Halogen compounds (A Level)

• The lone pair goes into the ring, so the C–Cl bond gets stronger and the nucleophile gets pushed away: chlorobenzene gives no reaction, not a slow one.

• 31.1.1 I can recall — recall the reactions by which halogenoarenes can be produced: substitution of an arene with Cl₂ or Br₂ in the presence of a catalyst, AlCl₃ or AlBr₃, to form a halogenoarene, exemplified by benzene to form chlorobenzene and methylbenzene to form 2-chloromethylbenzene and 4-chloromethylbenzene
• 31.1.2 I can explain — explain the difference in reactivity between a halogenoalkane and a halogenoarene as exemplified by chloroethane and chlorobenzene

What the Data section does and does not say. It gives C–Cl as 340 kJ mol⁻¹, an average over many compounds. Chlorobenzene’s bond lies above that average. No value for it is given, so quote the direction (“stronger than the average C–Cl bond”), never a number.

• “Chlorobenzene is hydrolysed by NaOH(aq), just more slowly than chloroethane.” Correct Chlorobenzene does not react with NaOH(aq), KCN or NH₃ under laboratory conditions — not slowly, not at all. The observation to write is “no reaction”, and with silver nitrate “no precipitate”.
• “The ring repels chlorine’s lone pair.” Correct The lone pair overlaps with the delocalised π system: electron density moves from chlorine into the ring. What the ring repels is the incoming nucleophile.
• “Chlorine and methylbenzene give chloromethylbenzene.” Correct Say which one. With AlCl₃ and no light: 2-chloromethylbenzene and 4-chloromethylbenzene, chlorine on the ring. In ultraviolet light: (chloromethyl)benzene, C₆H₅CH₂Cl, chlorine on the side-chain — a halogenoalkane.
• “340 kJ mol⁻¹ is the C–Cl bond energy in chlorobenzene.” Correct 340 kJ mol⁻¹ is the Data-section average for C–Cl. The aryl C–Cl bond is stronger than that average; no figure for it is given, so do not invent one.
• “Bromobenzene is made with iron filings.” Correct The syllabus names the catalyst as AlCl₃ or AlBr₃, the halogen carrier. Use those.
• “The curly arrow starts at the Cl⁺.” Correct The first arrow starts at the delocalised π system (the circle) and ends at the δ+ chlorine. No arrow ever starts at an atom label or a charge.
• “Chlorobenzene is made from benzene and HCl.” Repair The reagent is Cl₂, with the halogen carrier AlCl₃; HCl is the by-product.
• “Chlorine and methylbenzene give chloromethylbenzene.” Repair With AlCl₃ in the dark, 2-chloromethylbenzene and 4-chloromethylbenzene, on the ring. The side-chain product needs ultraviolet light and is (chloromethyl)benzene.
• “Chlorobenzene is hydrolysed by NaOH(aq), just more slowly.” Repair Under laboratory conditions it does not react at all. Write “no reaction”.
• “Chlorobenzene is unreactive because the C–Cl bond is less polar, and that is all.” Repair The bond is also stronger (partial double-bond character from lone-pair overlap) and the ring repels the nucleophile; give both, and say why SN1 and SN2 are blocked.
• “The chlorine’s lone pair is repelled by the ring.” Repair The lone pair overlaps with the π system; it is the incoming nucleophile that is repelled.
• “Chlorobenzene reacts by SN1 because the ring stabilises the carbocation.” Repair A positive charge on an sp² ring carbon cannot be stabilised, so Cl⁻ does not leave and there is no SN1.
• “Chlorobenzene gives a white precipitate with silver nitrate on warming.” Repair No precipitate: no Cl⁻ is released. That absence is the distinguishing observation.
• “340 kJ mol⁻¹ is the C–Cl bond energy of chlorobenzene.” Repair It is the Data-section average for C–Cl; chlorobenzene’s bond is stronger than that average, and no value is given for it.
• “(Chloromethyl)benzene is a halogenoarene, so it does not react with NaOH(aq).” Repair Its chlorine is on an sp³ side-chain carbon: it is a halogenoalkane and is hydrolysed to phenylmethanol.
• “Bromobenzene is made with iron filings.” Repair The syllabus names the catalysts AlCl₃ and AlBr₃; use AlBr₃ with Br₂.

• 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.
• Write the observation, not a rate. Chlorobenzene is attacked by nucleophiles only under conditions far beyond the laboratory. Under the conditions in this table the answer is “no reaction” or “no precipitate”, never “reacts slowly”.
• Interleave with the chapters that use this one. In chapter 33 (33.3.4), acyl chlorides are ranked against chloroalkanes and chloroarenes for ease of hydrolysis: re-answer worked example 3 before you start it. In chapter 36, halogenoarenes appear as steps in synthetic routes: re-answer retrieval question 12 so you never treat (chloromethyl)benzene as unreactive. 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 31 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 item on A Level content: topic 31 is examined in structured questions. Expect to give the reagents and conditions for making a halogenoarene, describe the chlorination or bromination mechanism with curly arrows, identify the products from methylbenzene, and explain why chlorobenzene does not react where chloroethane does. It also turns up inside synthesis questions (topic 36) as one step of a route.
• The topic produces no calculation of its own. The data are the Data-section average bond energies C–Cl 340, C–Br 280 and C–I 240 kJ mol⁻¹, which you are given in the examination and use to explain an order of reactivity. No value for the C–Cl bond in chlorobenzene is given, and none should be quoted.
• The silver nitrate experiment is a rate experiment timed to the first appearance of a precipitate. A Paper 5 question could ask you to plan it (variables, apparatus and its precision, what is held constant), to analyse a table of times, or to evaluate it: the largest error, its percentage, an improvement, and whether equal drops make a fair test.
• 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.