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Aldehydes Ketones and Carboxylic Acids Set-8 MCQ Online Practice Test | Class 12 | Quiz Tweek

Aldehydes Ketones and Carboxylic Acids Set-8

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

This chapter is a scoring backbone for CBSE and competitive exams because it links core concepts of carbonyl reactivity (nucleophilic additions), base-catalysed transformations (aldol/Cannizzaro), enolate chemistry, and acidity/equilibrium (p $K_a$ , hydration equilibria). Mastery here also improves speed in solving numericals and predicting major products in multi-step reaction questions commonly asked in JEE/NEET.

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Q1. A student needs to reduce 10.00 g of benzaldehyde ( $\mathrm{C_7H_6O}$ ; $M=106.12\ \mathrm{g\,mol^{-1}}$ ) to benzyl alcohol using sodium borohydride. Assuming one mole of $\mathrm{NaBH_4}$ delivers four hydride equivalents and the reaction goes to 100% yield, the mass of $\mathrm{NaBH_4}$ required (in grams) is approximately:

(A)

$0.22\ \mathrm{g}$

(B)

$1.78\ \mathrm{g}$

(C)

$0.89\ \mathrm{g}$

(D)

$0.45\ \mathrm{g}$

Q2. When benzaldehyde ( $\mathrm{PhCHO}$ ) reacts with acetone ( $\mathrm{CH_3COCH_3}$ ) in the presence of dilute $\mathrm{NaOH}$ (1:1 molar ratio) under typical crossed aldol conditions, the major isolated product is:

(A)

$\mathrm{CH_3COCH=CHPh}$ (the α,β‑unsaturated ketone from condensation)

(B)

$\mathrm{PhCH_2OH}$ (benzyl alcohol)

(C)

$\mathrm{PhCOOH}$ (benzoic acid)

(D)

$\mathrm{PhCH(OH)CH_2COCH_3}$ (β‑hydroxy ketone; incomplete dehydration intermediate)

Q3. For the conjugate (Michael) addition to an α,β‑unsaturated ketone such as cyclohexenone, which reagent is expected to give predominantly 1,4‑addition (conjugate addition) rather than 1,2‑addition at the carbonyl?

(A)

$\mathrm{NaBH_4}$

(B)

$\mathrm{LiAlH_4}$

(C)

$\mathrm{RMgBr}$ (Grignard reagent)

(D)

$\mathrm{R_2CuLi}$ (Gilman organocuprate reagent)

Q4. Given pKa values pKa $(\,$ 2,4‑pentanedione $)=9.0$ and pKa $(\,$ acetone $)=19.0$ , calculate the equilibrium constant $K$ for the proton‑transfer equilibrium

\begin{aligned} \mathrm{CH_3COCH_2COCH_3\ (HA)} + \mathrm{CH_3COCH_2^{-}\ (A'^{-})} &\rightleftharpoons \mathrm{CH_3COCH^{-}COCH_3\ (A^{-})} + \mathrm{CH_3COCH_3\ (HA')} \end{aligned}

(using $K=10^{\mathrm{p}K_a(\text{acid on right})-\mathrm{p}K_a(\text{acid on left})}$ )

(A)

$10^{-10}$

(B)

$10^{10}$

(C)

$1$

(D)

$10^{2}$

Q5. Which of the following carboxylic acids will undergo facile thermal decarboxylation on gentle heating to give the indicated neutral product?

(A)

$\mathrm{CH_3COCH_2COOH}$ (3‑oxobutanoic acid; decarboxylation → $\mathrm{CH_3COCH_3}$ , acetone)

(B)

$\mathrm{(CH_3)_3CCO_2H}$ (pivalic acid; decarboxylation → $\mathrm{(CH_3)_3C\!-\!H}$ )

(C)

$\mathrm{C_6H_5COOH}$ (benzoic acid; decarboxylation → $\mathrm{C_6H_6}$ )

(D)

$\mathrm{CH_3CH_2CH_2COOH}$ (butyric acid; decarboxylation → $\mathrm{CH_3CH_2CH_3}$ )

Q6. A $0.10\ \mathrm{M}$ aqueous solution of acetic acid is prepared. Given $K_a(\mathrm{CH_3COOH}) = 1.8\times10^{-5}$ , the approximate percentage dissociation ( $\alpha\times100$ ) is:

(A)

$0.13\%$

(B)

$1.34\%$

(C)

$13.4\%$

(D)

$0.0134\%$

Q7. For the reaction

ArCHO + HCN \rightarrow ArCH(OH)CN

which benzaldehyde derivative will give the faster initial rate of cyanohydrin formation under identical conditions and why?

(A)

p‑Methoxybenzaldehyde reacts faster because the $+M$ effect of OCH $_3$ increases the partial positive charge on the carbonyl carbon.

(B)

Both p‑nitro and p‑methoxy derivatives react at similar rates because HCN is a weak nucleophile and substituent effects are minimal.

