carbocation= racemization?

[sangria1986]

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can we assume carbocation = racemization?

also, carbanions are more stable with one carbon than 3 correct? because of inductive effects...

 

[Rabolisk]

I suppose yes to both questions.

 

[Chocolatebear89]

I thought tertiary carbanions are more stable, because of the inductive effects gives electron density to the radical, so it could more stable as octet

 

[sangria1986]

I thought tertiary carbanions are more stable, because of the inductive effects gives electron density to the radical, so it could more stable as octet

This would really depend on what the substituents are. but in general this doesnt happen. If the substituents are all carbon or other poor electornegative atoms then they will be electron DONATing....so more elctronegativity will be placed on the carbanion.

On the other hand, if you had CCl3 then I suppose that carbon could be a carbanion....but it doesnt seem likely. I doubt that would be on MCAT...correct me if im wrong

 

[Rabolisk]

OP said 3 carbons, which is why I interpreted a tertiary carbanion with 3 R groups versus just 3 hydrogens. Obviously stability is dependent on electron donating/withdrawing groups. There is no radical involved though...

 

[orgohacks]

This would really depend on what the substituents are. but in general this doesnt happen. If the substituents are all carbon or other poor electornegative atoms then they will be electron DONATing....so more elctronegativity will be placed on the carbanion.

On the other hand, if you had CCl3 then I suppose that carbon could be a carbanion....but it doesnt seem likely. I doubt that would be on MCAT...correct me if im wrong

Almost certainly not MCAT material - carbanion stability is much less straightforward than carbocation stability. For the curious, tert-butyllithium is a stronger base than n-butyllithium, it has to do with solvation effects. Again, not on the MCAT. Most of your organic TA's couldn't tell you that.

 

[Chocolatebear89]

Oops i was wrong. I mistaken carbanions as carbon with a radical

 

[collegestud2013]

For carbocations and carbon radicals, the order of stability is:

3* > 2* > 1* > methyl

For carbanions it's the opposite:

3* < 2* < 1* < methyl

The actual reason has to do with hyperconjugation, which by definition is electron density donation from a sigma bond to a nearby p-orbital (ie C-H or C-C). Hyperconjugation stabilizes carbocations the most, carbon radicals to a much lesser extent, and destabilizes carbanions. If you drew a molecular orbital diagram for each of the three cases, you would have 2 bonding e- and none antibonding e- in for the carbocation, 2 bonding e- and 1 antibonding e- for the carbon radical, and 2 bonding e- and 2 antibonding e- for the carbanion.

I know some of this info is probably beyond what's tested on the MCAT, but I guess it's never a bad idea to learn it...