Aromatic Chemistry

A Historical Perspective

The term "aromaticity" derives from the word aroma, which refers to the natural fragrance of certain compounds like benzaldehyde, that smells of almonds. Aromatic compounds have been designated as a class of their own due to their unusual stability and unreactivity, despite being primarily composed of unsaturated hydrocarbon.

During the 1800s, analysis of the combustion of benzene lead to several hypotheses as to its chemical structure. It was finally Kekulé who came up with the first correct structure for benzene. He correctly defined it as a resonant structure of alternating single and double bonds, a theory that supports the experimental data of benzene having six bonds of equal length. The bond lengths of benzene have been measured to be 1.39 Å (c.f. 1.34 Å for C=C and 1.54 Å for C-C ).

The stability of benzene was demonstrated by its unreactivity towards bromine water, a regent that typically adds across double bonds.
Instead, bromine can be added to benzene by means of the following Friedel-Crafts reaction:

Benzene: Chemical and Physical Properties

Heats of Hydrogenation

Quantifying the heats of hydrogenation of benzene-like structures demonstrates benzene's unique stability.

In other words, the resonance for hexadiene is worth about 8 kJmol^-1 (120x2 - 232)
and the resonance for benzene is worth about 152 kJmol^-1 (120x3 - 208)

NMR Ring Currents

Within an NMR is an applied field, that generates a ring current of electrons over the benzene molecule. This is due to the electrons being delocalised and being free to move around the plane of the molecule. The NMR magnetic field causes the electrons to start spinning in a distinct direction, thus inducing an electric field. This causes the protons (on the outside) of the benzene ring to become very deshielded (low electron density), while the inside of the benzene ring becomes shielded (high electron density). For larges aromatic molecules such as porphyrins, protons will be present on the inside of the ring - these protons will be shielded, and the outside ones will be deshielded, as they are in benzene.

On an NMR spectrum, this leads to the protons on the outside of an aromatic ring lying in the 6 - 8 parts per million (ppm), downfield from the standard.