JEE Chemistry Aldehydes Ketones and Carboxylic Acids MCQs Set 27

Multiple Choice Questions (MCQs) for JEE Chemistry: Aldehydes Ketones and Carboxylic Acids

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Multiple Choice Questions

Question 1: An aldehyde on oxidation gives
(a) An alcohol
(b) An acid
(c) A ketone
(d) An ether
Show Answer & Explanation

Answer: (b) An acid

Explanation:
1. RCHO is oxidised to RCOOH.

Teacher's Note:
1. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 2: The reaction of an aldehyde with hydroxylamine gives a product called
(a) Aminohydroxide
(b) Hydrazone
(c) Semicarbazone
(d) Oxime
Show Answer & Explanation

Answer: (d) Oxime

Explanation:
1. Aldehydes with NH2OH form aldoximes, \( \ce{RCH=NOH} \).

Teacher's Note:
1. Ammonia derivatives (NH2OH, NH2NH2, PhNHNH2, semicarbazide, 2,4-DNP) condense with C=O to give C=N products by addition then loss of water.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 3: When acetone is heated with hydroxylamine, the compound formed is
(a) Cyanohydrin
(b) Oxime
(c) Semicarbazone
(d) Hydrazone
Show Answer & Explanation

Answer: (b) Oxime

Explanation:
1. Acetone forms acetone oxime, \( \ce{(CH3)2C=NOH} \).

Teacher's Note:
1. Ammonia derivatives (NH2OH, NH2NH2, PhNHNH2, semicarbazide, 2,4-DNP) condense with C=O to give C=N products by addition then loss of water.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 4: The product of the reaction between ammonia and formaldehyde is
(a) Urotropine
(b) Formamide
(c) Paraformaldehyde
(d) Methanol
Show Answer & Explanation

Answer: (a) Urotropine

Explanation:
1. Methanal with ammonia gives hexamethylenetetramine.

Teacher's Note:
1. Urotropine, (CH2)6N4, comes from methanal and ammonia; it is a urinary antiseptic.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 5: Which product is obtained by the oxidation of propionaldehyde?
(a) Acetic acid
(b) Formic acid and acetic acid
(c) Propanoic acid
(d) n-Propyl alcohol
Show Answer & Explanation

Answer: (c) Propanoic acid

Explanation:
1. Oxidation of an aldehyde retains the carbon skeleton, so \( \ce{CH3CH2CHO} \) gives \( \ce{CH3CH2COOH} \).

Teacher's Note:
1. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 6: When acetaldehyde reacts with \( \ce{PCl5} \), the resulting compound is
(a) Ethyl chloride
(b) Ethylene chloride
(c) Ethylidene chloride
(d) Trichloro acetaldehyde
Show Answer & Explanation

Answer: (c) Ethylidene chloride

Explanation:
1. PCl5 replaces the carbonyl oxygen by two chlorine atoms, giving \( \ce{CH3CHCl2} \) (1,1-dichloroethane, ethylidene chloride).

Teacher's Note:
1. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 7: Benzaldehyde and acetaldehyde can be differentiated by
(a) HCN
(b) \( \ce{NH2OH} \)
(c) Hydrazine
(d) NaOH solution
Show Answer & Explanation

Answer: (d) NaOH solution

Explanation:
1. Both aldehydes add HCN, NH2OH and hydrazine.
2. With NaOH benzaldehyde gives Cannizzaro products, whereas acetaldehyde gives aldol and a brown resin.

Teacher's Note:
1. Cannizzaro reaction: an aldehyde with no alpha-hydrogen, in strong alkali, gives equal amounts of alcohol and carboxylate through hydride transfer.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 8: In the presence of a dilute base \( \ce{C6H5CHO} \) and \( \ce{CH3CHO} \) react together to give a product. The product is
(a) \( \ce{C6H5CH3} \)
(b) \( \ce{C6H5CH2CH2OH} \)
(c) \( \ce{C6H5CH2OH} \)
(d) \( \ce{C6H5CH=CHCHO} \)
Show Answer & Explanation

Answer: (d) \( \ce{C6H5CH=CHCHO} \)

Explanation:
1. Crossed aldol condensation, followed by dehydration, gives cinnamaldehyde.

