Example Infrared Spectra

The spectra that follow have been extracted chiefly from the database maintained by the National Institute of Standards and Technology (NIST). Any annotations are provided by me.

The first spectrum is 1-hexene:

The key peaks are marked; later, we will recognize the strong absorption about 890 cm-1 as the C-H out of plane bend for a terminal alkene.

Next is trans-2-hexene:

The C=C absorption is much weaker, and would get weaker still if we put a third alkyl group on the double bond. The C=C-H stretch is hidden; don't count on always seeing it. Again, later, we will recognize the strong peak at about 970 cm-1 as the alkene CH out of plane for a trans alkene.

For comparison, here is the cis isomer:

The C=C stretch is again weak, but the C=C-H stretch is visible. The peak at around 700 cm-1 is diagnostic for a cis alkene.

Next, a typical ketone, 2-hexanone:

The carbonyl stretch is the only significant absorption. Reduction of the ketone leads to the alcohol, 2-hexanol:

The small absorption at about 1700 cm-1 suggests that a very small amount of the ketone may be a contaminant.

Here is a typical aldehyde, butanal:

In addition to the carbonyl stretch, aldehydes have the very characteristic doublet at 2720 cm-1 and 2820 cm-1 for the aldehyde H out of plane bending.

Finally, an ester:

Now let's try a few problems. You probably will not be able to get a single correct answer for most of these, but try to generate at least one valid possible answer.

Compound 1: C5H10O

Compound 2: C8H8O

In addition to a C=O absorption near 1700 cm-1 and the benzene C-H stretch just above 3000 cm-1, the very good resolution allows us to see the requisite two peaks (730-770 and 690-710 cm-1) for a monosubstituted benzene.

Compound 3: C7H14O

Compound 4: C6H14O

Compound 5: C5H10O2

We will work some of these in recitation and post answers before the exam.


This page last modified 9:46 AM on Tuesday October 8th, 2013.
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