Analysis & spectroscopy

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

Infrared

• C=O 1680–1750 cm⁻¹ (sharp) · O–H alcohol 3230–3550 (broad) · O–H acid 2500–3000 (very broad) · fingerprint region <1500 identifies the exact compound

Mass spectra

• M⁺ peak → Mr. Common fragment losses: 15 (CH₃), 17 (OH), 29 (C₂H₅ or CHO), 45 (COOH).
• M and M+2 in ratio 3:1 → one Cl; 1:1 → one Br.

¹H NMR

• Reference TMS at δ = 0; solvent CDCl₃ (no ¹H signal).
• Number of signals = number of H environments; integration ratio = ratio of H atoms.
• n+1 rule: a signal splits into (n+1) peaks, where n = H atoms on adjacent carbons. Quartet ⇒ next to CH₃; triplet ⇒ next to CH₂.
• ¹³C NMR: 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

• 1: count ¹³C signals → carbon environments. 2: count ¹H signals → proton environments. 3: integration ratio → how many H in each. 4: splitting (n+1) → H on neighbouring carbons. 5: shift table → which functional groups.