Multi-step synthesis
A2 synthesis combines aliphatic and aromatic reactions. Key aromatic links: benzene → (conc. HNO₃/H₂SO₄) nitrobenzene → (Sn/HCl, NaOH) phenylamine → (NaNO₂/HCl < 10 °C) diazonium → (phenol/NaOH) azo dye, or (warm water) phenol. Benzene → (RCl/AlCl₃) alkylbenzene → (KMnO₄/OH⁻, heat) benzoic acid. Benzene → (RCOCl/AlCl₃) aryl ketone → (NaBH₄) secondary alcohol. Acyl chlorides make esters and amides. Plan by retrosynthesis, give reagents and conditions, and recognise each step's reaction type. Purify solids by recrystallisation and check purity by melting point.
≈ 50 minFunctional group routes
Organic synthesis links functional groups through known reactions. Alkene → (HBr) halogenoalkane → (NaOH aq) alcohol → (Cr₂O₇²⁻/H⁺, distil) aldehyde → (reflux) carboxylic acid → (alcohol, H₂SO₄) ester. Halogenoalkane → (KCN, ethanol) nitrile → (H⁺/H₂O) acid or (LiAlH₄) amine. Carbonyl → (HCN) hydroxynitrile; (NaBH₄) alcohol. Alcohol → (conc. H₂SO₄, heat) alkene. To lengthen a chain by one carbon, use CN⁻. Plan backwards from the target, and give reagents and conditions for each step.
≈ 45 minNMR spectroscopy
NMR detects nuclei (¹H, ¹³C) in a magnetic field; each environment gives a signal at a chemical shift δ (ppm) relative to TMS (δ = 0). Samples are dissolved in CDCl₃ (no ¹H signal). ¹³C NMR: number of peaks = number of carbon environments; shift identifies type (C=O 160–220; C–O 50–90; aromatic 110–160). ¹H NMR: number of peaks = H environments; integration gives the ratio of H atoms; splitting follows the n + 1 rule (n = H on neighbouring carbons): singlet, doublet, triplet, quartet. O–H/N–H peaks disappear on shaking with D₂O. Combine with IR and mass spectra to identify unknowns.
≈ 55 minChromatography techniques
Chromatography separates mixtures using a stationary phase and a mobile phase. Components that interact more strongly with the stationary phase move more slowly. TLC: silica/alumina on a plate (stationary), solvent (mobile); adsorption; ; colourless spots shown by UV, iodine or ninhydrin (amino acids). Gas chromatography (GC): liquid coating on a column (stationary), inert carrier gas (mobile); separation by partition and boiling point; retention time identifies compounds; peak area gives relative amount. GC–MS feeds each separated component into a mass spectrometer for identification. Column chromatography and HPLC separate larger quantities.
≈ 40 min