IR Functional Group Identification
This is IR used the way it is used at the bench: an unknown comes off the column, and the spectrum answers one question fast — what functional groups are here? You get a real spectrum with the structure hidden and call the group the bands demand. Note that the wrong options are not filler: each one is a group this spectrum actively rules out, and the explanation shows what evidence is missing.
Common Questions
Which functional groups can IR spectroscopy actually identify?
IR is decisive for the carbonyl family (acids, esters, amides, aldehydes, ketones near 1700 cm⁻¹), O–H and N–H groups (3100–3650), nitriles and alkynes (2100–2280), nitro groups (~1520 and 1345), and aromatic rings (~1600 and 1500). It is weak evidence for ethers, halides, and tertiary amines, whose characteristic bands sit in the crowded fingerprint region, which is why those groups are not asked here.
How do I tell the carbonyl compounds apart by IR?
Two handles: the exact C=O position and the companion bands. Esters sit high (1735–1750, higher for lactones), simple ketones and aldehydes near 1715–1725, conjugated carbonyls and amides lower (1620–1700). Then look for companions: a carboxylic acid drags a very broad O–H from 2500 to 3300, an aldehyde adds its C–H doublet near 2720 and 2850, an amide adds N–H bands above 3150.
Can the absence of a band prove a group is absent?
For strong bands, yes, and this is half the skill. A carbonyl stretch is intense: no band anywhere in 1650–1800 means no ordinary C=O in the molecule. The same logic holds for O–H and C≡N. It fails for intrinsically weak modes: an internal alkyne C≡C or a symmetric alkene C=C can be genuinely present yet invisible, so absence only excludes groups whose bands are reliably strong.