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The carbonyl group, C=O, is the single most important functional group in organic chemistry. It appears in aldehydes (RCHO), ketones (RR'CO), carboxylic acids (RCOOH), esters (RCOOR'), amides (RCONR'₂), and acyl chlorides (RCOCl) — collectively the carbonyl family that dominates A2 organic chemistry. This lesson focuses on aldehydes and ketones: the two simplest carbonyls in which C=O is bonded to H or to carbon substituents alone. We begin with the structure and bonding of C=O, then move to the four standard distinguishing tests (Fehling's solution, Tollens' reagent, acidified potassium dichromate, and 2,4-DNPH or Brady's reagent), and the iodoform test for methyl ketones. The second half develops the nucleophilic addition mechanism — the defining reaction of C=O — applied to reduction by sodium tetrahydridoborate(III) and addition of hydrogen cyanide to give 2-hydroxynitriles, including the stereochemical consequence (racemic mixture from an achiral carbonyl). Synthetic routes and a practical-skills box on Tollens' safety close the chemistry.
Spec mapping (AQA 7405): This lesson maps to §3.3.8 (aldehydes and ketones — nucleophilic addition, distinguishing tests, reduction, and hydroxynitrile formation). It depends on foundations-L7 (alcohols — primary alcohols oxidise to aldehydes then to carboxylic acids; secondary alcohols oxidise to ketones) and §3.3.6 (organic analysis — chemical tests for functional groups and 2,4-DNPH melting-point identification). It feeds forward into L1 of this course (carboxylic acids — products of aldehyde oxidation) and L2 (esters and amides — derivatives of carboxylic acids). Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: AO1 recall items include the colour changes of the four standard tests, the structures of Fehling's complex and Tollens' reagent, and the components of the nucleophilic addition mechanism (curly arrows, partial charges, intermediates). AO2 tasks dominate every Paper 2: distinguishing an aldehyde from a ketone using a named test with full colour-change observation, writing the curly-arrow mechanism for CN⁻ or H⁻ addition to a specified carbonyl, and identifying products of NaBH₄ reduction. AO3 reasoning appears in the iodoform-test specificity question (why only methyl ketones and CH₃CH(OH)-R alcohols give CHI₃), in stereochemistry-of-addition questions (why CN⁻ + propanal produces a racemate), and in synthetic-route design (using HCN to extend a carbon chain by one).
The carbonyl carbon is sp² hybridised — three sp² orbitals at 120° give a trigonal planar geometry, with the carbonyl C and its three attached atoms all in one plane. The remaining 2p orbital overlaps side-on with a 2p orbital on oxygen to form the π bond. The C=O bond length is ~0.122 nm (shorter than C-O single, 0.143 nm) and the bond enthalpy is ~743 kJ mol⁻¹.
The chemically key feature is polarity. Oxygen is significantly more electronegative than carbon (Pauling 3.44 vs 2.55), so the σ and π electrons are pulled towards oxygen:
The π bond is weaker than the σ bond (the π contribution is ~280 kJ mol⁻¹). When a nucleophile adds, the π bond breaks, leaving the σ bond intact and generating a tetrahedral sp³ alkoxide — analogous to alkene electrophilic addition, but with polarity reversed.
An aldehyde has the carbonyl carbon bonded to at least one hydrogen atom: RCHO (where R is H or any alkyl/aryl group). Methanal, HCHO, is the simplest member; ethanal CH₃CHO, propanal CH₃CH₂CHO, and benzaldehyde C₆H₅CHO are common examples. The systematic name uses the suffix -al, and the CHO carbon is always carbon-1 (so no locant is needed).
A ketone has the carbonyl carbon bonded to two carbon substituents: RR'CO. Propanone CH₃COCH₃ is the simplest. The suffix is -one and a locant is required from butanone onwards (butan-2-one, pentan-2-one, pentan-3-one). Methyl ketones — those with a CH₃ group adjacent to C=O — are a synthetically important sub-class (responsible for the iodoform test, below).
