THOMAS S. CROFTAND ROBERT J. HANRAHAN
2188
Vol. 66
TABLE IIr CHARACTERIZING PROPERTIES OF THE &BUTYL PERESTERS ASD INTERMEDIATES IN THEIR PREPARATION 4cyl Methyl ester
Acyl group
B.p., O C . (mm.1
Lauric Laurica Pelargonicb Capric Capric" Myristic h1yristic" I'almitic
139-1 4 1 15)
12%
1.4301
....
chloride B.p., "C. (mm.)
145 (18)
.....
..... .....
..*.
......
114 (15)
1,4235
53 (1.5)
1.4346
144(1.5)
..... 153-155 10)
.....
194 (12)
1.4355 (n=D)
Palmitic ..... .... Benzoicd ..... Perester supplied by Dr. L. S.Silbert. a t 10 p. N.B.S. benzoic acid was used.
......
Recrystn. temp., '(2.
Final method of purificn.
-40
Recryst. Recryst. Distill. Florisil, alumina R,ecryst. Recryst. Recryst. Recryst.
..
30-32' -45
..
-45
......
..
137(0.3)
-45
&Butyl perester n3Qn
1,4333 1,4337 1,4273 1.4297 1.4293 1.4373 1.4368
....
d3Q4
0.8783 ,8784 .8868
.8832 .8833 .8752 .8751 .
I
.
.
Available oxygen (% of theor.)
99.7 99.5 99.7 99.7 100.3 100 .o
100.0 99.6
...... .. Florisil, alumina .... .... 99.2 99.2 Recrvst. 39.5 (17) -45 1.00337 1.4957 Pelargonyl chloiride supplied by Dr. L. S. Silbert. c Boiling point of perester
alumina and "Florisil" ensured removal of water which might have occluded during recrystallization. The dipole moments of these samples agreed with the values for those obtained by recrystallization, indicating the absence of water in all the samples. Available oxygen was determined by the improved iodometric method of Silbert and Swern.24 Benzene.-Thiophene-free C.P. benzene was dried over phosphorus pentoxide for several days and distilled through a 1.5 X 130 cm. cylindrical column packed with 3/8 in. glass helices, then stored over Drierite until used. All transfers of solvent for weighing and washing were made using drying tubes to avoid contamination by atmospheric moisture. n-Hexane.-Phillips pure n-hexane was treated with concentrated sulfuric acid for 7 days and was then washed with water and 5% sodium carbonate solution. It finally was distilled over phosphorus pentoxide and stored in the same manner as the benzene.
Petroleum Ether.-Low boiling, olefin-free petroleum ether, used as solvent in the preparation of the peresters, was prepared in the same manner as was n-hexane. &Butyl Hydroperoxide.-The t-butyl hydroperoxide was prepared as described earlier.2 The fraction boiling a t 42.5" at 18 mm. was used in the preparation of the peresters. Apparatus and Methods.-Dielectric constants, refractive indices, and densities were measured as described earlier,z,a but for most of the capacitance measurements t-i General Radio Company 716 CS 1 radiofrequency bridge, 1330 A oscillator, and 1212 A null-detector were employed. Solute weight fractions were in the range 0.001 to 0.012. Temperatures were maintained at 30.00 i 0.005".
Acknowle-jgments.-~. D. TTerderame expresses his grateful ackno.cvledgment of the encouragementj given him by the Research Division of the Frankford Arsenal and of the support of the U. s. Army Research Office. The abundant aid given by Dr. Leonard S. Silbert also is gratefully acknowledged.
(24) L. S. Silbert and D. Swern, A d . Clrem., 80, 383 (1038).
IODIXE PlLODUCTION I N THE y-RL4D10LYSISOF CH'CT,OHEXANE-,aLI(YL IODIDE SOLUTIOSS
s.
B Y THOMASCROFT ANI)
ROBERT J. J h h l L i H l N
1)rpag tirient of Cheinislrg of the CimPrs2tU of Floi ida, Gaiiiesvzlle, F'lorada &celvcd .'iprd do, 1066
Iodine production induced by COG"7-radiolysis has been measured for solutions of methvl iodide in cvrlohexane and ethvl iodide in Cyclohexane, over the concentration range 0-95% cyclohexane by volume. In each case the addition of cvclohexane caused a smooth decrease in iodine production from the alkyl iodide, but net iodine production persisted until a t least 93 elcctron % cyclohexane was added. These observations are discussed in terms of a previously proposed mechanism.
