Ch-8: Aldehydes, Ketones and Carboxylic Acids12 Questions
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1. The characteristic functional group of aldehydes is:
A. -CHO
B. -COOH
C. >C=O only, without an attached H
D. -OH
Answer: A. -CHO
Explanation: Aldehydes are characterized by the -CHO group, where a carbonyl carbon is bonded to at least one hydrogen atom.
2. Tollens' reagent test is commonly used to distinguish:
A. Aldehydes from ketones
B. Alcohols from phenols
C. Acids from esters
D. Amines from amides
Answer: A. Aldehydes from ketones
Explanation: Aldehydes reduce Tollens' reagent (ammoniacal AgNO3) to form a silver mirror, while ketones generally do not react.
3. Aldol condensation occurs in carbonyl compounds that possess:
A. At least one α-hydrogen atom
B. No α-hydrogen atom
C. Only an aromatic ring
D. A carbon-carbon triple bond
Answer: A. At least one α-hydrogen atom
Explanation: α-hydrogens are essential for enolate formation, which then attacks another carbonyl carbon in aldol condensation.
4. The Cannizzaro reaction is characteristic of aldehydes that:
A. Lack an α-hydrogen atom
B. Possess an α-hydrogen atom
C. Are only aliphatic with α-H
D. Are actually ketones
Answer: A. Lack an α-hydrogen atom
Explanation: In the absence of α-hydrogens, aldehydes cannot undergo aldol reactions and instead undergo disproportionation (Cannizzaro reaction) in strong base.
5. Carboxylic acids are more acidic than phenols mainly because:
A. The carboxylate ion is more effectively stabilized by resonance
B. Carboxylic acids have a higher molecular weight
C. Phenol shows no resonance stabilization
D. Carboxylic acids are purely ionic
Answer: A. The carboxylate ion is more effectively stabilized by resonance
Explanation: In the carboxylate ion, the negative charge is delocalized equally over two oxygen atoms, giving greater stabilization than in the phenoxide ion.
6. The Hell-Volhard-Zelinsky (HVZ) reaction is used to introduce a halogen atom at the:
A. α-carbon of a carboxylic acid
B. Carbonyl carbon
C. β-carbon of the chain
D. Terminal carbon of the chain, regardless of position
Answer: A. α-carbon of a carboxylic acid
Explanation: The HVZ reaction uses red phosphorus and a halogen to selectively halogenate the α-carbon of a carboxylic acid.
7. Which test can distinguish acetaldehyde from acetone?
A. Tollens' test, since only acetaldehyde gives a silver mirror
B. Iodoform test, given by acetaldehyde only
C. Fehling's test, given only by acetone
D. Both give an identical positive result
Answer: A. Tollens' test, since only acetaldehyde gives a silver mirror
Explanation: Acetaldehyde, having a carbonyl carbon bonded to hydrogen, is oxidized by Tollens' reagent; the ketone acetone is not.
8. Nucleophilic addition reactions of carbonyl compounds are favoured due to the:
A. Electrophilic nature of the carbonyl carbon
B. Nucleophilic nature of the carbonyl carbon
C. Complete absence of polarity in C=O
D. High steric hindrance around the carbonyl group
Answer: A. Electrophilic nature of the carbonyl carbon
Explanation: The carbonyl carbon carries a partial positive charge due to oxygen's electronegativity, making it susceptible to nucleophilic attack.
9. Esterification of a carboxylic acid with an alcohol is typically carried out using:
A. Concentrated H2SO4 as a catalyst
B. NaOH as a catalyst
C. Zn dust
D. LiAlH4
Answer: A. Concentrated H2SO4 as a catalyst
Explanation: Conc. H2SO4 acts as an acid catalyst and dehydrating agent, driving the equilibrium of Fischer esterification towards ester formation.
10. Formic acid gives a positive Tollens' test because it contains:
A. A -CHO-like reducing group in addition to -COOH
B. Only a -COOH group with no reducing ability
C. No functional group capable of oxidation
D. A ketonic carbonyl group
Answer: A. A -CHO-like reducing group in addition to -COOH
Explanation: Formic acid (HCOOH) has an aldehydic hydrogen directly attached to the carbonyl carbon, giving it reducing properties similar to aldehydes.
11. Among compounds with a similar number of carbon atoms, which generally has the highest boiling point?
A. Carboxylic acid
B. Aldehyde
C. Ketone
D. Alkane
Answer: A. Carboxylic acid
Explanation: Carboxylic acids form strong intermolecular hydrogen-bonded dimers, giving them notably higher boiling points than aldehydes, ketones, or alkanes of similar size.
12. Decarboxylation of the sodium salt of a carboxylic acid with soda lime gives:
A. An alkane
B. An alkene
C. An alcohol
D. An aldehyde
Answer: A. An alkane
Explanation: Heating sodium salts of carboxylic acids with soda lime (NaOH + CaO) removes CO2, yielding the corresponding alkane.
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