Structurally, the difference is:
| Class | General formula | Carbonyl-C bonded to | Example |
|---|---|---|---|
| Aldehyde | RCHO | one H and one R | ethanal CH₃CHO |
| Ketone | RR'CO | two R groups | propanone CH₃COCH₃ |
The H on the aldehyde carbonyl is the structural feature that makes aldehydes oxidisable: the C-H bond can be broken to insert an O, giving a carboxylic acid (RCOOH). Ketones have no such H — they cannot be oxidised under mild conditions. This single fact underlies every test that distinguishes the two classes.
Four standard chemical tests are examinable. The first three exploit the oxidisability of aldehydes; the fourth (2,4-DNPH) detects any carbonyl and is used to identify which specific carbonyl is present via the melting point of the precipitate.
Fehling's solution is a deep blue solution containing Cu²⁺ ions complexed by sodium potassium tartrate (Rochelle salt) in alkaline aqueous solution. The tartrate ligand stabilises Cu²⁺ against precipitation as Cu(OH)₂ in the alkaline medium. The active oxidising agent is Cu²⁺, which is reduced to Cu⁺ (as Cu₂O).
Procedure: Add a few drops of the unknown to Fehling's solution in a test tube. Warm in a water bath at ~60-70 °C for a few minutes.
Observations:
Half-equation for the oxidation of the aldehyde:
RCHO + 3 OH⁻ → RCOO⁻ + 2 H₂O + 2 e⁻
Half-equation for the reduction of copper:
2 Cu²⁺ + 2 e⁻ + 2 OH⁻ → Cu₂O + H₂O
The aldehyde is oxidised to its carboxylate (carboxylic acid in acid; carboxylate in alkali). Aromatic aldehydes such as benzaldehyde do not give a positive Fehling's test — this is a common exam trap. Only aliphatic aldehydes are reliably detected.
Tollens' reagent is a colourless solution of the diamminesilver(I) complex [Ag(NH₃)₂]⁺, prepared by adding aqueous ammonia to silver nitrate solution until the brown Ag₂O precipitate just dissolves. The active oxidising agent is Ag⁺, which is reduced to metallic silver.
Procedure: Add a few drops of the unknown to freshly prepared Tollens' reagent in a clean test tube. Warm gently in a water bath at ~60 °C.
Observations:
Half-equations:
RCHO + 3 OH⁻ → RCOO⁻ + 2 H₂O + 2 e⁻
2 [Ag(NH₃)₂]⁺ + 2 e⁻ → 2 Ag + 4 NH₃
Aromatic aldehydes such as benzaldehyde do give a positive Tollens' test (unlike Fehling's). For exam purposes, Tollens' is therefore the most general test for any aldehyde.
Acidified K₂Cr₂O₇ is the standard A-Level oxidising agent for distinguishing primary and secondary alcohols (foundations course) and for oxidising aldehydes to carboxylic acids.
Procedure: Add a few drops of the unknown to acidified K₂Cr₂O₇ in a test tube. Warm gently. For full oxidation to the carboxylic acid, heat under reflux.
Observations:
Half-equations:
3 RCHO + 3 H₂O → 3 RCOOH + 6 H⁺ + 6 e⁻
Cr₂O₇²⁻ + 14 H⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O
The dichromate test is the same colour change as for primary and secondary alcohols, so it cannot distinguish an aldehyde from an alcohol — only from a ketone.
2,4-DNPH, also known as Brady's reagent after its developer, is a solution of 2,4-dinitrophenylhydrazine in dilute sulfuric acid in methanol. It is the standard A-Level test for the presence of any carbonyl group — both aldehydes and ketones give a positive result.
Procedure: Add a few drops of the unknown to 2,4-DNPH solution. A precipitate forms immediately at room temperature.
Observations:
Reaction (condensation):
RR'C=O + H₂N-NH-C₆H₃(NO₂)₂ → RR'C=N-NH-C₆H₃(NO₂)₂ + H₂O
The nitrogen of the hydrazine attacks the δ+ carbonyl carbon; loss of water gives a hydrazone with C=N in place of C=O. The conjugation between C=N and the two nitro groups on the dinitrophenyl ring gives the precipitate its characteristic orange colour.
