All of the following can be described as an aromatic heterocycle EXCEPT:

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11ecba2d_127d_ee07_a3bf_57734214b769_MD0008_00 Aromatic heterocycles are commonly found in biological systems. A heterocycle is any cyclic compound containing at least one heteroatom (ie, an atom other than carbon or hydrogen). To be classified as aromatic, a compound must have: Conjugated pi bonds in a cyclic structure Unhybridized p orbitals present in each atom in the ring Planar geometry, forming a continuous ring of parallel, overlapping unhybridized p orbitals 4n + 2 pi electrons (Hückel rule), where n is a non-negative integer Tryptophan, thymine, and adenine are all considered aromatic heterocycles because they have nitrogen-containing rings (indole, pyrimidine, and purine, respectively) with conjugated pi bonds and overlapping unhybridized p orbitals (Choices A, C, and D). Although thymine may not appear to have conjugated pi bonds, a resonance structure can be drawn to show a conjugated system similar to pyrimidine. Basic nitrogen atoms, such as in pyrimidine and purine, have a double bond placing the lone pair electrons in an sp² orbital perpendicular to the conjugated p orbitals containing the pi electrons. Electrons in orbitals perpendicular to the conjugated plane cannot undergo side-to-side overlap with the p orbitals and therefore do not participate in aromaticity. Nonbasic nitrogen atoms, such as in purine and indole, have three bonds, putting the lone pair of electrons into a p orbital parallel to other pi electron-containing p orbitals. Parallel orbitals permit side-to-side overlap; therefore, these electrons do participate in aromaticity. As a result, thymine has 6 pi electrons, and tryptophan and adenine each have 10 pi electrons; all satisfy Hückel rule. (Choice B) Proline can be classified as a heterocycle, but it is not considered aromatic because it is not conjugated and does not contain any pi electrons. Educational objective: To be considered aromatic, a compound must be planar and cyclic with conjugated pi bonds. Each atom must have parallel, overlapping unhybridized p orbitals. Finally, an aromatic compound will satisfy Hückel rule and have 4n + 2 pi electrons.

All of the Following Can Be Described as an Aromatic