Chemistry › Aromatic chemistry and amines
Aromatic chemistry and amines
Unit C21 · 4 lessons.
Hydrogenating benzene releases about 208 kJ mol⁻¹, against the 360 kJ mol⁻¹ that three separate double bonds would give, and aromatic chemistry follows from that difference. Benzene keeps its delocalised ring by substitution rather than addition, in nitration and Friedel–Crafts acylation. Amines are bases and nucleophiles, and amino acids, proteins and DNA apply the same chemistry to the molecules of life.
An account of benzene's stability needs that evidence: the scheme wants themeasured enthalpy of hydrogenation, −208 kJ mol⁻¹ against the −360 kJ mol⁻¹ expected, as well as the delocalised π system. For amines, phenylamine is a weaker base than ammonia because the lone pair on nitrogen is partly delocalised into the ring.
AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1, 2 and 4 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.3.10.1, 3.3.10.2, 3.3.11.1, 3.3.11.2, 3.3.11.3, 3.3.13.1, 3.3.13.5 · OCR A 6.1.1, 6.2.4, 6.2.1, 6.2.2 · Edexcel 18.1 to 18.22, 15.16 · CAIE 29.1, 29.3, 30.1, 31.1, 32.2, 29.2, 34.2, 12.1, 19.1, 34.1, 34.3, 34.4, 35.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Benzene: its structure and the evidence for delocalisation
The Kekulé structure of benzene and the three pieces of evidence against it, namely equal bond lengths, the absence of addition reactions and the enthalpy of hydrogenation; the delocalised model with a planar ring, overlapping p orbitals and six delocalised electrons; the delocalisation energy from a hydrogenation enthalpy cycle; drawing the ring with a circle; naming substituted benzenes with numbered positions; and, for the boards that set them, sp² hybridisation, the acidity of phenol against ethanol and ethanoic acid with its reactions with sodium hydroxide and sodium carbonate, esterifying phenol with an acyl chloride, the bromination and nitration of phenol, and why halogenoarenes resist hydrolysis.
- Electrophilic substitution: nitration and Friedel–Crafts acylation
Why benzene undergoes electrophilic substitution and not addition; nitration with concentrated nitric and sulfuric acids at 50 °C, the formation of the nitronium ion NO₂⁺, the mechanism with its horseshoe intermediate and the regeneration of the catalyst; the uses of nitration in explosives, dyes and the route to phenylamine; Friedel–Crafts acylation with an acyl chloride and aluminium chloride, the acylium ion and the mechanism; and why acylation is used; and, for the boards that set them, halogenation with a halogen carrier, Friedel–Crafts alkylation, the directing effects of groups on the ring, the bromination and nitration of phenol, and side-chain chlorination and oxidation.
- Amines: preparation, basicity and nucleophilic reactions
Primary, secondary and tertiary amines and quaternary ammonium salts, with their use as cationic surfactants; amines as Brønsted–Lowry bases and the order of base strength of phenylamine, ammonia and primary aliphatic amines; amines as nucleophiles with halogenoalkanes, including further substitution, and with acyl chlorides; and making amines from halogenoalkanes and excess ammonia, by reducing nitriles with LiAlH₄ or hydrogen and nickel, and by reducing nitrobenzene with tin and hydrochloric acid, then steam distillation; the yield over two steps; and, for the boards that set them, butylamine with copper(II) ions, why amides are barely basic, amide hydrolysis and reduction, and diazonium salts and azo dyes.
- Amino acids, proteins, enzymes and DNA
Amino acids with the amine and carboxylic acid groups on the same carbon, their chirality, zwitterions and the isoelectric point, and the ions they form in acid and alkali; peptide links, dipeptides and the hydrolysis of proteins with separation of the amino acids by thin-layer chromatography and by electrophoresis; esterifying an amino acid; primary, secondary and tertiary protein structure; enzymes as stereospecific catalysts and drugs as inhibitors; the structure of DNA from nucleotides with base pairing; and how cisplatin binds to DNA, with the risks of its use and why it is supplied as the cis isomer alone.
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