Identification of the specific carbonyl: Each carbonyl gives a hydrazone of a characteristic melting point. The precipitate is recrystallised from ethanol, dried, and its melting point measured. Comparison with a tabulated database of 2,4-DNPH derivative melting points identifies the original carbonyl. For example:
| Original carbonyl | 2,4-DNPH derivative mp / °C |
|---|---|
| Methanal | 167 |
| Ethanal | 168 |
| Propanal | 154 |
| Propanone | 126 |
| Butanal | 123 |
| Butan-2-one | 117 |
| Benzaldehyde | 237 |
| Cyclohexanone | 162 |
In a question that combines a 2,4-DNPH melting point with another test (Tollens', Fehling's, iodoform), the candidate is expected to use both results to narrow the identification to a single compound.
The iodoform test is specific to compounds containing the CH₃CO- group (methyl ketones) and the CH₃CH(OH)- group (methyl secondary alcohols, which are oxidised to methyl ketones in situ by the iodine).
Procedure: Add iodine solution in alkaline conditions (I₂ in aqueous NaOH, or potassium iodide and sodium chlorate(I)) to the unknown. Warm gently.
Observations:
Overall reaction (methyl ketone case):
CH₃COR + 3 I₂ + 4 OH⁻ → CHI₃ + RCOO⁻ + 3 I⁻ + 3 H₂O
Mechanistically, the three α-hydrogens of the CH₃ group are sequentially replaced by I in base, giving CI₃-CO-R. Hydroxide then attacks the carbonyl and the CI₃⁻ group leaves as the relatively stable triiodomethyl carbanion, which picks up a proton to give CHI₃.
Positive iodoform: all methyl ketones (propanone, butan-2-one, pentan-2-one); methyl secondary alcohols (propan-2-ol, butan-2-ol); ethanol (oxidised to ethanal in situ); and ethanal itself (R = H). Negative iodoform: primary alcohols beyond ethanol (propan-1-ol, butan-1-ol); pentan-3-one (no CH₃ adjacent to C=O); aldehydes beyond ethanal (propanal, butanal). The iodoform test detects a specific carbon skeleton (CH₃ adjacent to C=O), not a functional group — its discriminating power is what makes it examinable.
| Test | Aldehyde | Methyl ketone | Other ketone | Positive observation |
|---|---|---|---|---|
| Fehling's | Yes (aliphatic) | No | No | Brick-red Cu₂O precipitate |
| Tollens' | Yes (all) | No | No | Silver mirror |
| Acidified K₂Cr₂O₇ | Yes | No | No | Orange → green |
| 2,4-DNPH | Yes | Yes | Yes | Orange precipitate |
| Iodoform (I₂/OH⁻) | Only ethanal | Yes | No | Yellow CHI₃ precipitate |
A common exam strategy: use 2,4-DNPH to confirm any carbonyl is present, then use Tollens' to distinguish aldehyde from ketone, then use iodoform to test for the CH₃CO- structural feature. Three tests in sequence narrow the identification dramatically.
Nucleophilic addition is the defining reaction of the carbonyl group. The mechanism is the same regardless of the specific nucleophile — only the identity of Nu varies.
Step 1: Nucleophilic attack. The nucleophile Nu⁻ (or the lone-pair-bearing end of a polar molecule) attacks the δ+ carbonyl carbon. A new σ bond forms between Nu and C; simultaneously the π bond of C=O breaks heterolytically, with both π electrons moving onto the δ- oxygen.
A curly arrow shows the lone pair on Nu⁻ moving to form the new bond at C; a second curly arrow shows the C=O π bond moving onto O. The result is a tetrahedral sp³ carbon bonded to Nu, the original two R groups, and an O⁻ (alkoxide ion).
Step 2: Protonation. The alkoxide is a strong base. In aqueous or protic conditions it picks up a proton from H₂O, H₃O⁺, or H⁺ to give the neutral alcohol product. A curly arrow shows the lone pair on O attacking the H of H₂O (or H⁺), and a second curly arrow shows the O-H bond of water breaking to release OH⁻ (if H₂O is the proton source).
Net result: Nu and H add across the C=O double bond. The π bond is replaced by two new σ bonds (C-Nu and O-H), and the carbon is now sp³ tetrahedral.
