Infrared spectroscopy
Infrared spectroscopy
- Infrared (IR) spectroscopy helps find the functional group in a molecule.
- Each kind of bond absorbs IR at its own range of frequencies.
- Where a bond absorbs strongly, the spectrum shows a dip.
Practice
Infrared spectroscopy is mainly used to:
Each bond absorbs IR at its own wavenumber, so the dips reveal the functional groups present.
Reading the spectrum
- The position is measured in wavenumber (per cm). You're given a data table, so just match dips to bonds:
- broad dip ~$3200$–$3650$ → O–H in an alcohol.
- dip ~$1700$ → C=O (aldehyde, ketone, acid, ester).
- broad dip $2500$–$3000$ with a C=O dip → a carboxylic acid.
Practice
A strong dip near a wavenumber of 1700 (per cm) indicates a:
A dip around a wavenumber of 1700 (per cm) is the C=O bond (aldehyde, ketone, acid or ester).
Practice
A broad dip around a wavenumber of 3200–3650 (per cm) indicates an:
The broad O–H absorption sits in the 3200–3650 (per cm) wavenumber region.
Checking a reaction
- IR can confirm a reaction worked.
- E.g. if propene became propan-2-ol, the C=C dip should disappear and an O–H dip should appear.
Practice
How could IR confirm that an alkene has become an alcohol?
Losing the C=C and gaining an O–H dip shows the conversion to an alcohol.
You've got it
Key idea
- IR finds functional groups: each bond gives a dip at its own wavenumber
- key dips: O–H (broad ~3200–3650), C=O (~1700); a broad O–H plus C=O = a carboxylic acid
- use IR to check a reaction (a group's dip disappears or appears)