IB Chemistry›Structure 3›3.2›3.2.9
HL

Infrared (IR) Spectroscopy

Identifying functional groups by covalent bond vibrations.

Fingerprint vs Functional Group Region

Fingerprint Region (400–1500 cm⁻¹)

Complex, unique pattern for every molecule (like a fingerprint). Used for computer matching.

Functional Group Region (1500–4000 cm⁻¹)

Where specific bonds absorb. The area we analyze for identification.

Key Absorptions (Memorize These)

Bond Wavenumber (cm⁻¹) Appearance Found In
C=O 1700 – 1750 Strong, Sharp ("The Sword") Aldehydes, Ketones, Acids, Esters
O-H (Alcohol) 3200 – 3600 Strong, Broad ("The Tongue") Alcohols
O-H (Acid) 2500 – 3000 Very Broad, Messy ("The Beard") Carboxylic Acids
C-H 2850 – 3090 Strong, Sharp Everything organic (Ignore usually)
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