Although oxygen carries the δ- charge, it is not attacked by a nucleophile — that would put two negative species on the same atom. The δ+ carbon is the electrophilic site. Conversely, an electrophile (such as H⁺) would attack the δ- oxygen, not the δ+ carbon. This is the basis of acid catalysis: protonation of O makes the C even more δ+, accelerating nucleophilic attack.
Sodium tetrahydridoborate(III), NaBH₄, is the standard mild reducing agent for aldehydes and ketones. The hydride ion H⁻ (formally, though in practice it is delivered as part of BH₄⁻) acts as the nucleophile.
Step 1: H⁻ attacks the δ+ carbonyl carbon. Two curly arrows: one from the H-B bond of BH₄⁻ (or from a lone pair on H⁻) to the carbon, one from the C=O π bond onto O.
Step 2: The alkoxide is protonated by water (added on workup) to give the alcohol.
Why [H] rather than H⁻ in equations? The square-bracket notation [H] is the A-Level convention for reducing-agent-supplies-hydrogen. Two [H] are added — one as H⁻ from BH₄⁻ in the nucleophilic step, and one as H⁺ from water in the protonation step. The net addition is therefore H₂ across C=O.
Why NaBH₄ and not LiAlH₄? Lithium tetrahydridoaluminate(III) (LiAlH₄) is a much more powerful reducing agent — it reduces carboxylic acids and esters, not just aldehydes and ketones. NaBH₄ is selective for aldehydes and ketones, leaves esters and amides intact, and can be used in alcoholic or aqueous solvents (LiAlH₄ reacts violently with water). The A-Level specification names NaBH₄ as the reducing agent of choice for C=O.
Butan-2-one (CH₃COCH₂CH₃) + 2 [H] → butan-2-ol (CH₃CH(OH)CH₂CH₃).
Mechanism (in words): H⁻ from BH₄⁻ attacks the δ+ carbon of the C=O (the C2 carbon) from either face of the planar carbonyl; the π bond breaks to give an alkoxide; aqueous workup protonates O to give the alcohol. Because the carbonyl carbon was sp² (planar), attack can occur with equal probability from either face — the resulting butan-2-ol has a chiral centre at C2, but the product is a racemic 50:50 mixture of (R)- and (S)-butan-2-ol with no net optical activity.
The addition of HCN to a carbonyl is one of the most important synthetic transformations in A-Level organic chemistry, because it extends the carbon chain by one. The product is a 2-hydroxynitrile (also called a cyanohydrin).
HCN is generated in situ from KCN (or NaCN) and dilute H₂SO₄ — pure HCN is a volatile, extremely toxic gas (boiling point 26 °C) and is never used directly in a school laboratory. The reaction is run in aqueous or alcoholic solution at room temperature. The active nucleophile is the cyanide ion CN⁻; the proton source is HCN or H₃O⁺.
Step 1: CN⁻ attacks the δ+ carbonyl carbon. The lone pair on C of CN⁻ forms the new C-C bond; the C=O π electrons move onto O to give an alkoxide.
Step 2: The alkoxide is protonated (by HCN or H₂O) to give the 2-hydroxynitrile. The OH ends up on the former carbonyl carbon; the CN is now a new substituent on that same carbon.
CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN (2-hydroxybutanenitrile). Mechanism: (1) arrow from lone pair on C of CN⁻ to the δ+ aldehyde C; arrow from C=O π bond onto O, giving the tetrahedral alkoxide CH₃CH₂CH(CN)O⁻. (2) Arrow from a lone pair on O⁻ to H of HCN; H-CN bond breaks, regenerating CN⁻. Product is neutral, with a chiral centre at the former aldehyde C (now bonded to H, OH, CN, CH₂CH₃).
The carbonyl carbon of propanal is sp² and trigonal planar, so CN⁻ approaches either face with equal probability — there is no chiral influence in the substrate or reagent. Attack on the top face gives one enantiomer; attack on the bottom face gives its mirror image. The product is a 50:50 mixture of (R)- and (S)-2-hydroxybutanenitrile — a racemic mixture. A racemate is optically inactive: equal and opposite rotations from the two enantiomers cancel. Enantiopure product requires a chiral catalyst (e.g. an oxynitrilase enzyme) or a chiral substrate.
