Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs) and Explanations

Here are 40 MCQs based on the “Aldehydes, Ketones and Carboxylic Acids” notes, with detailed explanations.

Question 1: Which of the following statements is true regarding aldehydes and ketones? (A) Both undergo oxidation readily. (B) Aldehydes are more reactive than ketones towards nucleophilic addition. (C) Ketones have a hydrogen atom directly attached to the carbonyl carbon. (D) Both give positive Tollens’ test.

Explanation 1:

  • Correct Answer: (B)
  • Reasoning: Aldehydes are more reactive than ketones towards nucleophilic addition due to less steric hindrance and greater electrophilicity of the carbonyl carbon. Aldehydes are easily oxidized, but ketones are not. Only aldehydes give a positive Tollens’ test.

Question 2: Which reagent is used to convert an aldehyde to a primary alcohol? (A) KMnO4​ (B) PCC (C) LiAlH4​ (D) CrO3​

Explanation 2:

  • Correct Answer: (C)
  • Reasoning: LiAlH4​ (Lithium Aluminium Hydride) is a strong reducing agent that reduces aldehydes to primary alcohols. KMnO4​ and CrO3​ are oxidizing agents, and PCC is a mild oxidizing agent.

Question 3: What is the major product formed when propene undergoes ozonolysis followed by treatment with Zn/H2​O? (A) Propanal (B) Propanone (C) Ethanal and Methanal (D) Ethanoic acid and Methanoic acid

Explanation 3:

  • Correct Answer: (C)
  • Reasoning: Ozonolysis of propene (CH3​CH=CH2​) breaks the double bond. The carbons of the double bond become carbonyl carbons. This yields CH3​CHO (ethanal) from the CH3​CH= part and HCHO (methanal) from the =CH2​ part.

Question 4: Which of the following compounds will give a positive iodoform test? (A) Propanal (B) Propanone (C) Benzaldehyde (D) Ethanoic acid

Explanation 4:

  • Correct Answer: (B)
  • Reasoning: The iodoform test is positive for compounds containing the CH3​CO group or the CH3​CH(OH) group. Propanone (CH3​COCH3​) contains the CH3​CO group. Propanal and benzaldehyde lack this group, and ethanoic acid is a carboxylic acid.

Question 5: Carboxylic acids are stronger acids than phenols because: (A) Carboxylic acids have a higher molecular weight. (B) Carboxylate ion is more resonance stabilized than phenoxide ion. (C) Carboxylic acids form hydrogen bonds. (D) The carboxyl group contains a carbonyl group.

Explanation 5:

  • Correct Answer: (B)
  • Reasoning: The carboxylate anion (RCOO−) has two equivalent resonance structures where the negative charge is delocalized over two electronegative oxygen atoms. The phenoxide ion has resonance structures, but the negative charge is delocalized onto carbon atoms of the ring, and the resonance structures are not equivalent. This greater and equivalent resonance stabilization makes the carboxylate ion more stable, hence carboxylic acids are stronger acids.

Question 6: Which reaction involves the conversion of an acyl chloride to an aldehyde? (A) Etard reaction (B) Gattermann-Koch reaction (C) Rosenmund reduction (D) Stephen reaction

Explanation 6:

  • Correct Answer: (C)
  • Reasoning: Rosenmund reduction specifically uses H2​ over Pd/BaSO4​ (poisoned catalyst) to reduce acyl chlorides to aldehydes. Etard and Gattermann-Koch are for benzaldehyde synthesis from toluene and benzene, respectively. Stephen reaction is for nitriles.

Question 7: The product of reaction of an aldehyde with hydrazine (NH2​NH2​) is: (A) Oxime (B) Hydrazone (C) Semicarbazone (D) Phenylhydrazone

Explanation 7:

  • Correct Answer: (B)
  • Reasoning: Aldehydes and ketones react with hydrazine (NH2​NH2​) to form hydrazones (>C=N-NH2​).

Question 8: Which of the following is a common name for propanone? (A) Acetaldehyde (B) Formaldehyde (C) Acetone (D) Benzaldehyde

Explanation 8:

  • Correct Answer: (C)
  • Reasoning: Propanone is commonly known as Acetone (CH3​COCH3​).

Question 9: Which of the following reagents will convert a primary alcohol directly to a carboxylic acid? (A) PCC (B) Cu at 573 K (C) Acidified KMnO4​ (D) NaBH4​

Explanation 9:

  • Correct Answer: (C)
  • Reasoning: Acidified potassium permanganate (KMnO4​) is a strong oxidizing agent that can oxidize primary alcohols directly to carboxylic acids. PCC is a mild oxidant (stops at aldehyde), Cu at 573 K is for dehydrogenation, and NaBH4​ is a reducing agent.

