Spectroscopy Tutorial: Aromatics

IR Spectroscopy

Aromatics show a lot more bands in an IR spectrum than do alkanes, alkenes, and alkynes. The C-C stretching vibrations show up as several bands in the region 1600-1585 and 1500-1400.

One of the most “telling” bands is the presence of a band just to the left of 3000. Alkyl groups show up to the right of 3000, aromatic C–H stretches to the left of 3000.

NMR Spectroscopy

Aromatic protons show up from 6.5-8.5 ppm. Benzylic protons are from 2–3 ppm.

Monosubstituted rings will have 5 protons in the region 6.5-8.5 ppm; disubstituted rings will have 4 protons; trisubstituted rings will have 3 protons (and so on). Examples of the NMR of aromatics of mono-, di-, and tri-substituted aromatics are shown below. When interpreting the spectrum of an aromatic compound, remember to count the number of protons in the aromatic region to determine how many times the ring is substituted.

If an aromatic ring has more than one substituent, careful analysis of the shifts and splitting pattern of the protons in the aromatic region reveals the positions on the ring of the different substituents. However, the the shifts depend on the substituents and the splitting is not really first order. Most sophomore-level courses do not cover NMR spectroscopy in the depth required to analyze the aromatic region. Therefore, you will not be able to designate the exact ring substition in many cases. In the specific case of disubstituted aromatic rings, para-substituted rings usually show two symmetric sets of peaks that look like doublets. The para-substitution NMR aromatic region pattern usually looks quite different than the patterns for both ortho- and meta-substituted aromatic rings.

Examples of ortho, meta, and para substitution are illustrated in the NMR spectra of different isomers of chloronitrobenzene, below. The CDCl3 peak is pointed out in each spectrum. (The samples were run using CDCl3 as the solvent, and a small contaminant of this deuterated solvent is CHCl3, which shows up at 7.24 ppm. This is used to calibrate the spectrum.) Note the symmetry of the para substituted chloronitro benzene. (Click on each full-size image to view details of the region from 6.5-8.5 ppm.)

click on each NMR spectrum below to see an expanded view of the aromatic region



In the organic chemistry teaching labs at CU Boulder, you will NOT be held responsible for figuring out whether a disubstituted aromatic ring is ortho, meta, or para. You should take a stab at assigning it either para OR meta or ortho, but you probably have not been taught enough to properly analyze aromatic splitting patterns.

If you are interested in learning details about aromatic splitting and shifts, consult either your TA or one of the references listed in the reference section.