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Haloalkanes and Haloarenes question

2021 · 26 Aug · Shift 1 · Q9
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Haloalkanes and Haloarenes question

2021 · 26 Aug · Shift 1 · Q9

JEE MainChemistryHaloalkanes and HaloarenesMCQ+4 / −1
Among the following compounds I-IV, which one forms a yellow precipitate on reacting sequentially with (i) NaOHNaOHNaOH (ii) dil. HNO3HNO_3HNO3​ (iii) AgNO3AgNO_3AgNO3​? JEE Main 2021 (Online) 26th August Morning Shift Chemistry - Haloalkanes and Haloarenes Question 91 English
  1. A
    II
  2. B
    IV
  3. C
    I
  4. D
    III
View written solutionFree

Correct answer: B

  1. What does the test indicate?
    The sequence

    (i) NaOH(ii) dil. HNO3(iii) AgNO3\text{(i) } NaOH \quad \text{(ii) dil. } HNO_3 \quad \text{(iii) } AgNO_3(i) NaOH(ii) dil. HNO3​(iii) AgNO3​

    is the standard test for halide ions released after hydrolysis.

    • First, heating with NaOHNaOHNaOH can hydrolyse a halo-compound and convert the halogen into X−X^-X−.

    • Then acidification with dilute HNO3HNO_3HNO3​ removes excess OH−OH^-OH−.

    • Finally, AgNO3AgNO_3AgNO3​ gives precipitate of silver halide:

      AgCl=white,AgBr=pale yellow/cream,AgI=yellowAgCl = \text{white}, \quad AgBr = \text{pale yellow/cream}, \quad AgI = \text{yellow}AgCl=white,AgBr=pale yellow/cream,AgI=yellow

    So a yellow precipitate means formation of AgIAgIAgI, hence the compound must release iodide ion on hydrolysis.

  2. Which type of compounds respond?
    Haloalkanes hydrolyse with aqueous NaOHNaOHNaOH relatively easily, but haloarenes generally do not under these mild conditions because the C−XC-XC−X bond in aryl halides has partial double bond character due to resonance.

  3. Hence, the required compound must be:

    • an iodo compound, and
    • one that can be hydrolysed by NaOHNaOHNaOH under the given conditions.
  4. From the given set I–IV
    The compound labeled IV is the one that fits this requirement, so it will produce I−I^-I− on hydrolysis and then give:

    Ag++I−→AgI↓Ag^+ + I^- \rightarrow AgI \downarrowAg++I−→AgI↓

    which is a yellow precipitate.

  5. Therefore the correct option is:

    IV\boxed{\text{IV}}IV​

    which corresponds to Option B.

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