Question

In electrophilic aromatic substitution reaction of chlorobenzene, the ortho/para directing ability of chlorine is due to which of the following effects?
A.Positive Inductive Effect (+I)
B.Negative Inductive Effect (-I)
C.Positive Resonance Effect (+R)
D.Negative Resonance Effect (-R)

Answer 1

Hint: We know chlorine is ortho and para directing in electrophilic aromatic substitution reaction as it is an Electron withdrawing group.

Complete step by step answer:
Positive Resonance Effect- The +R effect or positive resonance effect is expressed by the electron donating groups. These Electron withdrawing groups withdraw electrons from the rest of the molecule by delocalization of electrons within the molecule.
Negative Resonance Effect- When groups withdraw elements from the molecule, due to the process of delocalization, negative resonance effect occurs. The groups are usually denoted by -R or -M.
Positive Inductive Effect- When a chemical specie, such as an alkyl group which have the tendency to release or donate electrons, is introduced to a carbon chain, the charge is relayed through the chain and this effect is called the Positive Inductive Effect or the +I Effect.
Negative Inductive Effect- The -I effect of chlorine withdraws electrons from the benzene ring. Hence tends to destabilize the intermediate carbocation formed during the electrophilic substitution. Conversely Chlorine donates its lone pair.
The electrons from the benzene ring are withdrawn by the -I effect of chlorine. Hence tends to destabilize the intermediate carbocation formed during the electrophilic substitution. Conversely Chlorine donates its lone pair of electrons to the aromatic ring and hence increases the electron density at ortho and para positions. Thus, the lone pair of electrons on the chlorine atom stabilize the intermediate carbocation due to resonance.
The -I effect of Chlorine is stronger than its +R effect, hence Chlorine causes net deactivation. Also further since resonance effect opposes the inductive effect, it makes deactivation less at o- and p- positions.
Therefore, in chlorobenzene, orientation of incoming substituent is controlled by the weaker resonance effect while reactivity is controlled by stronger -I effect.

Therefore, in electrophilic aromatic substitution reaction of chlorobenzene, the ortho/para directing ability of chlorine is due to positive resonance effect i.e. option B is correct and option A, C, D are incorrect.
Hence option B is correct.

Note:
During the inductive effect, partial positive and negative charges appear as the electron pair is slightly displaced towards the more electronegative atom. During the resonance effect, full positive and negative charges appear as the electron pair is completely transferred.