Question

How do you determine if a compound has an aromatic ring?

Answer 1

If you are talking about the experimental identification of a benzene ring, I would use a combination of infrared and NMR spectroscopy.

Explanation:

Infrared spectroscopy

IR spectroscopy alone may be sufficient to identify an aromatic ring.

Some of the characteristic absorptions are

`color(white)(m)barν"/cm"^"-1"color(white)(mll)"Vibration mode"`
`stackrel(———————————————-)("3100-3000"color(white)(m)"Aromatic C-H stretch")`
`"2000-1665"color(white)(m)"Weak overtones and combination bands"`
`"1650-1450"color(white)(m)"Medium to strong ring vibrations"`
`color(white)(ll)"900-675"color(white)(ml)"Out-of-plane (oop) C-H bending"`

The overtone and oop absorptions have characteristic appearances depending on the pattern of ring substitution.

Compare the IR spectrum of toluene.

(From Quora)

It shows

• Aromatic `"C-H"` absorptions at `barν > "3000 cm"^"-1"`
• 4 weak overtone peaks between `"2000 cm"^"-1" and "1665 cm"^"-1"` (monosubstituted phenyl
• Strong aromatic absorptions between `"1650 cm"^"-1" and "1450 cm"^"-1"`
• Strong oop absorptions at `"725 cm"^"-1" and "694 cm"^"-1"` (monosubstituted phenyl)

If IR spectroscopy is not conclusive, you can add NMR spectroscopy.

`""^1"H"` NMR spectroscopy

Hydrogens directly attached to an aromatic ring appear at about 7 ppm - 9 ppm, as in the `""^1"H"` NMR spectrum of toluene.

(From rod.beavon.org.uk)

`""^13"C"` NMR spectroscopy

Aromatic carbons usually absorb in the 125 ppm to 150 ppm range, as in the spectrum of toluene below.