Infrared spectroscopy exam tips for A-level Chemistry.
You can learn more about infrared spectroscopy by using my “Organic analysis” workbook, available here.
Infrared spectroscopy exam tips
When interpreting spectra, make use of all data you have about the compound. If you’re given the molecular formula, or information about any reactions that the compound does, it will give you important clues. If the question spans several pages you might find this info on an earlier page, so remember to look back.
Don’t suggest the presence of a bond unless the atoms in the bond are in the molecule. E.g. a lot of students make the mistake of suggesting that an N-H bond is present when the molecular formula doesn’t include nitrogen.
If there is an N-H bond present you will see a significant peak at the far left of the spectrum, usually made up of 2 tall, narrow peaks.
There will (nearly) always be a peak at around 3000 cm-1. This is because the C-H bond, present in most organic compounds, absorbs in that region of the spectrum. If water is present as an impurity it will also show in the spectrum in that region.
The alcohol and carboxylic acid O-H peaks are in a similar region of the spectrum but the alcohol O-H will be to the left of 3000cm-1, whereas the carboxyl O-H is usually centred around 3000 cm-1. The carboxylic acid O-H is usually much broader than the alcohol O-H. The alcohol O-H usually creates a peak made of 2 narrower peaks together. The shape of the peak is just as important as the position, so start looking out for this.
The C=C double bond peaks are in a similar position to C=O peak, so look for other clues in the question to help you decide which one is present.
When answering questions that ask you to distinguish between compounds, identifying what bonds are NOT present is often as identifying which bonds ARE present, so mention both in your answers.