Disproportionation and combination reactions of simple alkyl radicals: methyl, ethyl, n-propyl, and isopropyl

Abstract

Rates of the possible cross-disproportionation reactions of these radicals relative to their rates of cross-combination, have been measured and are compared with auto-disproportionation-to-combination ratios which were also determined for these radicals. Selected asymmetric azo alkanes RNNR′ and mixtures of symmetric azo alkanes RNNR were used as sources of the desired free radicals. Photolysis under relatively high intensity conditions (ca. 1018 quanta/cm2 s) at room temperature generated the radicals in sufficiently high concentration that competing reactions of abstraction and condensation were negligible. Both the alkene and alkane products of disproportionation reactions were measured to ensure the completeness of the mechanism invoked and to increase the accuracy of the results. Δ(Me, Et) = 0.036 ± 0.003, Δ(Me, Prn) = 0.058 ± 0.004, Δ(Me, Prs) = 0.163 ± 0.005, Δ(Et, Prn) = 0.065 ± 0.005, Δ(Et, Prs) = 0.18 ± 0.008, Δ(Prn, Et) = 0.057 ± 0.005, Δ(Prs, Et) = 0.12 ± 0.01, and Δ(Prn, Prs) = 0.41 ± 0.02 were deduced from the data, where Δ(A, B) refers to the ratio of disproportionation producing the alkane product derived from A to the combination of A with B.

Kinetics of the Mercury-Photosensitized Decomposition of Propane. Part II. Reactions of the Propyl Radicals

“…Insertion of the k7/k4 and k,/k, ratios into eq. 2 leads to which agrees very well with the value of Terry and Futrell (2).…”

Quantum yield of Hg 6(3P1) sensitization of hydrogen

“…Reaction [7] would require excess energy in Hz* of at least 65.8 kcal, the endothermicity of the reaction. Since, however, hydrogen atom addition to ethylene has an activation energy of a few kcal, and Hz addition should have an even Can.…”

“…The steric factor of reaction [7] cannot be higher than that of the abstraction reaction by ethyl radicals from Hz, l o p 2 (9) and therefore practically all Hz" would be relaxed by collisioll with Hz before reaction with ethylene could take place. Consequently, under the prevailing experimental conditions the possibility of reaction [7] can be dismissed.…”

“…Consequently, under the prevailing experimental conditions the possibility of reaction [7] can be dismissed.…”

“…'Reaction [7] can also be compared to the abstraction reaction by ethyl radicals from hydrogen. The Arrhenius parameters of this latter reaction are: log A = 9.6 (1 mole-' s-') and E, = 13.7 kcal(9).…”