Question 10: The Hell-Volhard-Zelinsky (HVZ) reaction is used for the halogenation of: (A) Aldehydes (B) Ketones (C) Carboxylic acids with α-hydrogen (D) Carboxylic acids without α-hydrogen

Explanation 10:

  • Correct Answer: (C)
  • Reasoning: HVZ reaction is a specific method for the halogenation of the α-carbon of carboxylic acids that have at least one α-hydrogen.

Question 11: The carbonyl carbon in aldehydes and ketones is: (A) sp3 hybridized (B) sp2 hybridized (C) sp hybridized (D) d2sp3 hybridized

Explanation 11:

  • Correct Answer: (B)
  • Reasoning: The carbon atom of the carbonyl group (>C=O) is sp2 hybridized, leading to a trigonal planar geometry around it.

Question 12: Which reducing agent is used in Clemmensen reduction? (A) LiAlH4​ (B) NaBH4​ (C) Zn-Hg/Conc. HCl (D) NH2​NH2​/KOH

Explanation 12:

  • Correct Answer: (C)
  • Reasoning: Clemmensen reduction uses amalgamated zinc and concentrated HCl (Zn-Hg/Conc. HCl) to reduce carbonyl compounds (>C=O) to methylene groups (>CH2​).

Question 13: Which of the following is a trihalo carboxylic acid? (A) Acetic acid (B) Chloroacetic acid (C) Trichloroacetic acid (D) Formic acid

Explanation 13:

  • Correct Answer: (C)
  • Reasoning: Trichloroacetic acid (CCl3​COOH) has three halogen atoms attached to the α-carbon of the carboxylic acid.

Question 14: Which of the following aldehydes will undergo Cannizzaro reaction? (A) Ethanal (B) Propanal (C) 2-methylpropanal (D) Benzaldehyde

Explanation 14:

  • Correct Answer: (D)
  • Reasoning: Cannizzaro reaction is characteristic of aldehydes that do not possess an α-hydrogen atom. Benzaldehyde (C6​H5​CHO) has no α-hydrogen. Ethanal, propanal, and 2-methylpropanal all have α-hydrogens.

Question 15: The most appropriate reagent for the conversion of carboxylic acid to an acyl chloride is: (A) PCl5​ (B) PCl3​ (C) SOCl2​ (D) HCl

Explanation 15:

  • Correct Answer: (C)
  • Reasoning: Thionyl chloride (SOCl2​) is generally considered the best reagent for converting carboxylic acids to acyl chlorides because the byproducts (SO2​ and HCl) are gases and escape, leaving a pure product.

Question 16: The boiling points of carboxylic acids are higher than those of corresponding alcohols primarily due to: (A) Greater molecular mass. (B) Formation of stronger intermolecular hydrogen bonds (dimerization). (C) Greater polarity. (D) Presence of carbonyl group.

Explanation 16:

  • Correct Answer: (B)
  • Reasoning: Carboxylic acids form strong hydrogen bonds and exist as dimers in both liquid and gaseous states. This effectively doubles their molecular weight in terms of intermolecular forces, leading to significantly higher boiling points compared to alcohols that form simple linear hydrogen-bonded chains.

Question 17: Which compound will give a red precipitate with Fehling’s solution? (A) Acetone (B) Acetic acid (C) Benzaldehyde (D) Phenol

Explanation 17:

  • Correct Answer: (C)
  • Reasoning: Fehling’s test is used to detect aliphatic aldehydes. While aromatic aldehydes like benzaldehyde also reduce Fehling’s solution, it’s generally a slower reaction or requires heating. Ketones and carboxylic acids do not give a positive Fehling’s test.

Question 18: The reagent used for Etard reaction is: (A) SnCl2​/HCl (B) Pd/BaSO4​ (C) CrO2​Cl2​ (D) CO/HCl

Explanation 18:

  • Correct Answer: (C)
  • Reasoning: Etard reaction uses chromyl chloride (CrO2​Cl2​) to oxidize toluene to benzaldehyde.

Question 19: What happens when benzoic acid is subjected to nitration? (A) o-nitrobenzoic acid is formed. (B) m-nitrobenzoic acid is formed. (C) p-nitrobenzoic acid is formed. (D) No reaction occurs.

Explanation 19:

  • Correct Answer: (B)
  • Reasoning: The −COOH group is a deactivating and meta-directing group in electrophilic aromatic substitution reactions. Therefore, nitration of benzoic acid predominantly yields m-nitrobenzoic acid.

