IR spectra

Below is a table of some typical covalent bonds and their characteristic wavenumber at which energy is absorbed.

Bond type
Type of molecule found in.
Wavenumber
C-O
Alcohols and esters
1000-1300
C=O
Carboxylic acids and esters
1680-1750
C=C
Alkenes
1600-1700
C-H
Alkanes
2850-3100
C-H
Alkenes
3000-3100
O-H
Carboxylic acids
2500-3300
O-H
Alcohols
3200-3600

Ethanol has the semi-structural formula CH3CH2OH. Its IR spectrum is shown on the right. We can see a number of troughs at wavenumbers:
- 3200-3500 representing the O-H bond;
- 2900-3000 representing the C-H bond;
- 1000-1100 representing the C-O bond.
The semi-structural formula of lactic acid is shown on the right. What similarities can you predict the IR spectrum of lactic acid will have with that of ethanol? Solution
Lets look at the IR spectrum of ethyl ethanoate. The molecule is shown on the right. Looking at the molecule we should be able to see troughs between wavenumbers:
- 1680 - 1750, representing the C=O bond;
- 2850 - 3100, representing the C-H bond;
- 1000`- 1300, representing the C-O bond.

C-H trough is seen at about 3000 .
C=O double bond is seen at about 1740 .
C-O single bond is seen at about 1240.

In the exam you may get a table of data and asked to identify the possible functional groups that are represented by each trough.
When comparing acids and alcohols it is easy to spot the difference using an IR spectrum.

The O-H bond of an alcohol absorbs at a wavenumber around 3400 cm- while the OH bond of an acid absorbs somewhere between 3230 - 3550 cm-. Visually you can identify an acid or an alcohol by the shape of the trough representing the OH absorbance. You will notice that the absorbance of the acid OH overlaps the absorbance of the C-H bond at around 2950 cm-, where as the OH of an alcohol is further to the left at around 3400 cm- and is slightly more narrow.
The semistructural formula of propanone is given on the right.
What are some of the similarities you would expect to find when comparing the IR spectra of propanone and ethyl ethanoate? Solution
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