Why/how do electron withdrawing groups stabalize carbocations?

[stester77s]

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I do not understand why a tertiary carbocation is more stabilized than a primary carbocation if that tertiary carbocation has a an R group that happens to be electron withdrawing, such as -CCl3.


I mean, that EWG pulls away negative charge making the positive charge on the carbocation even more positive, so why is that more stabilized than the primary carbocation? How?

 

[labqi]

I thought an alkyl group was electron donating? Weak activator and ortho-para directing. I'm just taking a stab at this. First and foremost, the Cl are weakly deactivating (yet ortho para) effecting the C most closely attached (making it more positive). But then you have two additional alkyl groups which are weakly activating. 2 weak activators vs one weak deactivator. Also the CCl3 is not super positive from my understanding as the cl are pulling electrons equally and the negative stays close to the C (that's bound to the the 3 cl). I may be wrong but it makes sense to me.

 

[medic86]

A tertiary carbocation is always more stable than a primary carbocation (hell, these practically don't exist). You need to brush up on your organic.

You should refer to here: http://www.masterorganicchemistry.com/2011/03/11/3-factors-that-stabilize-carbocations/

 

[stester77s]

Could someone else please attempt to answer this question?

It makes perfect sense that an electron donating group will stabilize the carbocation, but why would an electron withdrawing group (such as -CN, -CF3, whatever) stabilize it? The EWG should be making the positive charge on the carbocation even more positive, thus destabilizing it. Could someone provide an explanation to this?

 

[dudewheresmymd]

Could someone else please attempt to answer this question?

It makes perfect sense that an electron donating group will stabilize the carbocation, but why would an electron withdrawing group (such as -CN, -CF3, whatever) stabilize it? The EWG should be making the positive charge on the carbocation even more positive, thus destabilizing it. Could someone provide an explanation to this?

can you provide the question or screenshot?

 

[stester77s]

can you provide the question or screenshot?

I don't have the question or screenshot. I am just trying to get a better understanding of this.

 

[TheBoneDoctah]

I don't have the question or screenshot. I am just trying to get a better understanding of this.

I think you're reading the question wrong lol

 

[swivelj]

http://www.masterorganicchemistry.com/2011/03/11/3-factors-that-stabilize-carbocations/
From the site a previous poster linked. I asked myself the same question this pretty much sums up your problem, or least it did for me.


3) Carbocations are stabilized by adjacent lone pairs. The key stabilizing influence is a neighboring atom that donates a pair of electrons to the electron-poor carbocation. Note here that this invariably results in forming a double bond (π bond) and the charge will move to the atom donating the electron pair. Hence this often goes by the name of “π donation”.

The strength of this effect varies with basicity, so nitrogen and oxygen are the most powerful π donors. Strangely enough, even halogens can help to stabilize carbocations through donation of a lone pair. The fact that atoms that we normally think of as electron-wthdrawing (nitrogen, oxygen, chlorine) can actually be electron-donor groups is probably one of the most difficult factors to wrap your head around in Org 2.

This effect is tremendously important in the reactions of aromatic rings and also in enolate chemistry, where double bonds attached to donating groups (nitrogen and oxygen in particular) can be millions (or billions) of times more nucleophilic than alkenes that lack these groups.

 

[Doctor Dream]

I do not understand why a tertiary carbocation is more stabilized than a primary carbocation if that tertiary carbocation has a an R group that happens to be electron withdrawing, such as -CCl3.


I mean, that EWG pulls away negative charge making the positive charge on the carbocation even more positive, so why is that more stabilized than the primary carbocation? How?

Just learned about this a couple weeks ago, there is no EWG that would stabilize a carbocation. In fact, EWG's are often added to organic compounds to form partial positive charges on carbons and make them more unstable and willing to react.

Halogens themselves are EWG's, but I'm not sure if it's accurate to say an alkyl halide is

 

[getsome111]

although ortho/meta/para directing groups aren't on the mcat, knowing their trend is probably the best way to answer a question like this--its based on the exact same principle. Im sure you can already guess the standard trend, however halogens are an exception, though they are technically "deactivators", their ability to donate by resonance trumps their withdrawing characteristic. I am almost certain that whoever wrote this question is asking it as a theoretical extension of EAS chemistry and in the process likely went a little too far with their extrapolation of the theory, I think you would have to look up if this case happens in real life depending on the surrounding R groups. +CF3 is certainly destabilized by the flourines