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NVAMediumJEE 2026Electrophilic Substitution in Benzene

JEE Chemistry 2026 Question with Solution

Consider the following reaction of benzene. the percentage of oxygen is _____ %. (Nearest integer)

Reaction scheme showing benzene reacting with CH3COCH2CH2COCl in anhydrous AlCl3 to give P, followed by aq NaOH and heating to form Q.

Answer

Correct answer:23

Step-by-step solution

Standard Method

Given: Benzene undergoes Friedel–Crafts acylation followed by alkaline hydrolysis, leading to the formation of an aromatic carboxylic acid.

Find: The percentage of oxygen in the final product QQ.

The final product QQ is phenylacetic acid.

Q=C6H5CH2COOHQ = \mathrm{C_6H_5CH_2COOH}

Molecular formula of QQ:

C8H8O2\mathrm{C_8H_8O_2}

Molar mass of QQ:

=(8×12)+(8×1)+(2×16)= (8 \times 12) + (8 \times 1) + (2 \times 16)=96+8+32=136= 96 + 8 + 32 = 136

Mass of oxygen present:

=2×16=32= 2 \times 16 = 32

Percentage of oxygen:

% Oxygen=32136×100=23.52%\%\,\text{Oxygen} = \frac{32}{136} \times 100 = 23.52\%

Nearest integer:

23%23\%

Therefore, the percentage of oxygen in the product is 23%23\%. The answer key shows 1717, but the extracted solution concludes 23%23\%, so the solution result is used.

Common mistakes

  • Identifying the final product incorrectly as benzoic acid is wrong because the reaction sequence shown gives a side-chain containing CH2COOHCH_2COOH. First determine the product structure correctly, then calculate the oxygen percentage.

  • Using the wrong molecular formula is incorrect because the oxygen percentage depends directly on total molar mass. After identifying phenylacetic acid, use C8H8O2\mathrm{C_8H_8O_2} before substituting masses.

  • Forgetting that there are two oxygen atoms in the carboxylic acid group gives a smaller oxygen mass than required. Use oxygen mass =2×16=32= 2 \times 16 = 32, not 1616.

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