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Is $\ce{CF_3+}$ more stable than $\ce{CH_3+}$?

In $\ce{CF_3+}$, there is $\ce{C-F}$ back bonding that increases stability of the carbocations. But -I effect of $\ce{F}$ dominates +R effect and this decreases the carbocation stability. How can I compare these two competing effects to find the more stable carbocation?

Also, is $\ce{CF_3+}$ more stable than $\ce{CH3-CH+-CH_3}$ and tert-butyl carbocation?

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    $\begingroup$ Yeah, but your first sentence answers that question... You said that the -I effect outweighs the +R effect and therefore the $\ce{CF3+}$ cation is less stable. So your question is probably how to judge which of the inductive or resonance effect is larger. I mean, I think this could be a good question about competing effects if you are clear about what you are asking. $\endgroup$ – orthocresol Nov 2 '15 at 12:13
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    $\begingroup$ Is there evidence that CF3(+) is more stable than CH3(+)? My intuition would be that methyl cation is more stable. $\endgroup$ – jerepierre Nov 2 '15 at 16:20
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    $\begingroup$ But deprotonation forms the anion, not the cation. The $\ce{CF3-}$ anion is trigonal pyramidal, so there's no backdonation from fluorine, and it's stabilised by the -I effect of fluorine, which is what leads to the smaller $\mathrm{p}K_\mathrm{a}$. $\endgroup$ – orthocresol Nov 2 '15 at 17:03
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    $\begingroup$ In the gas phase, the trifluoromethyl carbocation is more stable than the methyl carbocation, See p. 170 here $\endgroup$ – ron Nov 3 '15 at 17:10
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    $\begingroup$ and p.54 here $\endgroup$ – ron Nov 3 '15 at 17:10
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$\ce{CF3+}$ is more stable than $\ce{CH3+}$. The comparison between -I and +R effect is made while talking about activation or deactivation of halogens when substituted on benzene. Fluorine always stabilises a carbocation to a large extent because of a very good overlap of $\ce{2p -~2p}$ orbitals.

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  • $\begingroup$ So should I consider that net electron donating tendency of halogens to carbocations is more than the net electron withdrawing tendency? $\endgroup$ – user1825567 Mar 20 '16 at 18:56

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