Organic Chemistry Solutions: #24

24.* (1991 1 3) Of the following two structures, which is more stable? Why?

For #1, the symmetry of the molecule dictates that the lone pairs on the two nitrogens must be in the same type of orbital. Either both lone pairs are in p orbitals (8 pi electrons) or they are both in sp³ orbitals (4 pi electrons). In the former case, the molecule is antiaromatic due to its 4n pi electrons. In the latter case, the molecule is non-aromatic because not all of the atoms in the ring have p orbitals.

For #2, each nitrogen is sp² hybridized. The lone pairs of the nitrogens are in sp² hybrid orbitals. The p orbital of each nitrogen contains one pi electron, as does the p orbital of each of the carbons. Therefore, the structure has a total of 6 pi electrons and is consequently aromatic.