Key IR Absorptions
This table is the core reference for IR interpretation. Memorize the ranges and peak shapes for the most common functional groups.
The Big Table
| Functional group | Wavenumber (cm⁻¹) | Shape | Notes |
|---|---|---|---|
| O-H (alcohol, H-bonded) | 3200-3550 | Broad, strong | The broader the signal, the stronger the H-bonding |
| O-H (carboxylic acid) | 2500-3300 | Very broad, strong | Extends lower due to dimer formation |
| N-H (primary amine) | 3300-3500 | 2 peaks, medium | Symmetric and asymmetric stretches |
| N-H (secondary amine) | 3300-3500 | 1 peak, medium | |
| N-H (amide) | 3200-3400 | Sometimes 2 peaks | Conjugated with C=O |
| C-H (sp³, alkane) | 2850-2960 | Strong, multiple peaks | Below 3000 |
| C-H (sp², alkene/aromatic) | 3000-3100 | Sharp, medium | Above 3000 |
| C-H (sp, terminal alkyne) | ~3300 | Sharp, strong | Terminal -C≡C-H only |
| C-H (aldehyde) | 2720 + 2820 | Two medium peaks | ”Doublet” - diagnostic for CHO |
| C≡C | 2100-2260 | Medium, sharp | Terminal: stronger; internal: weak |
| C≡N (nitrile) | 2210-2260 | Sharp, strong | Same region as alkyne |
| C=O (aldehyde) | 1720-1740 | Strong, sharp | |
| C=O (ketone) | 1705-1720 | Strong, sharp | |
| C=O (ester) | 1735-1750 | Strong, sharp | Higher than ketone |
| C=O (carboxylic acid) | 1700-1725 | Strong, sharp | With OH peak at 2500-3300 |
| C=O (amide) | 1630-1700 | Strong, sharp | Lowest due to N resonance |
| C=O (acyl chloride) | 1790-1815 | Strong, sharp | Highest due to Cl withdrawal |
| C=O (anhydride) | 1750 + 1820 | Two peaks | Sym and asym stretches |
| C=C (alkene) | 1620-1680 | Variable | Weak if symmetric |
| C=C (aromatic) | 1450-1600 | Multiple sharp | ”Aromatic finger” |
| N-O (nitro, asymmetric) | 1500-1570 | Strong | |
| C-O (alcohol, ether, ester) | 1000-1300 | Strong | Multiple peaks in this range |
| C-N | 1020-1220 | Variable | Fingerprint region |
Alcohol vs. Carboxylic Acid O-H
- Alcohol O-H: broad, 3200-3550 cm⁻¹. Does NOT extend to below 3000.
- Carboxylic acid O-H: VERY broad, 2500-3300 cm⁻¹. EXTENDS well below 3000, often overlapping with C-H stretches.
If the OH extends below 3000 cm⁻¹, suspect carboxylic acid (look for C=O at 1700-1720 to confirm).
Aldehyde “Doublet”
Aldehydes show TWO characteristic C-H stretches at 2720 and 2820 cm⁻¹ (the aldehyde C-H is unique because H is attached to the sp² carbonyl C). This “doublet” is diagnostic for aldehydes vs. ketones. If you see both peaks, it is an aldehyde. If not, it could still be an aldehyde (the peaks are weak), but ketones never have this doublet.
Alkyne vs. Nitrile
Both absorb in the 2100-2300 cm⁻¹ region. Distinguishing:
- Terminal alkyne (R-C≡C-H): paired with a SHARP C-H stretch at ~3300 cm⁻¹.
- Internal alkyne (R-C≡C-R’): no 3300 peak; alkyne peak may be weak or absent if symmetric.
- Nitrile (R-C≡N): no 3300 peak (no C-H on the triple bond); nitrile peak is usually stronger than alkyne.
Aromatic Fingerprint
Aromatic rings have distinctive features:
- sp² C-H stretches just above 3000 cm⁻¹.
- Two or three peaks in the 1450-1600 cm⁻¹ region (the “aromatic ring breathing” modes).
- Overtones in the 1650-2000 cm⁻¹ region (subtle but characteristic).
The exact pattern depends on the substitution pattern (mono-, di-, tri-substituted), and experienced chemists can distinguish ortho, meta, para from the IR. For MCAT, just recognizing that these features mean “aromatic” is sufficient.
Interpretation Flow
- Scan for OH (3200-3550, broad) or NH (3300-3500, sharper).
- Look at 3000-3100 for sp² C-H (alkene/aromatic).
- Is there a peak around 2100-2300? Alkyne or nitrile.
- Is there a peak around 1700? Carbonyl - narrow down type by exact position.
- Fingerprint region - use to confirm but not to identify from scratch.