Carbonyl compounds

• δ+ carbon, so nucleophiles add. Distil the aldehyde out before it can oxidise; then DNPH → Tollens’ → iodine: is it there, which is it, is there a CH₃CO–?
• δ+ carbon, so nucleophiles add. Distil for the aldehyde, reflux for the acid. DNPH, then Tollens’, then iodine: is it there, which is it, is there a CH₃CO–?

• 17.1.1 I can recall — recall the reactions (reagents and conditions) by which aldehydes and ketones can be produced: (a) the oxidation of primary alcohols using acidified K2Cr2O7 or acidified KMnO4 and distillation to produce aldehydes (b) the oxidation of secondary alcohols using acidified K2Cr2O7 or acidified KMnO4 and distillation to produce ketones
• 17.1.2 I can describe — describe: (a) the reduction of aldehydes and ketones using NaBH4 or LiAlH4 to produce alcohols (b) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat to produce hydroxynitriles as exemplified by ethanal and propanone
• 17.1.3 I can describe — describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones in 17.1.2(b)
• 17.1.4 I can describe — describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent) to detect the presence of carbonyl compounds
• 17.1.5 I can deduce — deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (Fehling's and Tollens' reagents; ease of oxidation)
• 17.1.6 I can deduce — deduce the presence of a CH3CO- group in an aldehyde or ketone, CH3CO-R, from its reaction with alkaline I2(aq) to form a yellow precipitate of tri-iodomethane and an ion, RCO2-

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.

• “Oxidise a primary alcohol with acidified dichromate(VI) to make the aldehyde.” Correct Distil for the aldehyde, reflux for the acid. The condition is part of the answer: the aldehyde is distilled off as it forms. Heated under reflux, the same mixture gives the carboxylic acid.
• “HCN is the nucleophile.” Correct The nucleophile is the cyanide ion, :CN⁻, bonding through its carbon. HCN is a weak acid that supplies very few ions, which is why KCN is added as the catalyst.
• “The CN⁻ attacks the oxygen, and the intermediate has its negative charge on carbon.” Correct The nucleophile attacks the δ+ carbon. The π electrons move onto the oxygen, so the intermediate carries O⁻, which then takes H from HCN.
• “Propanone and HCN give 2-hydroxypropanenitrile.” Correct Count the nitrile carbon as carbon 1. Propanone gives 2-hydroxy-2-methylpropanenitrile, (CH₃)₂C(OH)CN. 2-Hydroxypropanenitrile, CH₃CH(OH)CN, comes from ethanal.
• “An orange precipitate with 2,4-DNPH proves an aldehyde.” Correct It proves an aldehyde or a ketone. A second test is needed to tell them apart: Fehling’s or Tollens’ reagent, or ease of oxidation by warm acidified dichromate(VI).
• “Fehling’s gives a red precipitate of copper.” Correct An orange/red precipitate of copper(I) oxide, Cu₂O: copper goes from +2 to +1, not to the metal.
• “Any aldehyde gives a yellow precipitate with alkaline iodine.” Correct Only compounds with CH₃CO– (or CH₃CH(OH)–). Ethanal is the only aldehyde that responds; propanal does not.
• “Oxidising a primary alcohol gives an aldehyde.” Repair Only if the aldehyde is distilled off as it forms. Under reflux it is oxidised on to the carboxylic acid.
• “Ketones are oxidised to carboxylic acids by heating under reflux with dichromate(VI).” Repair A ketone has no hydrogen on its carbonyl carbon; it is not oxidised without a C–C bond breaking, and the dichromate(VI) stays orange.
• “Tertiary alcohols are oxidised to ketones.” Repair Tertiary alcohols are not oxidised by acidified dichromate(VI) or manganate(VII). Ketones come from secondary alcohols.
• “NaBH₄ reduces the C=C in an unsaturated ketone.” Repair NaBH₄ and LiAlH₄ supply H⁻, a nucleophile, which attacks only the δ+ carbon of the polar C=O. The non-polar C=C is left untouched.
• A curly arrow drawn from the C of CN⁻ to the O of C=O, or the intermediate drawn with the negative charge on carbon. Repair The nucleophile attacks the δ+ carbon. The π electrons move onto the oxygen, which carries O⁻ and then takes a proton from HCN.
• “HCN is the nucleophile, and KCN is a reactant.” Repair The nucleophile is CN⁻. HCN is a weak acid, so KCN is added to supply the ion, and the CN⁻ is regenerated in step 2: KCN is a catalyst.
• “Propanone + HCN gives 2-hydroxypropanenitrile.” Repair Count the nitrile carbon as C1: propanone gives 2-hydroxy-2-methylpropanenitrile, (CH₃)₂C(OH)CN. Ethanal gives 2-hydroxypropanenitrile.
• “The product from ethanal is a single compound because the reaction happens at one carbon.” Repair Carbon 2 of CH₃CH(OH)CN is chiral, and the planar C=O is attacked from either face with equal probability, so equal amounts of the two enantiomers form.
• “2,4-DNPH shows an aldehyde is present.” Repair It shows a carbonyl compound, an aldehyde or a ketone. Tollens’ or Fehling’s reagent then decides which.
• “Ethanoic acid gives an orange precipitate with 2,4-DNPH because it has C=O.” Repair Carboxylic acids and esters do not react. The test responds to the C=O of aldehydes and ketones only.
• “Fehling’s gives a red precipitate of copper.” Repair The precipitate is copper(I) oxide, Cu₂O, described as orange/red. Copper is reduced from +2 to +1, not to the metal.
• “Propanal gives a yellow precipitate with alkaline iodine.” Repair Propanal has CH₃CH₂CO–, not CH₃CO–. Ethanal is the only aldehyde that responds, and a positive result can also mean a CH₃CH(OH)– alcohol.
• “CH₃COR + I₂ → CHI₃ + RCOOH”, or the ion written with the same number of carbons as the ketone. Repair CH₃COR + 3I₂ + 4OH⁻ → CHI₃ + RCO₂⁻ + 3I⁻ + 3H₂O. The conditions are alkaline, so the product is the ion RCO₂⁻, with one carbon fewer than CH₃COR.
• Carrying out the Fehling’s, Tollens’ and iodine tests cold, or over a Bunsen flame. Repair All three are warmed in a water bath: cold tubes give false negatives, and the organic liquids are flammable.

