IR spectroscopy

In this lesson, we will learn:

• How IR spectroscopy works as a method of analysis.
• How to interpret an IR spectrum.
• How to find evidence of an organic compound or functional group using IR spectra.

• IR spectroscopy is another form of spectroscopy where the bonds in a chemical compound can be detected by the infrared (IR) light they absorb. Like some compounds absorb some wavelengths of visible light (hence they’re coloured), most bonds absorb some infrared light when it passes through them. A specific IR wavelength causes specific bonds to bend, vibrate or stretch, so functional groups in compounds can show up predictably like a fingerprint.

IR spectra are measured by:
• Wavenumber (a type of frequency) on the x-axis with the units cm^-1
• Percentage transmittance (%) on the y-axis, which is basically “how much of the light at that frequency wasn’t absorbed?”


• IR spectra of organic compounds have two main features:
• The fingerprint region, from around 500-1500 cm^-1. This region is complicated; it has lots of absorptions and isn’t great for identifying one specific bond or functional group. Most C-C vibrations are found here and like we discussed with NMR, when most of your compounds are carbon based, C-C bonds aren’t helpful for analysis! Its name comes from taking a ‘fingerprint’ of the whole compound rather than any individual bond or functional group in it.


• The functional group region, anything above 1500cm^-1. This is where you will find strong evidence of functional groups in your compounds. Some very distinct vibrations include:
• The carbonyl (C=O) stretch at around 1700 cm^-1. This varies depending on which functional group it is part of, but any sharp peak around this wavenumber is strong evidence of a carbonyl group being present.


• The O-H stretch at 3200-3500 cm^-1. This also varies depending on which functional group it is part of, such as an alcohol, phenol or carboxylic acid, which is extremely broad and occurs starting at around 2600 cm^-1.


• The C-H aromatic proton stretch at 3000-3100 cm^-1. This is an unusual stretch because it is only slightly higher than C-H stretches in alkanes (which are very common and therefore not very helpful) but is a lot sharper.

These are three of the most distinct absorptions found in the functional group region of an IR spectra which are used as strong evidence of functional groups in a molecule.



• Worked example: Propan-2-ol, CH₃CH(OH)CH₃


An IR spectrum of propan-2-ol is shown above.
• As mentioned already, the broad O-H absorption at around 3200-3400 cm^-1 is quite a distinct signal in the IR spectrum. This appears later than an O-H in a carboxylic acid, for example.
• The C-H alkyl absorption at 2900-3000cm^-1 is present but will be found in every organic molecule.
• There is an absorption at around 1200cm^-1 for the C-O bond, but it is better to point to the O-H bond for evidence of an alcohol.


• Worked example: methyl ethanoate, CH₃COOCH₃

Above is an IR spectrum of the ester methyl ethanoate :
• Compared to the alcohol above, the absorption at 3200 cm^-1 is gone, instead, we have a C=O carbonyl stretch at around 1750cm^-1.
• The C-H alkyl stretch is still present as it will be in virtually every organic molecule!
• Like in the alcohol, there is evidence of a C-O stretch near 1200cm^-1 but it is in/near the fingerprint region, so it is not the best idea to rely on this signal as evidence.


• Worked example: acetophenone, C₆H₅COCH₃

Above is an IR spectrum of acetophenone (phenyl ethenone):
• It has both C-H absorptions for alkyl chains at 2900-3000 cm^-1 and the aromatic C-H bonds at 3000-3100
• The C=O stretch at around 1700-1750 cm^-1 as is typical of a ketone.