Nitrogen compounds
Phenylamine, amides and azo dyes
Base strength depends on how available the N lone pair is: ethylamine > ammonia > phenylamine. Alkyl groups push electrons onto N; in phenylamine the lone pair delocalises into the ring, so it's a weaker base. Phenylamine is made by reducing nitrobenzene with Sn and conc. HCl (reflux), then NaOH. Its ring is activated: bromine water gives white 2,4,6-tribromophenylamine. Diazotisation: phenylamine + HNO₂ (NaNO₂ + HCl) below 10 °C gives benzenediazonium chloride, which couples with phenol in alkali to form an orange azo dye (–N=N–). Amides (RCONH₂) are neutral (lone pair delocalised onto C=O); they hydrolyse with acid or alkali and are reduced by LiAlH₄ to amines.
≈ 50 minAmino acids, peptides and DNA
α-Amino acids have NH₂ and COOH on the same carbon. In solids and at their isoelectric point they exist as zwitterions (high melting points, soluble in water). In acid they become cations; in alkali, anions. Two amino acids join by condensation, forming a peptide (amide) link –CONH– and water. Proteins are hydrolysed by hot 6 mol dm⁻³ HCl. Amino acids separate by electrophoresis (move by charge at a given pH) or chromatography. DNA: two sugar-phosphate strands held by hydrogen bonds between bases: A–T (2 H-bonds) and C–G (3). Cisplatin binds to guanine N atoms, stopping DNA replication in cancer cells.
≈ 50 min