Chapter 8: Aldehydes, Ketones and Carboxylic Acids Long Questions | chemistry Class 12 CBSE

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Chapter 8: Aldehydes, Ketones and Carboxylic Acids Long Questions | chemistry Class 12 CBSE | ByteNotes.in

Long Answer

Hard difficulty • Structured explanation

Hard

Question 1

Long Form

Compare the methods of preparation of aldehydes from acyl chlorides, nitriles, and aromatic hydrocarbons. Highlight the named reactions involved.

Rosenmund reduction: acyl chlorides are hydrogenated over Pd-BaSO₄ to yield aldehydes; the catalyst is partially poisoned to prevent over-reduction to alcohols.
Stephen reaction: nitriles react with SnCl₂/HCl to form imines which are hydrolysed to aldehydes; DIBAL-H alternatively reduces nitriles or esters selectively to aldehydes.
From aromatic hydrocarbons: (a) Etard reaction uses CrO₂Cl₂ to oxidise a methyl group to a chromium complex, hydrolysis of which gives benzaldehyde. (b) Gatterman-Koch reaction uses CO/HCl/AlCl₃ to introduce a formyl group directly onto the ring.
Side-chain chlorination of toluene with Cl₂/hν gives benzal chloride, hydrolysis of which at 373 K gives benzaldehyde — the commercial route.
Each method has selectivity advantages: Rosenmund for aliphatic, Etard/Gatterman-Koch for aromatic, and Stephen/DIBAL-H for α,β-unsaturated aldehydes without affecting the double bond.

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Long Answer

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Hard

Question 2

Long Form

Explain the mechanism of nucleophilic addition to the carbonyl group. How does the structure of the carbonyl compound affect reactivity?

The carbonyl carbon is sp²-hybridised and electrophilic due to the high electronegativity of oxygen; nucleophilic attack occurs perpendicular to the sp² orbital plane.
In step 1 (slow), the nucleophile attacks the electrophilic carbon, changing hybridisation from sp² to sp³ and generating a tetrahedral alkoxide intermediate.
In step 2 (fast), the alkoxide intermediate captures a proton from the reaction medium to give the neutral addition product.
Aldehydes are more reactive than ketones: sterically, the single substituent in aldehydes offers less hindrance than two groups in ketones; electronically, two alkyl groups in ketones reduce electrophilicity more than one.
Benzaldehyde is less reactive than propanal because resonance delocalisation from the aromatic ring reduces the electrophilicity of the carbonyl carbon.

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Hard

Question 3

Long Form

Discuss the Aldol condensation and Cannizzaro reaction — the conditions under which each occurs and the structural requirement of the carbonyl compound.

Aldol condensation requires the carbonyl compound to have at least one α-hydrogen; it is catalysed by dilute alkali (NaOH) or Ba(OH)₂.
The α-hydrogen is acidic due to electron withdrawal by the carbonyl group and resonance stabilisation of the enolate ion formed; the enolate attacks another carbonyl molecule.
The initial product is a β-hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol), which upon heating loses water to give an α,β-unsaturated carbonyl compound (aldol condensation product).
Cannizzaro reaction occurs only with aldehydes lacking α-hydrogen (e.g., HCHO, PhCHO, (CH₃)₃CCHO) when treated with concentrated alkali; one molecule is oxidised to carboxylate and another reduced to alcohol.
Cross aldol condensation between two different carbonyl compounds (both having α-H) gives a mixture of four products; a useful controlled version uses a non-enolisable aldehyde with an enolisable ketone.

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Question 4

Long Form

Analyse the various factors that determine the acidity of carboxylic acids, with examples comparing different substituents.

Acidity of carboxylic acids is measured by pKa; a smaller pKa means a stronger acid. Carboxylic acids are stronger than alcohols (pKa ~16) and phenols (pKa ~10) because the carboxylate ion has two equivalent resonance structures delocalising charge over two electronegative oxygens.
Electron-withdrawing groups (EWG) increase acidity by stabilising carboxylate anion through inductive and/or resonance effects, e.g., CF₃COOH (pKa 0.23) >> CH₃COOH (pKa 4.76).
Order of increasing acidifying effect: Ph < I < Br < Cl < F < CN < NO₂ < CF₃; multiple EWGs (e.g., CCl₃COOH > CHCl₂COOH > ClCH₂COOH) have additive effects.
Electron-donating groups (EDG) like alkyl groups destabilise the carboxylate anion, decreasing acidity; C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH.
Position of substituent matters: α-substituted acids are stronger than β or γ, as inductive effect decreases with distance; 4-nitrobenzoic acid (pKa 3.41) < benzoic acid (pKa 4.19) < 4-methoxybenzoic acid (pKa 4.46).

