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

1 Answer
Dec 26, 2016

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.

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.

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.

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