JEE Chemistry Aldehydes Ketones and Carboxylic Acids MCQs Set 36

Answer: (c) Butanal

Explanation:
1. Reductive ozonolysis of but-2-ene gives two molecules of acetaldehyde (P).
2. Aldol condensation with OH⁻/heat gives crotonaldehyde, CH₃CH=CHCHO (Q).
3. H₂ over Pd/C then reduces the C=C bond of crotonaldehyde under mild conditions, leaving the CHO group, to give butanal (R).
4. Answer: (c)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) The polarity of the carbonyl group is C(δ−)=O(δ+)

Explanation:
1. Oxygen is more electronegative than carbon, so the C=O dipole points towards oxygen: C(δ+)=O(δ−).
2. The carbon is electrophilic and the oxygen nucleophilic, and the dipole moment of C=O (about 2.3–2.7 D) is larger than that of an ether.
3. Statement (d) reverses the polarity, so it is not true.
4. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) a, b, c, d

Explanation:
1. (a) is the Etard reaction. (b) CrO₃ in acetic anhydride gives benzylidene diacetate, which hydrolyses to benzaldehyde.
2. (c) is the Gattermann-Koch reaction and (d) the Gattermann reaction.
3. All four give benzaldehyde.
4. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) 2-Methylbut-2-ene → acetone + acetaldehyde : KMnO₄/H⁺

Explanation:
1. PCC, DIBAL-H and CrO₃/Ac₂O all stop at the aldehyde stage, so (a)–(c) are correct.
2. KMnO₄/H⁺ is a strong oxidant: it cleaves the C=C bond but oxidises any aldehyde formed to a carboxylic acid (acetic acid).
3. The product would be acetone + acetic acid, so (d) is mismatched.
4. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) NH₂–CO–NH₂ : semicarbazone

Explanation:
1. NH₃, NH₂OH and 2,4-DNP give imines, oximes and 2,4-dinitrophenylhydrazones.
2. Semicarbazones need semicarbazide, NH₂NHCONH₂.
3. NH₂CONH₂ is urea and does not form a semicarbazone, so (d) is not matched correctly.
4. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) Phenyl acetate with H₃O⁺

Explanation:
1. Toluene and styrene give benzoic acid by side-chain oxidation, and benzonitrile gives it on hydrolysis.
2. Phenyl acetate is an ester of phenol: hydrolysis gives phenol and acetic acid, no benzoic acid.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) Phthalimide

Explanation:
1. Oxidation of both methyl groups gives phthalic acid (A).
2. Phthalic acid reacts with ammonia to form the ammonium salt, which on strong heating loses water to give phthalimide.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) (a) BH₃/THF, H₂O₂/OH⁻ (b) anhydrous CrO₃

Explanation:
1. Hydroboration-oxidation adds H and OH anti-Markovnikov, giving cyclohexylmethanol (a primary alcohol).
2. Anhydrous CrO₃ (or PCC) then oxidises the primary alcohol only as far as the aldehyde.
3. Ozonolysis would give cyclohexanone instead.
4. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (b) a, b, c

Explanation:
1. Aliphatic aldehydes and formic acid reduce Fehling's and Benedict's solutions.
2. Aromatic aldehydes reduce Tollens' reagent but not Fehling's solution.
3. So (a), (b) and (c) are correct, while (d) is not.
4. Answer: (b)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) Phenyl acetate + H₃O⁺

Explanation:
1. Friedel-Crafts acylation gives acetophenone, Markovnikov hydration of phenylacetylene gives acetophenone, and ozonolysis of a symmetric C(CH₃)(Ph)= alkene gives acetophenone.
2. Phenyl acetate hydrolyses to phenol and acetic acid, so no acetophenone forms.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) 1 and 3 both

Explanation:
1. Decarboxylation removes CO₂, and the Hofmann bromamide reaction converts RCONH₂ to RNH₂, losing the carbonyl carbon.
2. The HVZ reaction only introduces Br at the alpha carbon.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (b) Ethyl acetoacetate

Explanation:
1. Hydrolysis of A gives ethanol, so A is an ethyl ester.
2. B is a β-keto acid, which loses CO₂ on heating to give acetone: CH₃COCH₂COOH → CH₃COCH₃ + CO₂.
3. So A is ethyl acetoacetate.
4. Answer: (b)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) Stephen reaction for the preparation of aldehyde

Explanation:
1. SnCl₂/HCl reduces the nitrile to an imine salt, and hydrolysis gives acetaldehyde.
2. This is the Stephen reaction for the preparation of aldehydes.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (b) CH₃CH₂CHO

Explanation:
1. The iodoform test needs a CH₃CO– or CH₃CH(OH)– group.
2. Acetaldehyde, ethanol and acetone have it.
3. Propanal has CH₃CH₂CO– and no CH₃CO group, so it does not respond.
4. Answer: (b)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (b) Butan-2-one

Explanation:
1. A hydrazone needs a C=O group, so ethanol is ruled out.
2. The iodoform test needs CH₃CO–, which butan-2-one has; cyclohexyl ethyl ketone does not.
3. Tollens' test is positive for CH₃COCH₂CHO (it has CHO) but not for the ketone.
4. Answer: butan-2-one.
5. Answer: (b)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (b) CH₃CH₂NH₂

Explanation:
1. Propanoic acid gives propanoyl chloride (A), then propanamide (B).
2. The Hofmann bromamide reaction removes the carbonyl carbon and gives the primary amine with one carbon fewer.
3. C is CH₃CH₂NH₂, ethylamine.
4. Answer: (b)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (a) Tollens' reagent

Explanation:
1. Both aldehydes reduce Tollens' reagent to give a silver mirror.
2. Fehling's and Benedict's solutions are reduced only by the aliphatic aldehyde.
3. Only acetaldehyde gives the iodoform test.
4. Answer: (a)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.

Answer: (d) 2 and 3 both

Explanation:
1. Fehling's solution is reduced only by aliphatic aldehydes (and alpha-hydroxy ketones).
2. Benzaldehyde and acetone do not reduce it.
3. Answer: (d)

Teacher's Note:
1. Recall the reagent's role: nucleophilic addition-elimination for NH2-type reagents, enolate chemistry for aldol, and the reactivity order acid chloride > anhydride > ester > amide.