Aldehydes, Ketones and Carboxylic Acids — Class 12 MCQs with Answers

Aldehydes, Ketones and Carboxylic Acids — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Aldehyde functional group:

   A−COOHB−CHOC−OHDC=O within chain
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   Answer: −CHO

   Explanation: The −CHO (formyl) group defines aldehydes — the carbonyl carbon bonded to one H and one R group. −COOH is the acid group, and >C=O with two carbon substituents is a ketone, not an aldehyde.

2. Q2 of 6Easy

   Carboxylic acid functional group:

   A−OHB−CHOC−COOHD−NH₂
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   Answer: −COOH

   Explanation: The −COOH group (a carbonyl fused with a hydroxyl) makes a compound a carboxylic acid; −CHO is an aldehyde and −OH alone is an alcohol, both far less acidic since neither forms a resonance-stabilised carboxylate.

3. Q3 of 6Medium

   Tollens' reagent gives silver mirror with:

   AKetonesBAldehydesCAcidsDAlkanes
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   Answer: Aldehydes

   Explanation: Tollens' reagent [Ag(NH₃)₂]⁺ oxidises the aldehyde's C−H to −COOH while Ag⁺ is reduced to metallic Ag, depositing the silver mirror; ketones have no such H on the carbonyl carbon and give no reaction.

4. Q4 of 6Medium

   Carboxylic acids are more acidic than alcohols because:

   ALower massBSmallerCCarboxylate ion is resonance-stabilisedDMore polar only
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   Answer: Carboxylate ion is resonance-stabilised

   Explanation: Losing H⁺ from −COOH gives RCOO⁻, whose negative charge is delocalised equally over two O atoms by resonance; RO⁻ from an alcohol has no such stabilisation, so carboxylic acids ionise far more readily.

5. Q5 of 6Hard

   Aldol condensation needs:

   ANo hydrogenBα-hydrogen on the carbonyl compoundCStrong oxidiserDWater only
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   Answer: α-hydrogen on the carbonyl compound

   Explanation: Aldol condensation starts with base pulling off an α-hydrogen to generate a resonance-stabilised enolate, which then attacks a second carbonyl; without α-H there is no enolate, so the aldol step can't begin.

6. Q6 of 6Hard

   α-bromo acid from HVZ can be converted to α-amino acid by:

   ATreatment with HClBTreatment with NaOHCTreatment with excess NH₃DEsterification
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   Answer: Treatment with excess NH₃

   Explanation: Excess ammonia performs an SN2 substitution at the α-carbon bearing the good leaving group Br, displacing it with −NH₂ to give an α-amino acid; a shortage of NH₃ would instead favour over-alkylation of the product.

Aldehydes, Ketones and Carboxylic Acids notes: key concepts (Class 12 Chemistry)

Aldehydes, Ketones and Carboxylic Acids covers the carbonyl group, its nucleophilic addition, α-hydrogen reactions and oxidation/reduction, plus the acidity and reactions of carboxylic acids. Key named reactions include Rosenmund and Etard, the Aldol and Cannizzaro reactions, Clemmensen and Wolff–Kishner reductions, and the HVZ reaction.

The carbonyl group

The C=O group is polar (carbon δ+, oxygen δ−), planar and sp² hybridised; the electrophilic carbon is the site of nucleophilic attack.

Preparation of aldehydes/ketones

By controlled oxidation of alcohols, ozonolysis of alkenes, and from acyl chlorides (Rosenmund), toluene (Etard), or nitriles (Stephen reduction).

Nucleophilic addition

A nucleophile adds to the carbonyl carbon and a proton to oxygen; aldehydes are more reactive than ketones due to less steric hindrance and less +I electron donation.

Addition reactions

HCN gives cyanohydrins, NaHSO₃ gives bisulphite adducts (used to purify), and alcohols give hemiacetals/acetals; ammonia derivatives give imines/oximes/hydrazones.

Acidity of α-hydrogen

Hydrogens on the carbon next to C=O are acidic because the resulting enolate/carbanion is resonance-stabilised; this drives aldol and haloform reactions.

Aldol condensation

Two carbonyl molecules WITH α-H combine under dilute base to a β-hydroxy carbonyl (aldol), which on heating loses water to an α,β-unsaturated product.

Cannizzaro reaction

Aldehydes with NO α-H (HCHO, C₆H₅CHO) disproportionate in conc. alkali — one is oxidised to a carboxylate and another reduced to an alcohol.

Oxidation and distinguishing tests

Aldehydes are easily oxidised and give positive Tollens' (silver mirror) and Fehling's (red Cu₂O) tests; ketones do not — the key aldehyde/ketone distinction.

Reduction of carbonyls

C=O reduces to CH₂ by Clemmensen (Zn-Hg/HCl) or Wolff–Kishner (NH₂NH₂/KOH), and to alcohols by NaBH₄ or LiAlH₄.

Haloform reaction

Methyl ketones (and ethanal/ethanol) react with X₂/NaOH to give a haloform (CHX₃) plus a carboxylate; the iodoform test detects the CH₃CO– group.

Acidity of carboxylic acids

More acidic than phenols and alcohols because the carboxylate anion is stabilised by two equivalent resonance forms; electron-withdrawing groups raise acidity.

Reactions of carboxylic acids

Form salts, esters (with alcohols), acid chlorides (SOCl₂), anhydrides and amides; the –COOH group also undergoes decarboxylation with soda lime.

HVZ reaction

Hell–Volhard–Zelinsky: carboxylic acids with an α-H react with Br₂/red phosphorus to give α-halo acids, useful intermediates for amino acids.

Preparation of carboxylic acids

By oxidation of 1° alcohols/aldehydes/alkylbenzenes, hydrolysis of nitriles or esters, and carbonation of Grignard reagents (RMgX + CO₂).