Halogen compounds

• Aqueous NaOH substitutes, ethanolic NaOH eliminates; primary goes SN2, tertiary goes SN1; and the weakest C–X bond reacts fastest.
• Aqueous substitutes, ethanolic eliminates. Primary SN2, tertiary SN1. The weakest C–X bond reacts fastest.

• 15.1.1 I can recall — recall the reactions (reagents and conditions) by which halogenoalkanes can be produced: (a) the free-radical substitution of alkanes by Cl2 or Br2 in the presence of ultraviolet light, as exemplified by the reactions of ethane (b) electrophilic addition of an alkene with a halogen, X2, or hydrogen halide, HX(g), at room temperature (c) substitution of an alcohol, e.g. by reaction with HX(g); or with KCl and concentrated H2SO4 or concentrated H3PO4; or with PCl3 and heat; or with PCl5; or with SOCl2
• 15.1.2 I can classify — classify halogenoalkanes into primary, secondary and tertiary
• 15.1.3 I can describe — describe the following nucleophilic substitution reactions: (a) the reaction with NaOH(aq) and heat to produce an alcohol (b) the reaction with KCN in ethanol and heat to produce a nitrile (c) the reaction with NH3 in ethanol heated under pressure to produce an amine (d) the reaction with aqueous silver nitrate in ethanol as a method of identifying the halogen present as exemplified by bromoethane
• 15.1.4 I can describe — describe the elimination reaction with NaOH in ethanol and heat to produce an alkene as exemplified by bromoethane
• 15.1.5 I can describe — describe the SN1 and SN2 mechanisms of nucleophilic substitution in halogenoalkanes including the inductive effects of alkyl groups
• 15.1.6 I can recall — recall that primary halogenoalkanes tend to react via the SN2 mechanism; tertiary halogenoalkanes via the SN1 mechanism; and secondary halogenoalkanes by a mixture of the two, depending on structure
• 15.1.7 I can describe — describe and explain the different reactivities of halogenoalkanes (with particular reference to the relative strengths of the C-X bonds as exemplified by the reactions of halogenoalkanes with aqueous silver nitrates)

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.