• 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.
• Where every arrow starts and ends. Draw the lone pair on the carbon of CN⁻ and the δ+ and δ− on the C=O before any arrow. Arrow 1 starts at that lone pair and ends at the carbonyl carbon, not at the oxygen. Arrow 2 starts at the middle of the C=O double bond and ends at O. In step 2 the arrow starts at a lone pair on O⁻, and the arrow from the H–C bond ends at the carbon of CN. A mechanism with the negative charge on the carbon, or with HCN as the attacking species, has the chemistry wrong, not only the drawing.
• Warm every tube, in a water bath. Fehling’s and Tollens’ reagents react slowly in the cold, and a cold tube gives a false negative that makes an aldehyde look like a ketone. The compounds are flammable, so the heat comes from a beaker of hot water, not a flame. The syllabus notes that glucose may be used in place of an aldehyde when these tests are practised.
• Interleave with the chapters that use this one. Topic 18 takes aldehydes on to carboxylic acids and hydrolyses nitriles: re-answer the distil-or-reflux question there. Topic 21 builds synthetic routes: redraw the HCN addition as the step that adds one carbon. Topic 22 identifies C=O by infrared: re-answer the 2,4-DNPH questions beside the spectrum. 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 17 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 this topic can turn on a single precise fact: which alcohol gives which carbonyl compound, which reagent reduces C=O but not C=C, which compound gives a yellow precipitate with alkaline iodine, or how many carbons a hydroxynitrile has. Paper 2 asks you to give reagents and conditions, describe the mechanism of HCN addition with curly arrows, lone pairs, dipoles and charges, deduce a structure from a set of test results, and write balanced equations using [O] and [H].
• Topic 17 has no calculation of its own. The numbers you meet are the Data-section electronegativities of oxygen (3.5) and carbon (2.5), which explain the polar C=O, molecular formulae to count carbons in, and equations to balance in atoms and in charge, such as the tri-iodomethane equation. A yield calculation on a preparation is topic 2 applied to this chapter’s reactions.
• Paper 3 may ask you to carry out or interpret the organic tests the syllabus lists: Fehling’s and Tollens’ reagents for an aldehyde, alkaline aqueous iodine for CH₃CO– or CH₃CH(OH)–, and acidified manganate(VII) for a compound that can be oxidised. Every observation is recorded in the syllabus’s colour words, including “no change”. A Paper 5-style question can ask you to plan how to show that distillation and reflux give different products.
• 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.