Introduction The radiolysis of pure cyclohexane has been studied extensively because of its interest as a model compound, having only two types of chemical bonds. (Cf. recent papers by Freeman' and by Dyne and Stone2 for references to earlier work.) It also has been employed repeatedly as one component of the reactant mixture in energy transfer studies.3-6 Alkyl iodides have given especially G.R .
Freeman, J. Chem. P h p . , 83, 71 (1960). Can.J. Chem., 39,2381 (1961). (3) L. J. Forrestal and W. H. Hamill, J. Am. Chem. Soc., 88, 1535 (1961). (4) P. J. Dyne and W. M. Jenkinson, Can. J. Chem., 39, 2163 (1961). (1)
12) P. J. Dyne and J. A. Stone,
interesting results in such studies because they react with both electrons and hydrogen atom^.^^^^^ Previous studies of the radiolysis of cyclohexanealkyl iodide mixtures have been concerned with the production of radioactive alkyl iodides6 or gaseous products.~J Although the production of iodine in the radiolysis of such mixtures has been mentioned briefly,? no iodine yields were reported. In the experiments described here, G-values were obtained for the production of 1 2 in the radiolysis of methyl iodide-cyclohexane and ethyl iodide( 5 ) M. Burton, J. Chang, 9. Lipsky, and M. P. Reddy, Radiation Res., 8, 203 (1958). (6) R. R. Williams, Jr., and W. H. Hamill, ibad., 1, 158 (1954). (7) R. H. Schuler, J . Phys. Chem., 61, 1472 (1957).
NOV.,
1962
IODINE PRODUCTION IN
7-RADIOLYSIS OF CYCLOHEXANE-ALKYL IODIDE SOLUTIONS2189
cyclohexane solutions, over the concentration range 0-95ojO cyclohexane by volume in each case. Experimental
530
Phillip’ti “pure grade” cyclohexane was further purified 4 520 2 by stirring with fuming sulfuric acid, washing with water, 0) drying, dirJtilling on a 3-ft. column packed with glass helices (Todd still), and passing through SiOz before use. Alkyl iodides (Columbia Organic) were passed through alumina, distilled on the Todd still a t a reflux ratio of %5:1,and 510 p passed through alumina again before use. Individual 4V .. ml. samples were prepared volumetrically, dried with 1’206, degassed, transferred under vacuum to the irradiation vessels, and sealed off. The irradiation vessels were 13 X 500 ,“ 100-mm. test tubes with attached spectrophotometer cells. Irradiations werc performed a t room temperature using a ci modified Firestone-Willard type Cooo source of 400 c. which is described elsewhere.* The dose rate in the Fricke dosimeter was 0.895 x 1018 e.v./ml. min., takingG(Fe+s) = 490 15.6. Analysis of ferric solutions was done with a Beckman x DU spectrophotometer at 305 mp, using an extinction coefficient of 2201 a t 25” and a temperature coefficient of 0.7%/ deg.9 Dose rates were corrected for the differing values of 480 p , the linear absorption coefficient, for the various solutions. For CHJ and CzHsI, values of p(sample)/p(dosimeter) were calciilated to be 1.950 and 1.679, based on absorption 0 20 40 60 80 100 by Compton effect, and photoelectric effect. For cyclohexMole % cyclohexane. ane, the value 0.750 was obtained on the basis of electron density ratios. Relative absor tion figures for the interFig. 1.-Position of, , ,X for absorption of light by 1 2 mediate solutions were obtainejhy assuming an interpola- in various solutions of methyl iodide and ethyl iodide in tion linear in volume fraction. I n order to analyze for IZ cyclohexane. spectrophotometrically, the position of Xmsx and the corresponding extinction coefficients were determined, using a . a Beckman DU spectrophotometer with the cell compart5 1.4 ment maintained a t 25.0 S t 0.2’.
