question_answer

  Explain as to why haloarenes are much less reactive than haloalkanes towards nucleophilic substitution reactions.                                                                                                                                                                                                      [2] Or Which compound in each of the following pairs will react faster in reaction with?OH? Why? (i)                                            (ii)

Answer:

  Haloarenes are much less reactive than haloalkanes towardsreactions due to the following reasons (i) Resonance effect (C?Cl) bond is stabilised due to resonance. This resonance arises as a result of delocalisation of lone pair on Cl atoms and -elctrons in benzene sextet. (C?Cl) bond acquires partial double bond character. (ii) Electronegativity of sp2 > sp3 (C?Cl) bond length in aryl halide < alkyi halide Bond energy (C?Cl) > alkyi halide, Thus, sn reaction is not feasible in aryl halide.                        (iii) If reaction takes place, phenyl carbocation isvery less stable than alkyl carbocation.                                                  (iv) Nucleophile is repelled by electron rich aryl halide. Or (i) For reaction  [1] being larger in size causes steric hindrance for nucleophilic attack. (ii) Due to the presence of three alky] groups in (A), nucleophile cannot approach central carbon in reaction. Thus,  reaction is faster in (6).