Question 20: Which reducing agent is used to reduce a nitrile to an aldehyde in the Stephen reaction? (A) LiAlH4​ (B) NaBH4​ (C) SnCl2​/HCl (D) DIBAL-H

Explanation 20:

  • Correct Answer: (C)
  • Reasoning: The Stephen reaction specifically uses stannous chloride (SnCl2​) and concentrated HCl, followed by hydrolysis, to reduce nitriles to aldehydes. DIBAL-H can also do this, but SnCl2​/HCl is characteristic of Stephen reaction.

Question 21: The order of acidity for acetic acid, chloroacetic acid, and trichloroacetic acid is: (A) Acetic acid < Chloroacetic acid < Trichloroacetic acid (B) Trichloroacetic acid < Chloroacetic acid < Acetic acid (C) Chloroacetic acid < Trichloroacetic acid < Acetic acid (D) Acetic acid = Chloroacetic acid = Trichloroacetic acid

Explanation 21:

  • Correct Answer: (A)
  • Reasoning: Electron-withdrawing groups increase the acidity of carboxylic acids. Chlorine is an electron-withdrawing group via inductive effect. The more chlorine atoms present, the greater the electron-withdrawing effect, and thus stronger the acid. Therefore, CCl3​COOH (Trichloroacetic acid) is the strongest acid, followed by ClCH2​COOH (Chloroacetic acid), and then CH3​COOH (Acetic acid).

Question 22: Which of the following is a dihydric carboxylic acid? (A) Formic acid (B) Acetic acid (C) Oxalic acid (D) Benzoic acid

Explanation 22:

  • Correct Answer: (C)
  • Reasoning: Oxalic acid (HOOC-COOH) contains two carboxyl groups, making it a dihydric (dicarboxylic) acid. Formic, acetic, and benzoic acids are monocarboxylic acids.

Question 23: The reaction of an aldehyde with an alcohol in the presence of dry HCl gas forms: (A) Ester (B) Ether (C) Acetal (D) Ketone

Explanation 23:

  • Correct Answer: (C)
  • Reasoning: Aldehydes react with alcohols in the presence of dry HCl gas to form hemiacetals initially, which then react with another molecule of alcohol to form acetals. Ketones form ketals.

Question 24: Which of the following is used as a food preservative? (A) Formaldehyde (B) Acetaldehyde (C) Sodium benzoate (D) Acetone

Explanation 24:

  • Correct Answer: (C)
  • Reasoning: Sodium benzoate, the salt of benzoic acid, is commonly used as a food preservative, especially in acidic foods, to inhibit microbial growth. Formaldehyde (as formalin) is also a preservative but generally not for food.

Question 25: The general reactivity order towards nucleophilic addition for carbonyl compounds is: (A) Aldehydes > Ketones (B) Ketones > Aldehydes (C) Aldehydes = Ketones (D) Formaldehyde < other Aldehydes

Explanation 25:

  • Correct Answer: (A)
  • Reasoning: Aldehydes are generally more reactive than ketones towards nucleophilic addition due to less steric hindrance and greater electrophilicity of the carbonyl carbon. Formaldehyde is the most reactive aldehyde.

Question 26: The product of oxidation of an alkylbenzene with alkaline KMnO4​ followed by acidification is always: (A) Phenyl ketone (B) Phenol (C) Benzoic acid (D) Benzyl alcohol

Explanation 26:

  • Correct Answer: (C)
  • Reasoning: Alkylbenzenes (with at least one benzylic hydrogen) are oxidized by strong oxidizing agents like alkaline KMnO4​ to benzoic acid, regardless of the length of the alkyl chain. The entire alkyl chain is oxidized to the carboxyl group.

Question 27: The presence of a CH3​CH(OH)- group can be detected by: (A) Tollens’ test (B) Fehling’s test (C) Iodoform test (D) Benedict’s test

Explanation 27:

  • Correct Answer: (C)
  • Reasoning: The iodoform test gives a positive result (yellow precipitate of CHI3​) for compounds containing either a methyl ketone (CH3​CO-) group or a methyl secondary alcohol (CH3​CH(OH)-) group, as the latter can be oxidized to the former.

Question 28: Which of the following is a reagent used in the Wolff-Kishner reduction? (A) Zn-Hg/Conc. HCl (B) H2​/Pd (C) NH2​NH2​/KOH (D) LiAlH4​

Explanation 28:

  • Correct Answer: (C)
  • Reasoning: Wolff-Kishner reduction uses hydrazine (NH2​NH2​) and a strong base like KOH (in ethylene glycol, with heating) to reduce carbonyl compounds to hydrocarbons.

Question 29: Formalin is a: (A) 40% aqueous solution of formaldehyde. (B) 10% aqueous solution of formaldehyde. (C) 40% aqueous solution of acetaldehyde. (D) 10% aqueous solution of acetaldehyde.

Explanation 29:

  • Correct Answer: (A)
  • Reasoning: Formalin is a common term for a 40% aqueous solution of formaldehyde, widely used as a disinfectant and preservative.

