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Wize University Chemistry Textbook > Introduction to Organic Chemistry

IR Spectroscopy

Properties of Functional GroupsPrevious Section
Nomenclature [hydrocarbons]Next Section
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IR Spectroscopy

Since we measure absorptions characteristic to certain bonds, IR spectroscopy is a great way to determine which functional groups are present in a molecule.
Wize Concept
We will just focus on memorizing a few functional groups from this table. If you see a more complicated problem on your exam, your prof will provide you with a table similar to the one shown below (usually found on a formula sheet).

Important functional groups to know:
  • OH
  • C=O

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General Trends in IR Peaks

Hybridization affects bond strength! Stronger bonds have vibrate at higher frequencies (they have higher wavenumbers).
sp(C−H)3300 cm−1>sp2(C−H) 3100 cm−1>sp3(C−H) 2900 cm−1\rm sp(C-H) 3300\ cm^{-1} > sp^2(C-H)\ 3100\ cm^{-1} > sp^3(C-H)\ 2900\ cm^{-1}

Vibrations of bonds involving heavier atoms have lower vibrational frequencies (they have lower wavenumbers).
v(C−H)>v(C−C)>v(C−O)>v(C−Br)v({\rm C-H}) > v({\rm C-C}) > v({\rm C-O}) > v({\rm C-Br})

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  • We often use a unit called wavenumber (cm−1\rm cm^{-1}) to represent the energy of vibrational absorbances
ν~=1λ\tilde\nu=\dfrac{1}{\lambda}
  • IR spectroscopy generally covers the 500-4000 cm−1\rm cm^{-1} range.
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Example: IR Spectra

The IR spectrum for cyclohexanone is given below. Which functional groups can you identify from this data? How?

There are sp3 C-H bonds which we can see for the peak(s) around 2900cm-1.
There is a C=O which we can see by the peak at 1700 cm-1

Practice: IR Spectra

Based on the IR spectrum below, what is the most likely empirical formula for this molecule?


Practice: IR Spectra

Identify all the functional groups in the following IR Spectrum:

Practice: IR Spectroscopy

The "Brazilian blowout" hair treatment, now renamed the "keratin smoothing" treatment after bad press, contains the chemical methylene glycol. This treatment was found to release a toxic chemical called formaldehyde due to a reversible reaction between methylene glycol and water. The reaction is detailed below.



IR Spectra Cheatsheet

O-H vs C-H vs COOH

OH is broad and to the left of 3000 cm-1
C-H is the sharp band just right of 3000 cm-1

COOH is much more broad than an O-H and overlaps all of the C-H stretch,
We can also see the C=O stretch at ~1800 cm-1 . This is a key feature of all carbonyl compounds.
Do not mistake this for C=C which are small and subtle (and easier to identify using NMR anyway)