Hybrid orbitals in aromatic heterocycles

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. 

Pyrimidine is a six-membered heterocycle with two nitrogen atoms situated in a 1,3- arrangement. Both nitrogen atoms are like the pyridine nitrogen. Each has its lone pair of electrons in the sp2 hybrid orbital in the plane of the aromatic ring. These lone pairs are not needed for the aromatic sextet, and they are basic, like the lone pair of pyridine. 

Important criteria for heterocycles include hybridization, electronegativity, and stereochemistry. For thiophene, the matter is further complicated by the question whether the 3d-orbital of sulfur also participates in bond formation. If so, then the two resonance structures 119 and 120 in Scheme 7 should also be considered in determining the degree of aromaticity. 

We can visualize these heterocycles as similar to the simpler aromatic systems pyrrole, furan and thiophene. For example, in imidazole, each carbon and nitrogen will be sp hybridized, with p orbitals contributing to the aromatic rt system. The carbon atoms will each donate one electron to the rt system. Then, as in pyrrole, the NH nitrogen supplies two electrons, and, as in pyridine, the =N- supplies one electron and retains a lone pair. Oxygen or sulfur would also supply two electrons, as we saw in furan and thiophene. 

Hiickel s rule (1931) for planar species states that if the number of tt electrons is equal to 2 + 4n, where n equals zero or a whole number, the species is aromatic. The rule was first applied to carbon-containing monocyclics in which each C is capable of being -hybridized to provide a p orbital for extended it bonding it has been extended to unsaturated heterocyclic compounds and fused-ring compounds. Note that benzene corresponds to n = 1. 

Furan, thiophene, pyrrole, and pyridine are all examples of heterocyclic aromatic compounds (heteroaromatic compounds). The heteroatoms in some of these compounds (furan, thiophene, pyrrole) contribute one lone pair to the aromatic system, whereas in others (pyridine) they contribute none. You can determine how many lone pairs a heteroatom contributes to the aromatic system by examining the effect of lone-pair donation on the hybridization of the heteroatom. For example, if the N atom of pyridine used its lone pair to participate in resonance, it would have to be sp-hybridized (one p orbital required for the N=C n bond, one for the lone pair used in resonance), but sp hybridization requires 180° bond angles, which are not possible in this compound. Therefore the N atom must be sp2-hybridized, and the N lone pair must be in a hybrid orbital that is orthogonal to the cyclic array of p orbitals. In pyrrole, by contrast, if the N atom uses its lone pair in resonance, the N atom must be sp2-hybridized, which is reasonable. Therefore, there is a cyclic array of p orbitals in pyrrole occupied by six electrons (two from each of the C=C it bonds and two from the N lone pair), and pyrrole is aromatic. 

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