CBSE Class 12 Chemistry (2026–27)
Chapter 9: Aldehydes, Ketones and Carboxylic Acids
20 Important Questions with Answers
This chapter covers nomenclature, preparation, physical properties, chemical reactions, named reactions, and applications of aldehydes, ketones, and carboxylic acids. Important reactions include Aldol condensation, Cannizzaro reaction, Rosenmund reduction, Stephen reaction, Clemmensen reduction, Wolff-Kishner reduction, Haloform reaction, and HVZ reaction.
Q1. What is the carbonyl group? Explain its nature.
Answer:
The carbonyl group (>C=O) is the functional group present in aldehydes and ketones. In aldehydes, the carbonyl carbon is attached to at least one hydrogen atom, while in ketones it is bonded to two carbon atoms. The carbon atom and oxygen atom are sp² hybridized, giving the group a planar structure. Oxygen is more electronegative than carbon and attracts the shared electrons towards itself, creating a polar bond. As a result, the carbonyl carbon acquires a partial positive charge and behaves as an electrophile. This polarity makes aldehydes and ketones highly reactive towards nucleophilic addition reactions.
Q2. Why are aldehydes more reactive than ketones towards nucleophilic addition?
Answer:
Aldehydes are more reactive towards nucleophilic addition than ketones because of both electronic and steric factors. In aldehydes, the carbonyl carbon is attached to one alkyl group and one hydrogen atom. The hydrogen atom does not donate electrons, so the carbonyl carbon remains highly positive and susceptible to nucleophilic attack. In ketones, two alkyl groups donate electron density to the carbonyl carbon, reducing its positive character. Additionally, the presence of two bulky alkyl groups creates steric hindrance, making it difficult for nucleophiles to approach the carbonyl carbon. Therefore, aldehydes undergo nucleophilic addition reactions more readily than ketones.
Q3. Describe the preparation of aldehydes from primary alcohols.
Answer:
Aldehydes can be prepared by the controlled oxidation of primary alcohols. When a primary alcohol is treated with a mild oxidizing agent such as PCC (Pyridinium Chlorochromate), the alcohol is converted into the corresponding aldehyde without further oxidation. For example, ethanol is oxidized to ethanal. Strong oxidizing agents are generally avoided because they oxidize aldehydes further to carboxylic acids. Aldehydes may also be prepared by dehydrogenation of primary alcohols using heated copper at about 573 K. This method removes hydrogen from the alcohol molecule to form an aldehyde. Controlled oxidation is one of the most important laboratory methods for preparing aldehydes.
Q4. What is Rosenmund reduction?
Answer:
Rosenmund reduction is a method used for the preparation of aldehydes from acid chlorides. In this reaction, an acyl chloride is hydrogenated in the presence of palladium supported on barium sulfate (Pd/BaSO₄), which acts as a poisoned catalyst. The catalyst slows down the reaction and prevents the aldehyde from being further reduced to an alcohol. For example, benzoyl chloride on Rosenmund reduction gives benzaldehyde. This method is especially useful for preparing aromatic aldehydes in high purity. Rosenmund reduction is an important named reaction in the Class 12 organic chemistry syllabus and is frequently asked in board examinations.
Q5. Explain Stephen reaction.
Answer:
Stephen reaction is used for the preparation of aldehydes from nitriles. In this method, a nitrile is reduced using stannous chloride (SnCl₂) and hydrochloric acid to form an iminium salt. The intermediate is then hydrolysed to produce the corresponding aldehyde. For example, ethanenitrile on Stephen reduction gives ethanal after hydrolysis. This reaction is valuable because nitriles can be easily converted into aldehydes without forming alcohols. It provides a convenient route for synthesizing aldehydes from compounds containing one additional carbon atom. Stephen reaction is one of the important named reactions included in the CBSE Class 12 syllabus.
Q6. What is Aldol condensation?
Answer:
Aldol condensation occurs when aldehydes or ketones containing at least one α-hydrogen atom react in the presence of dilute alkali. Two molecules combine to form a β-hydroxy aldehyde or β-hydroxy ketone known as an aldol. For example, acetaldehyde undergoes self-condensation to form 3-hydroxybutanal. On heating, the aldol loses a molecule of water and forms an α,β-unsaturated carbonyl compound. Aldol condensation is an important carbon-carbon bond-forming reaction in organic chemistry. The reaction is widely used in industrial and laboratory synthesis and is frequently tested in board and competitive examinations.
