Previous year questions: Aldehydes, Ketones and Carboxylic Acids

The questions below come from official CBSE board papers and official Sample Question Papers. The year label in each heading is printed on that source, and the marks are the marks printed for that question or part.

Q1 (CBSE Board 2023, 5 marks)

(a)(i) Write the reaction involved in Cannizzaro's reaction. (ii) Why is the boiling point of aldehydes and ketones lower than that of the corresponding carboxylic acids? (iii) An organic compound A with molecular formula C5H8O2 is reduced to n-pentane with hydrazine followed by heating with NaOH and glycol. A forms a dioxime with hydroxylamine and gives a positive iodoform and Tollens test. Identify A and give its reactions.

Answer: (i) Two molecules of an aldehyde with no alpha hydrogen react with concentrated NaOH; one is oxidised to the carboxylate and the other reduced to the alcohol, for example benzaldehyde gives sodium benzoate and benzyl alcohol. (ii) A carboxylic acid has an OH group and forms strong intermolecular hydrogen bonds, which aldehydes and ketones cannot donate. The acid therefore boils higher. (iii) A is CH3COCH2CH2CHO, 2-oxopentanal. The Wolff Kishner reduction of both carbonyl groups gives n-pentane, hydroxylamine gives a dioxime because there are two carbonyls, the methyl ketone end gives iodoform and the aldehyde end reduces Tollens reagent. A five mark answer needs the Cannizzaro equation with both products, the hydrogen bonding comparison for the boiling points, and the structure of A with the dioxime, iodoform and Tollens evidence explained.

Q2 (CBSE Board 2024, 1 mark)

The formation of a cyanohydrin from an aldehyde is an example of (A) nucleophilic addition (B) electrophilic addition (C) nucleophilic substitution (D) electrophilic substitution

Answer: (A) nucleophilic addition. The cyanide ion is a nucleophile and it attacks the electron poor carbonyl carbon. The pi bond opens to give a tetrahedral alkoxide, which then picks up a proton. The exact name of the mechanistic class is the mark, so write nucleophilic addition rather than addition.

Q3 (CBSE Board 2024, 2 marks)

Write the chemical equation when (a) butan-2-one is treated with Zn(Hg) and concentrated HCl, (b) two molecules of benzaldehyde are treated with concentrated NaOH.

Answer: (a) This is the Clemmensen reduction. The carbonyl oxygen is removed and butane forms, CH3COCH2CH3 to CH3CH2CH2CH3. (b) This is the Cannizzaro reaction. One molecule is oxidised and the other reduced, so the products are sodium benzoate and benzyl alcohol, C6H5COONa plus C6H5CH2OH. One mark for each equation, so balance both and name the alkane and the two Cannizzaro products.

Q4 (CBSE Board 2025, 1 mark)

Assertion (A): The solubility of aldehydes and ketones in water decreases with increase in the size of the alkyl group. Reason (R): Aldehydes and ketones have dipole dipole interaction.

Answer: Both statements are true but the reason is not the correct explanation of the assertion. The dipole dipole interaction is real, but solubility falls because the growing hydrocarbon part cannot hydrogen bond with water and takes up space in the water network. The correct explanation is the increasing hydrophobic character of the alkyl group. Assertion and reason questions need the two judgements stated separately, then the correct explanation in one sentence about the hydrophobic group.

Q5 (CBSE Board 2025, 2 marks)

(a) How can acetaldehyde be prepared from acetyl chloride? (b) Propanal is more reactive than propanone towards nucleophilic addition. Give a reason.

Answer: (a) Use the Rosenmund reaction: acetyl chloride with hydrogen over palladium on barium sulphate, poisoned with quinoline, gives acetaldehyde. The poison stops the aldehyde from being reduced further to the alcohol. (b) Propanone has two methyl groups on the carbonyl carbon, which crowd the carbon and release electron density inductively, so the carbon is less positive. Propanal has one methyl and one hydrogen, so it is less crowded and more reactive. Part (a) needs the Rosenmund reagent with the inhibitor, and part (b) needs the steric and inductive points stated as two causes.

