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Aromaticity emerged in the mid-nineteenth century to distinguish unsaturated hydrocarbons from benzene. By the late nineteenth century, cyclicity was thought to be essential for this differentiation. However, early in the twentieth century, it was discovered that cyclooctatetraene displayed properties typical of polyenes, challenging this assumption. A key characteristic of benzene-like compounds, often linked to aromaticity, is their low reactivity. This led to the definition of thermodynamic stability as resonance energy, the first quantitative measure of aromaticity. Various theoretical approaches have since been developed to estimate this quantity, which is now considered fundamental. Around the same time, magnetic susceptibility was also employed to describe aromaticity, resulting in concepts such as the nucleus independent chemical shift (NICS) and Fowler’s maps of ring currents, which effectively assess aromaticity. Schleyer defined aromaticity based on these criteria, stating that compounds with significantly elevated diamagnetic susceptibility are aromatic. Cyclic delocalization can lead to bond length equalization, abnormal chemical shifts, magnetic anisotropies, and unique chemical and physical properties reflecting energetic stabilization.
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Aromaticity in heterocyclic compounds, Tadeusz Marek Krygowski
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- Année de publication
- 2009
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