Szwarc Transition

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

The transition states of free radical reactions generally show evidence of polar character wherein electron transfer to or from the radical has occurred (20). Thus, the electron affinity or ionization potential of the radical involved should affect the reaction. The much higher electron affinity (16) of ROo than CH3 radicals no doubt alters the transition state so that the reactivities toward it show less selectivity. The results of Szwarc and Binks (22) center around the fact that only carbon radicals were used for the correlation, and thus the electron affinity does not vary sufficiently to show in the correlation any deviation from the expected reactivity-selectivity relationship. 

The configuration of each monomer unit in the chain is thus fixed at its point of entry. A similar transition state was suggested by Szwarc (106) where the incoming monomer molecule approaches above the plane of the last unit of the polymer chain. Only isoprene in the cis-form would be capable of reaction. 1,2 addition is also possible with this mechanism but its occurrence is considered less likely owing to the greater electron availability at the terminal carbon atoms, and to the lower stability of the olefin formed. 

Another series of closely related reactions for which transition-state calculations have greatly helped in providing an understanding of the observed trends is the addition to deuterium-substituted alkenes. Szwarc and co workers (Feld et al., 1962) have determined secondary deuterium isotope effects for methyl and trifluoromethyl radicals by comparing the rate of addition to a terminal alkene with the rate for the deuterium-substituted alkene (25). Isotope effects for cyclopropyl radical addition have been measured by Stefani and coworkers (1970). For these three radicals a small inverse isotope effect (kJkK)... 

The approaches of Gloss, Kaptein-Oosterhoff, and Adrian are based on quantitative treatments of the microscopic behaviour of radical pain. Very siiAilar results can also be obtained from a simple kinetic model which involves formal rate constants for the processes of pair reaction from singlet states k, pair escape k, and singlet-triplet transitions kf . Replacement of the actual pair behaviour by a simple kinetic scheme may be an oversimplification, thou it seems justified by the results of Szwarc and co-workers who showed that pair reaction and escape may be described to a good approximation in terms of simple first-order processes. 

There is another question that has to be discussed the direction of an attack in the S), 2 reaction vs. the position of the anion in the onium salts. Indeed, the simple and attractive picture proposed by Szwarc in order to explain the differences between the reactivities of maaoanions and macroion pairs in the anionic homopropagation of styrene is based on the assumption that an ion pair has to dissociate partially when the transition state of propagation is reached ). Szwarc, after observing a similar reactivity of the polystyryl anion and polystyryl cesium ion pair, also assumed that no partial dissociation was needed for the large Cs cation. The... 

Matsuoka and Szwarc (1961) photolyzed diazomethane in the presence of isooctane and styrene in one case, and in the presence of isooctane and styrene-a,j8,j8-d3 in the other case, and thus determined the methyl affinities of styrene and of trideuteriostyrene with respect to isooctane by the method of competing reactions. The isotope effect ( d/ h) was 1-07-1 ll. Matsuoka and Szwarc used these data to justify the conclusion that in radical addition reactions the initial structure of a reactive center is preserved in the transition state. In styrene and in trideuteriostyrene the reactive centers are both terminal carbon atoms doubly bonded to another carhon. In the product the reactive carbon atom becomes tetrahedral, and thus the transition state could conceivably resemble either reactant (trigonal) or product (tetrahedral). It can be calculated, however, that a change of configuration from trigonal to tetrahedral in the transition state should exhibit an isotope effect kj)jk ) of about 1-8 in the reactions studied by Matsuoka and Szwarc the tetrahedral configuration was therefore excluded by these authors. 

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