THE Hg 6(3P1) PHOTOSENSITIZATION OF

T H E Hg 6(3P1) PHOTOSENSITIZATION O F SPIROPENTANE

G. R. DE MAR& L. G. WALKER,0. P. STRAUSZ, AND H. E. GUNNING

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 117.169.1.37 on 08/26/15 For personal use only.

Departinetzt of Clzenzistry, University of Alberta, Edmonton, Alberta Received October 13, 1965 ABSTRACT The triplet mercury photosensitization of spiropentane affords ethylene, allene, and methylenecyclobutane a s major reaction products. This indicates a n olefinic-type interaction with the quaternary C-C u-bond of spiropentane, the cleavage of which results in a triplet state diradical. The initially formed diradical may undergo isomerization through ring cleavage or deactivation by collision. T h e open chain diradical then either dec~mposesunimolecularly into CeI-14 C3H4 or on collisional relaxation isomerizes to methylenecyciobutane. The paraffinic-type interaction between Hg* and spiropentane which would lead to C-H bond scission appears to have minor significance.

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INTRODUCTION

Our interest in the mercury photosensitization of spiropentane arose from our current investigations on the analogous reactions of cyclopropane. This topic has recently been reviewed (1); therefore only a very brief outline will be given here. Cyclopropane, in most of its reactions, exhibits an intermediate behavior between olefins and paraffins. In its interaction with I-Ig* atoms the final reaction products can equally well be accounted for by an olefinic- or paraffinic-type reaction (2,3). T h e olefinic interaction would imply quenching into the C-C a-bond with the formation of a vibrationally excited triplet trimethylene which then may partly decompose into allyl H. Alternately paraffinic interaction through C-H bond cleavage could lead to the formation of vibrationally excited cyclopropyl radicals which may subsequently isomerize to allyl either uni~nolecularlyor on combination with other radicals. This study was undertaken in the hope that with a more complex cyclopropane derivative such as spiropentane some of the complications mentioned above could be circumvented and a clear distinction made between the two possibilities.

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EXPERIMENTAL T h e reaction system and the method used to determine the fractional absorption of the resonance radiation have been described in a previous paper (4). T h e light intensity, measured by propane actinometry a t 730 Torr propane pressure, was 1.42 X 1016 quanta/s. Spiropentane was prepared by the method of Applequist et al. (5) and purified extensively. A neat sample gave a single resonance on a n A60 nuclear magnetic resonance instrument a t maximuin intensity, verifying the excellent purity. Nitric oxide (Matheson C.P.) was purified a s previously described. Propylene (Phillips research grade) and propylene-1,l-dz (Merck) were degassed a t -196" and distilled from -130". Ethylene and allene were identified by their mass spectra and relative gas chromatographic retention times. They were determined quantitatively on a 16 f t column of 30% tricresyl phosphate on 30160 mesh firebrick a t 75'. Methylenecyclobutane was identified by its relative gas chromatographic retention time a n d mass and nuclear magnetic resonance spectra and was determined quantitatively on a dimethylsulfolane column. RESULTS A N D DISCUSSION

The major products of the reaction were ethylene, allene, and methylenecyclobutane. Minor products were hydrogen (@ E 0.04 a t 50 Torr), two isomers of the substrate, and a t longer exposure times, a product of mass 70, and a Clo compound. T h e yields of ethylene and allene were examined as a function of exposure time a t a substrate pressure of 53.0 f 1.5 Torr (see Fig. 1) and a s a function of substrate pressure Canadian Journal of

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(see Fig. 2). The yield of methylenecyclobutane as a function of substrate pressure was measured in separate experiments and is also shown in Fig. 2, along with the sum of the yield of ethylene and methylenecyclobutane.