Q7. What is Cannizzaro reaction?
Answer:
Cannizzaro reaction is shown by aldehydes that do not possess an α-hydrogen atom. When such aldehydes are treated with concentrated sodium hydroxide solution, one molecule is oxidized to a carboxylic acid while another molecule is reduced to an alcohol. This is an example of disproportionation. For example, benzaldehyde reacts with concentrated NaOH to produce benzyl alcohol and sodium benzoate. The reaction proceeds through the transfer of a hydride ion from one aldehyde molecule to another. Cannizzaro reaction is important because it helps distinguish aldehydes lacking α-hydrogen from those that undergo aldol condensation.
Q8. What is the Haloform reaction?
Answer:
Haloform reaction is characteristic of compounds containing the CH₃CO– group. Such compounds react with halogens in the presence of alkali to produce haloforms such as chloroform, bromoform, or iodoform. Acetaldehyde and methyl ketones give this reaction. In the laboratory, the iodoform test is commonly used for identification. A yellow precipitate of iodoform (CHI₃) indicates a positive test. Ethanol also gives this reaction because it is oxidized to acetaldehyde under the reaction conditions. Haloform reaction is widely used for the identification of methyl ketones and related compounds in qualitative organic analysis.
Q9. How can aldehydes and ketones be distinguished using Tollens’ test?
Answer:
Tollens’ reagent is an ammoniacal solution of silver nitrate. Aldehydes readily reduce Tollens’ reagent to metallic silver while themselves getting oxidized to carboxylic acids. As a result, a bright silver mirror is deposited on the inner walls of the test tube. Ketones generally do not respond to this test because they are resistant to oxidation under these conditions. Therefore, the silver mirror test is used to distinguish aldehydes from ketones. For example, ethanal gives a positive Tollens’ test whereas propanone does not. This reaction is a very important qualitative test for aldehydes in organic chemistry.
Q10. Explain Fehling’s test.
Answer:
Fehling’s solution consists of Fehling A (copper sulfate solution) and Fehling B (alkaline sodium potassium tartrate solution). When an aliphatic aldehyde is heated with Fehling’s solution, it reduces Cu²⁺ ions to cuprous oxide (Cu₂O), producing a brick-red precipitate. The aldehyde is simultaneously oxidized to a carboxylate ion. Ketones generally do not give this test. Aromatic aldehydes usually fail to reduce Fehling’s solution because of their lower reactivity. Fehling’s test is therefore useful for distinguishing aliphatic aldehydes from ketones and is commonly employed in qualitative analysis.
Q11. What is Clemmensen reduction?
Answer:
Clemmensen reduction converts aldehydes and ketones into hydrocarbons by reducing the carbonyl group to a methylene group. The reaction is carried out using zinc amalgam (Zn-Hg) and concentrated hydrochloric acid. For example, acetone is reduced to propane. This reaction is especially useful for compounds that are stable under strongly acidic conditions. Clemmensen reduction is widely used in synthetic organic chemistry to remove oxygen from carbonyl compounds. It serves as an important method for converting aldehydes and ketones into alkanes and is often compared with Wolff-Kishner reduction in examinations.
Q12. What is Wolff-Kishner reduction?
Answer:
Wolff-Kishner reduction converts aldehydes and ketones into hydrocarbons under strongly basic conditions. The carbonyl compound first reacts with hydrazine to form a hydrazone. On heating with potassium hydroxide in a high-boiling solvent such as ethylene glycol, nitrogen gas is eliminated and the hydrocarbon is formed. This reaction is useful for compounds sensitive to acidic conditions. For example, acetone is converted into propane through Wolff-Kishner reduction. The reaction is widely used in organic synthesis and is often discussed alongside Clemmensen reduction because both reactions accomplish the same overall transformation through different reaction conditions.
Q13. Why are carboxylic acids acidic in nature?
Answer:
Carboxylic acids are acidic because they can donate a proton from the carboxyl group (–COOH). After losing a proton, they form a carboxylate ion (RCOO⁻). This ion is highly stable due to resonance, as the negative charge is delocalized over two oxygen atoms. The stability of the conjugate base makes proton release easier and increases acidity. In contrast, alcohols form alkoxide ions that lack such resonance stabilization. Therefore, carboxylic acids are much stronger acids than alcohols. Their acidic strength is influenced by substituents attached to the carbon chain, particularly electron-withdrawing groups that further stabilize the carboxylate ion.
Q14. Explain the effect of substituents on the acidity of carboxylic acids.
