Q.5 The correct order of stability in water is
Option (C) is correct: Me-NMe₂ > Me-OMe > Me-Cl > Me-O-Me.
These represent para-substituted benzyl carbocations (Ph-CH₂⁺ with substituents at para position), where stability in water follows the electron-donating ability via resonance (+R > +I), modified slightly by solvation. NMe₂ provides the strongest +R stabilization due to its lone pair resonance delocalizing the charge effectively into the ring and carbocation.
Substituent Effects
NMe₂ (-σ_p = -0.83) is a powerful +R donor with a neutral nitrogen lone pair forming strong resonance with the empty p-orbital, making its carbocation most stable. OMe (-σ_p = -0.27) donates via oxygen lone pair but weaker (+R less effective, oxygen more electronegative), ranking second. Cl (-σ_p = +0.23) offers +R from lone pairs outweighing -I in carbocation stabilization, better than Me (+I only, -σ_p = -0.17). In water, solvation reduces differences but order persists as resonance dominates for benzylic ions.
Option Analysis
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(A) NMe₂ > OMe > OMe > Cl: Incorrect; duplicates OMe and ignores Me, misorders Cl vs Me.
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(B) OMe > NMe₂ > OMe > Cl: Wrong; OMe less stabilizing than NMe₂, duplicate error.
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(C) NMe₂ > OMe > Cl > Me: Correct per +R strength: NMe₂ > OMe > Cl > Me.
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(D) NMe₂ > Me > OMe > Cl: Flawed; underrates OMe and Cl relative to Me.
The order of stability of carbocations in water is crucial for CSIR NET Life Sciences and organic chemistry exams, especially for benzyl carbocations with substituents like NMe₂, OMe, Cl, and Me. This ranking—NMe₂ > OMe > Cl > Me—stems from electron donation to the positive charge.
Key Factors Influencing Stability
Carbocation stability increases with hyperconjugation, inductive (+I), and resonance (+R) effects. In aqueous medium, hydration stabilizes all but preserves relative order due to dominant +R in benzylic systems.
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Resonance Donors (NMe₂, OMe, Cl): Lone pairs conjugate with the carbocation p-orbital via the phenyl ring.
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Inductive Donor (Me): Provides +I but no +R.
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Water’s Role: Polar protic solvent solvates ions, slightly favoring less basic donors, yet resonance prevails.
Detailed Stability Order
| Substituent | σ_p Value | Effect | Relative Stability |
|---|---|---|---|
| p-NMe₂ | -0.83 | Strong +R | Highest |
| p-OMe | -0.27 | Moderate +R | High |
| p-Cl | +0.23 | Weak +R > -I | Medium |
| p-Me | -0.17 | +I only | Lowest |
NMe₂ excels as its dialkylamino group maximizes resonance without charge repulsion; OMe follows but oxygen’s electronegativity weakens donation; Cl’s +R trumps -I for carbocations; Me relies solely on hyperconjugation.
Exam Relevance for CSIR NET
For competitive exams, memorize: +R order NRR₂ > OR > Hal > Alkyl in benzylic carbocations. Practice drawing resonance structures to visualize charge delocalization.