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Long Answer

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Medium

Question 5

Long Form

Describe the preparation of carboxylic acids from primary alcohols, alkylbenzenes, nitriles, and Grignard reagents. Compare the suitability of each method.

From primary alcohols: oxidation with KMnO₄ (neutral/acidic/alkaline) or K₂Cr₂O₇/CrO₃ (acidic, Jones reagent); straightforward but requires controlled conditions to avoid over-oxidation.
From alkylbenzenes: vigorous oxidation with KMnO₄-KOH or CrO₃; entire side chain oxidised to –COOH regardless of chain length; tertiary alkyl groups are not oxidised.
From nitriles: hydrolysis with H⁺ or OH⁻ catalyst via amide intermediates gives carboxylic acids; mild conditions stop reaction at amide stage; nitriles are prepared from alkyl halides via SN2.
From Grignard reagents: R–MgX + CO₂ → carboxylate salt → RCOOH on acidification; adds one carbon to the chain, enabling series ascent.
Grignard and nitrile methods are preferred for chain extension; alkylbenzene oxidation is specific to aromatic acids; alcohol oxidation is general but requires avoiding over-reduction.

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Question 6

Long Form

Describe the various reactions of carboxylic acids involving cleavage of the C–OH bond and the –COOH group. Give balanced equations wherever possible.

Anhydride formation: two molecules of carboxylic acid heated with H₂SO₄ or P₂O₅ lose water to give the anhydride (e.g., 2CH₃COOH → (CH₃CO)₂O + H₂O).
Esterification: RCOOH + R'OH ⇌ RCOOR' + H₂O, catalysed by H⁺; a reversible nucleophilic acyl substitution where protonation of carbonyl oxygen activates it towards nucleophilic attack by alcohol.
Acid chloride formation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl; SOCl₂ preferred as gaseous by-products make purification easy.
Amide formation: RCOOH + NH₃ → RCOONH₄ (ammonium salt) → RCONH₂ + H₂O on strong heating.
Reduction: LiAlH₄ or diborane (B₂H₆) reduces –COOH to –CH₂OH; diborane is selective (does not reduce esters, halogens, nitro groups); NaBH₄ does not reduce carboxylic acids.

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Long Answer

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Medium

Question 7

Long Form

Discuss the nucleophilic addition-elimination reactions of aldehydes and ketones with ammonia and its derivatives. Tabulate the reagents and product names.

Ammonia and its derivatives (H₂N-Z) add to the carbonyl group in two stages: nucleophilic addition of H₂N-Z forms a carbinolamine intermediate (–C(OH)(NHZ)–), which then undergoes acid-catalysed dehydration to give >C=N-Z.
The general reaction is reversible and acid-catalysed; equilibrium favours the product due to rapid dehydration of the intermediate.
Hydroxylamine (NH₂OH, Z = –OH) gives oximes (>C=N-OH); hydrazine (NH₂NH₂, Z = –NH₂) gives hydrazones (>C=N-NH₂); phenylhydrazine gives phenylhydrazones.
2,4-Dinitrophenylhydrazine (2,4-DNP) gives coloured (yellow/orange/red) crystalline 2,4-DNP derivatives useful for characterising and identifying carbonyl compounds by melting point.
Semicarbazide (NH₂-NHCONH₂) gives semicarbazones; primary amines (R–NH₂) give Schiff's bases (substituted imines); all reactions are acid-catalysed and involve elimination of water.

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Question 8

Long Form

Compare and contrast the oxidation reactions of aldehydes and ketones, including the role of Tollens' reagent, Fehling's reagent, and the haloform reaction.

