How to tell when a carbocation or carbanion forms?

[Astra]

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For example, in B-dicarboxylic acid, the loss of the alpha hydrogen causes a formation of carbanion.

Where as, tertiary carbons form carbocations.


Here is my guess:

The carbocation or anion formation can be determined by the groups nearby and attached to the carbon.

If the groups are electron withdrawing, then the carbanion will form.

If the groups are electron donating, then the carbocation will form.

Correct?

 

[aldol16]

You're not even talking about the same reaction. Loss of alpha hydrogen will form a carbanion, yes. It forms a nucleophile which can attack an electrophile.

Tertiary carbons lose a leaving group - note that they do not lose hydrides because they are not hydridic - and end up with a positive charge. So that's an electrophile and it will be reactive towards a nucleophile, like in SN1.

You will not get something that will form either a carbocation or a carbanion with anywhere near the same degree of spontaneity. That's because the factors required to stabilize each form are directly contradictory. EWG stabilizes carbanions and destabilize carbocations. EDG stabilizes carbocations and destabilize carbanions.

At this point, you should have a feel for what sort of reactions can happen with which compounds. Another principle to guide you is charge conservation. If you lose a proton, the resulting R group must have a negative charge on it. If you lose a halide, the resulting R group must have a positive charge on it. If you ever have a mechanism where you end up with a positive R group without there being any indication of there having been a leaving group, you've done something wrong (except in limited cases like NADH and NADPH).