The 2-hydroxynitrile RCH(OH)CN is a versatile intermediate: acid hydrolysis of CN gives a 2-hydroxycarboxylic acid (e.g. lactic acid from ethanal); reduction of CN gives a 2-amino alcohol. The carbon-chain extension by one is rare in A-Level chemistry — most reactions preserve or shorten the chain — which makes HCN addition synthetically valuable.
Fehling's solution. Stocked as two solutions — Fehling's A (CuSO₄ in dilute H₂SO₄) and Fehling's B (sodium potassium tartrate in NaOH) — mixed equal-volume immediately before use to give the active deep-blue reagent (it decomposes over hours). Warm in a water bath (60-70 °C), never with a direct flame.
Tollens' reagent. Prepared by adding NaOH(aq) to AgNO₃(aq) to precipitate brown Ag₂O, then adding dilute aqueous ammonia drop-by-drop until the precipitate just dissolves. Excess ammonia must be avoided.
Critical safety hazard: Tollens' reagent must not be stored. On standing it slowly decomposes to silver fulminate AgONC and/or silver nitride Ag₃N — both friction-sensitive primary explosives that can detonate on a glass surface. Prepare fresh, use immediately, and dispose of residues with dilute HCl and copious water — never store in a stoppered bottle. Serious laboratory injuries have been documented over the past century from old Tollens' bottles exploding. The test tube must also be scrupulously clean (NaOH-washed) for a good silver mirror; grease gives a grey-black colloidal suspension instead.
Problem: From propan-1-ol, make 2-hydroxybutanoic acid (CH₃CH₂CH(OH)COOH). Step 1: Oxidise propan-1-ol with acidified K₂Cr₂O₇ under controlled distillation → propanal CH₃CH₂CHO. Step 2: Add KCN / dilute H₂SO₄ → 2-hydroxybutanenitrile CH₃CH₂CH(OH)CN (racemic, chain extended by one C). Step 3: Reflux with dilute H₂SO₄ → 2-hydroxybutanoic acid CH₃CH₂CH(OH)COOH (CN hydrolysed via amide intermediate, NH₄⁺ released).
Question 1. [13 marks total]
(a) A student has two unlabelled bottles, one containing propanal (CH₃CH₂CHO) and one containing propanone (CH₃COCH₃). Describe one chemical test the student could carry out to distinguish the two compounds. State the reagent, the conditions, and the observations for both bottles. [4 marks]
(b) Write the mechanism for the reaction of propanal with cyanide ion (CN⁻) in the presence of dilute acid. Use curly arrows to show the movement of electron pairs, and show all intermediates and the final product. [5 marks]
(c) State why the product formed in (b) is described as a racemic mixture, and explain why an aqueous solution of the product does not rotate plane-polarised light. [2 marks]
(d) An unknown carbonyl compound X gives an orange precipitate with 2,4-DNPH of melting point 126 °C, and gives a yellow precipitate when warmed with I₂ in aqueous NaOH. Identify X and justify your answer. [2 marks]
(a) Distinguishing test [4 marks, AO2]
Iodoform is not a valid answer here because propanone gives a positive iodoform test (it is a methyl ketone), so iodoform cannot distinguish the two. 2,4-DNPH is not valid because both give an orange precipitate. The candidate must avoid these two tests.
(b) Nucleophilic addition mechanism [5 marks, AO2]
Common errors: arrow drawn from CN⁻ as a whole rather than from the lone pair on C; arrow drawn to O instead of to C; missing intermediate; wrong final product (e.g. CN drawn on O instead of C).
(c) Racemic mixture and optical activity [2 marks, AO3]
Award the second mark if the candidate states clearly that the enantiomers have opposite signs of rotation that cancel; partial credit if they merely say the mixture is optically inactive without explaining why.
(d) Identification of X [2 marks, AO3]
The three responses below cover the meaningful A-Level range: Grade C (the borderline-pass floor), Grade B (solid mark-scheme coverage), and Grade A* (top-band synthesis with synoptic depth). No Grade D or E responses are shown — no A-Level student is aiming for those bands, and modelling failure adds nothing pedagogically. The commentary after each response names the marks earned and the specific moves that differentiate from adjacent bands.
