Chapter 12: Infrared Spectroscopy
IR is a method useful in determining the functional groups of a molecule.
IR radiation causes bonds in a molecule to absorb energy and leads to vibrational excitation, such as bonds stretching and bending. Different types of bond will absorb a characteristic frequency which will allow us to determine which types of bonds are present in a molecule we are working with.
ALDEHYDES AND KETONES
Note: if =O is next to another double bond, the IR frequency will be LOWER.
Solving IR problems
Check around 3000 cm-1 (CH stretching). If you see peaks to the right, then we have an alkane. If you see peaks to the left, then we have an alkene or a benzene ring.
Check the region around 3500cm-1. If there is a strong, broad band, we have an OH. If there is one or two peaks in the region at somewhat lower frequency than for OH may indicate an amine.
Check for a strong, sharp peak in the region 1630–1820 cm-1. If there is one, there must be a =O group present on the molecule