Question

What are they typical stretches, absorbances, peaks and resonances you would expect to see on the NMR, IR, UV/Vis and Mass Spec of a molecule containing an aromatic ring?

Answer 1

Aromatic compounds spectroscopy:

>NMR - The NMR shift for the aromatic hydrogen is around 7-9 ppm. This is due to the ring current of benzene that creates local magnetic field that has the same direction of the external magnetic field. Hence the chemical shift value is downfield.

>IR -

The C-H stretching for C=C-H is observed at 3000-3100 cm^-1.

The C-C stretching vibrations of the same will be around 1600-1585 cm^-1 and 1500-1400 cm^-1.

Overtone (weak) that distinguishes aromatics is 2000-1665 cm^-1.

>UV/Vis- Comparing lower to higher conjugated aromatic rings, as the conjugation and resonance increases, the bands will show a Bathochromic or Red shift.

>Mass Spectroscopy- Aromatic nucleus created m/e around 77, 65, 63, etc generally.