The first thing to look for: 1700
A strong, sharp absorption near 1700 cm⁻¹ means a carbonyl — C=O. The exact position narrows it down: aldehydes ~1720–1740, ketones ~1705–1720, esters ~1735–1750, amides ~1640–1690 (lower because of N lone pair conjugation), carboxylic acids ~1700–1725 (paired with a broad O–H from 2500–3300).
O–H and N–H
A broad bump from 3200–3550 is an O–H stretch (alcohol or phenol). A very broad O–H from 2500–3300 with a 1700-ish C=O is a carboxylic acid. Sharp peaks around 3300 are N–H — primary amines show two peaks (asym + sym stretch), secondary amines show one, tertiary none.
C–H tells
sp³ C–H: 2850–3000, the workhorse, almost every organic molecule has these. sp² C–H (=C–H, aromatic): just above 3000. sp C–H (≡C–H, terminal alkyne): sharp peak around 3300, easily mistaken for N–H but the alkyne is sharper and accompanied by a weaker peak around 2100.
The triple bond region (2100–2260)
C≡C alkynes give a weak peak around 2100–2260, often barely visible. C≡N nitriles give a sharper, stronger peak around 2210–2260. Both are in a region that's usually empty in IR spectra, so any peak there is worth investigating.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.