(a) Use Tollens' reagent. Warm the two bottles separately with a few cm³ of Tollens' in a water bath at about 60 °C. Propanal gives a silver mirror on the inside of the test tube. Propanone gives no reaction — the solution stays colourless and no silver appears.
(b) Mechanism: CN⁻ attacks the δ+ carbon of the C=O in propanal. Curly arrow from the lone pair on C of CN⁻ to the carbonyl C. Curly arrow from the C=O π bond onto O. This gives an intermediate alkoxide CH₃CH₂CH(CN)O⁻ with a negative charge on O and a tetrahedral carbon bearing CN, H, and CH₂CH₃. The O⁻ then picks up a proton from HCN (or from dilute acid in the reaction mixture) — curly arrow from a lone pair on O to the H of HCN, and the H-C bond of HCN breaks to give CN⁻ back. The final product is 2-hydroxybutanenitrile, CH₃CH₂CH(OH)CN.
(c) The product is racemic because the carbonyl C in propanal is planar (sp²), so CN⁻ can attack from either face equally. Half the molecules end up as the (R) enantiomer and half as the (S) enantiomer. The two enantiomers rotate plane-polarised light in opposite directions, so the rotations cancel and the mixture is optically inactive.
(d) The 2,4-DNPH precipitate has mp 126 °C, which matches propanone (CH₃COCH₃) from the data book. Propanone is a methyl ketone, so it gives a positive iodoform test with I₂/NaOH (yellow CHI₃ precipitate). Both tests are consistent with X = propanone.
Editorial commentary (Grade C): All four parts answered with the right reagent, the right colour change, and the right product. The mechanism in (b) names the curly arrows and the intermediate but is sparse — there is no labelling of partial charges (δ+, δ-) on the starting carbonyl, no explicit statement that the carbonyl carbon becomes sp³ in the intermediate. The stereochemistry answer in (c) is correct but compressed. To progress to B, the candidate should add the δ+/δ- labels to the carbonyl, name the intermediate as an alkoxide, and add one line in (c) about the geometry of the trigonal-planar carbonyl carbon.
(a) Add a few drops of each bottle's contents to freshly prepared Tollens' reagent (made by adding aqueous NH₃ drop-by-drop to AgNO₃ solution until the brown Ag₂O precipitate just dissolves). Warm both test tubes in a water bath at 60 °C for two to three minutes. Propanal (the aldehyde) reduces [Ag(NH₃)₂]⁺ to metallic silver, giving a silver mirror on the inside of a clean test tube. Propanone (the ketone) has no oxidisable C-H on the carbonyl carbon, so no reaction occurs — the solution remains colourless.
(b) The carbonyl C of propanal carries a δ+ partial charge because O is more electronegative than C; O carries δ-. Step 1: a lone pair on the carbon of CN⁻ attacks the δ+ carbonyl C — curly arrow from the lone pair on C of CN⁻ to the carbonyl C. Simultaneously the π bond of C=O breaks heterolytically: curly arrow from the C=O π electrons onto O. The intermediate is an alkoxide: CH₃CH₂C(H)(CN)O⁻, with a tetrahedral sp³ carbon and a negative charge on O. Step 2: the alkoxide O⁻ picks up a proton from HCN (or H₃O⁺). Curly arrow from a lone pair on O⁻ to the H of HCN; curly arrow from the H-C bond of HCN onto the C of CN, regenerating CN⁻. Final product: 2-hydroxybutanenitrile, CH₃CH₂CH(OH)CN.
(c) The product C2 carbon has four different groups (H, OH, CN, CH₂CH₃) — it is a chiral centre. The carbonyl C of propanal was sp² planar, so CN⁻ can attack with equal probability from either face. The two faces lead to mirror-image products: (R)- and (S)-2-hydroxybutanenitrile, in a 50:50 ratio — a racemic mixture. The two enantiomers rotate plane-polarised light by equal and opposite amounts, so the net rotation is zero and the racemate is optically inactive.
(d) 2,4-DNPH mp 126 °C matches propanone (data-book value 126 °C). Propanone CH₃COCH₃ has the CH₃CO- group, so it gives a positive iodoform test (yellow CHI₃). X = propanone.