eo
3
!? -8 .e
ai
p
X
Results Graphs of A,, for iodine in methyl iodidecyclohexane and ethyl iodide-cyclohexane solutions are shown in Fig. 1. Extinction coefficients in the pure solvents were found t o be 1277 for CHJ, 1295 for CzHsI, and 940 for cyclo-CsHiz, in good agreement with earlier values.lO.ll Several experiments on the mixed solvent systems gave graphs of extinction coefficients us. mole fraction alkyl iodide-cyclohexane which deviated from an assumed linear interpolation by less than experimental error; extinction coefficients used in calculating G-values were obtained from the linear relations hip. Initial experiments t o determine 1 2 production vs. dose in methyl iodide-cyclohexane solutions gave very irreproducible results, especially for solutions with compositions in the vicinity of 80% cyclohexane, 20% methyl iodide by volume. However, it appears that this irreproducibility was caused by a little residual moisture, since more careful drying on the vacuum line cleared up the difficulty. It can be seen from Fig. 2 that graphs of iodine concentration vs. dose were essentially linear for all of the methyl iodide-cyclohexane solutions studied. The corresponding graph for the ethyl iodide-cyclohexane system looked very similar, and is not shown. Iodine production was linear with dose in that case also. The slopes of the various lines were converted into G-values (81 R. J. Hanrahan, Intern. J. A p p l . Radiation Isotopes, 13, 254 (1962). (9) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” I). Van Nostrand Co., Princeton, New Jersey, 1961, p. 21. (IO) E. 0. Hornig and J. E. Willard, J . Am. Chem. Soc., 79, 2429 (1957). (11) E. N. Weber, P. F. Forsyth, and R. H. Schuler, Radiation Res., 8, 68 (1955).
$
E“ 1.0
3
0.0
0
40
20
Dose,
60
e.v./ml. X
80
100
IO-’*,
Fig. 2.-Iodine production in the radiolysis of methyl iodide-cyclohexane solutions, measured by light absorption at wave lengths given in Fig. 1. Figures by each line indicate % cyclohexane by volume. Lines for solutions of 5, 10, 40, 60, and 95% were omitted to avoid crowding the diagram.
for iodine production, and the resulting values are shown as a function of reactant concentration in Fig. 3 . Discussion It can be seen from Fig. 3 that G(IJ is a smooth function of concentration (given in terms of electron % cyclohexane) for both of the alkyl iodide-cyclohexane systems studied, varying from a maximum value for the pure alkyl iodide t o zero when the alkyl iodide is diluted with 93 to 100 electron yo cyclohexane. Although the curves of Fig. 3 are similar to those which would result if iodine were produced directly from the alkyl iodide in proportion to its concentration, with the cyclohexane acting only as a diluent, such cannot be the case because free radicals from cyclohexane would certainly react with iodine from the alkyl iodide. It then becomes necessary to determine whether
2190
$
c
0.8
*\
2 0.0 -
0
10
20
30 40 Electron
THOMAS 5. CROFTASD ROBERT 8.HAYRAHAK
HI yield is so low that it may be ignored to a good
\
50
60
70
80
90 100
7a cyclohexane. Fig. 3.-G-values for Iz production as a function of composition for solutions of methyl iodide and ethyl iodide in cyclohexane.
p J
F 4 0
3 2
1 A 0
20
Vol. 66
G 40 60 Electron % cyclohexane.
80
100
Fig. 4.-Schematic representation, based on a simple free radical mechanism, of the dependence of radical yields (lines BC, AD, and CD) and total iodine yield (BE) for solutions of methyl iodide in cyclohexane. Ket iodine production is given by EB minus CD, and falls to zero for solutions richer in cyclohexane than point G. See text.
