Organic Chemistry by Brown, Iverson, Anslyn, Foote | Mayya's Study Guide | Chapter 12

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.

ALKANES

IR Spectroscopy of alkanes

ALKENES

IR Spectroscopy of alkenes

ALKYNES

IR Spectroscopy of alkynes

ALCOHOLS

IR Spectroscopy of alcohols

AMINES

IR Spectroscopy of amines

ALDEHYDES AND KETONES

IR Spectroscopy of aldehydes and ketones

Note: if =O is next to another double bond, the IR frequency will be LOWER.

CARBOXYLIC ACIDS

IR Spectroscopy of carboxylic acids

Solving IR problems

Steps:

1. 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.

2. 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.

3. 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