Pyrovus
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Stability of halogenated carbocations
Hi all,
I've been wondering for a while what effect the presence of halogen atoms on the positive carbon (i.e. in carbocations of the form R2XC+) has on the
overall stability of the carbocation. All the material I can find on carbocation stability is purely concerned with the effect of alkyl groups, and
says nothing about the effects of other possible substituents..
As I see it, there will be two competing factors involved.
- The halogen, being more electronegative than carbon will withdraw electron density, decreasing the stability of the cation.
- The positive charge will be partly delocalised onto the halogen atom by resonance eg. H2C(+)-Cl <-> H2C=Cl(+), which will increase the
stability of the cation.
Which of these factors can be expected to win out, and in general, how will the magnitude of the stabilising/destabilising effect of halogen
substitution compare to that caused by the presence of alkyl groups on the carbocation?
Never accept that which can be changed.
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DrP
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We THINK that the first case is true - The Cl withdraws electron density, destablising the cation even further. The Cl is too e-withdrawing for the
second case to happen. R-groups rather than X groups would stablise the cation - the larger the R group and the more of them, the more the cation
is stabilised. Therefore tertiary carbocations are the most stable.
[Edited on 10-5-2006 by DrP]
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Pyrovus
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Thanks, that's in line with what I expected for chlorine, given it's high electronegativity. What about if we go down to iodine? The electronegativity
of iodine is only slightly higher than that of carbon, so it shouldn't be too electron withdrawing, and iodine is a very large atom, making it quite
polarizable and thereby able to spread the charge out some more. Would iodine therefore be expected to have a net stabilising effect?
Also, in qualitative terms, how strong is the destabilising effect of a chlorine atom, compared to the stabilising effect of alkyl groups? For
instance, if I were to start with the methyl cation, and replace one of the hydrogens with a methyl group and another with a chlorine (to give
(CH3)ClHC(+)), would this ion be more or less stable than the methyl cation?
Never accept that which can be changed.
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DrP
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"Also, in qualitative terms, how strong is the destabilising effect of a chlorine atom, compared to the stabilising effect of alkyl groups? For
instance, if I were to start with the methyl cation, and replace one of the hydrogens with a methyl group and another with a chlorine (to give
(CH3)ClHC(+)), would this ion be more or less stable than the methyl cation? "
In this case the effect from the chlorine predominates as it is much more electronegative. So it would still be less stable. Even if the size of the
R groups were to be increased, any extra stablisation would be negligable. I would expect that phenyl groups would stablize alot more due to the
conjuation - but I'm speculating now really. As for the iodene situation - I don't know what the net effect is, but as you said - obviously no where
near as destablising as the Cl. I would expect you'd have to ballance the electronegitivity against the slightly electron donating properties of the
hydrogens on the R groups.
PS: if it has a tertiary cation with larger R groups, you can get some steric protection of the cation - it is still reactive, but the reaction
ingredients wont be able to physically get into where the charge is due to long/large R groups physically blocking the way.
[Edited on 7-6-2006 by DrP]
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