Ncert Exercises & Solutions

Question 13.22:-

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, $E^{+}$

(a)Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b)Toluene, $p - H_{3} C - C_{6} H_{4} - N O_{2}, p - O_{2} N - C_{6} H_{4} - N O_{2} .$

Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles.
The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile,

E^{+}

(Electrophilic reaction)

(a) The presence of an electron-withdrawing group (i.e.,

{NO}_{2}^{-}

and

{Cl}^{-}

) deactivates the aromatic ring by decreasing the electron density.
Since

{NO}_{2}^{-}

the group is more electron-withdrawing (due to resonance effect) than the

{Cl}^{-}

group (due to inductive effect), the decreasing order of reactivity is as follows: Chlorobenzene > p – nitro chlorobenzene > 2, 4 – dinitrochlorobenzene

(b) While

{CH}_{3}^{-}

is an electron-donating group,

{NO}_{2}^{-}

group is electron-withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by

E^{+}

{NO}_{2}^{-}

is an electron-withdrawing group. Hence, when the number of

{NO}_{2}^{-}

substituents is
greater, the order is as follows:

Toluene.

> p - C H_{3} - C_{6} H_{4} - N O_{2,} p - O_{2} N - C_{6} H_{4} - N O_{2}

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