Free-Radical Reactivity with Theoretical Calculations

Comparison of Free-Radical Reactivity with Theoretical Calculations 

Free-radical reactivity of thiazole has been calculated by semiempirical methods, and results free valence and localization energy) have been compared with experimental data. For mono- and dimethylthiazoles the radical localization energy of the unsubstituted position may be correlated with the logarithm of experimental reactivity (180, 200). The value of the slope shows that a Wheland-type complex is involved in the transition state. 

The thiazolyl radicals are, in comparison to the phenyl radical, electrophilic as shown by isomer ratios obtained in reaction with different aromatic and heteroaromatic compounds. Sources of thiazolyl radicals are few the corresponding peroxide and 2-thiazolylhydrazine (202, 209, 210) (see Table III-34) are convenient reagents, and it is the reaction of an alky] nitrite (jsoamyl) on the corresponding (2-, 4-, or 5-) amine that is most commonly used to produce thiazolyl radicals (203-206). The yields of substituted thiazole are around 40%. These results are summarized in Tables III-35 and IIT36. 

TABl E UI-34. PRODUCTS FROM 2-THIAZOLYLHYDRAZlNF, AND THlAZOTE CARBONYI PEROXIDES (2 D 

TABLE 111-35. ISOMER RATIOS ANP RELATIVES RATES FOR HOMOLYTIC 2-THlAZOLYLATTON OF ALKYLBENZENES (207) 

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D. Comparison of Free-Radical Reactivity with Theoretical Calculations... 

This order of reactivity was observ for add dedeuteration, but for acetylation, formylation, and chlorination it was slightly different thieno[3,2-h]thiophene (2) > thieno[2,3-h]thiophene (1) > thiophene thieno[3,4-6]thiophene (3) was not studied. A substantially greater discrepancy between theoretical and experimental data was observed for nucleophilic substitution from the data on base dedeuteration and competitive metalation reactions/ the order of decreasing reactivity was as follows thieno[2,3-h]thiophene (1) > thieno[3,2-h]thiophene (2) > thiophene. To a certain extent this may be explained by differences in the mechanism of metalation and deuterium exchange with a base. A discrepancy between calculation and experiment was also found for free-radical substitution. ... 

The theoretical models discussed above are frequently employed in the description of the kinetics of gas-phase reactions, especially reactions of atoms and free radicals. This class of reactions is of interest in a broader scientific context, and a better understanding of their mechanism is of primary importance for the development of chemical modeling. Free atoms and radicals are very reactive species, which occur in and take part in many different reaction systems. Therefore, a radical reaction usually proceeds in competition with a few parallel or subsequent processes. The kinetic behavior of the reaction system may be very complicated and difficult for quantitative description. Theoretical investigations of the reaction kinetics provide information useful for a better understanding and correct interpretation of experimental findings. Results of ab initio calculations are employed to evaluate the rate constant in terms of the computational methods of the reaction rate theory. 

Quantum chemistry provides a powerful tool for studying kinetic and mechanistic problems in free-radical polymerization. Provided a high level of theory is used, ah initio calculations can provide direct access to accurate values of the barriers, enthalpies, and rates of the individual reactions in the process, and also provide useful related information (such as transition structures and radical stabilization energies) to help in understanding the reaction mechanism. In the following, some of the applications of quantum chemistry are outlined. This is not intended to be a review of the main contributions to this field, nor is it intended to provide a theoretical account of reactivity in free-radical polymerization (108). Instead, some of the types of problems that quantum chemistry can tackle are described, with a view to highlighting the potential of quantum-chemical calculations as a tool for studying free-radical polymerization (see Radical Polymerization). 

The experimental values of V- and M-centred triads from [ H] triads were compared with the theoretical values from Harwood s [171] statistical model using copolymerisation reactivity ratios. The reactivity ratios for free-radical solution copolymerisation of V with M were calculated using the Kelen-Tudos (KT) [172] and the nonlinear error in variables (EVM) [173] methods using the RREVM [174] program. Homonuclear H-2D-COSY and 2D-NOESY NMR of the copolymer sample were recorded for determining the interactions between different protons in the copolymer chain. 

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