Aim

To record and interpret the infrared (IR) spectrum of benzoic acid and identify its characteristic functional group absorption bands.

Principle

Infrared spectroscopy is used to identify functional groups present in organic compounds. Molecules absorb infrared radiation at specific wavenumbers (cm⁻¹) due to vibrational transitions of chemical bonds.

Each functional group shows characteristic absorption bands, allowing the identification of groups such as carboxylic acid, aromatic ring, and C–H bonds present in benzoic acid.

Apparatus

FTIR spectrophotometer
Sample holder / KBr pellet

Chemicals

Benzoic acid sample

Procedure

Place a small quantity of benzoic acid in the sample holder or prepare a KBr pellet.
Insert the sample into the FTIR spectrophotometer.
Record the IR spectrum in the range 4000–400 cm⁻¹.
Note the important absorption peaks and assign them to functional groups.

Observation Table

Sr. No.	Wavenumber (cm⁻¹)	Type of Vibration	Functional Group
1	2500–3300	O–H stretching (very broad)	Carboxylic acid
2	1680–1710	C=O stretching	Carboxylic acid
3	3000–3100	C–H stretching	Aromatic ring
4	1450–1600	C=C stretching	Aromatic ring
5	1200–1300	C–O stretching	Carboxylic acid
6	690–900	C–H bending	Aromatic ring

Interpretation of Spectrum

A very broad absorption band at 2500–3300 cm⁻¹ indicates O–H stretching of carboxylic acid group.
A strong peak around 1700 cm⁻¹ corresponds to C=O stretching of the carboxyl group.
Peaks near 3000–3100 cm⁻¹ indicate aromatic C–H stretching.
Bands between 1450–1600 cm⁻¹ represent C=C stretching of the benzene ring.
Absorption near 1200–1300 cm⁻¹ confirms C–O stretching.

Result

The IR spectrum of benzoic acid shows characteristic peaks of carboxylic acid (O–H, C=O) and aromatic ring vibrations, confirming the structure of benzoic acid containing a benzene ring with a –COOH group.