Hydrocarbons (A Level)

• The ring is kept, not opened: an electrophile swaps for a hydrogen, and the catalyst comes back.
• Arrow from the π system, arrow from the C–H bond, catalyst back. Carrier in the dark: ring. Ultraviolet light: side-chain. Lone pair or alkyl: 2,4. δ+ atom on the ring: 3.

• 30.1.1 I can describe — describe the chemistry of arenes as exemplified by the following reactions of benzene and methylbenzene: (a) substitution reactions with Cl₂ and with Br₂ in the presence of a catalyst, AlCl₃ or AlBr₃, to form halogenoarenes (aryl halides) (b) nitration with a mixture of concentrated HNO₃ and concentrated H₂SO₄ at a temperature between 25 °C and 60 °C (c) Friedel–Crafts alkylation by CH₃Cl and AlCl₃ and heat (d) Friedel–Crafts acylation by CH₃COCl and AlCl₃ and heat (e) complete oxidation of the side-chain using hot alkaline KMnO₄ and then dilute acid to give a benzoic acid (f) hydrogenation of the benzene ring using H₂ and Pt/Ni catalyst and heat to form a cyclohexane ring
• 30.1.2 I can describe — describe the mechanism of electrophilic substitution in arenes: (a) as exemplified by the formation of nitrobenzene and bromobenzene (b) with regards to the effect of delocalisation (aromatic stabilisation) of electrons in arenes to explain the predomination of substitution over addition
• 30.1.3 I can predict — predict whether halogenation will occur in the side-chain or in the aromatic ring in arenes depending on reaction conditions
• 30.1.4 I can describe — describe that in the electrophilic substitution of arenes, different substituents direct to different ring positions (limited to the directing effects of –NH₂, –OH, –R, –NO₂, –COOH and –COR)

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “The first arrow starts at a carbon atom of the ring, and the second at the H.” Correct A curly arrow moves a pair of electrons, so it starts where a pair is: arrow 1 at the delocalised π system (the circle); the arrow in the second step, which removes H⁺, at the C–H bond (arrow 2 in nitration, arrow 3 in bromination, where the Br–Br arrow is arrow 2). Never at an atom label or a charge.
• “The intermediate is benzene with a + sign next to it.” Correct The circle is broken. One carbon is sp³ and carries both H and the new group; the other five share four π electrons and the positive charge, drawn as an open arc with + inside.
• “Nitrate benzene at 25 °C, and no warmer.” Correct The syllabus condition is concentrated HNO₃ and concentrated H₂SO₄ at a temperature between 25 °C and 60 °C. Quote the range, not a single number inside it.
• “Chlorine and light chlorinate the ring of methylbenzene.” Correct Ultraviolet light makes radicals, and radicals substitute the side-chain. The ring needs a halogen carrier, AlCl₃ or AlBr₃, in the absence of ultraviolet light.
• “Oxidise methylbenzene with acidified KMnO₄.” Correct Hot alkaline KMnO₄, and then dilute acid: the alkaline oxidation gives the benzoate ion, and the acid turns it into benzoic acid.
• “–NO₂ is a big group, so it pushes the next one to the far side of the ring.” Correct Size is not the reason. –NO₂ withdraws electron density, and the 3-position is the least depleted. –OH, –NH₂ and alkyl groups donate density and direct to 2 and 4.
• “Benzene reacts with bromine like an alkene.” Repair It needs a halogen carrier, it substitutes rather than adds, and it does not decolourise bromine water.
• “The electrophile in nitration is HNO₃.” Repair It is the nitronium ion, NO₂⁺, made from HNO₃ by concentrated H₂SO₄: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.
• The first curly arrow drawn from a ring carbon atom, or from the + of NO₂⁺. Repair Arrows start at an electron pair: the first at the delocalised π system, and the one in the second step (which removes H⁺) at the C–H bond.
• The intermediate drawn with the circle intact and a + sign outside the ring. Repair The circle is broken: an open arc over five carbons with + inside, and the sp³ carbon carrying both H and the new group.
• “H₂SO₄ is used up in nitration.” Repair HSO₄⁻ takes the H⁺ back, so H₂SO₄ is a catalyst and is regenerated — as AlBr₃ is from AlBr₄⁻ in bromination.
• “Substitution occurs because the ring is too stable to react.” Repair The ring does react. It substitutes because losing H⁺ restores the delocalisation energy that addition would lose.
• “Nitration is carried out at 25 °C only.” Repair The syllabus’s condition is a temperature between 25 °C and 60 °C, with both acids concentrated.
• “Methylbenzene is oxidised by acidified KMnO₄.” Repair Hot alkaline KMnO₄, then dilute acid to release the acid from its salt.
• “Ethylbenzene is oxidised to phenylethanoic acid.” Repair The whole side-chain becomes one –COOH on the ring: benzoic acid.
• “Chlorine in ultraviolet light chlorinates the ring.” Repair In methylbenzene, light gives radicals, and radicals attack the side-chain; ring chlorination needs a halogen carrier in the absence of ultraviolet light.
• “2-chloromethylbenzene” written for C₆H₅CH₂Cl. Repair That is (chloromethyl)benzene; 2-chloromethylbenzene has the Cl on the ring, next to the CH₃.
• “–NO₂ directs to 2 and 4 because it is a big group.” Repair It withdraws electron density, so the new electrophile enters at the 3-position; size is not the reason.
• “–OH directs to 3.” Repair –OH and –NH₂ have a lone pair to donate into the ring and direct to 2 and 4 (and 6).
• Hydrogenation of benzene written up as a substitution. Repair It is the addition of three H₂; the product, cyclohexane, has no π system at all.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 31 starts from the chloromethylbenzene pair made here; topic 32 explains why phenol substitutes so much more easily than benzene; topic 33 recalls benzoic acid from methylbenzene; topic 34 reduces nitrobenzene to phenylamine; topic 36 builds routes in which the order of steps fixes the product. At each, re-answer: which group is on the ring when the electrophile arrives, and where does it send it? Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 30 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• There is no multiple-choice paper on A Level content, so topic 30 is met in Paper 4 structured questions. They ask you to give reagents and conditions for a reaction of benzene or methylbenzene, describe the electrophilic substitution mechanism with curly arrows for nitration or bromination, explain why an arene substitutes rather than adds, predict whether halogenation happens in the ring or the side-chain, and deduce where a second group enters a substituted ring. Topic 36 then asks you to put these reactions into routes, where the order of steps matters.
• The arithmetic is percentage yield from a preparation: moles of the arene, a 1 : 1 equation, the theoretical mass and the yield, to the significant figures of the data. Relative atomic masses come from the Periodic Table you are given (C 12.0, H 1.0, N 14.0, O 16.0, Br 79.9). The one energy figure this chapter uses, the 240 kJ mol⁻¹ estimate of benzene’s delocalisation energy, comes from chapter 29’s Data-section argument and is not a value to recall.
• Benzene itself is not used in school laboratories, so the procedure this topic supplies is methylbenzene’s side-chain oxidation: reflux with alkaline KMnO₄, filter off MnO₂, acidify, collect and dry the white benzoic acid, and check its melting point. A Paper 5 question can ask you to plan it, to set out the results table, to compute the yield, to identify the largest source of error and to explain a yield above 100%.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.