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Haloalkanes and Haloarenes — Chemistry STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceChemistryHaloalkanes and Haloarenes4 Marks
Question
Read the passage given below and answer the following questions:
Haloarenes are less reactive than haloalkanes. The low reactivity of haloarenes can be attributed to:
  • Resonance effect.
  • $sp^2$ hybridisation of $C - X$ bond.
  • Polarity of $C - X$ bond
  • Instability of phenyl cation $($formed by self$-$ionisation of haloarene$).$
  • Repulsion between the electron rich attacking nucleophiles and electron rich arenes.
Reactivity of haloarenes can be increased or decreased by the presence of certain groups at certain positions for example, nitro $(-NO_{^2})$ group at o/ p positions increases the reactivity of haloarenes towards nucleophilc substitution reactions.
The following questions are multiple choice questions Choose the most appropriate answer:
  1. Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to
  1. The formation of less stable carbonium ion.
  2. Resonance stabilisation.
  3. Larger carbon$-$halogen bond.
  4. Inductive effect.
  1. Which of the following aryl halides is the most reactive towards nucleophilic substitution?
  1. Which one of the following will react fastest with aqueous $\text{NaOH}$?
  1. Which chloro derivative of benzene among the followings would undergo hydrolysis most readily with aqueous sodium hydroxide to furnish the corresponding hydroxy derivative?
  1. $\ce{C_6H_5Cl}$
  1. The reactivity of the compounds $\ce{(i)\ MeBr, (ii)\ PhCH_2Br, (iii)\ MeCI, (iv)\ p-MeOC_6H_4Br}$ decreases as:
  1. $\ce{(i) > (ii) > (iii) > (iv)}$
  2. $\ce{(iv) > (ii) > (i) > (iii)}$
  3. $\ce{(iv) > (iii) > (i) > (ii)}$
  4. $\ce{(ii) > (i) > (iii) > (iv)}$
✓

Answer

  1. $(b)$ Resonance stabilisation.
  2. $(d)$ 

When in aryl halides the electron withdrawing groups are attached at ortho and para positions to the chlorine atom then the removal of chlorine atom as $Cll-$ ion becomes easy, therefore$, 2,4,6-$trinitro chlorobenzene is the most reactive among given aryl halides.
  1. $(d)$

  1. $(a)$

$Cl$ in $2,4,6-$trinitrochlorobenzene is activated by three $NO_2$ groups at $o,$ and $p-$positions and hence undergoes hydrolysis most readily.
  1. $(d)\ \ce{(ii) > (i) > (iii) > (iv)}$
The order of reactivity follows the sequence: benzyl halides $>$ alkyl halides $>$ aryl halides. Out of chlorides and bromides, bromides are more reactive. Therefore, the correct order of reactivity is $\ce{PhCH_2Br (ii)\ > MeBr (i)\ > MeCl (iii)\ > p - MeOC_6H_4Br (iv).}$

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  1. $1.95m, 0.120$
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  4. $\text{x}_2=\frac{1-\text{mM}_1}{\text{mM}_1}$
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