JEE Challenger
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Stability Order of Carbanions and Electronic Effects

Given below are two statements:

Statement I: On the basis of inductive effect, the order of stability of alkyl carbanions is CH3−>CH3−CH2−>(CH3)2CH−>(CH3)3C−\text{CH}_3^- > \text{CH}_3-\text{CH}_2^- > (\text{CH}_3)_2\text{CH}^- > (\text{CH}_3)_3\text{C}^-.

Statement II: Allyl and benzyl carbanions are more stabilised by inductive effect and not by resonance effect.

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

Options

A

Both Statement I and Statement II are correct

B

Both Statement I and Statement II are incorrect

C

Statement I is correct but Statement II is incorrect

Correct
D

Statement I is incorrect but Statement II is correct

Topics & Concepts

Organic Chemistry - Basic PrinciplesInductive EffectResonance

Step-by-Step Solution

To evaluate the given statements:

  1. Analysis of Statement I: Alkyl groups exert an electron-releasing inductive effect (+I+I effect). In a carbanion, the carbon atom carries a negative charge. The presence of +I+I groups increases the electron density on this negatively charged carbon, thereby intensifying the negative charge and destabilizing the carbanion.

    • CH3−\text{CH}_3^- has no alkyl groups directly attached to supply electron density via +I+I effect.
    • CH3CH2−\text{CH}_3\text{CH}_2^- (primary carbanion) has one +I+I methyl group.
    • (CH3)2CH−(\text{CH}_3)_2\text{CH}^- (secondary carbanion) has two +I+I methyl groups.
    • (CH3)3C−(\text{CH}_3)_3\text{C}^- (tertiary carbanion) has three +I+I methyl groups.

    Therefore, as the number of alkyl groups increases, the stability decreases. The stability order on the basis of inductive effect is: CH3−>CH3−CH2−>(CH3)2CH−>(CH3)3C−\text{CH}_3^- > \text{CH}_3-\text{CH}_2^- > (\text{CH}_3)_2\text{CH}^- > (\text{CH}_3)_3\text{C}^- Hence, Statement I is correct.

  2. Analysis of Statement II: In allyl (CH2=CH−CH2−\text{CH}_2=\text{CH}-\text{CH}_2^-) and benzyl (C6H5−CH2−\text{C}_6\text{H}_5-\text{CH}_2^-) carbanions, the unshared electron pair on the carbanionic carbon is conjugated with a double bond or an aromatic ring, respectively. This allows for delocalization of the negative charge through resonance, which provides significant stabilization to these carbanions compared to simple inductive effects.

    Hence, allyl and benzyl carbanions are stabilized primarily by the resonance effect, not by the inductive effect. Therefore, Statement II is incorrect.

Conclusion: Statement I is correct, but Statement II is incorrect.

Thus, the correct answer is Option C.

Stability Order of Carbanions and Electronic Effects | Chemistry PYQ Solution - JEE Challenger