Analytical chemistry-infrared spectroscopy-spectra of different molecules-solution to ethanol-lactic acid problem.

Solution

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 wavenumber:
- 3200-3500 representing the O-H bond;
- 2900-3000 representing the C-H bond;
- 1000-1100 reperesnting the C-O bond.

The IR spectrum of lactic acid is shown on the right. We can clearly see the trough at 2600-3500 representing the O-H bond.
Why is the trough representing the O-H bond so wide?

Solution

This is interesting because it contains two different sorts of O-H bond - the one in the acid and the simple "alcohol" type in the chain attached to the -COOH group.
The O-H bond in the acid group absorbs between 2500 and 3300, the one in the chain between 3230 and 3550. Taken together, that gives this immense trough covering the whole range from 2500 to 3550 . Lost in that trough as well will be absorptions due to the C-H bonds.

Hide solution

You will also find troughs at 1000-1100 representing the C-O bond. The trough at about 1700 represents the C=O bond and is not found in the IR spectrum of ethanol.