Answer:
The acidity of carboxylic acids depends on the stability of the carboxylate ion formed after proton loss. Electron-withdrawing groups such as chlorine, fluorine, or nitro groups increase acidity by stabilizing the negative charge through the inductive effect. For example, chloroacetic acid is more acidic than acetic acid. On the other hand, electron-donating groups such as alkyl groups decrease acidity because they destabilize the carboxylate ion. The effect is stronger when the substituent is closer to the carboxyl group. Thus, acidity increases with electron-withdrawing substituents and decreases with electron-donating substituents. This concept is important for comparing acidic strengths of organic compounds.
Q15. What is esterification?
Answer:
Esterification is the reaction between a carboxylic acid and an alcohol in the presence of concentrated sulfuric acid to form an ester and water. For example, acetic acid reacts with ethanol to produce ethyl acetate and water. Concentrated sulfuric acid acts as both a catalyst and a dehydrating agent. Esters generally possess pleasant fruity odours and are widely used in perfumes, flavouring agents, and cosmetics. The reaction is reversible and reaches equilibrium. To obtain a higher yield of ester, excess alcohol or removal of water is employed. Esterification is one of the most important reactions of carboxylic acids.
Q16. What is Hell-Volhard-Zelinsky (HVZ) reaction?
Answer:
The Hell-Volhard-Zelinsky reaction is used to introduce a halogen atom at the α-position of a carboxylic acid. In this reaction, a carboxylic acid containing an α-hydrogen atom is treated with chlorine or bromine in the presence of red phosphorus. The α-hydrogen is replaced by a halogen atom, producing α-halo carboxylic acids. These products are important intermediates in organic synthesis because they can be converted into amino acids and other useful compounds. The HVZ reaction demonstrates the reactivity of the α-carbon atom adjacent to the carboxyl group and is frequently included in CBSE board examinations.
Q17. How are aldehydes converted into alcohols?
Answer:
Aldehydes are converted into primary alcohols by reduction. Common reducing agents include sodium borohydride (NaBH₄), lithium aluminium hydride (LiAlH₄), and catalytic hydrogenation using hydrogen gas with nickel or platinum catalysts. During reduction, the carbonyl group gains hydrogen and is transformed into a hydroxyl group. For example, ethanal is reduced to ethanol. This reaction is important in both laboratory and industrial chemistry because alcohols are valuable intermediates and solvents. The conversion demonstrates the ability of aldehydes to undergo addition reactions due to the electrophilic nature of the carbonyl carbon atom.
Q18. What are the important physical properties of aldehydes and ketones?
Answer:
Aldehydes and ketones are polar compounds due to the presence of the carbonyl group. Their boiling points are higher than those of hydrocarbons and ethers of similar molecular masses because of strong dipole-dipole interactions. However, they have lower boiling points than alcohols since they cannot form intermolecular hydrogen bonds among themselves. Lower members such as methanal, ethanal, and propanone are soluble in water because they form hydrogen bonds with water molecules. Solubility decreases with increasing molecular size. Many aldehydes and ketones possess characteristic odours and are used in perfumes, flavouring agents, and industrial solvents.
Q19. Give two methods for the preparation of carboxylic acids.
Answer:
Carboxylic acids can be prepared by several methods. One common method is the oxidation of primary alcohols or aldehydes using strong oxidizing agents such as potassium dichromate or potassium permanganate. For example, ethanol is oxidized to ethanoic acid. Another method involves the hydrolysis of nitriles. When nitriles are heated with dilute acids or alkalis, they form carboxylic acids. For example, ethanenitrile on hydrolysis yields ethanoic acid. Both methods are widely used in organic synthesis and are important for preparing various industrially significant carboxylic acids used in food, pharmaceuticals, and chemical manufacturing.
Q20. Write the uses of aldehydes, ketones and carboxylic acids.
Answer:
Aldehydes, ketones, and carboxylic acids have numerous industrial and biological applications. Formaldehyde is used in the manufacture of plastics, resins, and disinfectants. Benzaldehyde is used in perfumes and flavouring agents. Acetone, an important ketone, is widely employed as a solvent in paints, nail polish removers, and pharmaceuticals. Carboxylic acids such as acetic acid are used in vinegar, food preservation, and chemical synthesis. Benzoic acid is used as a food preservative, while fatty acids play important roles in biological systems. These compounds are essential raw materials in the production of dyes, drugs, polymers, and fragrances.