JEE Challenger
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Stability of Carbocation and Carbanion with Resonance Effects

Given below are two statements :

Statement I : In O2N−C6H4−C⊕H−C6H4−OCH3\text{O}_2\text{N}-\text{C}_6\text{H}_4-\overset{\oplus}{\text{C}}\text{H}-\text{C}_6\text{H}_4-\text{OCH}_3, the carbocation is stabilised by +R+\text{R} effect of −OCH3-\text{OCH}_3 group.

Statement II : In O2N−C6H4−C⊖H−C6H4−OCH3\text{O}_2\text{N}-\text{C}_6\text{H}_4-\overset{\ominus}{\text{C}}\text{H}-\text{C}_6\text{H}_4-\text{OCH}_3, the carbanion is stabilised by −R-\text{R} effect of −NO2-\text{NO}_2 group.

In the light of the above statements, choose the correct answer from the options given below :

Question Diagram 1
Question Diagram 2

Options

A

Both Statement I and Statement II are true

Correct
B

Both Statement I and Statement II are false

C

Statement I is true but Statement II is false

D

Statement I is false but Statement II is true

Topics & Concepts

Organic Chemistry - Basic PrinciplesInductive EffectResonance

Step-by-Step Solution

To determine the correctness of Statements I and II, let us analyze the resonance effects present in both species:

1. Analysis of Statement I

  • Structure: O2N−C6H4−C⊕H−C6H4−OCH3\text{O}_2\text{N}-\text{C}_6\text{H}_4-\overset{\oplus}{\text{C}}\text{H}-\text{C}_6\text{H}_4-\text{OCH}_3
  • The central carbon atom is a carbocation with a vacant pp-orbital (sp2\text{sp}^2 hybridized).
  • The −OCH3-\text{OCH}_3 group attached at the para-position relative to the carbocation contains unshared lone pairs on the oxygen atom.
  • The oxygen atom donates its lone pair of electrons via resonance (+R+\text{R} effect) through the conjugated π\pi-system of the benzene ring into the empty pp-orbital of the carbocation: H3C−O¨−C6H4−C⊕H−Ar⟷H3C−O⊕=C6H4=CH−Ar\text{H}_3\text{C}-\ddot{\text{O}}-\text{C}_6\text{H}_4-\overset{\oplus}{\text{C}}\text{H}-\text{Ar} \longleftrightarrow \text{H}_3\text{C}-\overset{\oplus}{\text{O}}=\text{C}_6\text{H}_4=\text{CH}-\text{Ar}
  • This leads to a canonical form where all atoms (including carbon) possess a complete octet of electrons, providing immense thermodynamic stabilization to the carbocation.

Thus, Statement I is true.

2. Analysis of Statement II

  • Structure: O2N−C6H4−C⊖H−C6H4−OCH3\text{O}_2\text{N}-\text{C}_6\text{H}_4-\overset{\ominus}{\text{C}}\text{H}-\text{C}_6\text{H}_4-\text{OCH}_3
  • The central carbon atom is a carbanion bearing a negative charge and a lone pair of electrons.
  • The nitro group (−NO2-\text{NO}_2) attached at the para-position is a powerful electron-withdrawing group by resonance (−R-\text{R} effect).
  • The negative charge on the central carbanion delocalizes through the π\pi-system of the benzene ring directly onto the electronegative oxygen atoms of the nitro group: O=N⊕(O⊖)−C6H4−C⊖H−Ar′⟷⊖O−N⊕(O⊖)=C6H4=CH−Ar′\text{O}=\overset{\oplus}{\text{N}}(\text{O}^\ominus)-\text{C}_6\text{H}_4-\overset{\ominus}{\text{C}}\text{H}-\text{Ar}' \longleftrightarrow {^\ominus\text{O}}-\overset{\oplus}{\text{N}}(\text{O}^\ominus)=\text{C}_6\text{H}_4=\text{CH}-\text{Ar}'
  • Delocalizing the excess electron density onto the oxygen atoms significantly lowers the energy and stabilizes the carbanion.

Thus, Statement II is true.

Conclusion

Both Statement I and Statement II are correct statements.

Correct Option: A (Both Statement I and Statement II are true)

Stability of Carbocation and Carbanion with Resonance Effects | Chemistry PYQ Solution - JEE Challenger