Chapter 8: Aldehydes, Ketones and Carboxylic Acids Short Questions | chemistry Class 12 CBSE

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Chapter 8: Aldehydes, Ketones and Carboxylic Acids Short Questions | chemistry Class 12 CBSE | ByteNotes.in

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Question 1

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What is the Rosenmund reduction? State the reagents used and the type of compound it produces.

Rosenmund reduction is the hydrogenation of an acyl chloride (acid chloride) to give the corresponding aldehyde.
The reagents used are hydrogen gas (H₂) with palladium on barium sulphate (Pd-BaSO₄) as the catalyst.
The barium sulphate support partially poisons the catalyst, preventing over-reduction of the aldehyde to alcohol.

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Question 2

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State two differences in the boiling point behaviour of aldehydes/ketones compared to alcohols and hydrocarbons of similar molecular mass.

Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of comparable molecular mass due to dipole-dipole interactions arising from the polar carbonyl group.
Their boiling points are lower than alcohols of similar molecular mass because aldehydes and ketones cannot form intermolecular hydrogen bonds with each other (no O–H bond present).
For example, propan-1-ol (M=60, b.p. 370 K) boils much higher than acetone (M=58, b.p. 329 K) and n-butane (M=58, b.p. 273 K).

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Question 3

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What is the Gatterman-Koch reaction? Give the conditions and product.

The Gatterman-Koch reaction is the formylation of benzene or its derivatives to give benzaldehyde or substituted benzaldehyde.
It involves treating benzene with carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of anhydrous aluminium chloride (AlCl₃) or cuprous chloride (CuCl).
The reaction is an electrophilic aromatic substitution where a formyl cation (CHO⁺) acts as the electrophile.

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Question 4

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Why are aldehydes more reactive than ketones in nucleophilic addition reactions? Give two reasons.

Sterically, aldehydes have only one alkyl/aryl substituent on the carbonyl carbon, offering less hindrance to the approach of the incoming nucleophile compared to ketones, which have two such groups.
Electronically, in ketones two alkyl groups donate electron density to the carbonyl carbon, reducing its electrophilicity more than the single alkyl group in aldehydes.
Hence aldehydes are more susceptible to nucleophilic attack and undergo addition reactions faster than ketones.

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Question 5

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What is Tollens' test? What does a positive test indicate, and which class of compound does NOT respond to it?

Tollens' test involves warming an organic compound with freshly prepared ammoniacal silver nitrate solution (Tollens' reagent); a bright silver mirror on the inner surface of the test tube indicates a positive result.
A positive Tollens' test indicates the presence of an aldehyde group, as the aldehyde is oxidised to the corresponding carboxylate anion while Ag⁺ is reduced to Ag metal.
Aromatic aldehydes do not respond to Fehling's test, but all aldehydes (both aliphatic and aromatic) give a positive Tollens' test; ketones do not respond to Tollens' test.

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Question 6

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Distinguish between Clemmensen reduction and Wolff-Kishner reduction.

Clemmensen reduction uses zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl) to reduce the carbonyl group (C=O) to a methylene group (–CH₂–).
Wolff-Kishner reduction uses hydrazine (N₂H₄) followed by heating with sodium or potassium hydroxide in a high-boiling solvent such as ethylene glycol to achieve the same C=O to CH₂ conversion.
Clemmensen is used for acid-stable substrates, while Wolff-Kishner is suitable for base-stable substrates.

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Question 7

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How is the carbonyl carbon of aldehydes and ketones hybridised? Describe its geometry.

The carbonyl carbon is sp²-hybridised, forming three sigma (σ) bonds in a trigonal planar arrangement.
The fourth valence electron of the carbonyl carbon remains in its p-orbital and forms a π-bond with the p-orbital of oxygen.
The carbonyl carbon and the three atoms directly attached to it all lie in the same plane, with bond angles of approximately 120°.

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Question 8

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What is the Stephen reaction? Name the reagents involved.

The Stephen reaction is the reduction of a nitrile (RCN) to an imine (RCH=NH) using stannous chloride (SnCl₂) in the presence of hydrochloric acid (HCl).
The imine intermediate is then hydrolysed with water (H₃O⁺) to give the corresponding aldehyde (RCHO).
This reaction is particularly useful for preparing aliphatic and aromatic aldehydes from nitriles.

