both addition and (9) S. Toby and H. I. Schiff, Can. J . Chem., 3 4 , 1061 (1956). (10) II. W. Melville J . Chem. Snc., 1243 (1934). (11) (12) (13) (14)
R. Klein and M. D. Scheer, J . Phgs. Chem., 6 2 , 1011 (1958). R. Klein, M. D. Scheer and J. G. Waller, ibzd., 64, 1247 (1960). R. Klein and M. D. Scheer, abzd., 65, 324 (1961). N.D. Scheer and R. Klem, ibzd., 65, 375 (1961).
T. J. HARDWICK
292
hydrogen abstraction were found for butene-l and isopentene on reaction in the solid state.14 The present work concerns the reactivity of hydrogen atoms with olefins in the liquid phase. In the technique used, as described previo~sly,l6-'~ it is required that the olefins be liquid at room temperature. As this precludes the use of olefins of fewer than five carbon atoms, the only work with which direct comparison may be made is that of Allen, et al.7 In measuring the reactivity of hydrogen atoms in the liquid phase, the hydrogen atoms are formed by the radiolysis of an alkane solvent (RH), and react competitively with solvent and added solute (S). The kinetic expression derived from the steady state kinetics is15J6 1
- 1
-7
GH~(o)- G H ~ W AGH,
=
[RHI
1
G
[SI
x2+
shown in Table I, where the olefins are listed by structural type. Addition of H Atoms to Olefins.-For all olefins of a simiIar structure, e.g., RCH = CHR, the rates of addition of hydrogen atoms are the same. A minor variation is found with R2C = CHR. This rate of addition varies, however, from one structural group t o another, RZC = CH2 having the fastest rate, RzC = CR2 the slowest. TABLE I REACTIVITYOF HYDROGEN ATOMSWITH OLEFINSIN WHEXANE(2' = 23 * 1")
where GH,(~)and G H ~ ( are ~ ) the radiolytic hydrogen gas yields without and with added solute. GZ is the thermal hydrogen atom yield ( = 3.16 for nhexane); IC,, IC4 and ks are the rate constants for the Pentene-1 reactions H
Hexene-1 Heptene-1 Octene-1 Decene-1
+ RB +Hz + R k4
H+S+HS
H
+ S +Hz + R' k6
If the kinetics are followed, a plot of l/AGFa us. [RH]/ [SIgives a straight line, the slope of which is (k3/k4)(1/G2), and the intercept ( k ~ / k d 1) (l/Gz). It is the purpose of this paper to study the reac-
+
tivity of hydrogen atoms on olefins in n-hexane solution, with particular attention to the effect of olefin structure on the rates of reaction. Experimental Materials.-The following olefins were Phillips Pure Grade: 2-methylbutene-1, pentene-1 , pentene-2, hexene-1, hexene-2, heptene-1, heptene-2, octene-1, cyclohexene. Cyclopentene was Phillips Reagent Grade. Decene-1 , 2,4,4-trimethylpentene-1, 2-methylpentene-1, 2-methylheptene-1, 2-methylpentene-2, trimethylethylene, tetrarnethylethylene, 2-ethylbutene-1, 3-ethylpentene-2, and vinylcyclohexane were obtained from K and K laboratories. All olefins tested to 99-101yo unsaturation. %-Hexane was Phillips Pure Grade, and was further purified by sulfuric acid to lower the unsaturation below B measured value of 0.15 mM/1. Normal hexane was used as the solvent and source of hydrogen atoms. The techniques of sample preparation, irradiation and analysis have been described previously.16s18 Corrections were made to the measured hydrogen gas yields to allow for direct absorption of the radiation by the solute.'? I n general the solute concentration ranged between 0.3 and 1.5% by volume. All irradiations were made at 23 & 1
ks (E, (addition) abstraction) cc. mole-' set.-* X 1011
k4
Olefin
Cy clopentene Cyclohexene Pentene-2 Hexene-2 Heptene-2
ka
Vol. 66
RCH-CHR 5.3 4.9 5.5 5.2 5.1
k6/k4
1.33 1.12 1.23 1.27 1.35
0.25 .23 * 22 .25 .27
1.26
0.24
-
2.3 2.4 2.9 2.0 2.5
-
0.30 .31 .35 .27 .32
7.9
2.4
0.30
10.8 10.8 11.7 11.o 11.5
3.4 3.4 3.7 3.1 3.7
0.32 .32 .32 .28 .33
11.2
3.4
0.31
7.6
2.1
0.28
RZC=CHR 7.5 6.6 7.1
0.6 1.2 1.8
0.085 .17 .26
RzC=CRz 5.5
