Quick question about Halogens

[vk79]

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Just got done with the AAMC self assessment and I had a question about one of the answer explanations. It said:

"Halogens stabilize carbocations through resonance
and Halogens stabilize carbanions via induction"

Can anyone give me an idea about what this means, and why there is a difference?

Thanks so much!

 

[Trayshawn]

halogens have lone pairs that it can donate to adjacent carbocations thus removing the positive charge on that carbon and gaining a positive (formal) charge on itself. This is the other "resonance form." [In reality, the electrons are distrubuted among both of these "forms," they are not seperate structures].
So by spreading that charge around, it stabilizes the carbocation. Remember, nature hates localized charge!

Carbanions are unstable for another reason. They have charge which is bad, yes, but the charge is a NEGATIVE one on the relatively electropositve carbon. Carbon hates negative charge, it has a really low electronegativity that tells you this. So having the more electronegative halogen atoms next to it to pull away some of that charge density (aka induction) will stabilize it since the carbon will feel less of that oh-so-horrible negative charge on it.

 

[vk79]

halogens have lone pairs that it can donate to adjacent carbocations thus removing the positive charge on that carbon and gaining a positive (formal) charge on itself. This is the other "resonance form." [In reality, the electrons are distrubuted among both of these "forms," they are not seperate structures].
So by spreading that charge around, it stabilizes the carbocation. Remember, nature hates localized charge!

Carbanions are unstable for another reason. They have charge which is bad, yes, but the charge is a NEGATIVE one on the relatively electropositve carbon. Carbon hates negative charge, it has a really low electronegativity that tells you this. So having the more electronegative halogen atoms next to it to pull away some of that charge density (aka induction) will stabilize it since the carbon will feel less of that oh-so-horrible negative charge on it.

Ah I get it! Thank you so much, you've definitely helped clear this up for me