JEE Advanced 2022 Paper 1 · Q07 · Named Reactions & Reagents
The weight percentage of hydrogen in Q, formed in the following reaction sequence, is ________.
[Figure: starting material is chlorobenzene (benzene ring with a single -Cl substituent).]
chlorobenzene $\xrightarrow[\text{(i) NaOH, 623 K, 300 atm}]{\text{(ii) conc. H}_2\text{SO}_4\text{ and then conc. HNO}_3}$ Q (major product)
[Given: Atomic mass of H = 1, C = 12, N = 14, O = 16, S = 32, Cl = 35]
Reveal answer + step-by-step solution
Correct answer:1.31
Solution
Step (i): chlorobenzene + NaOH at 623 K and 300 atm is the Dow process — the chloride is displaced to give sodium phenoxide, which on workup gives phenol.
Step (ii): phenol with conc. H$_2$SO$_4$ followed by conc. HNO$_3$ performs successive electrophilic nitration. -OH is strongly o,p-directing, so the major polynitrated product is the 2,4,6-trinitrophenol (picric acid).
Q = picric acid, C$_6$H$_3$N$_3$O$_7$. Molar mass = 6(12) + 3(1) + 3(14) + 7(16) = 72 + 3 + 42 + 112 = 229 g mol$^{-1}$.
Weight % of H = (3/229) $\times$ 100 = 1.310% $\approx$ 1.31%.
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