Question

The most reactive towards electrophilic reagent is :
(A)

(B)

(C)

(D)

Answer 1

Hint: Electrophiles are electron deficient species. Benzene is nucleophilic (electron rich) in nature due to the presence of $\pi $ electron cloud and thus shows electrophilic substitution reactions. Electron donating groups (EDG) increase the electron density in the benzene ring and activate it towards electrophilic reagents.


Complete step by step answer:
- Reactivity of substituted benzene depends on the type of substituent present in the ring.
- Electron releasing groups increase the overall electron density and hence the reactivity of the benzene rings. Such groups mainly increase the electron density at ortho and para positions and direct the incoming group to substitute at these positions. Hence, they are called as o, p- directing groups.
- Since presence of these groups facilitates further electrophilic substitutions in the ring, they are also known as activating groups.

- Reactivity of a benzene ring substituted with o, p- directing group depends on the electron donating ability of the group attached. The electron donating ability of some EDG follows the given order:

\[-N{{H}_{2}}>-N{{R}_{2}}>-OH>-OR>-NHCOR>-R\]

Applying the above order in substituted toluene, we know that hydroxyl ($-OH$) group is more electron donating than $-NHCOC{{H}_{3}}$, $-OC{{H}_{3}}$ and $-C{{H}_{2}}OH$.

Therefore, o-hydroxy toluene is most reactive towards electrophilic reagent.

So, the correct answer is “Option D”.

Additional Information:
Electron Withdrawing groups (EWG) withdraw electron density from the ring (particularly from the meta-position) and deactivate the ring toward electrophones. They are also called as m-directing groups. $-{{N}^{\oplus }}{{R}_{3}}$, $-N{{O}_{2}}$, -CN, etc are strong EWG.

Note: Although EDG equally directs the incoming groups at o and p- positions. However, if the incoming group or the EDG already present on the ring is bulky, the incoming group generally attaches at the p-position to avoid steric hindrance with EDG which may arise if it is attached at the o-position.