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Why is a 3° Carbanion unstable than 1° Carbanion? Carbon atom with negative charge will become little electropositive relative to a neutral carbon. So if a Carbanion is attached to three neutral carbons, essentially it should donate it's electrons distributing the negative charge of -1, thus stabilizing it. By this logic, more carbon atoms attached means more stability. But this does not happen. 1° Carbanion is most stable. Why?

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  • $\begingroup$ You are half way through explaining the question yourself. You just need to realise that alkyl groups themselves are electron donating groups. $\endgroup$
    – user88957
    Commented Jun 22, 2020 at 1:26
  • $\begingroup$ Why that is my question they should be more Electronegative than the negatively charged carbon, thereby accepting electrons. $\endgroup$ Commented Jun 22, 2020 at 3:04
  • $\begingroup$ To make it common knowledge, I am a high school student. For my reasoning I have considered only the Inductive effect which is basically electron density movement from less to more Electronegative atom. I have even not considered C-H bond polarity in alkyl group nor the hyperconjugation effect. Please tell me what I am missing particularly and why my logic is wrong? $\endgroup$ Commented Jun 22, 2020 at 3:16
  • $\begingroup$ Methyl group is EDG and exerts +I effect due to Hyperconjugation. Related: Why is the methyl group electron-donating via the inductive effect? and What is Hyperconjugation? $\endgroup$ Commented Jun 22, 2020 at 3:19
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    $\begingroup$ Does this answer your question? Why is the methyl group electron-donating via the inductive effect? $\endgroup$ Commented Jun 22, 2020 at 3:21

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Carbanions and Carbocations are perfectly opposite in order of stability because they are opposite in order of charge. The order is preferred of carbocations the same as acidity by Lewis' nucleophile/electrophile definition. Therefore, because a positive charge is stabilized better by a tertiary carbon, it is preferred to host a carbocation because the electron shells of its substituent carbons help stabilize the charge.

In the case of a carbanion, the negative charge is best stabilized when not in contact with neighboring nuclei that would resonate the negative charge away from the nucleophilic site. If you think about it like sterics, the further away the negative charge can be from other substituents, the better in the case of carbanions.

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