approximation. Then assuming that the various yields are directly proportional t o the electron % of the parent compound (Fig. 4), the total iodine yield (in equivalents) would decrease linearly from its value of 7.7 in pure methyl iodide and the radical yield would decrease from 5.3,12,14 both going to zero a t 0% methyl iodide. At the same time, the free radical yield from cyclohexane would increase from zero to its value of 5.6 for pure cyclohexane,l5 and the total radical yield from t)he two compounds would follow a linear interpolation between 5.3 and 5.6. I n Fig. 4 a graph constructed on this basis is shown, and it can be seen that the total free radical yield should exceed the iodine yield, eo that no net iodine could be produced, for all solutions richer in cyclohexane than about 30 electron %. The experimental results in Fig. 3 show that net iodine production persists until methyl iodide is diluted -with 93 electron % cyclohexane. I n a recent paper, Forrestal and Hamil13 presented considerable data on methyl iodide-cyclohexane and related systems, and reviewed earlier work. They measured CH4 and Hz yields as a function of CHJ concentration, both with added HI and Iz,and also measured H I and cyclohexene with Izpresent as a scavenger. Water was used to prevent HI from back-reacting. They concluded that hydrogen arises from cyclohexane in three ways: an ionic mechanism giving molecular H,, a hot atom abstraction reaction, and a thermal radical abstraction reaction. The latter both are assumed to result from initially excited cyclohexanel' CaHlz C6H12'
---t
CsHiz'
e-
+ e- --+C6H12***
C6H12*** -+ Hz
+ olefin
(1)
(2) (3)
C6H12 -3 C~HIZ** (4) the situation could be explained by assuming that the primary yields from the alkyl iodide and from CeH12** +C6IIii. E. (5) cyclohexane are proportional to the fraction of the energy absorbed by each (given presumably by its H - C6111z+Hz CsH1l. (ea) respective electron fraction), followed by interH a ?ti+H. Rl (Bb) action of radicals from both compounds with iodine. He C6Hiz +Hz CeIJii. (7) In considering possible mechanisms, the case of methyl iodide-cyclohexane will be discussed in Reaction 7 has an appreciable activation energy detail because it has been investigated previously and is subject to competition if other hydrogen by Forrestal and Hamill,3 and also because of com- atom scavengers are present. With Iz, €11, and plications arising in the radiolysis mechanism of CH,I reactions 8 -10 can occur other iodides. It has been shown p r e v i o ~ s l y l ~ ~ ~ ~ that the observed yield of 1 2 in irradiated alkyl H. 1 2 --+ HI 1. (8) iodides represents the excess of I. production (14) R. J. Hanrahan, Ph.D. Thesis, University of Wisconsin, 1957. over alkyl radical production in spurs (the diffrom University Mtcrofilms, Ann Arbor, Michigan. ference corresponding to the production of stable available (15) Based on iodme scavenBing work in this Laboratory by W. C. hydrocarbons), plus an added component due to Blasky, and ref. 16. (he reaction of thermal free radicals with HI. (16) R. W. Fessenden and R. €1. Schuler, J . A m . Chem. SOC.,79, In general, the role of H I must be considered in 273 (1957). (17) The asterisk designations on excited states are those of Fordetail, but in the particular case of CHJ, the restal and Hamill, ref. 3 . -4 further excited state, designated with
+
+
+
+
+
+
A m . Chem. Soo., 79, 2434 (12) R. J. Hanrehan and J. E. Willard (1957). (13) H. A. Gillis, R,R. Williams, Jr., and FV. H, Hamill, zbzd., 88, 17 f1961).
+ +
+
one asterisk by Forrestal and Ramill, is postulated by them to be produced with G = 0.75 and t o transfer energy either t o HI or CHsI causing them t o dissociate. It 18 postulated to deactivate with no net effect in pure cyclohexane.
Nov., 1962
IODINE PRODUCTIOX I N
R RADIOLYSIS
+ HI +Ha + 1. H. + CHJ +H I + CH3. H.
(9) (10)
A portion of reaction 10 may give methane by a diffusion controlled reaction between H I and CH,., If CH31 is the H . atom scavenger present in highest concentration, then reaction 10 is the predominant fate of the H . atoms. The fate of the resulting CH,. radicals, and of cyclohexyl or other radicals present, depends on what other scavengers are present. If 1 2 is the other scavenger, whether added initially or produced by radiolysis, these rl2actions occur
+ I, -+ CH,* +
CsH,,.
12
and if HI
LS
+ CH,I + 1.
C6H111 1.
(lla) (llb)
the other scavenger besides CHJ
OF
CYCLOHEXANE-ALKYL IODIDE SOLUTIONS 2191
the basis of mass spectroscopic evidence by Gillis, Williams, and Hamill13 CHJ+
+ CH3I + (C&)zIf + I .
