IR Structure Matching Practice
One real spectrum, four candidate structures, and only one survives the data. The other three are systematic edits of the real compound — a carbonyl deleted, an O–H capped, a nitrile swapped for an alkyne — and each is kept as an option only because the measured bands genuinely rule it out. Work by elimination: what band does each candidate demand, and is it there?
Common Questions
How can IR distinguish two similar structures?
IR works at the functional-group level: it distinguishes structures that differ in which groups they carry. Consider an alcohol versus its ketone, a nitrile versus a terminal alkyne, or an ester versus its ether: each pair differs by a strong, characteristic band. What IR cannot do is separate isomers with identical functional groups. That is NMR's job.
What is the fastest way to eliminate a candidate structure?
Two one-line checks do most of the work. First, does the structure demand a strong band the spectrum lacks? A candidate with a C=O needs a band near 1700, so a spectrum with nothing there eliminates it. Second, does the spectrum show a band the structure cannot produce? A broad 3350 cm⁻¹ band on a candidate with no O–H or N–H is just as disqualifying.
Where do these spectra and structures come from?
The spectra are real: measured ATR-FTIR curves from the Chemotion open repository (CC BY-SA), with the source dataset cited on every problem. The wrong structures are systematic edits of the real one (a carbonyl removed, an O–H methylated, a nitrile swapped for an alkyne), kept only when the real band data genuinely rules them out.