Hydroxy compounds

• Distil for the aldehyde, reflux for the acid; a tertiary alcohol is not oxidised; only CH₃CH(OH)– gives tri-iodomethane.
• Distil for the aldehyde, reflux for the acid. Tertiary: no H on the –OH carbon, stays orange. Only CH₃CH(OH)– gives yellow CHI₃, and RCO₂⁻ has one carbon fewer.

• 16.1.1 I can recall — recall the reactions (reagents and conditions) by which alcohols can be produced: (a) electrophilic addition of steam to an alkene, using H2O(g) and H3PO4 catalyst (b) reaction of alkenes with cold dilute acidified potassium manganate(VII) to form a diol (c) substitution of a halogenoalkane using NaOH(aq) and heat (d) reduction of an aldehyde or ketone using NaBH4 or LiAlH4 (e) reduction of a carboxylic acid using LiAlH4 (f) hydrolysis of an ester using dilute acid or dilute alkali and heat
• 16.1.2 I can describe — describe: (a) the reaction with oxygen (combustion) (b) substitution to form halogenoalkanes, 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 (c) the reaction with Na(s) (d) oxidation with acidified K2Cr2O7 or acidified KMnO4 to: (i) carbonyl compounds by distillation (ii) carboxylic acids by refluxing (primary alcohols give aldehydes which can be further oxidised to carboxylic acids, secondary alcohols give ketones, tertiary alcohols cannot be oxidised) (e) dehydration to an alkene, by using a heated catalyst, e.g. Al2O3 or a concentrated acid (f) formation of esters by reaction with carboxylic acids and concentrated H2SO4 as catalyst as exemplified by ethanol
• 16.1.3 I can — (a) classify alcohols as primary, secondary and tertiary alcohols, to include examples with more than one alcohol group (b) state characteristic distinguishing reactions, e.g. mild oxidation with acidified K2Cr2O7, colour change from orange to green
• 16.1.4 I can deduce — deduce the presence of a CH3CH(OH)- group in an alcohol, CH3CH(OH)-R, from its reaction with alkaline I2(aq) to form a yellow precipitate of tri-iodomethane and an ion, RCO2-
• 16.1.5 I can explain — explain the acidity of alcohols compared with water

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.

• “Heat ethanol under reflux with acidified dichromate(VI) to make ethanal.” Correct Reflux returns the aldehyde to the oxidant, so it is oxidised on to ethanoic acid. To stop at ethanal, distil it off as it forms.
• “A tertiary alcohol is oxidised to a ketone.” Correct A tertiary alcohol is not oxidised. Oxidation to C=O removes a hydrogen from the –OH carbon, and a tertiary –OH carbon has none. The dichromate(VI) stays orange.
• “Any secondary alcohol gives a yellow precipitate with alkaline iodine.” Correct Only an alcohol with the CH₃CH(OH)– group does, and that includes ethanol, where R = H. Pentan-3-ol is secondary and gives nothing.
• “NaOH, heat: halogenoalkane to alcohol.” Correct NaOH(aq), heat. NaOH in ethanol gives an alkene instead, so the solvent is part of the answer.
• “NaBH₄ reduces a carboxylic acid to an alcohol.” Correct Only LiAlH₄ reduces a carboxylic acid. NaBH₄ and LiAlH₄ both reduce aldehydes and ketones.
• “2-Methylpropan-1-ol is secondary, because it is branched.” Correct Count the carbons bonded to the –OH carbon. It is a CH₂ bonded to one carbon: primary.
• “The alkyl group withdraws electrons, so ethanol is a stronger acid than water.” Correct The alkyl group releases electron density, which destabilises the ethoxide ion. Ethanol is the weaker acid.
• “2-Methylpropan-1-ol is secondary because the chain is branched.” Repair Count the carbons bonded to the –OH carbon. CH₂OH is bonded to one carbon, so it is primary.
• “Refluxing a primary alcohol with dichromate(VI) gives the aldehyde.” Repair Reflux gives the carboxylic acid; the aldehyde is obtained by distilling it off as it forms.
• “A tertiary alcohol is oxidised to a ketone.” Repair Its –OH carbon has no hydrogen, so it is not oxidised, and the dichromate(VI) stays orange.
• “The solution turns green because the product is green.” Repair The green is Cr³⁺, the reduced form of orange Cr₂O₇²⁻. The organic product is colourless.
• “NaBH₄ reduces ethanoic acid to ethanol.” Repair Only LiAlH₄ reduces a carboxylic acid; NaBH₄ reduces aldehydes and ketones.
• “Hydration of propene gives propan-1-ol.” Repair The –OH goes to the more substituted carbon: propan-2-ol is the major product.
• “Ethene + KMnO₄ gives ethane-1,2-diol.” Repair Cold dilute acidified KMnO₄. Hot concentrated manganate(VII) breaks the C=C instead.
• “Dehydrating butan-2-ol gives but-2-ene.” Repair It gives three alkenes: but-1-ene, cis-but-2-ene and trans-but-2-ene.
• “Ethanol is a stronger acid than water because it reacts with sodium.” Repair Water reacts more vigorously with sodium; ethanol is the weaker acid.
• “The alkyl group withdraws electrons, so the ethoxide ion is stabilised.” Repair The alkyl group releases electron density, which destabilises the ethoxide ion by concentrating the charge on oxygen.
• “Any alcohol gives a yellow precipitate with alkaline iodine.” Repair Only alcohols with the CH₃CH(OH)– group, including ethanol (R = H). Methanol, propan-1-ol, pentan-3-ol and every tertiary alcohol give nothing.
• “Ethanol + propanoic acid gives propyl ethanoate.” Repair The alcohol supplies the alkyl part, named first: ethyl propanoate, CH₃CH₂COOCH₂CH₃.

• 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 17 (carbonyl compounds): re-answer “which alcohol gives this aldehyde or ketone, and how?”. Topic 18 (carboxylic acids and esters): re-answer the esterification equation and the naming rule. Topic 21 (organic synthesis): re-draw the ethanol reaction map from memory, because almost every route passes through an alcohol. Topic 32, the A Level twin, builds on the acidity argument in section H. 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 16 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 item on alcohols turns on one exact detail: the class of an alcohol, the one reagent that works (LiAlH₄ for an acid), the product of distillation versus reflux, which alcohols give tri-iodomethane, or how many alkenes a dehydration gives. A Paper 2 structured question asks you to give reagents and conditions, write balanced equations with [O], describe the observations of a test, deduce a structure from observations, or explain the acidity of an alcohol compared with water.
• The numbers here are stoichiometry: the percentage yield of an ester or an oxidation product, the volume of hydrogen from an alcohol and sodium (Vm = 24.0 dm³ mol⁻¹ at room conditions), combustion equations, and the balanced ionic equation for dichromate(VI) oxidation. The Data section supplies the relative atomic masses, the molar gas volume, the electronegativities (O 3.5, C 2.5, H 2.1) and the Cr₂O₇²⁻/Cr³⁺ half-equation.
• Paper 3 may ask you to carry out and record two organic tests this topic explains: alkaline aqueous iodine and acidified potassium manganate(VII), recording every observation, including “no change”. Reflux and distillation are planning material in the Paper 5 style: which apparatus gives which product, the direction of the cooling water, the thermometer position, the controls, and the largest source of error.
• 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.