Infrared (IR) Spectroscopy (Edexcel AS Chemistry)

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Interpreting an IR Spectrum

  • 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 stimulate 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

Bond Functional groups containing the bond Characteristic infrared absorption range
Wavenumber (cm-1)
C–H
(stretching)
Alkane
Alkene
Alkyne
Arene
Aldehyde
2962 - 2853
3095 - 3010
3300
3030
2900-2820 and 2775 - 2700
C–H
(bending)
Alkane
Arene (5 adjacent H)
Arene (4 adjacent H)
Arene (3 adjacent H)
Arene (2 adjacent H)
Arene (1 adjacent H)
1485 - 1365
750 and 700
750
780
830
880
N–H
(stretching)
Amine
Amide
3500 - 3300
3500 - 3140
O–H
(stretching)
Alcohols and phenols
Carboxylic acids
3750 - 3200
3300 - 2500
C=C
(stretching)
Isolated alkene
Arene
1669 - 1645
1600, 1580, 1500, 1450
C=O
(stretching)
Aldehydes, saturated alkyl
Ketones, alkyl
Ketones, aryl
Carboxylic acids, alkyl
Carboxylic acids, aryl
Carboxylic acids, anhydrides
Acyl halides, chlorides
Acyl halides, bromides
Esters, saturated
Amides
1740 - 1720
1720 - 1700
1700 - 1680
1725 - 1700
1700 - 1680
1850 - 1800 and 1790 - 1740
1795
1810
1750 - 1735
1700 - 1630
Cidentical toN
(stretching)
Nitrile 2260 - 2215
Cidentical toC
(stretching)
Alkyne 2260 - 2100

Worked example

Analysing IR Spectra

Look at the two infrared spectra below and determine which one corresponds to propanone and which one to propan-2-olAnalytical 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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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.