Pyridine, electronic structure

Pseudothiohydantoine, as acyclic intermediate, in reaction of o-halogeno acids or esters with thiourea, 232. See also 2-Amino-4-hydroxythiazole Pyridazines (4,7-dioxo-4,5,6,7-tetrahydro-thiazolo[4,5d]), preparation of, 206 Pyridine, electronic structure, 36, 39, 46 ultraviolet absorption, 47 thiazolylation of, 373 2-(4-Pyridyl)-4-carboxyethylthiazoie, from thioisonicotinamide and ethyl bromopyruvate, 198... 

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

Theoretical calculations of the electronic structure of phosphorin indicate that the Tr-charge-distribution is different to that of pyridine and cannot be explained by simple resonance theory. 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

Pyridine has tt electron structure similar to that of benzene. Each of the five 5 p -hybridized carbons has a p orbital perpendicular to the plane of the ring. Each p orbital has one tt electron. The nitrogen atom is also sp -hybridized and has one electron in the p orbital. So, there are six tt electrons in the ring. The nitrogen lone pair electrons are in an sp orbital in the plane of the ring and are not a part of the aromatic tt system. 

Dewar and Maitlis143 discussed quite successfully the course of nitration in series of pyridine-like heterocycles in terms of the Dewar reactivity numbers. There is a continuing interest in the electronic structure of pyridine65, 144-140 a model of this compound has been studied by the ASP MO LCAO SCF (antisymmetrized products) method in the 77-electron approxition.146 The semi-empirical parameters146 were obtained from the most recent values of ionization potentials and electron affinities, and bicentric repulsion integrals were computed theoretically. 

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