Kinetic study of the cyclohexadienyl radical. Part 2.—Patterns of combination and disproportionation with the methyl, ethyl, sec-propyl and tert-butyl radicals

Abstract

The pattern of interaction of the cyclohexadienyl radical with each of a series of primary, secondary products CH3. MeCHz-MezCH. M e P and tertiary alkyl radicals R* is given by the percentages of the products :28 34 57The almost constant ratio of the products of combination implies that this pattern is determined almost exclusively by the properties of the cyclohexadienyl radical, and we have proposed that combination at the para position is favoured because the spin density is highest there. Conversely, the monotonic increase of the percentage of disproportionation with the degree of substitution at the a carbon atom of the allcyl radical parallels the behaviour of the alkyl radicals in mutual interaction. The significance of these results is discussed in relation to current models of the transition state.

A kinetic study of the reactions of the hydrogen atom with the cyclohexadiene isomers in the gas phase

“…1.0 X lo4, and 0.7 x lo4 l./mole-sec at 25°C for metathesis between the methyl, ethyl, and isopropyl radicals, respectively, and cyclohexadiene-l,4 [23]. The comparatively high values of kl!…”

“…and ks probably reflect the usual trend that a reaction of the hydrogen atom has a higher preexponential factor and a lower energy of activation than the corresponding reaction of an alkyl radical. The ratio of the metathetical rate constants k5/k5, = 6.8 is remarkable as AH," -AHSO = -0.3 kcal/mole, but it is paralleled by the values of 2.8 and 2.5 for the ratios of the rate constants for matathesis between the ethyl and isopropyl radicals, respectively, and cyclohexadiene-l,4 and cyclohexadiene-l,3 at 25°C [23,29,30].…”

“…Table I1 shows that the change in each of the five main parameters is minor wheny is increased from 0 to 0.10, but the decrease in Rs/R7 is more rapid as y is increased, and may be used in a subsidiary test. The corresponding ratios Rsa/R7, were measured by James and Suart [22,23] for the generalized reactions of the cyclohexadienyl radical ( 6 4 t 7a)…”

“…If the pattern were ruled solely by the ratio of the number of equivalent sites, ks/k7 would be 0.5; and for similar reasons each value of ks,/k7, for all the systems would be 0.5. The actual values are significantly greater, and the unconsidered factor is probably the electrophilic nature of the hydrogen atom [24] and the alkyl radicals [23]. Evidence for the electrophilic nature of the hydrogen atom is derived from quantitative studies of its addition reactions, both in the gas phase [25] and in aqueous solution [26].…”

“…The interaction of the conjugated isomer cyclohexadiene-1,3 with the hydrogen atom involves 84% of addition; this offers a striking parallel to the interactions of this isomer with the ethyl radical and the isopropyl radical, which involve 83% and 87% of addition, respectively [28,29]. Abstraction accounts for 8'2% of the interaction of the unconjugated isomer cyclohexadiene-l,4 with the hydrogen atom, and predominates in the interactions of methyl, ethyl, isopropyl, and tert-butyl radicals with this isomer, although the percentage contributions are unknown [22,23,30,31]. Cyclohexadiene-1,4 is thus a notable exception to the usual pattern of predominant addition observed for the interaction of hydrogen atoms with olefins at room temperature.…”