(14)
Upon neutralization of the product ion there results a ‘(pocket)’ containing tn o methyl radicals and two iodine atoms which can undergo diffusion controlled recombination, giving a net yield of C2Hs and Iz. We now postulate that the rather efficient production of Iz in dilute solutions in cyclohexane is due to this same mechanism, reaction 14 and its consequences, and that it is able to occur under these circumstances with considerable efficiency because of charge transfer from cyclohexane C6H12+ -t CHJ +CaH12
+ CHJ+
(15)
Between 10 and 100 electron % methyl iodide, the residual free radical production from cyclohexane would fall to zero, and the efficiency of reaction 14 would approach its value in pure methyl iodide. (12b) -4lthough Forrestal and Hamill considered the In all caset?I atoms combine to form I,. charge exchange reaction (15) and found it comForrestal and Hamill found that their results patible with their data, they preferred to postulate with 0.1 electron 70 or less added CHSI could be that the effects observed in the 0.1-10 electron % explained by competitive hydrogen atom scaveng- CHJ range were to be attributed to the electron ing-reactions 7 and 10. They found that the capture process, reaction 13. The present results decrease in G(HJ is balanced by an increase in seem to require that the charge exchange process G(H1) as would be predicted from the reactions (15) definitely occurs to account for net iodine postulated. As the CHJ concentration is in- production, but do not exclude the possibility creased into the l-lOTo range, however, the H . that electron capture is occurring also. The most atom yield remains constant ’but the molecular likely situation, suggested also by Forrestal and Hz yield drops and is matched by an increase in Hamill, is that electron capture and charge exG(CH3.)--measured by converting it to CH4 change both develop in the 0.1-10% concentration with H I . They postulate that this behavior is range, and tend to promote each other. due to an interference in the ionic mechanism The situation regarding the alkyl radicalfor production of molecular hydrogen, reaction iodine balance in et>hyliodide must be similar to sequence (1,2,3) abox-e, due to dissociative electron the methyl iodide ease discussed above. Howcapture by C H d ever, an examination of the corresponding plot of G(12) us. concentration (Fig. 3) shows two regions eCH31--+ I- -I- CH3. (13) of markedly different slope. Furthermore, the The iodide ion presumably is neutralized without iodine yield apparently extrapolates to zero only at 0% of ethyl iodide. It is reasonable to assume giving any further effect. The present results concerning IZproduction are that these differences are due to differences in the not incompatible with the above reaction scheme. decomposition mechanism of methyl iodide as However, no combination of reactions 1 through compared with other alkyl iodides. I n the case of 13 gives a net production of I,. Most sequences methyl iodide, the key step leading to net I, of reactions which can be derived from (1-13) are production must be a bimolecular step, either hot neutral in 1 2 over-all, and the hot atom sequence radical or ion-molecule. Although such processes (5,6a) must consume Iz. The yield of cyclohexyl are possible and presumably also occur for ethyl radicals from this source is given as 2 X 0.85 = iodide and the higher iodides, in these cases it is 1.7 for pure cyclohexane, and if, as argued by possible to get a unimolecular production of HI Forrestal and Hamill, no energy transfer from the (either by a true molecular elimination, or by a parent excited state occurs, the yield in a solution caged radical-I atom reaction) which ultimately containing 7 electron VGmethyl iodide could be can lead to net 1 2 formation. Thus the persistence taken as 0.93 X 1.7 = 1.6. Since this value is of iodine production to lower concentrations of nearly 60% of the net iodine atom production rate ethyl than of methyl iodide, observed in the presin pure methyl iodide, it is clear that methyl iodide ent work, may be correlated with the much higher must be decomposing with high efficiency to give quantum yields obtained for ethyl than for methyl a break even point in iodine concentration a t 7 iodide in various photochemical experiments. electron % methyl iodide. -4s noted in the Introduction, the measurements The moi3t satisfactory proposal to date for ex- of 11 production reported here are related to some plaining 12 produetion in the radiolysis of pure earlier experiments of Schuler,’ in which it was CHJ is thLe ion-molecule mechanism proposed on found that dilute solutions of methyl or ethyl
+ H I +CsH12+ 1. CH,. + H I +CII, + 1.
C61-Il,.
+
(12a)
2192
RALPHE. WESTOS, JR.,AKD
iodide in cyclohexane produce methane and C p hydrocarbons by chain reactions under radiolysis, but that the chains do not develop in the presence of iodine, whether added initially or produced during radiolysis. In the case of methyl iodide a t 2 mole %, the chain reaction occurred uninhibited, but i t was quenched by 6 mole %. I n the case of ethyl iodide, on the other hand, as little as 0.3 mole yo dropped the yield of the chain process drastically. Converting from mole % to electron %, these results imply that net iodine production occurs with ca. 8 electron % or more methyl iodide, and with as little as 0.4 electron % ethyl iodide in cyclohexane, in good agreement with
Vol. A6
STSKLEY SELTZER
the present work. It should be noted that all of the experiments reported here were conducted with alkyl iodide concentrations sufficiently high to prevent the chain reaction. Acknowledgments.-Some preliminary data on the methyl iodide-cyclohexane system were taken by Miss Geraldine Restmoreland. Mr. Wm. C. Blasky aided in several phases of the experimental work. This research was supported by the State of Florida Nuclear Science Program. It was presented in Paper No. 119, Division of Physical Chemistry, 140th Kational Meeting of the American Chemical Society, Chicago, Illinois, September, 1961.
