Stable Free Radicals. III. Rates of Reaction of l-Ethyl-4-carbomethoxypyridinyl with Halocarbons

Edward M. Kosower, Irving Schwager

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The rate of halogen abstraction by the stable free radical, l-ethyl-4-carbomethoxypyridinyl, has been measured for the halocarbons, CH2Cl2, CH2Br2, BrCH2Cl, ICH2C1, CH3I, CHCl3, CDCl3, and CCl4. The rates are extremely sensitive to the nature of the halocarbon, a factor of 108 separating CH2Cl2 and CCl4. The sensitivity of the rate to the nature of the halogen, the yield of pyridinium halide (ca. 50%), the small secondary isotope effect (kH/kF = 1.21 for CHCl3), the small solvent polarity effect on the rate, and the formation of dihydropyri-dines support a three-step mechanism: (1) Py- + XC → PyX(covalent) + -C; (2) Py- + C → PyC; and (3) PyX → Py+X-. The negligible solvent effect on the reaction of the radical with CH2Br2 over a Z-value range of 15 demonstrates that the transition state for bromine atom transfer has the same charge separation as the initial state. The contrast between this conclusion and that expected for a contribution of "pyridinium ion stabilization" to the transition state led to the re-evaluation of polar factors in some radical reactions as being attributable to ground state (initial state) effects, as suggested by Semenov, as well as transition state effects. The kinetic data and some simple assumptions lead to maximum values for the stabilization of the product radicals relative to the methyl radical: CH2C1, 6 kcal./mole; -CHCl2, 10 kcal./mole; and-CCl3, 17 kcal./mole.

Original languageEnglish
Pages (from-to)5528-5535
Number of pages8
JournalJournal of the American Chemical Society
Issue number24
StatePublished - 1 Dec 1964
Externally publishedYes


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