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Question 9

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Why do carboxylic acids react with sodium bicarbonate (NaHCO₃) to evolve CO₂, but alcohols do not?

Carboxylic acids are stronger acids than carbonic acid (H₂CO₃), so they can protonate the bicarbonate ion and liberate carbon dioxide gas.
Alcohols are weaker acids than carbonic acid and cannot donate a proton to the bicarbonate ion, so no reaction occurs.
This difference is used as a chemical test to distinguish carboxylic acids (or phenols vs. carboxylic acids, since phenols also do not react with NaHCO₃) from other functional groups.

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Question 10

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What is Fehling's test? Which class of aldehydes does NOT respond to it?

Fehling's test uses a mixture of Fehling's solution A (aqueous CuSO₄) and Fehling's solution B (alkaline sodium potassium tartarate); a reddish-brown precipitate of Cu₂O on heating indicates a positive result.
The positive test shows the presence of an aliphatic aldehyde, which reduces Cu²⁺ to Cu⁺ (Cu₂O) while being oxidised to the corresponding carboxylate anion.
Aromatic aldehydes (like benzaldehyde) do not respond to Fehling's test, making it useful for distinguishing aliphatic from aromatic aldehydes.

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Question 11

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What are cyanohydrins? State their synthetic importance.

Cyanohydrins are compounds formed by the nucleophilic addition of HCN to aldehydes or ketones, containing both a –CN (cyano) and a –OH (hydroxyl) group on the same carbon atom.
The reaction is catalysed by a base (which generates CN⁻, the actual nucleophile), as pure HCN reacts very slowly.
Cyanohydrins are valuable synthetic intermediates because the –CN group can be hydrolysed to –COOH or reduced to –CH₂NH₂, allowing carbon-chain extension.

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Question 12

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Explain why carboxylic acids are more acidic than phenols.

The conjugate base of carboxylic acid (carboxylate ion) is stabilised by two equivalent resonance structures where the negative charge is delocalised over two electronegative oxygen atoms.
The conjugate base of phenol (phenoxide ion) has non-equivalent resonance structures where the negative charge is partly on less electronegative carbon atoms, providing less effective stabilisation.
Greater stabilisation of carboxylate ion over phenoxide ion means carboxylic acid loses its proton more readily, making it the stronger acid.

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Question 13

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How are ketones prepared by Friedel-Crafts acylation? Give the reaction conditions.

Friedel-Crafts acylation involves treating benzene or a substituted benzene with an acyl chloride (RCOCl) or acid anhydride in the presence of anhydrous aluminium chloride (AlCl₃) as a Lewis acid catalyst.
The AlCl₃ activates the acyl chloride by forming an electrophilic acylium ion (RCO⁺), which then attacks the aromatic ring in an electrophilic substitution.
The product is an aromatic ketone; for example, benzene reacted with acetyl chloride/AlCl₃ gives acetophenone (phenyl methyl ketone).

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Chapter 8: Aldehydes, Ketones and Carboxylic Acids | Short Questions

What is the Rosenmund reduction? State the reagents used and the type of compound it produces.

State two differences in the boiling point behaviour of aldehydes/ketones compared to alcohols and hydrocarbons of similar molecular mass.

What is the Gatterman-Koch reaction? Give the conditions and product.

Why are aldehydes more reactive than ketones in nucleophilic addition reactions? Give two reasons.

What is Tollens' test? What does a positive test indicate, and which class of compound does NOT respond to it?

Distinguish between Clemmensen reduction and Wolff-Kishner reduction.

How is the carbonyl carbon of aldehydes and ketones hybridised? Describe its geometry.

What is the Stephen reaction? Name the reagents involved.

Why do carboxylic acids react with sodium bicarbonate (NaHCO₃) to evolve CO₂, but alcohols do not?

What is Fehling's test? Which class of aldehydes does NOT respond to it?

What are cyanohydrins? State their synthetic importance.

Explain why carboxylic acids are more acidic than phenols.

How are ketones prepared by Friedel-Crafts acylation? Give the reaction conditions.

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