Bottles containing C6H5I and C6H5CH2I lost their original labels. They were labelled A and B for testing. A and B were separately taken in test tubes and boiled with NaOH solution. The resulting solution in each tube was made acidic with dilute HNO3, and some AgNO3 solution was added. Solution B gave a yellow precipitate. Which one of the following statements is true for the experiment?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A was C6H5I
The test works only if boiling with NaOH sets the iodide ion free; the iodide ion then gives a yellow precipitate of silver iodide with \( \ce{AgNO3} \).
Benzyl iodide, \( \ce{C6H5CH2I} \) — a benzylic halide, highly reactive towards SN1 substitution (its carbocation is stabilised by resonance). The hydroxide ion replaces iodine:
\( \displaystyle \ce{C6H5CH2I + NaOH -> C6H5CH2OH + NaI} \)
\( \displaystyle \ce{NaI + AgNO3 -> AgI v + NaNO3} \)
Result: yellow precipitate, so this is B.
Iodobenzene, \( \ce{C6H5I} \) — an aryl halide. The C–I bond has partial double bond character and does not break on boiling with NaOH, so no iodide ion forms. Result: no precipitate, so this is A.
Why the other options are incorrect
Option (1) Addition of HNO3 was unnecessary — Incorrect. The solution still holds excess NaOH. The acid removes the hydroxide ions, which would otherwise precipitate silver ions as well and spoil the test.
Option (3) A was C6H5CH2I — Incorrect. Benzyl iodide releases iodide ions and gives the precipitate, so it is B, not A.
Option (4) B was C6H5I — Incorrect. Iodobenzene does not react with NaOH under these conditions, so it could not give the precipitate seen with B.
NCERT (2026-27 edition):
6.7.2 Reactions of Haloarenes: 1. Nucleophilic substitution – “Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons: … C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.”