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Wiadomości Chemiczne

Tytuł artykułu

Reakcje przeniesienia protonu od C-kwasów do zasad organicznych w rozpuszczalnikach aprotonowych

Autorzy Jarczewski, A. 
Treść / Zawartość
Warianty tytułu
EN Proton transfer reactions between C-acids and organic bases in aprotic solvents
Języki publikacji PL
EN The proton transfer reactions in solutions seem to be simple, yet a number of consecutive steps can be distinguished. The useful tool in determining the mechanisms of these reactions was, and still is the primary deuterium kinetic isotope effect]. Therefore the factors influencing the rate constants and then KIE of these reactions as: steric hindrance of reactants, polarity of solvents and symmetry of transition state were widely discussed. The commonly acepted view on the deuterium kinetic isotope effects is that they are large when bulky substituents are crowded around the reaction site], and also the reactions are carried out in low polarity solvent]. However, there was found that even in polar aprotic solvents as acetonitrile large KIE values take place]. The KIE values are not very sensitive on the steric hindrance, showing in some extreme cases reduced KIE values, what is incompatible with previous papers. Likewise some other effects as "scrambling effect" simulating very large KIE are considered . The products of proton transfer reactions between C-acids and strong organic bases in polar aprotic solvents in contrary to previous findings appeared to be highly dissociated into free ions. The reason and consequences of erroneous fulfillment of Benesi-Hildebrand equation are discussed. It was essential to carry out the kinetic study of the proton transfer reactions, in particular those of low equilibrium constants in a BH+/B buffer. Then the association effect should be considered. Also a number of homoconjugation constants for selected organic bases and equilibrium acidities of some C-acids in acetonitrile solvent are given. The study of proton transfer reactions in mixed H2O-Me2SO solution prevented the homoconjugation effects and gave the thermodynamic and kinetic acidity, what enabled to evaluate the intrinsic barriers or intrinsic reactivity. To assess the aci-nitro behavior of C-acids the protonation reaction of 4-nitrophenylnitromethane carbanion at different pH values were performed. The influence of residual water which cannot be removed completely from the reaction systems of C-acids and strong organic bases in aprotic solvent was a subject of interest for many years. After careful examination it appeared that the traces of water in reaction medium, in aprotic solvents, caused a marked decrease of reaction rate instead of its acceleration [97]. It was clamed that the reactivity of ion pairs is negligible compared to free ions. However, in some cases the reactions are going via ion pair reagents with distinct differentiation of reactivity between loose and tight ion pairs . Using literature values of the fractionation of cesium n-propoxide for ions and ion pairs, 2.5 times larger reactivity has been found for ion pairs than free ions in proton abstraction from 1-(4-nitrophenyl)-1-nitroethane. The recent progress in the study of proton transfer reactions indicates new aspects in understanding the mechanism and theory of these processes.
Słowa kluczowe
PL reakcja przeniesienia protonu  
Wydawca Polskie Towarzystwo Chemiczne
Czasopismo Wiadomości Chemiczne
Rocznik 2000
Tom [Z] 54, 3-4
Strony 203--226
Opis fizyczny wykr., bibliogr. 118 poz.
autor Jarczewski, A.
  • Zakład Chemii Ogólnej, Wydział Chemii, Uniwersytet im. A.Mickiewicza ul. Grunwaldzka 6, 60-780 Poznań
Kolekcja BazTech
Identyfikator YADDA bwmeta1.element.baztech-article-BUS1-0008-0060