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Aldehydes, Ketones and Carboxylic Acids Set-1 MCQ Online Practice Test | Class 12 | Quiz Tweek

Aldehydes, Ketones and Carboxylic Acids Set-1

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

Aldehydes, ketones and carboxylic acids form a core part of Class 12 Chemistry because they cover major reaction patterns (nucleophilic addition, enol/enolate chemistry, aldol/Cannizzaro, esterification, decarboxylation) that frequently appear in both board exams and competitive tests. Mastery of acidity/basicity trends, electronic effects, and mechanism-based reasoning is essential to score well in this chapter.

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Q1. Among the following carbonyl compounds, which will react fastest with hydrogen cyanide (HCN) to give the corresponding cyanohydrin under identical kinetic conditions (assume nucleophilic addition rate is governed by electrophilicity and steric hindrance)?

(A)

(A) $CH_3COCH_3$ (acetone)

(B)

(B) $HCHO$ (formaldehyde)

(C)

(D)

(D) $CH_3CHO$ (acetaldehyde)

Q2. In aqueous solution, which of the following benzaldehyde derivatives will have the largest equilibrium constant for hydration (formation of gem‑diol $R\!-\!CH(OH)_2$ )?

(A)

(A) $p$ -OCH $_3$ C $_6$ H $_4$ CHO (p-methoxybenzaldehyde)

(B)

(B) C $_6$ H $_5$ CHO (benzaldehyde)

(C)

(D)

(D) $p$ -NO $_2$ C $_6$ H $_4$ CHO (p-nitrobenzaldehyde)

Q3. Which of the following carbonyl compounds will have the largest fraction of enol tautomer in a non‑polar solvent (e.g., benzene) at room temperature?

(A)

(A) $2,4$ -pentanedione ( $CH_3COCH_2COCH_3$ , acetylacetone)

(B)

(B) $CH_3COCH_3$ (acetone)

(C)

(D)

(D) $C_6H_5COCH_3$ (acetophenone)

Q4. Which of the following aldehydes can undergo both the Cannizzaro reaction and aldol condensation under strongly basic conditions (no other reagents present)?

(A)

(A) $HCHO$ (formaldehyde)

(B)

(B) $CH_3CHO$ (acetaldehyde)

(C)

(D)

(D) none of the above

Q5. For $2$ -methylcyclohexanone, which set of conditions will predominantly generate the thermodynamically more stable (more substituted) enolate before trapping with an electrophile?

(A)

(A) $\mathrm{LDA}$ in THF at $-78^\circ\mathrm{C}$ (fast, irreversible deprotonation)

(B)

(B) $\mathrm{LDA}$ in THF at $-78^\circ\mathrm{C}$ followed by immediate quench with electrophile

(C)

(D)

(D) NaH in THF at $-78^\circ\mathrm{C}$

Q6. Calculate the pH of a $0.020\ \text{M}$ solution of acetic acid (Ka = $1.8\times10^{-5}$ ). Assume $x\ll c$ .

(A)

$3.04$

(B)

$3.22$

(C)

$2.78$

(D)

$3.74$

Q7. Arrange the following carbonyl compounds in order of increasing reactivity toward nucleophilic addition (e.g., reduction by $NaBH_4$ ): formaldehyde ( $HCHO$ ), acetone ( $CH_3COCH_3$ ), benzaldehyde ( $C_6H_5CHO$ ), and p-nitrobenzaldehyde ( $p$ - $NO_2$ - $C_6$ H $_4$ CHO$). Which order is correct?

(A)

$CH_3COCH_3 < C_6H_5CHO < p\text{-}NO_2\text{-}C_6H_4CHO < HCHO$

(B)

$C_6H_5CHO < CH_3COCH_3 < p\text{-}NO_2\text{-}C_6H_4CHO < HCHO$

(C)

$CH_3COCH_3 < p\text{-}NO_2\text{-}C_6H_4CHO < C_6H_5CHO < HCHO$

(D)

$HCHO < p\text{-}NO_2\text{-}C_6H_4CHO < C_6H_5CHO < CH_3COCH_3$

Q8. Which of the following substituted benzoic acids is the least acidic (i.e., has the largest $pK_a$ )?

(A)

$p$ - $NO_2$ - $C_6H_4$ COOH$

(B)

$m$ - $NO_2$ - $C_6H_4$ COOH$

(C)

$m$ - $OCH_3$ - $C_6H_4$ COOH$

(D)

$p$ - $OCH_3$ - $C_6H_4$ COOH$

Q9. Assertion (A): Acetyl chloride ( $CH_3COCl$ ) reacts rapidly with methanol at $25^\circ\text{C}$ to give methyl acetate, whereas acetic acid ( $CH_3COOH$ ) does not convert to methyl acetate under the same mild conditions.
Reason (R): The chloride in acetyl chloride is a much better leaving group than the hydroxyl in acetic acid; additionally the carbonyl carbon in $CH_3COCl$ is more electrophilic because electron density is pulled by the electronegative Cl and resonance stabilization is less than in a carboxylic acid.

(A)

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

(B)

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

(C)

A is true but R is false.

(D)

A is false but R is true.

Q10. On heating, which of the following carboxylic acids undergoes decarboxylation to give acetone ( $CH_3COCH_3$ )?

(A)

$CH_3CH_2COOH$

(B)

$HOOCCH_2COOH$

(C)

$CH_3COCH_2COOH$

(D)

$C_6H_5COOH$

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