Teacher's Note:
1. Aldol addition needs at least one alpha-hydrogen; dilute alkali gives the beta-hydroxy carbonyl compound, and heating dehydrates it to an enone.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 9: Grignard's reagent reacts with ethanal and propanone to give
(a) Higher aldehydes with ethanal and higher ketones with propanone
(b) Primary alcohols with ethanal and secondary alcohols with propanone
(c) Ethers with ethanal and alcohols with propanone
(d) Secondary alcohols with ethanal and tertiary alcohols with propanone
Show Answer & Explanation

Answer: (d) Secondary alcohols with ethanal and tertiary alcohols with propanone

Explanation:
1. RMgX adds to the carbonyl; ethanal gives a secondary and propanone a tertiary alcohol after hydrolysis.

Teacher's Note:
1. Grignard reagents give a secondary alcohol with aldehydes (primary with HCHO) and a tertiary alcohol with ketones.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 10: Benzaldehyde reacts with ammonia to form
(a) Benzaldehyde ammonia
(b) Urotropine
(c) Hydrobenzamide
(d) Aniline
Show Answer & Explanation

Answer: (c) Hydrobenzamide

Explanation:
1. Three molecules of benzaldehyde and two of ammonia give hydrobenzamide, \( \ce{(C6H5CH=N)2CHC6H5} \).

Teacher's Note:
1. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 11: Glucose with Tollens' reagent gives a silver mirror, which shows
(a) Presence of acidic group
(b) Presence of alkaline group
(c) Presence of ketonic group
(d) Presence of aldehyde group
Show Answer & Explanation

Answer: (d) Presence of aldehyde group

Explanation:
1. Silver is deposited when Tollens' reagent oxidises an aldehyde group.

Teacher's Note:
1. Tollens' reagent, [Ag(NH3)2]+, is reduced to silver by aldehydes and not by ketones.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 12: Fehling's solution is
(a) Ammoniacal cuprous chloride solution
(b) Acidified copper sulphate solution
(c) Copper sulphate and sodium hydroxide with Rochelle salt
(d) None of these
Show Answer & Explanation

Answer: (c) Copper sulphate and sodium hydroxide with Rochelle salt

Explanation:
1. Fehling's solution is alkaline copper(II) sulphate with sodium potassium tartrate (Rochelle salt) as complexing agent.

Teacher's Note:
1. Fehling's solution is reduced by aliphatic aldehydes (red Cu2O) but not by ketones or aromatic aldehydes.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 13: Reduction of an aldehyde produces
(a) Primary alcohol
(b) Monocarboxylic acid
(c) Secondary alcohol
(d) Tertiary alcohol
Show Answer & Explanation

Answer: (a) Primary alcohol

Explanation:
1. RCHO + 2[H] gives RCH2OH.

Teacher's Note:
1. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 14: Which of the following on reaction with conc. NaOH gives an alcohol?
(a) Methanal
(b) Ethanal
(c) Propanal
(d) Butanal
Show Answer & Explanation

Answer: (a) Methanal

Explanation:
1. Methanal has no alpha-hydrogen and undergoes Cannizzaro reaction to give methanol.
2. The others undergo aldol reactions and form resins.

Teacher's Note:
1. Cannizzaro reaction: an aldehyde with no alpha-hydrogen, in strong alkali, gives equal amounts of alcohol and carboxylate through hydride transfer.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 15: Schiff's reagent is
(a) Magenta colour solution decolourised with sulphurous acid
(b) Ammoniacal cobalt chloride solution
(c) Ammoniacal manganese sulphate solution
(d) Magenta solution decolourised with chlorine
Show Answer & Explanation

Answer: (a) Magenta colour solution decolourised with sulphurous acid

Explanation:
1. Schiff's reagent is a solution of magenta (rosaniline hydrochloride) whose colour is discharged by sulphur dioxide.