(C)

p‑Nitrobenzaldehyde reacts faster because the $-M$ / $-I$ effect of NO $_2$ withdraws electron density, increasing electrophilicity of the carbonyl carbon.

(D)

p‑Methoxybenzaldehyde reacts faster because the methoxy group stabilizes the developing negative charge in the transition state by hydrogen bonding with HCN.

Q8. An equimolar mixture of benzaldehyde ( $\mathrm{PhCHO}$ ) and acetaldehyde ( $\mathrm{CH_3CHO}$ ) is treated with dilute NaOH at $0^\circ\mathrm{C}$ . After work-up and dehydration, the major isolated product is most likely:

(A)

Cinnamaldehyde, $\mathrm{PhCH=CHCHO}$

(B)

Crotonaldehyde, $\mathrm{CH_3CH=CHCHO}$

(C)

Benzyl alcohol, $\mathrm{PhCH_2OH}$

(D)

A complex mixture of self‑aldol condensation products of both aldehydes

Q9. Assertion (A): Organocuprates ( $\mathrm{R_2CuLi}$ ) add to $\alpha,\beta$ ‑unsaturated carbonyl compounds predominantly by 1,2‑addition to give allylic alcohol derivatives.
Reason (R): Organocuprates are soft nucleophiles and therefore preferentially undergo conjugate (1,4) addition to the $\beta$ ‑carbon of $\alpha,\beta$ ‑unsaturated carbonyls, whereas hard nucleophiles (e.g. organolithiums) favor 1,2‑addition.

(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. Consider the proton‑transfer equilibrium at 25°C:

\mathrm{CH_3COOH + C_6H_5NH_2 \rightleftharpoons CH_3COO^- + C_6H_5NH_3^+}

Given $pK_a(\mathrm{CH_3COOH})=4.76$ and $pK_a(\mathrm{C_6H_5NH_3^+})=4.63$ , the equilibrium constant $K$ for the reaction (products/reactants) is approximately:

(A)

$0.53$

(B)

$0.37$

(C)

$0.74$

(D)

$1.35$

Q11. Which of the following aldehydes will not undergo self-aldol condensation under base-catalysed conditions?

(A)

$CH_3CHO$ (ethanal)

(B)

$CH_3CH_2CHO$ (propanal)

(C)

$C_6H_5CHO$ (benzaldehyde)

(D)

$(CH_3)_2CHCHO$ (isobutyraldehyde)

Q12. Which of the following compounds has the most acidic $\alpha$ -hydrogen (lowest pKa) and thus forms the most stabilized enolate on deprotonation?

(A)

$CH_3COCH_2COCH_3$ (acetylacetone)

(B)

$CH_3COCH_3$ (acetone)

(C)

$CH_3COOCH_2CH_3$ (ethyl acetate)

(D)

$CH_3CHO$ (acetaldehyde)

Q13. Which of the following carbonyl compounds will react fastest with sodium borohydride ( $NaBH_4$ ) in ethanol at 25°C?

(A)

$CH_3COCH_3$ (acetone)

(B)

$C_6H_5CHO$ (benzaldehyde)

(C)

$CH_3CH_2CH_2CHO$ (butanal)

(D)

$CH_3CHO$ (ethanal)

Q14. 2‑Methylcyclohexanone is treated with methylation under two different conditions: (i) $LDA$ , $-78^\circ\text{C}$ then $CH_3I$ (kinetic conditions) and (ii) $NaOEt$ , $25^\circ\text{C}$ then $CH_3I$ (thermodynamic conditions). Which pair gives the major products for (i) and (ii), respectively?

(A)

(i) $\text{2,2-dimethylcyclohexanone}$ ; (ii) $\text{2,6-dimethylcyclohexanone}$

(B)

(i) $\text{2,6-dimethylcyclohexanone}$ ; (ii) $\text{2,2-dimethylcyclohexanone}$

(C)

(i) $\text{2,6-dimethylcyclohexanone}$ ; (ii) $\text{2,6-dimethylcyclohexanone}$

(D)

(i) $\text{2,2-dimethylcyclohexanone}$ ; (ii) $\text{2,2-dimethylcyclohexanone}$

Q15. Assertion: Aldehydes ( $RCHO$ ) are readily oxidized to carboxylic acids by mild oxidising agents (e.g. Tollens' reagent, $Ag_2O$ ), whereas ketones ( $R_2CO$ ) are generally resistant to such mild oxidation.
Reason: Aldehydes have a hydrogen on the carbonyl carbon and can form a gem‑diol intermediate which is easily oxidized; ketones lack that hydrogen and so are less susceptible to mild oxidation.

(A)

Both Assertion and Reason are true, and Reason correctly explains the Assertion.

(B)

Both Assertion and Reason are true, but Reason does not correctly explain the Assertion.

(C)

Assertion is true but Reason is false.

(D)

Assertion is false but Reason is true.

...and 5 more challenging questions available in the interactive simulator.

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