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Aldehydes, Ketones and Carboxylic Acids Set-3 MCQ Online Practice Test | Class 12 | Quiz Tweek

Aldehydes, Ketones and Carboxylic Acids Set-3

Practice Important MCQs for Aldehydes, Ketones and Carboxylic Acids — Chemistry, Class 12.

This chapter is high-scoring for both board and competitive exams because it links core concepts like carbonyl reactivity, hydration/tautomerism, acid–base equilibria, and characteristic reduction/oxidation reactions. Mastery of enolate formation, nucleophilic addition patterns, and decarboxylation/electrochemical reactions helps you solve both direct fact-based questions and reasoning-based MCQs efficiently.

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Q1. (A 0.10 M solution of acetic acid ( $CH_3COOH$ ) with $pK_a = 4.76$ is adjusted to $pH = 6.76$ . What fraction of acetic acid molecules are present as the deprotonated form $CH_3COO^-$ at equilibrium?)

(A)

$90.9\%$

(B)

$99.01\%$

(C)

$9.09\%$

(D)

$0.99\%$

Q2. (At 25°C, equal molar solutions of acetone ( $CH_3COCH_3$ ), benzaldehyde ( $C_6H_5CHO$ ) and p-nitrobenzaldehyde ( $p$ - $NO_2$ - $C_6H_4CHO$ ) are treated with excess $NaBH_4$ . Which carbonyl compound will be reduced the fastest? Consider electronic and steric effects.)

(A)

Acetone ( $CH_3COCH_3$ )

(B)

Benzaldehyde ( $C_6H_5CHO$ )

(C)

All three react at comparable rates

(D)

p-Nitrobenzaldehyde ( $p$ - $NO_2$ - $C_6H_4CHO$ )

Q3. (Which of the following carbonyl compounds will exist to the greatest extent as its hydrate (gem-diol) in aqueous solution at equilibrium: formaldehyde ( $HCHO$ ), acetaldehyde ( $CH_3CHO$ ), benzaldehyde ( $C_6H_5CHO$ ) or acetone ( $CH_3COCH_3$ )?)

(A)

Formaldehyde ( $HCHO$ )

(B)

Acetaldehyde ( $CH_3CHO$ )

(C)

Benzaldehyde ( $C_6H_5CHO$ )

(D)

Acetone ( $CH_3COCH_3$ )

Q4. (Consider cyclohex-2-en-1-one (an $\alpha,\beta$ -unsaturated ketone). Which reagent below will predominantly give 1,4-conjugate addition (Michael/1,4-addition) rather than 1,2-addition to the carbonyl carbon?)

(A)

Sodium borohydride ( $NaBH_4$ )

(B)

Lithium aluminium hydride ( $LiAlH_4$ )

(C)

Gilman reagent (organocuprate, $R_2CuLi$ )

(D)

Grignard reagent ( $RMgX$ )

Q5. (In the Kolbe electrolysis of an equimolar mixture of sodium acetate ( $CH_3COONa$ ) and sodium propionate ( $CH_3CH_2COONa$ ) the carboxylate ions undergo decarboxylation to give methyl and ethyl radicals which dimerise. Assuming random radical coupling, what is the expected statistical ratio of the hydrocarbon products ethane:propane:butane?)

(A)

$1:1:1$

(B)

$1:2:1$

(C)

$2:1:2$

(D)

$1:3:1$

Q6. A 0.10 M solution of acetic acid ( $CH_3COOH$ , $pK_a=4.76$ ) is adjusted to pH $5.76$ . Using the Henderson–Hasselbalch relationship, what percentage of acetic acid exists as acetate ( $CH_3COO^-$ ) at this pH?

(A)

$9.09\%$

(B)

$90.9\%$

(C)

$50.0\%$

(D)

$99.0\%$

Q7. A mixture containing equal amounts of benzoic acid ( $C_6H_5COOH$ ), $p$ ‑nitrophenol ( $p$ ‑ $NO_2$ ‑ $C_6H_4$ OH) and aniline ( $C_6H_5NH_2$ ) in diethyl ether is extracted sequentially with (i) 5% NaHCO $_3$ , (ii) then 5% NaOH, and (iii) finally 1 M HCl. Which species will be present in the aqueous extracts after steps (i), (ii) and (iii) respectively?

(A)

(i) $C_6H_5COO^-$ ; (ii) aniline ( $C_6H_5NH_2$ unchanged) ; (iii) $p$ ‑nitrophenolate ( $p$ ‑ $NO_2$ ‑ $C_6H_4$ O $^-$ )

(B)

(i) none ; (ii) $p$ ‑nitrophenolate ( $p$ ‑ $NO_2$ ‑ $C_6H_4$ O $^-$ ) ; (iii) $C_6H_5COO^-$

(C)

(i) $C_6H_5COO^-$ ; (ii) $C_6H_5COO^-$ + $p$ ‑nitrophenolate ; (iii) $C_6H_5NH_3^+$

(D)

(i) $C_6H_5COO^-$ ; (ii) $p$ ‑nitrophenolate ( $p$ ‑ $NO_2$ ‑ $C_6H_4$ O $^-$ ) ; (iii) anilinium ion ( $C_6H_5NH_3^+$ )

Q8. For 2‑methylcyclohexanone, which enolate predominates under (i) treatment with LDA at $-78^\circ\mathrm{C}$ (strong, non‑nucleophilic base, low temperature) and (ii) treatment with NaOEt at $25^\circ\mathrm{C}$ (weaker base, thermodynamic conditions)?

(A)

(i) Kinetic (less‑substituted) enolate formed by deprotonation at the less‑hindered α‑carbon; (ii) Thermodynamic (more‑substituted) enolate formed by deprotonation α to the methyl group

(B)

(i) Thermodynamic (more‑substituted) enolate; (ii) Kinetic (less‑substituted) enolate

(C)

(i) and (ii) both give the same (less‑substituted) enolate

(D)

(i) and (ii) both give the same (more‑substituted) enolate

Q9. Assertion (A): Acetylacetone ( $CH_3COCH_2COCH_3$ ) exists predominantly in the enol form in non‑polar solvents.
Reason (R): The enol form is stabilized by an intramolecular six‑membered hydrogen bond and by conjugation (resonance) between the enolic double bond and the adjacent carbonyl, lowering its energy relative to the diketo form.

(A)

Both A and R are true but R is NOT the correct explanation of A.

(B)

A is true but R is false.

(C)

Both A and R are true and R is the correct explanation of A.

(D)

A is false but R is true.

Q10. One mole of acetic acid ( $CH_3COOH$ ) and one mole of ethanol ( $C_2H_5OH$ ) are mixed in a $1.0\ \mathrm{L}$ vessel and allowed to reach equilibrium for the reaction

CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O

The equilibrium constant at $25^\circ\mathrm{C}$ is $K_c=4.0$ . Assuming initial $[H_2O]=0$ , what is the equilibrium concentration (in mol L $^{-1}$ ) of ethyl acetate ( $CH_3COOC_2H_5$ )?

(A)

$0.50\ \mathrm{M}$

(B)

$0.667\ \mathrm{M}$

(C)

$0.40\ \mathrm{M}$

(D)

$0.333\ \mathrm{M}$

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