Spectroscopy (IR, NMR, MS)
Spectroscopy is pattern recognition. Start broad (functional group, unsaturation) and converge to a unique structure. Use IR to identify functional groups, NMR to assemble carbon/hydrogen environments, and MS to check molecular weight/fragments.
IR: fast functional-group detection
| Band | Range (cm⁻¹) | Shape | Interpretation |
|---|---|---|---|
| O–H (alcohol) | 3200–3600 | broad | hydrogen bonding |
| O–H (carboxylic acid) | 2500–3300 | very broad | often overlaps C–H region |
| N–H | 3300–3500 | medium | 1° amines show two peaks |
| C=O | 1650–1800 | strong | exact position depends on conjugation/derivative |
| C≡N | 2210–2260 | sharp | nitrile |
| sp C–H | ~3300 | sharp | terminal alkyne |
¹H NMR: the 3-step method
- Integration → how many hydrogens in each signal.
- Chemical shift (δ) → what environment (alkyl, allylic, aromatic, next to O/N, aldehyde, acid).
- Splitting → neighbors (n+1 rule; watch for nonequivalent couplings).
| δ (ppm) | Typical protons |
|---|---|
| 0.8–1.8 | alkyl (sp³ C–H) |
| 1.8–3.0 | allylic/benzylic/α to C=O |
| 3.0–4.5 | next to O/N/halogen |
| 5.0–6.5 | alkenyl |
| 6.5–8.5 | aromatic |
| 9.0–10.5 | aldehyde |
| 10–13 | carboxylic acid (broad) |
Reference charts (interactive)
Hover the plots; they’re tuned for memorization.IR band mapranges by group
¹H NMR shift bandstypical windows
Mass spectrometry: quick rules
| Clue | Interpretation |
|---|---|
| M⁺ peak | approx. molecular weight (may be weak/absent for fragile compounds) |
| M and M+2 pattern | Cl (3:1) or Br (1:1) isotopes |
| Base peak | most stable fragment (not necessarily molecular ion) |
| Common fragments | benzyl → m/z 91; acylium ions from carbonyl compounds |