How to Deal with Local Aromaticity in Organic Chemistry – Part 1

Global aromaticity refers to measures that characterize the entire molecule while local aromaticity is used to assess the aromatic character of individual rings within a polycyclic system.


Local aromaticity is crucial because different rings can have vastly different levels of stability and reactivity.

Global aromaticity measurements assess the collective π-e─ system and are useful for comparing the overall stability of different aromatic compounds and include energetic measures of aromaticity (resonance energy, later aromatic stabilization energy), anisotropy of magnetic susceptibility and magnetic susceptibility exaltation.

Clar’s rule (empirical) states that given a polycyclic aromatic hydrocarbon, the resonance structure most important to characterize its properties is that with the largest number of aromatic π-sextets i.e. benzene-like moieties.

The local aromaticity is evaluated through the geometry-based harmonic oscillator model of aromaticity (HOMA), calculated nucleus-independent chemical shift (NICS), measured proton nuclear magnetic resonance (1H-NMR) chemical shifts.

In cheminformatics toolkits, the distinction of global vs. local aromaticity also affects the detection of aromaticity.

Question:

Which of the rings in Phenanthrene is less aromatic?

Solution infographic:



Click/tap to enlarge the image

How to deal with annulation in organic chemistry? Click HERE!

Affinity for aromaticity❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1) Wikipedia Contributors. Aromaticity. Wikipedia. https://en.wikipedia.org/wiki/Aromaticity.

(2) Graham, T. W.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry.; Hoboken, Nj John Wiley Et Sons, Inc, 2016.

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