Pyridines orbital picture

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

Problem 1S.4 Pyridine is a flat, hexagonal molecule with bond angles of 120°. It undergoes electrophilic substitution rather than addition and generally behaves like benzene. Draw an orbital picture of pyridine to explain its properties. Check your answer by looking ahead to Section 15.7. 

The aromatic five-membered heterocyde imidazoie is important in many biological processes. One of its nitrogen atoms is pyridine-like in that it contributes one w electron to the aromatic sextet, and the other nitrogen is pyrrole-like in that it contributes two IT electrons. Draw an orbital picture of imidazole, and account for its aromaticity. Which atom is pyridine-like and which is pyrrole-like ... 

The molecular orbital picture of pyridine is shown in Figure 6.7. There is a difference between this picture and that for benzene. In benzene, the HOMO consists of two filled orbitals of equal energy (hence degenerate), but in pyridine there is only one filled HOMO. Importantly, none of the orbitals are degenerate. 

The orbital pictures for benzene (Sec. 4.4 and Figure 4.2) and pyridine are similar. The nitrogen atom, as with the carbons, is sp -hybridized, with one electron in a p orbital perpendicular to the ring plane. Thus, the nitrogen contributes one electron to the six electrons that form the aromatic pi cloud above and below the ring plane. On the other hand, the unshared electron pair on nitrogen lies in the ring plane (as with the C—H bonds) in an sp orbital. 

As with pyridine, these heterocycles can be thought of as derived by replacing an aromatic CH group by N. The unshared electron pair on this nitrogen (at position 3) is therefore not part of the 677 aromatic system, as seen in the orbital picture for imidazole in Figure 13.4. 

Figure 25-2 (A) Orbital picture of pyridine. The lone electron pair on nitrogen is in an sp -hybridized orbital and is not part of the aromatic 7T system. (B) The electrostatic potential map of pyridine reveais the location of the lone electron pair on nitrogen (red) in the molecular plane and the electron-withdrawing effect of the electronegative nitrogen on the aromatic tt system (green compare to the electrostatic potential map of pyrrole in Section 25-3).

We can draw Frost circles (see Section 2.9.3) to show the relative energies of the molecular orbitals for pyridine and pyrrole. The picture for pyridine is essentially the same as for benzene, six jt electrons forming an energetically favourable closed shell (Figure 11.1). For pyrrole, we also get a closed shell, and there is considerable aromatic stabilization over electrons in the six atomic orbitals. 

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... 

Pyridine is the prototypical electron-poor six-membered ring heterocycle. The aromaticity originally found in the benzene framework is maintained in pyridine via overlap with the unhybridized p orbital found on the sp hybridized nitrogen atom that is parallel to the Ji-system of the carbon framework. The resonance pictures, as well as, the natural atomic charges of pyridine (Fig. 4.2), predict its electron deficient nature. 

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