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Zjawiska dysocjacji i asocjacji towarzyszące reakcjom przeniesienia protonu między C-kwasami i zasadami organicznymi w rozpuszczalnikach aprotonowych

Nowak I., Binkowska I., Stańczyk-Dunaj M., Jarczewski A.

Wiadomości Chemiczne

2008

[Z] 62, 1-2

67-95

The proton transfer reactions in solutions have attracted the attention of a number of laboratories for many years. These processes seem to be quite simple as they proceed without perturbation of bounding electrons and have low steric demands what enables study of their mechanism. However, many factors influences the mechanisms of these reactions. Among them and not the trifling ones, are dissociation-association effects accompanying them. The definition, behavior and reactivity of carbon acids (C-acids) depending on the type of electron withdrawing group are given [1-57]. Also the values of dissociation constants of C-acids in protic and aprotic solvents are collated in Tables 4, 5 and 7 [11, 38, 49]. The nature of products, free ions and ion pairs, of proton transfer reactions of various C-acids and strong organic bases are carefully discussed. Also their spectral characteristics are given. The equilibria and the possible routes between the substrates and the products of these reactions are also shown [58-66]. The reasons of the homoconjugation effects as a result of an association of the ionic products of the proton transfer reactions in different solvents are discussed in the context of their solvation abilities [67-86]. The formation constants of the homoconjugation complexes of amidine and guanidine bases in relation to their pK? values are quoted in Table 6 [87-95]. The nature of heteroconjugation complexes formed as the result of hydrogen bonding of different entities in the system of proton transfer reactions is discussed [96-116]. The pK? values of a number of C-acids, derivatives of nitromethane, measured in acetonitrile by potentiometric method, are given in Table 7. The pK? values of these derivatives measured in DMSO/H2O systems are compared with those obtained in acetonitrile [18, 38]. The general view of these problems is discussed and carefully reviewed.

tert-Butylimino-tris(dimethylamino)phosphorane as a Proton Acceptor in the System of Reaction of 1-Nitro-1-(4-nitrophenyl)alkanes in THF

Wisłocka I., Nowak I., Jarczewski A.

Polish Journal of Chemistry

2006

Vol. 80, nr 12

2017-2029

Unexpectedly the reaction of deprotonation of carbon acids with phosphazene bases occurred very slowly. The kinetic study of the proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of R = H, Me, Et, i-Pr substituents as 4-nitrophenylnitromethane (1), 1-nitro-1-(4-nitrophenyl)ethane (2), 1-nitro-1-(4-nitrophenyl) propane (3), 2-methyl-1-nitro-1-(4-nitrophenyl)propane (4) and the tert-butylimino-tris(dimethylamino)phosphorane (5) named P1-t-Bu phosphazene is elaborated. The reactions have been studied in THF under pseudo-first-order conditions. The product of the proton transfer reactionwith P1-t-Bu in THF appeared to be associated into ion pairs. The equilibrium constants range from >100000 to 11.8 decreasing along with growing bulk of alkyl substituent in the reacting C-acid. The second order rate constants (k2H) are rapidly declining: 9360, 2.31, 0.66, 0.09 dm3 mol–1 s–1for 1, 2, 3, and4 respectively, and could not be accounted for the small values of the enthalpies of activation HH = 6.1, 18.0, 20.7 and 11.1 kJ mol–1. The reactions show negative and relatively large values of the entropies of activation SH = –149.7, –176.5, –178.7, –227.8 J mol–1 deg–1. The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, kH/kD = 15.8, 13.6, 13.2 for 1,2, and 3, respectively. The results have been discussed in terms of the influence of steric effects brought by the bulk of alkyl substituents in the C-acid and the base on formation of the transition state. Also the influence of traces of residual water present in the reaction system has been taken into consideration.