Teacher's Note:
1. Aldol needs alpha-hydrogen; Cannizzaro needs none.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 16: Pyrolysis of acetone gives \( \ce{CH2=C=O} \), called
(a) Methylene oxide
(b) Methyl carbon monoxide
(c) Ketene
(d) Methane
Show Answer & Explanation

Answer: (c) Ketene

Explanation:
1. \( \ce{CH2=C=O} \) is ketene.

Teacher's Note:
1. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 17: Acetal is obtained by reacting in the presence of dry HCl and alcohol with
(a) Aldehyde
(b) Ketone
(c) Ether
(d) Carboxylic acid
Show Answer & Explanation

Answer: (a) Aldehyde

Explanation:
1. An aldehyde and two molecules of alcohol give an acetal under dry HCl.

Teacher's Note:
1. Aldol needs alpha-hydrogen; Cannizzaro needs none.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 18: The reagent with which both aldehyde and acetone react easily is
(a) Fehling's reagent
(b) Grignard reagent
(c) Schiff's reagent
(d) Tollens' reagent
Show Answer & Explanation

Answer: (b) Grignard reagent

Explanation:
1. Aldehydes and ketones both undergo nucleophilic addition with RMgX. Fehling's, Tollens' and Schiff's reagents react only with aldehydes.

Teacher's Note:
1. Tollens' reagent, [Ag(NH3)2]+, is reduced to silver by aldehydes and not by ketones.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 19: Which of the following is used in the manufacture of thermosetting plastics?
(a) Formaldehyde
(b) Acetaldehyde
(c) Acetone
(d) Benzaldehyde
Show Answer & Explanation

Answer: (a) Formaldehyde

Explanation:
1. Formaldehyde with phenol or urea gives thermosetting resins (Bakelite, urea-formaldehyde).

Teacher's Note:
1. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 20: Which compound undergoes the iodoform reaction?
(a) \( \ce{HCHO} \)
(b) \( \ce{CH3CHO} \)
(c) \( \ce{CH3OH} \)
(d) \( \ce{CH3COOH} \)
Show Answer & Explanation

Answer: (b) \( \ce{CH3CHO} \)

Explanation:
1. Only ethanal has the \( \ce{CH3CO-} \) group.

Teacher's Note:
1. The iodoform test is positive for CH3CO- groups and CH3CH(OH)- groups (ethanol, propan-2-ol, ethanal, methyl ketones); methanal, benzophenone and 3-pentanone do not respond.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 21: Which of the following does not react with Fehling solution?
(a) Acetaldehyde
(b) Benzaldehyde
(c) Glucose
(d) Formic acid
Show Answer & Explanation

Answer: (b) Benzaldehyde

Explanation:
1. Aromatic aldehydes do not reduce Fehling's solution.
2. Formic acid, being also an aldehyde, does.

Teacher's Note:
1. Fehling's solution is reduced by aliphatic aldehydes (red Cu2O) but not by ketones or aromatic aldehydes.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 22: Which of the following compounds will react with ethanolic KCN?
(a) Ethane
(b) Acetyl chloride
(c) Chlorobenzene
(d) Benzaldehyde
Show Answer & Explanation

Answer: (b) Acetyl chloride

Explanation:
1. Acetyl chloride is attacked by cyanide, giving acetyl cyanide.
2. Benzaldehyde would also add cyanide (benzoin condensation), but ethane and chlorobenzene do not react.

Teacher's Note:
1. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 23: Schiff's reagent gives pink colour with
(a) Aldehydes
(b) Ethers
(c) Ketones
(d) Carboxylic acid
Show Answer & Explanation

Answer: (a) Aldehydes

Explanation:
1. Aldehydes restore the magenta colour of Schiff's reagent.

