Heterolytic and homolytic bond dissociation energies of the N-H bonds in Ylide precursors and properties of the corresponding nitrogen-centered radicals  

作　　者：程津培 刘博 李钟 还振威 

机构地区：[1]Department of Chemistry. Nankai University. Tianjin 300071, China

出　　处：《Chinese Science Bulletin》1995年第16期1338-1341,共4页

基　　金：Project supported by the National Natural Science Foundation of China and the Fund for Excellent Young University Teachers,SEDC.China.

摘　　要：It is well known that the reaction pattern and reactivity of Ylides are related to thestabilities of the corresponding organic anions and therefore to the pK_a values of theirprecursor molecules. However, water, the most commonly used medium for deriving theacidities for most inorganic acids, is not a good solvent for determining the pK_a’s ofIt is well known that the reaction pattern and reactivity of Ylides are related to the stabilities of the corresponding organic anions([1,2]) and therefore to the pK(a) values of their precursor molecules. However, water, the most commonly used medium for deriving the acidities for most inorganic acids, is not a good solvent for determining the pK(a)'s of Ylide precursors because of the differential solvation effect([3]) and sometimes of unexpected side-reactions of water with the substrates. It was established only in recent years that dimethyl sulphoxide (DMSO) is an excellent medium for precisely determining the intrinsic free-ion pK(a) values, which covers a much wider pK range (0 to 33) than that in water (0 to 14)([3]). In this note we report, for the first time, the equilibrium acidities (eq.(1)) of a group of nitrogen acids in DMSO solution, where the nitrogen atom is adjacent to triphenylphosphonium moiety. No pK(a)'s of the nitrogen-centered Ylide precursors of the type R(3)P(+)-NHR (R=alkyl) appeared to be available in literature. The pK(a) values determined in this work therefore not only serve as the first collection of the data for this general type of compounds, but also provide a base for investigating stabilities of the corresponding Ylide anions. [Ph(3)P - NHR](+) + DMSO reversible arrow(Ka) Ph(3)P(+) - N(-)R + DMSOH(+) The homolytic bond dissociation energies (BDE) of the N-H bonds in the Ylide precursors are also a much desired quantity and can easily be obtained using the method (eg. (2)) reported in an early paper where the pK(a) value is utilized([4]): BDE(kJ/mol) = 5.707pK(a) + 96.48E(ox)(N-) + 308.4. The thermodynamic stabilities of the corresponding nitrogen-centered radicals are discussed accordingly based on the relative BDE values.

关 键 词：PKA Wide bond DISSOCIATION energy NITROGEN radical. 

分 类 号：O621[理学—有机化学]