Question

Identify incorrect statements:
I. Halo group activates benzene ring by mesomeric effect and destabilizes it by inductive effect.
II. Halo group is a deactivating group.
III. Benzene is \[{{10}^{4}}\] times more reactive than nitrobenzene toward the nucleophile.
IV. \[C{{F}_{3}}\] is a strongly deactivating group.

A. I, II, III
B. III Only
C. II only
D. II and IV

Answer 1

Hint: We must know that to decide the kind of inductive effect group will show we see the electronegativity difference between the groups. If it has high electronegativity it will withdraw the electron density via sigma bond and show –I effect. For a group to show +M effect it must have a lone pair of electrons.

Complete answer:
Halo group activates benzene ring by mesomeric effect and destabilizes it by inductive effect. This statement is correct as halogens have lone pairs of electrons which they can donate to the ring and increase the electron density of the ring this is +M effect or mesomeric effect. Also, halogens have high electronegativity values which allows them to withdraw electron density via sigma bonds and apply –I or inductive effect.
Halo group is a deactivating group. This statement is true as for the halogens like chlorine, bromine and iodine the –I or inductive effect dominates over +M effect or mesomeric effect and thus they deactivate the group they are attached to.
Benzene is \[{{10}^{4}}\] times more reactive than nitrobenzene toward the nucleophile. This statement is incorrect because benzene is less reactive than nitrobenzene towards nucleophilic reactions. In nucleophilic aromatic substitution reaction, the nitro group which is electron withdrawing decreases electron density on benzene ring and this in turn increases electrophilic character of aromatic nucleus and facilitates nucleophilic attack
\[C{{F}_{3}}\] is a strongly deactivating group. This statement is true as the halogens like fluorine have high electronegativity values which allows them to withdraw electron density via sigma bonds and apply –I or inductive effect thereby deactivating the group.

So, the correct answer is “Option B”.

Note: When the substituents like -OH have an unshared pair of electrons, the resonance effect is stronger than the inductive effect which make these substituents stronger activating groups, since this resonance effect direct the electron toward the ring and mostly resonance effect dominates over inductive effect expect in case of halogens like chlorine, bromine and iodine.