Analysis & spectroscopy

IR, mass spectra, ¹H and ¹³C NMR, chromatography

Infrared

A sharp C=O absorption sits at 1680–1750 cm⁻¹. An alcohol O–H is broad at 3230–3550, while an acid O–H is very broad at 2500–3000. The fingerprint region below 1500 is unique to each molecule and identifies the exact compound.

Mass spectra

The M⁺ peak gives Mr directly. Learn the common fragment losses: 15 (CH₃), 17 (OH), 29 (C₂H₅ or CHO) and 45 (COOH). If M and M+2 appear in a 3:1 ratio the molecule contains one Cl; a 1:1 ratio means one Br.

¹H NMR

Shifts are referenced to TMS at δ = 0, and the solvent is CDCl₃ because it gives no ¹H signal. The number of signals equals the number of H environments, and the integration ratio gives the ratio of H atoms.

Splitting follows the n+1 rule: a signal splits into (n+1) peaks, where n = H atoms on adjacent carbons. So a quartet means the signal sits next to CH₃, and a triplet means next to CH₂.

¹³C NMR is simpler still — one signal per carbon environment. Count them first in any structure question.

Chromatography

TLC (required practical 12): separation depends on relative affinity for the stationary vs mobile phase. Never let the solvent reach the top of the plate.

3.3.15The NMR problem-solving routine

Run the same five steps every time. First count the ¹³C signals for the carbon environments, then count the ¹H signals for the proton environments. Integration ratios tell you how many H sit in each, splitting via (n+1) reveals the H on neighbouring carbons, and the shift table pins down which functional groups are present.