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.
Nov., 1962
SECOXDARY
DEUTERIUM ISOTOPE EFFECT I N PYROLYSIS OF bIMETHYLMERCURY
both carbon-mercury bonds are broken simult aneously . One can measure the isotope effect in step 1. by allowing a mixture of two isotopic species of dimethylmercury to pyrolyze competitively, and then comparing the isotopic composition of the methane produced with that of the original dimethylmercury. Experimenedl Dimethylmercury was prepared according to the method of Gilman and Brown.6 After the solvent (ether) was dis-
tilled through a W i n . bubble-plate column, the residue was distilled through a 12-in. platinum-spiral vacuum-jacketed cohmn. The fraction boiling at 92" was collected. Examination of this fraction with the Perkin-Elmer vapor fractomet,er, Model 154C, disclosed the presence of two impurities: ether (0.4%) and an unknown material with a boiling point at approximately 60" (0.1 7 , ) . Further purification was necessary; therefore, the dimethylmercury was chromatographed in 4-cc. portions on a 12-ft., 5/8-in. i.d. column composed of 35y0 Dow Corning 710 silicone oil on ,Johns-Manville C 22 firebrick (30-70 mesh) a t 110' and a helium pressure of 20 p.s.i. Under thePo conditions, dimethylmercury a,ppeared after 23.5 min. The effluent vapor waEi condensed in a trap cooled t o -78". Some decomposition occurred on the passage through the hot thermal conductivity cell, m evidenced by the appearance of droplets of mercury. The organic decomposition products are, however, non-condensable gases. A further distillation was performed to free the dimethylmercury of any silicone oil that might have been carried through. An infrared spectrum, after this purification, compared favorably with that reported by Gutowsky.6 Dimethylmercury-d6 was prepared from methyl-& iodide (99.370 D, Merck and Co. Ltd.) and purified in the same manner as described above. Cyclopeintane (Phillips research grade) was chromatographed in 5-cc. portions a t 75" on the same macro column as that used for the purification of dimethylmercury. The first 60% of each sample was distilled, so that the cyclopentane was freed of small amounts of impurities, as shown by chromatography in the Perkin-Elmer vapor fractometer. Reaction Vessel.-The reaction vessel was a 440-cc. Pyrex bulb of roughly cylindrical shape, connected to the vacuum manifold through a section of 1-mm. capillary tubing and. a mercury cut-off valve of the type described by Miller, et al.' The vessel was contained inside a copper rylinder, on which a heating element was wound, and this in turn was inside an insulated box. The temperaturecontrolling circuit depended on a copper-constantan thermocouple as the sensing element, and has been described rlsewhere.8 Temperature fluctuations at the center of the vessel were &0.3", but there was also a spatial inhomogeneity of temperature, with a maximum difference of 3" between th.e end of the vessel and the center. This should not produce a measurable error in the isotope effect determinations. S o correction to the rate constant was made for the volume of the reaction vessel outside the oven, which amounted to