Q6 (CBSE Board 2025, 3 marks)

An organic compound A with formula C8H8O gives the 2,4-DNP test. It does not give the Tollens test, but gives a yellow precipitate B with NaOH and I2. On drastic oxidation it gives a carboxylic acid C with formula C7H6O2. Identify A, B and C and write the reactions.

Answer: A is acetophenone, C6H5COCH3. It is a methyl ketone, which explains the iodoform test and the yellow precipitate B of iodoform, CHI3. It has no aldehyde group, so Tollens gives no silver mirror. Drastic oxidation of the methyl group and the side chain gives benzoic acid, C6H5COOH, which is C. The iodoform reaction and the oxidation both confirm the methyl ketone structure. The three identifications carry one mark each, and the iodoform reaction and the oxidation to benzoic acid should be written out for the third.

Q7 (CBSE SQP 2023-24, 1 mark)

What is the IUPAC name of the ketone A which undergoes the iodoform reaction to give CH3CH=C(CH3)COONa and a yellow precipitate of CHI3?

Answer: A is 3-methylpent-3-en-2-one, CH3COC(CH3)=CHCH3. The iodoform reaction needs a CH3CO group, and the remaining carbon skeleton is a five carbon chain with a double bond and a methyl branch. The yellow precipitate is iodoform, CHI3, and the other product is the sodium salt of the acid formed by cleavage. The CH3CO group is the giveaway, so explain the cleavage that gives the sodium salt and iodoform and name the ketone carefully.

Q8 (CBSE SQP 2023-24, 2 marks)

(a) Out of p-tolualdehyde and p-nitrobenzaldehyde, which one is more reactive towards nucleophilic addition, and why? (b) Write the structure of the product formed when acetone reacts with 2,4-DNP reagent.

Answer: (a) p-Nitrobenzaldehyde is more reactive. The nitro group withdraws electron density from the carbonyl carbon through the ring, which makes that carbon more positive and easier for a nucleophile to attack. The methyl group of p-tolualdehyde does the opposite. (b) Acetone reacts with 2,4-dinitrophenylhydrazine to give acetone 2,4-dinitrophenylhydrazone, a yellow orange solid, with loss of water. Part (a) turns on the electron withdrawing nitro group, and part (b) needs the hydrazone name along with the loss of water.

Q9 (CBSE SQP 2022-23, 3 marks)

Write the reaction and the IUPAC name of the product formed when 2-methylpropanal is treated with ethylmagnesium bromide followed by hydrolysis.

Answer: The Grignard reagent adds to the carbonyl carbon of 2-methylpropanal and the adduct is hydrolysed. Ethyl and the aldehyde hydrogen end up on the same carbon, so the product is CH3CH(CH3)CH(OH)CH2CH3, 2-methylpentan-3-ol. The reaction shows the general rule that an aldehyde gives a secondary alcohol with a Grignard reagent, and the carbon chain grows by the number of carbons in the reagent. The product name and the class of alcohol both carry marks, so state that an aldehyde gives a secondary alcohol with a Grignard reagent.

Q10 (CBSE SQP 2024-25, 1 mark)

When C6H5COOCOCH3 is treated with water, the products obtained are (a) benzoic acid and ethanol (b) benzoic acid and ethanoic acid (c) acetic acid and phenol (d) benzoic anhydride and methanol

Answer: (b) benzoic acid and ethanoic acid. The compound is a mixed acid anhydride, made from benzoic acid and ethanoic acid, so hydrolysis breaks the anhydride linkage and returns the two acids. For comparison, aspirin is an ester of salicylic acid and gives salicylic acid and ethanoic acid on hydrolysis. Both acid names must appear, and the hydrolysis should be explained as cleavage of the anhydride linkage to the two acids.

How marks are usually split

A two mark question usually allows one mark for the equation and one for the reason. A three mark identification question carries about one mark for each of the three species, so write the structure and the name together. The five mark question on this chapter combines a named reaction, a property comparison and a structure identification, so divide the time in the same ratio and answer the parts you know first.

When a mechanism is asked for, the marks go to the two steps and to the proton transfer, not to a description of the arrows. Write step one as the nucleophilic attack and the tetrahedral intermediate, step two as the protonation, and add the acid catalysis point if the question mentions acidic conditions.