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

$\ce{CF3+}$ 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.

  • $\begingroup$ 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
  • $\begingroup$ @user1825567: I guess the extent of effect follows, Mesomeric > Resonance > Inductive. No? $\endgroup$ – Rahul Verma Dec 31 '19 at 13:39
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    $\begingroup$ @Rahul Verma Sorry, I am no longer fluent in this topic. BTW...All the best for JEE 😉! $\endgroup$ – user1825567 Dec 31 '19 at 17:20
  • $\begingroup$ @RahulVerma: Aren't 'mesomeric' and 'resonance' effects the same thing? According to the Gold Book 'mesomeric effect' is an alternate term for 'resonance effect'. If so, can you tell what did you mean by 'Mesomeric > Resonance'? $\endgroup$ – Vishnu Jul 2 '20 at 3:45
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    $\begingroup$ @234ff, have a look at this answer. It has a basic explanation of the trend. If you find it useful, don't forget to vote it $\endgroup$ – Rahul Verma Jul 21 '20 at 11:00

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