Editorial commentary (Grade B): Mechanism now fully labelled with partial charges, sp² → sp³ rehybridisation explicit, and the alkoxide named. The stereochemistry answer in (c) explicitly identifies the chiral centre and gives the (R)/(S) labels. To progress to A*, the candidate could discuss synoptic links — the iodoform reaction's mechanistic basis (sequential α-iodination followed by haloform cleavage), the IR/NMR confirmation of propanone (C=O stretch at 1715 cm⁻¹, single CH₃ singlet at δ 2.1 in ¹H NMR, carbonyl C at δ 207 in ¹³C), or the enantiopure-synthesis alternative using a chiral oxynitrilase enzyme.
(a) Tollens' reagent is the most general test (works on aromatic and aliphatic aldehydes). Prepare fresh: NaOH(aq) to AgNO₃(aq) gives brown Ag₂O, then dilute NH₃(aq) drop-by-drop until the precipitate just dissolves, giving colourless [Ag(NH₃)₂]⁺. Warm each unknown with Tollens' in a clean (NaOH-washed) test tube at ~60 °C for 2-3 min. Propanal: silver mirror, RCHO + 3 OH⁻ → RCOO⁻ + 2H₂O + 2e⁻; [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃. Propanone: no reaction (no oxidisable C-H on the carbonyl C). Safety: discard Tollens' immediately — slow decomposition gives silver fulminate AgONC and silver nitride Ag₃N, friction-sensitive primary explosives.
(b) Propanal: CH₃CH₂-C(δ+)(H)=O(δ-). Step 1 (rate-determining): lone pair on C of CN⁻ attacks the δ+ carbonyl C from either face of the trigonal-planar sp² carbon (arrow 1); C=O π electrons move onto O (arrow 2). Product: alkoxide CH₃CH₂C(H)(CN)O⁻ (sp³, negative on O). Step 2 (fast): protonation. Lone pair on O⁻ to H of HCN (arrow 3); H-C bond of HCN onto C of CN, regenerating CN⁻ (catalytic, arrow 4). Final product: 2-hydroxybutanenitrile CH₃CH₂CH(OH)CN.
(c) C2 of the product carries H, OH, CN, CH₂CH₃ — four different groups, a chiral centre. The carbonyl C of propanal is sp² planar with no facial bias, so CN⁻ attacks the top and bottom faces with equal probability, giving (R) and (S) in 50:50 — a racemate, (±)-2-hydroxybutanenitrile. By Cahn-Ingold-Prelog priority CN > OH > CH₂CH₃ > H. The mixture is optically inactive because [α]_D and −[α]_D from equal (R) and (S) populations cancel exactly.
(d) 2,4-DNPH mp 126 °C matches propanone (literature 126 °C; nearest alternatives ethanal 168, propanal 154, butan-2-one 117, butanal 123 °C — all ruled out by ≥2 °C, the resolution limit of a Thiele-tube determination). Positive iodoform confirms the CH₃CO- group, present in propanone. X = propanone. Independent confirmation: IR C=O stretch ~1715 cm⁻¹, ¹H NMR singlet δ 2.17 ppm (6H equivalent CH₃), ¹³C carbonyl δ 207 ppm.
Editorial commentary (Grade A):* Genuinely A*: Tollens' safety hazard explicitly named (silver fulminate, silver nitride); iodoform-test specificity justified with melting-point resolution data; CIP priorities applied; identification cross-checked against IR and NMR. Mechanism fully labelled, rate-determining step identified. Demonstrates the synoptic integration of carbonyl tests, mechanism, stereochemistry, and spectroscopy that characterises the top band.
Three undergraduate-adjacent extensions:
This lesson aligns with AQA A-Level Chemistry specification §3.3.8 (aldehydes and ketones — nucleophilic addition, distinguishing tests, reduction, and hydroxynitrile formation), and equips candidates to identify carbonyl compounds by chemical and spectroscopic methods, to write fully labelled curly-arrow mechanisms for nucleophilic addition, and to rationalise the stereochemical outcome of addition to a planar prochiral carbonyl.