An organic compound (A) with the molecular formula C_9H10O forms 2, 4-DNP derivative, reduces Fehling solution and undergoes Cannizzaro reaction. On vigorous oxidation, it gives 1, 2-benzene dicarboxylic acid. (i) Identify the compound (A) and write its IUPAC name. (ii) Write the reaction of compound (A) with (1) 2, 4-Dinitrophenyl hydrazine and (2) Fehling solution (iii) Write the equation of compound (A) when it undergoes Cannizzaro reaction.

ChemistryCBSE Class 12 · Board exam 2024, set 56/5/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2024: question 33(a)

Set in the CBSE Class 12 board exam of 2024, paper 56/5/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerAldehydes, Ketones and Carboxylic Acids

An organic compound (A) with the molecular formula $C_{9} H_{10} O$ forms 2, 4-DNP derivative, reduces Fehling solution and undergoes Cannizzaro reaction. On vigorous oxidation, it gives 1, 2-benzene dicarboxylic acid.

(i)
Identify the compound (A) and write its IUPAC name.
2 marks
(ii)
Write the reaction of compound (A) with
(1) 2, 4-Dinitrophenyl hydrazine and
(2) Fehling solution
2 marks
(iii)
Write the equation of compound (A) when it undergoes Cannizzaro reaction.
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Each test is a clue: 2,4-DNP shows a carbonyl group, Fehling's shows an aldehyde, and Cannizzaro shows an aldehyde with no α-H.

Hint 2

Phthalic acid from vigorous oxidation means two side chains sit ortho to each other on the benzene ring.

Hint 3

Subtract C $_{6}$ H $_{4}$ and CHO from C $_{9}$ H $_{10}$ O to find the second side chain.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(i) Deduction:
- It forms a 2,4-DNP derivative, so it has a C=O group.
- It reduces Fehling's solution, so the C=O is an aldehyde (–CHO).
- It undergoes the Cannizzaro reaction, so the –CHO has no α-hydrogen and is attached directly to the ring.
- Vigorous oxidation gives 1,2-benzenedicarboxylic acid (phthalic acid), so the ring carries two side chains in ortho positions.
C $_{9}$ H $_{10}$ O minus C $_{6}$ H $_{4}$ and –CHO leaves –C $_{2}$ H $_{5}$ . Hence (A) is $o$ -ethylbenzaldehyde, $C_{6} H_{4} (CHO) (C_{2} H_{5})$ with the two groups at positions 1 and 2.
1 mark
IUPAC name of (A): 2-Ethylbenzaldehyde
1 mark
(ii)(1) With 2,4-dinitrophenylhydrazine:
$C_{2} H_{5} C_{6} H_{4} CHO + H_{2} N - NH - C_{6} H_{3} (N O_{2})_{2} H^{+} C_{2} H_{5} C_{6} H_{4} CH = N - NH - C_{6} H_{3} (N O_{2})_{2} + H_{2} O$
The product is the orange-yellow 2-ethylbenzaldehyde 2,4-dinitrophenylhydrazone.
1 mark
(ii)(2) With Fehling's solution:
$C_{2} H_{5} C_{6} H_{4} CHO + 2C u^{2 +} + 5O H^{-} \to C_{2} H_{5} C_{6} H_{4} CO O^{-} + C u_{2} O ↓ (red-brown) + 3 H_{2} O$
Note: aromatic aldehydes usually do not respond to Fehling's test. The scheme therefore awards this mark for any reasonable attempt.
1 mark
(iii) Cannizzaro reaction (conc. NaOH):
$2 C_{2} H_{5} C_{6} H_{4} CHO + NaOH (conc.) Δ C_{2} H_{5} C_{6} H_{4} C H_{2} OH + C_{2} H_{5} C_{6} H_{4} COONa$
The products are 2-ethylbenzyl alcohol, i.e. (2-ethylphenyl)methanol, and sodium 2-ethylbenzoate.
1 mark

Answer: (A) = 2-ethylbenzaldehyde (o-C $_{2}$ H $_{5}$ C $_{6}$ H $_{4}$ CHO); it gives the 2,4-DNP hydrazone, reduces Fehling's solution to Cu $_{2}$ O, and in the Cannizzaro reaction gives 2-ethylbenzyl alcohol and sodium 2-ethylbenzoate.

Where the marks are

Identifies (A) as o-ethylbenzaldehyde with its structure (1 mark)
Gives the IUPAC name 2-ethylbenzaldehyde (1 mark)
Writes the reaction with 2,4-DNP forming the hydrazone (1 mark)
Writes the Fehling's reaction; per the scheme, 1 mark is given for any attempt (1 mark)
Writes the Cannizzaro equation giving the alcohol and the carboxylate salt (1 mark)

Revise the chapter

Chapter: Aldehydes, Ketones and Carboxylic Acids (notes, formulas, where marks are lost and every past question)

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CBSE Class 12 Chemistry 2024: question 33(a)

Hints

Step-by-step solution

Revise the chapter

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