Hybrid orbitals in oxygen and nitrogen

Phosphorus and sulfur are the third-row analogs of nitrogen and oxygen, and the bonding in both can be described using hybrid orbitals. Because of their positions in the third row, however, both phosphorus and sulfur can expand their outer-shell octets and form more than the typical number of covalent bonds. Phosphorus, for instance, often forms five covalent bonds, and sulfur occasionally forms four. 

Qualitative application of VB theory to molecules containing second-row elements such as carbon, nitrogen, and oxygen involves the concept of hybridization, which was deveioped by Linus Pauling. The atomic orbitals of the second-row elements include the spherically symmetric 2s and the three 2p orbitals, which are oriented perpendicularly to one another. The sum of these atomic orbitals is equivalent to four sp orbitals directed toward the corners of a tetrahedron. These are called sp hybrid orbitals. In methane, for example, these orbitals overlap with hydrogen Is orbitals to form CT bonds. 

The empty oxygen 2p orbital is made available when the oxygen electrons themselves become sp hybridized we get three filled sp hybrid orbitals, and an empty 2p atomic orbital, just as in the case of nitrogen. 

The molecular orbital description for the nitrite ion just presented was developed without the aid of symmetry considerations and as a starting point, it assumed that a bonds were formed from sp2 hybrid orbitals on the nitrogen and oxygen atoms. Lei us now see how we could have obtained a similar end result by using a method that involves a more formal application of symmetry and does not invoke hybridization. (For a review of symmetry in bonding, see Chapter 3.) 

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. 

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