IR Band Assignment Practice
An IR spectrum is read band by band: each absorption reports a bond type vibrating in a characteristic wavenumber window. Here one band is highlighted on a real measured spectrum and you name the stretch. The structure appears after you answer.
Common Questions
What order should I read an IR spectrum in?
By region, not left to right. Check near 1700 cm⁻¹ for a C=O first: it is the strongest and most reliable band in the spectrum. Then the O–H/N–H region above 3100 (broad means hydrogen bonding), then the triple-bond window 2100–2280, then the aromatic ring stretches near 1600 and 1500. The C–H stretches near 2900 appear in nearly every organic compound and rarely decide anything.
Why does this site ignore the fingerprint region below 1500 cm⁻¹?
Below about 1500 cm⁻¹ the spectrum fills with bending and skeletal modes that overlap heavily. This fingerprint region is excellent for matching a spectrum against a known reference, but individual bands there cannot be assigned honestly to single groups, so our practice problems only quiz the diagnostic region at 1500 cm⁻¹ and above.
What do the s, m, w, and br labels next to IR bands mean?
They are the intensity descriptors chemists report with each band: strong, medium, weak, and broad. Broad almost always signals hydrogen bonding, which is why an alcohol O–H is a wide hill near 3350 while an N–H is a narrower spike. Intensity carries real information: a carbonyl stretch is always strong, and an internal alkyne C≡C can be too weak to see at all.