SECONDARY ISOTOPE EFFECT IN THE ADDITION OF HYDROGEN ATOMS TO PROPYLENE-d₆

Abstract

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Reaction of H and D atoms with deuterated propylenes

“…Using an extrapolation technique, Takahasi and Cvetanovic [3] found abstraction from CD3CDCD2 to be favored over CH3CHCH2, in contrast to the present observations, and also found 20-fold more abstraction than the present experiments indicate. However, using direct mass-spectrometric determination of the H D to H 2 ratios in the same study, Takahasi and Cvetanovic [3] found 20 times smaller rates of abstraction than by the kinetic extrapolations.…”

“…Although only small differences, within experimental uncertainty, exist among CH3CHCH2, CD3CHCH2, and CD3CDCD2 in which either none or all of the olefinic hydrogen atoms have been substituted, the direction of the slight shifts is consistent with those observed for CH&DCH2 and CHaCHCD2 if substitution of the methyl group has an effect paralleling 2-substitution. These changes are in the same direction and of the same magnitude as those found by Takahasi and Cvetanovic [3] for the overall rate of addition.of H atoms to fully deuterated propylene, by Penzhorn and Darwent [14] for H atom addition to CzD4, and by Matsuoka and Szwarc [15] for the addition of methyl radicals to CsH5CDCD2. The smallness of the isotope effect for methyl radical addition was cited as evidence that the configuration of the transition state was changed only slightly from that of the initial state [ 151.…”

“…However, using direct mass-spectrometric determination of the H D to H 2 ratios in the same study, Takahasi and Cvetanovic [3] found 20 times smaller rates of abstraction than by the kinetic extrapolations. This reduced role for abstraction is more consistent with the present results.…”

“…Deuterium substitution at the @-carbon atoms, known to be the principal sites of abstraction [l], will reduce the rate of abstraction from the 0-carbon atoms, reflecting the slight increase in bond strength of C-D bonds as compared with C-H bonds [2]. Further, the rate of addition to the double bond will be enhanced by deuteration owing to a secondary isotope effect [3] which will most likely be maximized if substitution is at the a-carbon atoms.…”

“…Since Takahasi and Cvetanovic [3] found complete deuteration of propylene to enhance the rate of addition by 8y0 relative to light propylene, this might be expected to be the order of magnitude for changes in the relative rates of addition to the two olefinic sites.…”