Question: 74: Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Chlorine is an electron withdrawing group but it is ortho, para directing in electrophilic aromatic substitution.
Reason (R) : Inductive effect of chlorine destabilises the intermediate carbocation formed during the electrophilic substitution, however due to the more pronounced resonance effect, the halogen stabilises the carbocation at ortho and para positions.
In the light of the above statements, choose the most appropriate answer from the options given below :
(1) Both (A) and (R) are correct and (R) is the correct explanation of (A)
(2) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
(3) (A) is correct but (R) is not correct
(4) (A) is not correct but (R) is correct.
Answer: Option (1)
Explanation:
Chlorine is more electronegative than carbon and hence withdraws electrons from the benzene ring through the inductive effect,
which makes the ring less reactive toward electrophilic substitution.
Therefore, chlorine behaves as an electron withdrawing group.
Despite being electron withdrawing by inductive effect, chlorine has lone pairs of electrons that can be donated to the benzene ring through resonance.
This resonance effect increases the electron density at the ortho and para positions of the ring.
During electrophilic aromatic substitution, the carbocation intermediate formed at ortho
and para positions is stabilised by resonance donation of lone pair electrons from chlorine.
Although the inductive effect destabilises the carbocation,
the resonance effect is more pronounced at ortho and para positions.
As a result, chlorine directs the incoming electrophile to ortho and para positions.
Thus, both the assertion and the reason are correct, and the reason correctly explains the assertion.