25

50

REACTION TIME, MIN

75

100

SPIROPENTANE PRESSURE, TORR

FIG. I. Quantum yield of ethylene V and allene 0 as a function of reaction time. FIG.2. Dependence of the product yields on spiropentane pressure: ethylene V, allene 0, methylenecyclobutane 0 ,and (ethylene methylenecyclobutane) 0.

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The effect of added olefin and nitric oxide on the reaction was checked and it was found that besides their function as an inert gas they have no effect on the major reaction products. The following mechanism is proposed to explain the results.

Steady-state treatment gives the following relation for the ratio of product yields:

A plot of the left-hand side of [I] vs. substrate pressure is shown in Fig. 3. A satisfactory straight line is obtained. From the slope then kS/k4 = 5.55 X mm Hg-1. If k5 is set

DE

MAR^

ET XL.: ~g

0(3~,)PI-IOTOSEKSITIZATIOS

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OF S P I R O P E X T A ~ ~ E

equal to 1 X 10-lo cc/molecule s (ca. the collision frequency) then k4 = 5.8 X l o 8 s-' and T ~ ,the lifetime of the 1,4-diradical, 2-methylenebutane in this system, is equal to 1.7 x S. AISO [I11

l/@(C21-14) = l/@(C3H4) = (k4

+ k~[?VI])(k3+ ks[R/ll)/@(l)k3k4

where @(I)is the quantum yield of step [I]. Equation [I]]reduces to


l/(a(CzH,)

=

l/@(C3EI4) = 1/@(1)

a t zero pressure. Plots of l/@(C2H4)and l/@(CsH4) VS. substrate pressure are shown in Fig. 4. Both lines extrapolate t o 2.28 a t zero pressure giving @(I)= 0.44, which agrees well with the experimental value of 0.45 obtained from the sum of molecular split and methylenecyclobutane formation (cf. Fig. 2 ) , and which is unaffected by pressure over the range studied. The actual value of @ ( I ) ,o~vingto its exposure time dependence (cf. Fig. I ) , is somewhat uncertain and is probably about 10-15% higher than given above, t h a t is, ca. 0.50.

100

200

300

100

SPIROPENTANE PRESSURE, TORR

200

300

SPIROPENTANE PRESSURE. TORR

vs. spiropentane pressure. FIG. 3. Plot of R(~neth~lellec~clobutane)/R(ethylene) FIG. 4. Plot of l/+(C?H.,)and l/+(C3H4)0 VS. spiropentane pressure.

If now i t is assumed that k5 = kg = 10-lo cc/molecule then from [II] a value of ca. 10-lo s is obtained for the lifetime of the diradical, and therefore collisional stabilization should not be important for this speciks over the pressure range studied. The quantum yield of step [2] is ca. 0.04 as determined from the yield of the Hz product. Step [7] is probably more rapid with spiropentane than with cyclopropane but selfscavenging of 1-1 atoms by C2I-I4 and C3H4 still inay be significant. In the cyclopropane reaction the main mode of f-I-atom disappearance is self-scavenging followed by polymerization, and EIz actually is a very minor product (2,3). T h e estimation of (a(2) from the Hz yield alone in the present system is quite uncertain since self-scavenging may also lead t o polymer formation. However, in contrast to the cj,clopropane case polymer forination could not be observed in the spiropentane system, consequently polymerization via selfscavenging cannot be important. The two unidentified isomers of spiropentane formed in very small yields (@(combined) < 0.05) could have been either vinylcyclopropane, cyclopentene, or a pentadiene. Both of the major reactions, the molecular decomposition into ethylene and allene, [4], and the isoinerization to methylenecyclobutane, [5], require no hydrogen migration in contrast t o other possible but apparently less probable reactions. The thermal reaction of spiropentane in the temperature interval of 360 and 410 OC has also been reported (6) to yield the same kind of products, CZH4 C3H4 and methylenecyclobutane, as the photosensitization. The isomerizatioil is truly unimolecular xith an

[)\;

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activation energy of 57.6 kcal/mole and a pre-exponential factor of 1015.8