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Infrared Spectroscopy: Identifying Functional Groups, High school final essays of Chemistry
An overview of the characteristic infrared (ir) absorption bands associated with various functional groups, including alcohols, amines, aldehydes, ketones, carboxylic acids, esters, and nitriles. It explains how the position and shape of these absorption bands can be used to identify the presence and type of functional groups in organic compounds. The o-h and n-h stretching vibrations, as well as the carbonyl (c=o) stretching vibrations, and provides examples of ir spectra for different functional groups. This information can be useful for students and researchers in organic chemistry, analytical chemistry, and related fields to interpret ir spectroscopic data and determine the structural features of unknown compounds.
Typology: High school final essays
2022/2023
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O-H and N-H Stretching
- Both of these occur around 3300 cm
- 1
, but
they look different.
- Alcohol O-H, broad with rounded tip.
- Primary amine (RNH 2 ), broad with two sharp spikes.
- Secondary amine (R 2 NH), broad with one sharp spike.
- No signal for a tertiary amine (R 3
N)
An Alcohol IR Spectrum
=>
Amines
The N–H stretches of amines are in the region
3300 - 3000 cm
- 1
. (weaker and sharper than alcohol
O–H stretches).
- In primary amines (RNH 2
Secondary amines (R
2
NH) show only a single weak
band , only one N–H bond.
- Tertiary amines (R 3
N) do not show any band in this
region since they do not have an N–H bond.
Structural unit Range, cm
- 1 Aldehydes and ketones 1710 - 1750 Carboxylic acids 1700 - 1725 Acid anhydrides 1800 - 1850 and 1740- 1790 Esters 1730 - 1750 Amides 1680 - 1700 C (^) O Stretching vibrations (Carbonyl groups)
Aldehydes and ketones
A useful diagnostic band for aldehydes
is the O=C–H stretch, in the region
2800 - 2700cm
- 1 .
O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm
- 1