Teacher's Note:
1. Aldol needs alpha-hydrogen; Cannizzaro needs none.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 24: The compound which reacts with Fehling solution is
(a) \( \ce{C6H5COOH} \)
(b) \( \ce{HCOOH} \)
(c) \( \ce{C6H5CHO} \)
(d) \( \ce{CH2ClCH3} \)
Show Answer & Explanation

Answer: (b) \( \ce{HCOOH} \)

Explanation:
1. Formic acid has an aldehyde group and reduces Fehling's solution; benzoic acid, benzaldehyde and chloroethane do not.

Teacher's Note:
1. Fehling's solution is reduced by aliphatic aldehydes (red Cu2O) but not by ketones or aromatic aldehydes.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 25: Which one of the following undergoes reaction with 50% sodium hydroxide solution to give the corresponding alcohol and acid?
(a) Butanal
(b) Benzaldehyde
(c) Phenol
(d) Benzoic acid
Show Answer & Explanation

Answer: (b) Benzaldehyde

Explanation:
1. Benzaldehyde has no alpha-hydrogen, so it undergoes Cannizzaro reaction.

Teacher's Note:
1. Cannizzaro reaction: an aldehyde with no alpha-hydrogen, in strong alkali, gives equal amounts of alcohol and carboxylate through hydride transfer.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 26: Which one of the following is reduced with zinc and hydrochloric acid to give the corresponding hydrocarbon?
(a) Acetamide
(b) Acetic acid
(c) Ethyl acetate
(d) Butan-2-one
Show Answer & Explanation

Answer: (d) Butan-2-one

Explanation:
1. Clemmensen reduction converts the carbonyl of a ketone to a methylene group, giving butane.

Teacher's Note:
1. Clemmensen (Zn-Hg/HCl, acid) and Wolff-Kishner (N2H4/KOH, base) both convert C=O to CH2; choose by the acid or base sensitivity of the rest of the molecule.
2. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.

Question 27: Which of the following is used to detect aldehydes?
(a) Millon's test
(b) Tollens' reagent
(c) Neutral ferric chloride solution
(d) Molisch's test
Show Answer & Explanation

Answer: (b) Tollens' reagent

Explanation:
1. Tollens' reagent gives a silver mirror with aldehydes.

Teacher's Note:
1. Tollens' reagent, [Ag(NH3)2]+, is reduced to silver by aldehydes and not by ketones.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

Question 28: Which of the following aldehydes give a red precipitate with Fehling's solution?
(a) Benzaldehyde
(b) Salicylaldehyde
(c) Acetaldehyde
(d) None of these
Show Answer & Explanation

Answer: (c) Acetaldehyde

Explanation:
1. Only aliphatic aldehydes reduce Fehling's solution to red \( \ce{Cu2O} \).

Teacher's Note:
1. Fehling's solution is reduced by aliphatic aldehydes (red Cu2O) but not by ketones or aromatic aldehydes.
2. Aldol needs alpha-hydrogen; Cannizzaro needs none.

Question 29: A gives \( \ce{(CH3)2C=CHCOCH3} \). A is
(a) Acetone
(b) Acetaldehyde
(c) Propionaldehyde
(d) Formaldehyde
Show Answer & Explanation

Answer: (a) Acetone

Explanation:
1. Two molecules of acetone give mesityl oxide, \( \ce{(CH3)2C=CHCOCH3} \), on condensation with loss of water.

Teacher's Note:
1. Compare reactivity with steric hindrance and electron donation: HCHO > RCHO > ArCHO > R2CO > Ar2CO.
2. Use the tests: Tollens' and Fehling's for aldehydes, iodoform for CH3CO- and CH3CH(OH)-, 2,4-DNP for any carbonyl.

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