• “Bromoethane with sodium hydroxide gives ethanol.” Correct Only with aqueous NaOH and heat. With NaOH dissolved in ethanol and heat the same reagent acts as a base and gives ethene. The solvent is part of the answer every time you write NaOH.
• “Chloroalkanes react fastest because the C–Cl bond is the most polar.” Correct Polarity predicts that order and the experiment refutes it. The observed order is iodo > bromo > chloro, the order in which the C–X bond energy falls (C–Cl 340, C–Br 280, C–I 240 kJ mol⁻¹, Data section). Bond strength controls the reactivity.
• “1-Bromo-2-methylpropane is branched, so it is secondary.” Correct Classify by the carbon that carries the halogen. Here it is a CH₂ bonded to one other carbon: primary. The branch is on the next carbon.
• “Bromoethane and KCN give ethanenitrile.” Correct The cyanide carbon joins the chain, so the product has three carbons: propanenitrile, CH₃CH₂CN. And the KCN is dissolved in ethanol, not water.
• “In SN1 the hydroxide ion knocks the bromine off.” Correct In SN1 the C–Br bond breaks on its own in the slow first step; the nucleophile arrives only in the second step, at the carbocation. Drawing OH⁻ in step 1 turns it into SN2.
• “The silver nitrate reacts with the halogen in the halogenoalkane.” Correct A covalently bonded halogen gives no precipitate. Water first hydrolyses the C–X bond and releases X⁻(aq); only then does Ag⁺(aq) precipitate it.
• “1-Bromo-2-methylpropane is secondary because it is branched.” Repair Classify by the carbon carrying the Br. It is a CH₂ bonded to one carbon: primary.
• “NaOH, heat → alcohol.” Repair Aqueous NaOH, heat under reflux, substitutes; NaOH in ethanol, heat, eliminates. Without the solvent the answer names neither reaction.
• “Bromoethane + KCN → ethanenitrile.” Repair The cyanide carbon joins the chain: propanenitrile, CH₃CH₂CN, three carbons.
• “Reagent: aqueous KCN, heat.” Repair KCN in ethanol, heat under reflux. In water, hydroxide ions and water would compete as nucleophiles and give the alcohol.
• “NH₃, heat → amine.” Repair NH₃ in ethanol, heated under pressure, with excess ammonia; and the equation needs 2NH₃, the second one to remove the proton and form NH₄Br.
• “Silver nitrate reacts with the halogenoalkane to give the precipitate.” Repair Water first hydrolyses the C–X bond and releases X⁻; then Ag⁺ precipitates it. A covalently bonded halogen gives no precipitate.
• “AgBr is a white precipitate.” Repair AgCl white, AgBr cream, AgI pale yellow: the syllabus’s colour words.
• SN2 drawn with OH⁻ approaching from the same side as the bromine. Repair Attack is from the side opposite the leaving group, through a bracketed transition state with ‡, and the configuration inverts.
• SN1 step 1 drawn with OH⁻ pushing the bromine off. Repair In SN1 the C–Br bond breaks on its own; the nucleophile attacks the carbocation in step 2.
• A carbocation drawn tetrahedral, or with the + floating beside the ion. Repair It is trigonal planar, three bonds at 120°, with the + on the carbon; the nucleophile attacks either face.
• “Tertiary halogenoalkanes use SN1 because they are bigger.” Repair The tertiary carbocation is stabilised by the electron-releasing (inductive) effect of three alkyl groups, and the crowded carbon blocks SN2 attack. Both reasons, not “bigger”.
• “Chloroalkanes are the most reactive because C–Cl is the most polar.” Repair Reactivity follows bond strength: C–I (240 kJ mol⁻¹) is the weakest and breaks fastest, so iodoalkanes react fastest.

• 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.
• Why concentrated H₃PO₄ is offered. Concentrated sulfuric acid is an oxidising agent: topic 11 shows it oxidising bromide and iodide ions to the halogens. Concentrated phosphoric acid is not, so it is the non-oxidising alternative. The syllabus names the reaction with KCl only; do not extend route (2) to other halide salts in an answer.
• Where every arrow starts. The first arrow starts at a lone pair on oxygen, not at the letter O or at the minus sign, and ends at the carbon. The second starts at the middle of the C–Br bond and ends at Br. Draw the lone pairs and the dipole before the arrows; they are part of the mechanism, not decoration.
• Interleave with the chapters that use this one. Topic 16 meets the alcohol substitutions from the other side; re-answer the PCl₅ and SOCl₂ equations there. Topic 19 takes nitriles and amines further; re-answer the KCN and ammonia conditions there. Topic 21 uses the nitrile to lengthen a chain; re-draw the bromoethane reaction map there. 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 15 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 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.
• A Paper 1-style item on this topic can turn on one word: the solvent for NaOH or KCN, the class of a branched halogenoalkane, the number of carbons in a nitrile, or which mechanism has a carbocation. A Paper 2 structured question asks you to give reagents and conditions, write equations, describe a mechanism with every dipole, lone pair and curly arrow, and explain the SN1/SN2 choice and the reactivity order.
• The C–Cl, C–Br and C–I bond energies (340, 280 and 240 kJ mol⁻¹) and the electronegativities are given in the Data section, not recalled; you are expected to use them to explain the reactivity order. Mole calculations appear as percentage yields of a substitution (topic 2 skills in a topic 15 setting).
• The silver nitrate comparison is a rate experiment by timing an observation: the time to the first precipitate, with the halogen as the independent variable. A planning or evaluation question asks for the variables, the controlled conditions, the precision of the thermometer and stop-clock, the observations in the syllabus’s colour words, and the largest source of error with a realistic improvement.
• 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.