Carbanion or carbocation?

[stitchattack]

I'm probably missing the concept but I'm confused on when carbocation forms versus carbanion. This is what I understand:
1. carbocation: C is positively charged; electron deficient; electron donating group stabilizes it
2. carbanion: C is negatively charged; not electron deficient

In Aldol Condensation, a carbanion forms for the alpha C when the hydroxyl takes an alpha H away. Why does a carbanion forms instead of a carbocation?

In Pinacol Rearrangement, a carbocation forms when water leaves. So basically, I'm confused because my understanding is that when a C loses a bond, it becomes a carbocation.

So, can someone explain to me when a carbocation and carbanion forms?

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[labqi]

For the Aldol condensation, please correct me if I am Wrong, the hydroxyl group (oh-) removed a proton aka alpha hydrogen. A proton being a positive charge or in others a hydrogen. Remove a proton leaves the carbon with a pair of electrons left over. This making it negative and it becomes nucleophilic.

The way I think about it, if you have a nucleophile, it wants positives only, and not the electrons. Therefore it steals the proton/positive charges while leaving the negatives behind.

 

[labqi]

A carbocation forms when electrons are removed from the carbon. A carbocation is sp2 hybridized and trigonal planar. When a SN1 reaction occurs, a stable leaving group must form before an attack can occur. What makes a stable leaving group - something that becomes neutrally charged usually or something that has it's electrons, octet (cl-). In order for water to leave, it has to take the electrons shared in the covalent bond so it can form it's own neutral species and leave. Leaving the carbon with a deficit in electrons. I think that's right sorry I couldn't explain better

 

[stitchattack]

The part about H (proton) leaving; thus, leaving the C leftover with a pair of electrons so the C is negatively charged (carbanion) makes sense. However, when water leaves, the C is deficient of electrons so it's positively charged still confuses me. Even when H2O leaves, I thought that the C would still have a lone pair on it so why would this part not make the C negatively charged?

 

[stitchattack]

Ok, correct me if I'm wrong...I just want to get all these carbo intermediates down.
1. Carbocation forms when bond between C-X is broken and X is more electronegative than C.
2. Carbanion forms when bond between C-X is broken and X is less electronegative than C.

Another intermediate..free radical is formed during photolysis action/light or when bond between C and another C is broken? Just throwing this in there since it's another carbon intermediate.

 

[labqi]

The part about H (proton) leaving; thus, leaving the C leftover with a pair of electrons so the C is negatively charged (carbanion) makes sense. However, when water leaves, the C is deficient of electrons so it's positively charged still confuses me. Even when H2O leaves, I thought that the C would still have a lone pair on it so why would this part not make the C negatively charged?

Before water leaves for example, how many bonds are there? 4 in most cases. This when water leaves it takes the electrons that belonged to the carbon and itself. Thus that's where you get a +1 charge (carbon loses it's original electron). Someone please correct if I'm incorrect.

 

[labqi]

I believe a free radical acts an intermediate but not in the usual sense. Iirc - it's an sp2 intermediate. Not too sure. It has all it's electron except one. Yet it's neutrally charged. The lone electron has the ability to attack.

 

[labqi]

A free radical can form anytime a covalent bond is broken imo. Each species takes back it's original electron. It doesn't have to be between a carbon carbon. It could also be between a c chlorine, bromine, hydrogen. Or it could be between a chlorine chlorine etc. oxygen could also be a radical