Pyridine atom polarization

Af-Oxidation of pyrazines appears to result in increased shielding of the a and a carbon resonances by 6-11 p.p.m., whereas the /3 and /3 carbon atoms are deshielded by 3-4 p.p.m., a trend similar to that observed with substituted pyridines. These results have been qualitatively explained in terms of resonance polar effects (80OMR(l3)l72). 

Predict whether each of the following molecules is likely to be polar or nonpolar (a) C HjN (pyridine, a molecule like benzene except that one —CH— group is replaced by a nitrogen atom) (b) C2H6 (ethane) (c) CHC1, (trichloromethane, also known as chloroform, a common organic solvent and once used as an anesthetic). 

Inductive or polar effects. These effects involve electron displacements that are transmitted along a chain of atoms without any reorganization of the formal chemical bonds in the molecule. For example, the introduction of a methyl group in a pyridine ring involves a displacement of electrons to the nitrogen atom from the methyl group. This effect falls off rapidly with separation distance. 

The dipole moment of phosphabenzene is reinforced by the methyl group shown in (147) and increases it from 1.46 to 1.77 D thus it resembles pyridine, which has the heteroatom at the negative end of the dipole.181 The magnitudes and trends of the dipole moments of the methylphosphines have been investigated by MO studies and the dipoles partitioned into bond moments, bond polarization, and lone-pair moments.182 The reciprocal effects of the double bond and the phosphorus atom in... 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

It should be mentioned that in many cases it turns out to be impossible to isolate the bimetallic mixed-ligand complex. The isolable reaction products appear to be homometallic heteroleptic species. The composition of the reaction products is often dependent on the polarity and donor properties of the solvent as the molecules of the latter compete with the bridging alkoxo-, car-boxylato-, or p-diketonatoligands for the place in the coordination sphere of metal atoms. Thus if the reaction of Pb(OAc)2 with Sn(OBu )4 mentioned above is carried out in pyridine, it gives Sn(OBu )3(OAc)Py and Pb(OAc)(OBu ) as the major products and not the bimetallic complex [298]. Sn(OBu )3(OAc)Py turns also to be the major product of the reaction of Sn(OBu )4with Me3SiOAc... 

---