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 |
CN (stretching) |
Nitrile | 2260 - 2215 |
CC (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-ol
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.