- Radical ions in photochemistry. Carbon-carbon bond cleavage of radical cations in solution: Theory and application
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The cleavage of radical cations of two series of alkanes, 1,1,2-triaryl- and 1,1,2,2-tetraarylalkanes, generated by photoinduced single electron transfer in acetonitrile-methanol, occurs with formation of radical and carbocation fragments. The radical cations of some unsymmetrically substituted alkanes cleave to give all four of the possible products, two hydrocarbons emanating from the radicals and two methyl ethers from the carbocations, in proportion to the oxidation potentials of the two possible radical fragments. There is an excellent linear correlation between the logarithm of the observed ratio of products and that calculated from the reported electrochemically determined oxidation potentials (r = 0.998, 5 points). The proportionality constant (1.27) for this relationship is close to unity which indicates that the product ratio is determined by the relative rates of cleavage in the two possible modes or by equilibration of the radicals and carbocations before separation of the geminate radical carbocation pair and not by equilibration upon reencounter of freely solvated radical and carbocation fragments. The effect of temperature on the relative oxidation potentials of the radicals studied is small and can be neglected when radicals of the same order (i.e., both secondary or both tertiary) and of similar size are compared. The ratio of products obtained upon cleavage of the radical cation at 25 °C can be used to determined standard oxidation potentials of radicals. The oxidation potential of the diphenylmethyl radical (0.350 V vs SCE) has been accepted as the primary standard and the (4-methyl-phenyl)phenylmethyl (0.265 V) and bis(4-methylphenyl)methyl (0.188 V) radicals are established as secondary standards against which the oxidation potentials of other radicals can be measured. Oxidation potentials of several 4-substituted cumyl radicals have been determined by this photochemical method. There is a good (r = 0.987, 5 points) linear correlation between the measured oxidation potentials and the σ+ substituent constants. The reaction constant is appropriately negative and large (p = -6).
- Popielarz,Arnold
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p. 3068 - 3082
(2007/10/02)
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- Azole derivatives
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Azole derivatives of the formula SPC1 Wherein R1 is free or esterified carboxyl or other functionally modified carboxyl group, R2 and R3 each are aryl; A is Cn H2n in which n is an integer from 1 to 10, inclusive; and Z is O or S; and the physiologically acceptable salts thereof, possess, with good compatibility, excellent antiphlogistic activity and, in particular, influence favorably the chronic progressive diseases of the joints, e.g., arthritis. They can be prepared from compounds of the formula SPC2 Wherein X1 is a group convertible into the group --S--A--R1, and R2 and R3 have the values given above.
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