Infrared spectroscopy

In infrared (IR) spectroscopy, a molecule is exposed to infrared radiation. Covalent bonds absorb IR energy and vibrate (stretch and bend). Each type of bond absorbs at a characteristic wavenumber (measured in cm⁻¹), so the absorptions act as a fingerprint for the bonds — and hence the functional groups — present.

An IR spectrum plots % transmittance (downwards) against wavenumber (cm⁻¹). An absorption shows as a dip (trough) in transmittance. You read off the position (wavenumber), shape (sharp or broad) and presence/absence of key peaks.

Bond	Functional group	Wavenumber / cm⁻¹	Notes
O–H	alcohols	~3200–3550	broad
O–H	carboxylic acids	~2500–3000	very broad
N–H	amines, amides	~3300–3500
C–H	most organics	~2850–3100
C≡N	nitriles	~2200–2280
C=O	aldehydes, ketones, acids, esters	~1630–1820	strong, sharp
C=C	alkenes	~1610–1680
C–O	alcohols, esters	~1040–1300

You don't need to memorise exact figures — the Data Booklet provides them. The skill is matching peaks to functional groups and using the shape (broad O–H vs sharp C=O) as a clue.

The power of IR is distinguishing functional groups by which peaks are present or absent:

• Alcohol — broad O–H (~3200–3550) and C–O (~1040–1300), but no C=O.
• Aldehyde/ketone — strong C=O (~1700), but no broad O–H.
• Carboxylic acid — both a very broad O–H (~2500–3000) and a strong C=O (~1700).
• Ester — strong C=O plus C–O, but no O–H.

Worked. A spectrum shows a strong peak at ~1715 cm⁻¹ and a very broad absorption from ~2500–3300 cm⁻¹. The C=O plus the very broad O–H → a carboxylic acid.

Monitoring reactions. IR can confirm a change has happened — e.g. oxidising an alcohol to a carboxylic acid: the broad alcohol O–H is replaced by an acid's broad O–H and a new C=O appears.