IR is the fast functional-group detector: it confirms a carbonyl, O–H, N–H, or triple bond in seconds, even though it won't hand you a full structure the way NMR can.
Three functional groups, three IR signatures — each read from just one or two peaks. Structures drawn live.
1. IR Detects Which Functional Groups Are Present, Not the Whole Structure
Scan the wavenumber axis (4000–400 cm⁻¹) for a few yes/no answers — carbonyl? O–H? N–H? triple bond? — each of which names one group and rules others out.
2. A Bond's Frequency Is Set by Bond Stiffness and Atom Mass
Treat each bond as a ball-and-spring: stiffer bonds (triple > double > single) and lighter atoms (any X–H) both vibrate faster and absorb higher up the axis.
3. The Carbonyl C=O Is a Strong Peak Near 1700 cm⁻¹ — Look Here First
The strong, sharp carbonyl stretch near 1700 is the most useful peak, and its exact position identifies the group: ester ~1735, aldehyde/ketone ~1715, acid ~1710, amide ~1650.
4. O–H and N–H Stretches Sit High and Broad, Above 3200 cm⁻¹
Read shape and count: broad-and-rounded O–H sits 3200–3550 (very broad 2500–3300 for an acid), while sharp N–H at 3300–3500 gives two spikes for –NH₂ and one for –NH–.
5. C–H Position Reveals Hybridization, and the Triple-Bond Window Is 2100–2260
C–H straddles 3000 as a hybridization tell (sp³ below, sp²/aromatic above), and the otherwise-empty 2100–2260 window flags a weak C≡C alkyne or a sharper C≡N nitrile.
6. Read IR Alongside Degrees of Unsaturation and NMR
Run IR after computing degrees of unsaturation and alongside ¹H NMR: DoU counts the unsaturation, IR names the groups, NMR maps the skeleton.
7. IR Frequency Reference Table
Scan left to right and note both position and peak shape — breadth and intensity are as diagnostic as the number.
| Bond / group | Region (cm⁻¹) | Shape & intensity |
|---|---|---|
| O–H alcohol / phenol | 3200–3550 | broad, rounded |
| O–H carboxylic acid | 2500–3300 | very broad, spans C–H |
| N–H amine / amide | 3300–3500 | sharp; 2 spikes (1°), 1 (2°) |
| C–H sp³ | 2850–3000 | just below 3000 |
| C–H sp² / aromatic | 3000–3100 | just above 3000 |
| ≡C–H terminal alkyne | ~3300 | sharp, thin |
| C≡C alkyne | 2100–2260 | weak, often faint |
| C≡N nitrile | 2210–2260 | medium, sharp |
| C=O carbonyl | 1700–1750 | strong — most useful |
| C=C alkene | 1620–1680 | medium |
| Aromatic C=C | 1450–1600 | several medium bands |
| Fingerprint region | <1500 | complex — for matching |
8. Summary
Stiffer bonds and lighter atoms absorb higher · C=O ~1700 first look · O–H broad 3200–3550 · N–H sharp 3300–3500 · C–H straddles 3000 · C≡C/C≡N 2100–2260 · below 1500 is the fingerprint region.
Worked example
- A strong ~1715 cm−1 band is a C=O (carbonyl) stretch.
- A very broad band across 2500–3300 cm−1 is the O–H of a carboxylic acid (H-bonded).
- C=O together with that broad O–H is diagnostic of –COOH — not a plain alcohol or ketone.
Answer. A carboxylic acid (–COOH).
Quiz yourself
Tap a question to reveal the answer — free, no login.
A carboxylic acid. The strong ~1710 band is the C=O stretch, and the very broad hump sprawling from 2500–3300 (often swallowing the C–H peaks) is the hydrogen-bonded acid O–H. That O–H shape is unique to carboxylic acids — an alcohol O–H would be higher and less extreme (3200–3550).
Frequency depends on both bond stiffness and atom mass. Hydrogen is the lightest atom, and a lighter "ball" on the spring vibrates faster — pushing the absorption to high wavenumber. Bond order (stiffness) explains why triple > double > single elsewhere, but the low mass of H is what lifts every X–H stretch up near 3000–3500 despite the single bond.
The 1650 cm⁻¹ peak is the amide. The nitrogen lone pair conjugates into the C=O, giving the bond partial single-bond character — a weaker, longer, less stiff bond vibrates at lower wavenumber. The 1735 peak, absorbing higher, fits an ester. (An N–H peak near 3300 would confirm the amide.)
With four degrees but no carbonyl and no O–H, suspect an aromatic ring — which alone accounts for all four degrees (three π bonds plus one ring). Confirm it with aromatic C–H just above 3000 cm⁻¹ and several medium C=C bands between 1450 and 1600 cm⁻¹. IR names the ring; NMR (aromatic protons near 7 ppm) then maps the substitution.
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.