4.1.1 Arenes Structure of benzene The development and acceptance of models for the structure of benzene. Candidates should be able to: (a) compare the Kekulé and delocalised models for benzene in terms of p-orbital overlap forming bonds; (b) review the evidence for a delocalised model of benzene in terms of bond lengths, enthalpy change of hydrogenation and resistance to reaction; Electrophilic substitution of arenes Halogen carriers include iron, iron halides and aluminium halides. (c) describe the electrophilic substitution of arenes with (i) concentrated nitric acid in the presence of concentrated sulfuric acid, For nitration, candidates should include equations for formation of NO2+ For halogenation, candidates should include equations for formation of X+ or +X–AlX3–. (ii) a halogen in the presence of a halogen carrier;(d) outline the mechanism of electrophilic substitution in arenes, using the mononitration and monohalogenation of benzene as examples (e) explain the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the bonds in benzene compared with the localised electron density of the C=C bond in alkenes; Phenols (f) describe the reactions of phenol: (i) with aqueous alkalis and with sodium to form salts, (ii) with bromine to form 2,4,6-tribromophenol; (g) explain the relative ease of bromination of phenol compared with benzene, in terms of electron-pair donation to the benzene ring from an oxygen porbital in phenol; (h) state the uses of phenols in production of plastics, antiseptics, disinfectants and resins for paints. 4.1.2 Carbonyl Compounds Reactions of carbonyl compounds In equations for organic redox reactions, [O] and [H] should be used. The nucleophile can be considered as being the hydride ion, H–, with subsequent protonation of the organic intermediate from H2O. Candidates should be able to: (a) describe the oxidation of alcohols using Cr2O7 2–/H+ (ie K2Cr2O7/H2SO4), including: (i) the oxidation of primary alcohols to form aldehydes and carboxylic acids; the control of the oxidation product using different reaction conditions, (ii) the oxidation of secondary alcohols to form ketones; (b) describe the oxidation of aldehydes using Cr2O7 2–/H+ to form carboxylic acids; (c) describe the reduction of carbonyl compounds using NaBH4 to form alcohols; (d) outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides, such as NaBH4 Characteristic tests for carbonyl compounds (e) describe the use of 2,4-dinitrophenylhydrazine to: (i) detect the presence of a carbonyl group in an organic compound, In equations involving Tollens’ reagent, [O] is acceptable. (ii) identify a carbonyl compound from the melting point of the derivative; (f) describe the use of Tollens’ reagent (ammoniacal silver nitrate) to: (i) detect the presence of an aldehyde group, (ii) distinguish between aldehydes and ketones, explained in terms of the oxidation of aldehydes to carboxylic acids with reduction of silver ions to silver. 4.1.3 Carboxylic Acids and Esters Properties of carboxylic acids Candidates should be able to: (a) explain the water solubility of carboxylic acids in terms of hydrogen bonding and dipole–dipole interaction; (b) describe the reactions of carboxylic acids with metals, carbonates and bases; Esters, triglycerides, unsaturated and saturated fats Link between unsaturated and saturated fats and current concerns about heart disease and obesity. Use of biodiesel as a fuel to increase contribution to energy requirements from renewable fuels. (c) describe esterification of carboxylic acids with alcohols, in the presence of an acid catalyst, of acid anhydrides with alcohols; (d) describe the hydrolysis of esters: (i) in hot aqueous acid to form carboxylic acids and alcohols, (ii) in hot aqueous alkali to form carboxylate salts and alcohols; (e) state the uses of esters in perfumes and flavourings; (f) describe a triglyceride as a triester of glycerol (propane-1,2,3-triol) and fatty acids; (g) compare the structures of saturated fats, unsaturated fats and fatty acids, including cis and trans isomers, from systematic names and shorthand formulae; (h) compare the link between trans fatty acids, the possible increase in ‘bad’ cholesterol and the resultant increased risk of coronary heart disease and strokes; (i) describe and explain the increased use of esters of fatty acids as biodiesel. 4.1.4 Amines Basicity of amines Candidates should be able to: (a) explain the basicity of amines in terms of proton acceptance by the nitrogen lone pair; (b) describe the reactions of amines with acids to form salts; Preparation of amines (c) describe the preparation of: (i) aliphatic amines by substitution of halogenoalkanes with excess ethanolic ammonia, (ii) aromatic amines by reduction of nitroarenes using tin and concentrated hydrochloric acid; Synthesis of azo dyes Nitrous acid is generated in situ from NaNO2/HCl. (d) describe the synthesis of an azo dye by reaction of an aromatic amine with nitrous acid ( with formation of a diazonium ion, followed by coupling with a phenol under alkaline conditions; (e) state the use of reactions, such as (d), in the formation of dyestuffs.