Pyridine

Structure of Pyridine :

The structure of Pyridine is completely analogous to that of benzene, being related by replacement of CH by N. The key differences are : (i) the going away from perfectly regular hexagonal geometry caused by the presence of the heteroatom, in particular the shorter carbon-nitrogen bonds, (ii) the replacement of a hydrogen in the plane of the ring with an unshared  electron pair, likewise in the plane of the ring, located in an sp2 hybrid orbital and not at all involved in the aromatic π - electron sextet; It is this nitrogen lone pair which is responsible for the basic properties of Pyridines, and (iii) a strong permanent diople, discovered to the greater electronegativity of nitrogen compared with carbon.

It is important to realise that the electronegative nitrogen causes inductive polarisation, mainly in the σ - bond framework, and additionally stabilize those polarised mesomeric contributors in which nitrogen is negatively charged - 8,9, and 10 - which, together with contributors 6 and 7, which are strictly analogous to the Kekul'e contributors to benzene, represent Pyridine. The polarised contributors also imply a permanent polarisation of the π - electron system.

The polarisations resulting from inductive and mesomeric effects are in the same direction in pyridine,, resulting in a permanent diople towards the nitrogen atom. This also means that there are fractional positive charges on the carbons of the ring, located mainly on the α - and γ - positions. It is because of this general electron-deficiency at carbon that pyridine and similar heterocycles are referredd to as 'electron-poor' , or somethings 'π - deficient'. A comparison with the diople moment of piperidine, which is due wholly to the induced polarisation of the σ - skeleton, gives an idea of the additional polarisation associated with distortion of the π - electron system.

Thank you