THE SECONDARY DEUTERIUM ISOTOPE EFFECT I N THE PYROLYSIS OF DIMETHYLMERCURY1 BY RALPHE. WESTON,JR.,AKD STASLEYSELTZER Chemistry Department, Brookhaven National Laboratovy, Upton, hrew York Received Mau 8 , 1908
The relative rates of the cyclopentane-inhibited pyrolysis of dimethylmercury and dimethylmercury-de have been measured a t 366'. The rate constant ratio is kn/kx = 1.072 & 0.009, This inverse isotope effect is attributed to an increase in C-H stretching frequencies of the transition state compared with the normal molecule, while the C-H bending frequencies remain unchanged. Rate constants for the pyrolysis of dimethylmercury a t 303-366" have been determined, and are in good agreement with values obtained previously. This is also true of the C13 isotope effect, which a t 366" is klz/kls = 1.0386 i 0.0007. It was found that methyl radicals did not exchange with dimethylmercury under the conditions of these experiments.
Introduction Within the past few years, several investigations of secondary &-deuterium kinetic isotope effects have been made.2 Such an effect is defined as the effect of deuterium substitution on the rate of a reaction of the type RR'C(H or D)X +RR'C(H or D)
+X
The observed effects have been attributed to a change in the hybridization of the carbon atom (to which the deuterium is bonded) from sp3 (tetrahedral) to sp2 (trigonal) in going from the ground state to the transition state.3 On the basis of the vibrational frequencies of stable molecules (such as aldehydes and alkenes) in which the carbonhydrogen bond is sp2 in nature, a large decrease in one of the C-H bending frequencies is expected, and this will produce a normal isotope effect. It was our expectation that a similar isotope effect mould be found in a reaction producing methyl radicals, and that the magnitude of this effect might give some information about the vibrational frequencies or configuration of the methyl radical. The reaction chosen for this study was the pyrolysis of dimethylmercury in the presence of excess cyclopentane. Both the kinetics of this reaction and the C13 isotope effect have been previously (1) Research performed under the auspices of t h e U. S. Atomio Energy Commission. t2) For a rpcent review, cf. R. E. Weston, Jr., A n n . Rev. Nuclear Sci., 11, 439 (1961). (3) Other interpretations of secondary a-deuterium effects have been presented hy: (a) M. Wolfsherg, 9. Seltzer, and R. 8. Freund, private communication; (b) L. S. Bartell, J . A m . Chem. Soc., 83, 3567 (1961).
studied by Russell and B e r n ~ t e i n . ~The mechanism proposed by these authors on the basis of their kinetic results is
+ CH3
Hg(CH3)Z + HgCH3 +Hg
+ Hg(CH3)Z CI-18 + CsHio CH3 + Hg(CH3)z
CH3
+ CH,
(1) (2)
+ CH3HgCHz (3) CH4 + CDHS (4) CzH6 + HgCH3 (5)
+CH4 +
+
+ CHs +CzHa CH3HgCHz +HgCH3 + CHz CH3
+
-
(6)
(7)
CJL
(8)
CsHg +Products
(9)
C H ~ C~IL
In the presence of a ten- to twentyfold excess of cyclopentane, it can be shown that reactions 3, 5, 6, 7, and 8 are negligible.4 The rate expression becomes simply -d [Hg(CHa)z]/dt
=
d [CH4]/2dt
=
ki [Hg(CH3)2]
so that the rate-determining step is the unimolecular decomposition of dimethylmercury. It is not possible to decide, on the basis of the kinetic results, whether step 2 is distinct from step 1, or whether (4) M. E. Russell and R . B. Bernstein, J . Chem. Pkvs., 30, 607, 613 (1959). These papers contain references t o earlier literature on the kinetics.
