IB Chemistry SL

Revision Notes

11.1.4 Infrared Spectroscopy

How IR Spectroscopy works

  • Infrared (IR) spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies
  • A spectrophotometer irradiates the sample with electromagnetic waves in the infrared region and then detects the intensity of the wavelength of IR radiation which goes through the sample
  • All organic molecules absorb IR radiation and depending on which energies of radiation are absorbed, bonds between atoms will vibrate by stretching, bending and twisting
  • The molecules will only vibrate at a specific frequency
  • The resonance frequency is the specific frequency at which the molecules will vibrate to stimular larger vibrations
  • Depending on the rest of the molecule, each vibration will absorb specific wavelengths of IR radiation which are also shown as the reciprocal of the wavelength
    • This unit is called the wavenumber (cm-1)
  • Particular absorbance have characteristic widths (broad or sharp) and intensities (strong or weak)
    • For example, hydrogen bonds cause the O-H bonds in alcohols and carboxylic acids to be broad whereas the C-O bond in carbonyl (C=O) groups have a strong, sharp absorbance peak
  • The energies absorbed by different functional groups are given as a range and an unknown compound can be identified by comparing its IR spectrum to the IR spectrum of a known compound

Absorption range of bonds table

Analytical Techniques Table 1_ Interpreting IR Spectra, downloadable AS & A Level Chemistry revision notes

 

  • Due to some absorption bands overlapping each other, other analytical techniques such as mass spectroscopy should be used alongside IR spectroscopy to identify an unknown compound

Interpreting an IR Spectrum

Worked Example

Examine the two spectra shown and determine which one belongs to propan-2-ol and which one belongs to propanone

Analytical Techniques Question Worked Example - Analysing IR Spectra, downloadable AS & A Level Chemistry revision notes

Answer

IR spectrum A is propanone and spectrum B is propan-2-ol.

In IR spectrum A the presence of a strong, sharp absorption around 1710 cm-1 corresponds to the characteristic C=O, carbonyl, group in a ketone.

In spectrum B the presence of a strong, broad absorption around 3200-3500 cm-1 suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2-ol.

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