JEE Main 2025 Jan 23 Shift 1, Chemistry Q23: Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

JEE Main2025Jan 23, Shift 1Chemistry

Consider the following sequence of reactions to produce major product (A)

Molar mass of product (A) is ______ g mol $^{- 1}$ .

(Given molar mass in g mol $^{- 1}$ of C : 12, H : 1, O : 16, Br : 80, N : 14, P : 31)

Your answer

Solution

Walk through the steps on m-nitrotoluene.

(i) $B r_{2}$ /Fe: electrophilic bromination. The $- C H_{3}$ group is o/p-directing and the $- N O_{2}$ group is m-directing; their joint preference puts Br at the position ortho to $- C H_{3}$ and para to itself relative to $- N O_{2}$ . Product: 2-bromo-4-nitrotoluene (Br on C2, $N O_{2}$ on C4, $C H_{3}$ on C1).

(ii) Sn/HCl reduces $- N O_{2}$ to $- N H_{2}$ , giving 2-bromo-4-methylaniline.

(iii) $N a N O_{2}$ /HCl at 273 K converts the $- N H_{2}$ to a diazonium salt, $- N_{2}^{+} C l^{-}$ .

(iv) $H_{3} P O_{2} / H_{2} O$ replaces the diazonium group by H (deamination).

Final product A: 3-bromotoluene, $C_{7} H_{7} B r$ .

Molar mass $= 7 (12) + 7 (1) + 80 = 84 + 7 + 80 = 171$ g/mol.

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JEE Main 2025 Jan 23 Shift 1, Chemistry Q23: Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

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