Aldehydes are readily oxidised to carboxylic acids by mild or strong oxidising agents (HNO₃, KMnO₄, K₂Cr₂O₇); ketones require vigorous conditions and undergo C–C bond cleavage to give a mixture of carboxylic acids with fewer carbons.
Tollens' test (ammoniacal AgNO₃): aldehydes produce a silver mirror; both aliphatic and aromatic aldehydes respond; ketones do not.
Fehling's test (CuSO₄ + alkaline sodium potassium tartarate): aliphatic aldehydes produce reddish-brown Cu₂O precipitate; aromatic aldehydes and ketones do not respond.
Haloform reaction (NaOX): methyl ketones (CH₃CO–) or compounds with CH₃CH(OH)– group are oxidised to give haloform (CHX₃) and sodium salt of carboxylic acid with one fewer carbon; used to detect CH₃CO group.
Iodoform test (NaI/I₂/NaOH): gives yellow CHI₃ precipitate with methyl ketones and secondary alcohols of type CH₃CHOH–; useful for distinguishing acetaldehyde and methyl ketones from other carbonyl compounds.

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Long Answer

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Medium

Question 9

Long Form

Describe the physical properties of aldehydes, ketones, and carboxylic acids in relation to their intermolecular forces, giving a comparative analysis.

Aldehydes and ketones are polar molecules with substantial dipole moments due to the C=O bond; they have higher boiling points than non-polar hydrocarbons and ethers of comparable molecular mass through dipole-dipole interactions.
Carboxylic acids have even higher boiling points than alcohols of similar molecular mass due to very strong intermolecular hydrogen bonding forming dimers; these dimers persist even in the vapour phase and in aprotic solvents.
Lower aldehydes and ketones are miscible with water (hydrogen bonding with water); solubility decreases with chain length. Lower carboxylic acids (up to C₄) are miscible with water; higher members are insoluble.
Lower aldehydes (methanal, ethanal) have sharp pungent odours; larger aldehydes and many ketones have fragrant odours and are used in perfumery. Carboxylic acids up to C₉ are liquids with unpleasant odours; higher acids are wax-like solids practically odourless.
Methanal is a gas (b.p. 254 K), ethanal a volatile liquid; ketones like acetone (b.p. 329 K) are widely used as solvents; carboxylic acids like acetic acid (b.p. 391 K) boil much higher than comparable alcohols.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 10

Long Form

Explain the mechanism of esterification of carboxylic acids with alcohols. Why is the reaction reversible, and how can the yield be improved?

Esterification is a nucleophilic acyl substitution reaction; protonation of the carbonyl oxygen by H⁺ increases the electrophilicity of the carbonyl carbon.
The alcohol (nucleophile) attacks the activated carbonyl carbon, forming a tetrahedral intermediate with two –OH groups.
Proton transfer within the intermediate converts one –OH to –+OH₂ (a better leaving group); water is then eliminated regenerating the C=O.
The protonated ester loses a proton to give the neutral ester product; because every step is reversible, the reaction reaches an equilibrium (RCOOH + R'OH ⇌ RCOOR' + H₂O).
Yield can be improved by Le Chatelier's principle: removing water or ester as it forms, or using excess of one reactant (usually the cheaper alcohol); the mineral acid catalyst (H₂SO₄ or HCl) speeds up equilibrium attainment but does not shift it.

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Chapter 8: Aldehydes, Ketones and Carboxylic Acids | Long Questions

Compare the methods of preparation of aldehydes from acyl chlorides, nitriles, and aromatic hydrocarbons. Highlight the named reactions involved.

Explain the mechanism of nucleophilic addition to the carbonyl group. How does the structure of the carbonyl compound affect reactivity?

Discuss the Aldol condensation and Cannizzaro reaction — the conditions under which each occurs and the structural requirement of the carbonyl compound.

Analyse the various factors that determine the acidity of carboxylic acids, with examples comparing different substituents.

Describe the preparation of carboxylic acids from primary alcohols, alkylbenzenes, nitriles, and Grignard reagents. Compare the suitability of each method.

Describe the various reactions of carboxylic acids involving cleavage of the C–OH bond and the –COOH group. Give balanced equations wherever possible.

Discuss the nucleophilic addition-elimination reactions of aldehydes and ketones with ammonia and its derivatives. Tabulate the reagents and product names.

Compare and contrast the oxidation reactions of aldehydes and ketones, including the role of Tollens' reagent, Fehling's reagent, and the haloform reaction.

Describe the physical properties of aldehydes, ketones, and carboxylic acids in relation to their intermolecular forces, giving a comparative analysis.

Explain the mechanism of esterification of carboxylic acids with alcohols. Why is the reaction reversible, and how can the yield be improved?

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