Question 30: Which of the following is formed when an amide undergoes hydrolysis? (A) Alcohol (B) Aldehyde (C) Carboxylic acid (D) Amine

Explanation 30:

  • Correct Answer: (C)
  • Reasoning: Amides, upon hydrolysis (in acidic or basic medium), yield carboxylic acids and ammonia or an amine.

Question 31: What type of reaction is the Aldol condensation? (A) Nucleophilic substitution (B) Electrophilic addition (C) Nucleophilic addition followed by elimination of water (D) Free radical substitution

Explanation 31:

  • Correct Answer: (C)
  • Reasoning: Aldol condensation involves the nucleophilic addition of an enolate ion (from one carbonyl compound) to the carbonyl carbon of another, followed by elimination of a water molecule to form the α,β-unsaturated carbonyl compound.

Question 32: An aldehyde which does not give positive Tollens’ test is: (A) Formaldehyde (B) Acetaldehyde (C) Benzaldehyde (D) All aldehydes give positive Tollens’ test.

Explanation 32:

  • Correct Answer: (D)
  • Reasoning: All aldehydes (both aliphatic and aromatic) give a positive Tollens’ test (silver mirror formation) because the aldehyde group is easily oxidized to a carboxylate group. The question might be tricky, but technically all aldehydes will show a positive test.

Question 33: The boiling points of aldehydes and ketones are lower than corresponding alcohols because: (A) They are less polar than alcohols. (B) They cannot form intermolecular hydrogen bonds. (C) They have lower molecular weights. (D) They are more volatile.

Explanation 33:

  • Correct Answer: (B)
  • Reasoning: Aldehydes and ketones cannot form intermolecular hydrogen bonds with each other (as they lack a hydrogen atom directly bonded to oxygen), unlike alcohols, which can. This leads to weaker intermolecular forces and thus lower boiling points than alcohols of comparable molecular mass.

Question 34: The reaction of an aldehyde with an alcohol to form an acetal is catalyzed by: (A) Base (B) Acid (C) Light (D) Peroxide

Explanation 34:

  • Correct Answer: (B)
  • Reasoning: The formation of acetals (and ketals) from aldehydes (and ketones) and alcohols is an acid-catalyzed reaction, typically using dry HCl gas.

Question 35: Which of the following will react with CO2​ (dry ice) followed by hydrolysis to give a carboxylic acid? (A) Alkene (B) Alkane (C) Grignard reagent (D) Alcohol

Explanation 35:

  • Correct Answer: (C)
  • Reasoning: Grignard reagents react with carbon dioxide (dry ice) to form an adduct which, upon hydrolysis, yields a carboxylic acid. This is an important method for increasing the carbon chain length.

Question 36: The IUPAC name for CH3​CH2​COOH is: (A) Acetic acid (B) Formic acid (C) Propanoic acid (D) Butanoic acid

Explanation 36:

  • Correct Answer: (C)
  • Reasoning: The longest carbon chain containing the carboxyl group has three carbon atoms. Therefore, its IUPAC name is Propanoic acid.

Question 37: Which of the following is an example of an aromatic aldehyde? (A) Ethanal (B) Propanone (C) Benzaldehyde (D) Methanal

Explanation 37:

  • Correct Answer: (C)
  • Reasoning: Benzaldehyde (C6​H5​CHO) has an aldehyde group directly attached to a benzene ring, making it an aromatic aldehyde. The others are aliphatic aldehydes or ketones.

Question 38: The common name of CH3​COOH is: (A) Formic acid (B) Oxalic acid (C) Benzoic acid (D) Acetic acid

Explanation 38:

  • Correct Answer: (D)
  • Reasoning: CH3​COOH is commonly known as Acetic acid.

Question 39: Which product is formed when benzaldehyde undergoes Cannizzaro reaction? (A) Benzyl alcohol and sodium benzoate (B) Benzoic acid and methanol (C) Toluene (D) Benzene

Explanation 39:

  • Correct Answer: (A)
  • Reasoning: Benzaldehyde, lacking an α-hydrogen, undergoes disproportionation in the Cannizzaro reaction. One molecule is oxidized to sodium benzoate, and another is reduced to benzyl alcohol.

Question 40: The reaction to convert an aldehyde or ketone to a hydrocarbon using hydrazine and KOH is called: (A) Clemmensen reduction (B) Rosenmund reduction (C) Wolff-Kishner reduction (D) Stephen reaction

Explanation 40:

  • Correct Answer: (C)
  • Reasoning: This is the definition of the Wolff-Kishner reduction. Clemmensen reduction uses Zn-Hg/HCl, and Rosenmund and Stephen are for aldehyde synthesis.

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