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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. $\endgroup$ Nov 2, 2015 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, 2015 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 $\mathrm{p}K_\mathrm{a}$. $\endgroup$ Nov 2, 2015 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, 2015 at 17:10
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    $\begingroup$ and p.54 here $\endgroup$
    – ron
    Nov 3, 2015 at 17:10

2 Answers 2

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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? $\endgroup$ Mar 20, 2016 at 18:56
  • $\begingroup$ @user1825567: I guess the extent of effect follows, Mesomeric > Resonance > Inductive. No? $\endgroup$ Dec 31, 2019 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 😉! $\endgroup$ Dec 31, 2019 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, 2020 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 $\endgroup$ Jul 21, 2020 at 11:00
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Despite the strong $\sigma$-electron withdrawing effect of fluorine, $\ce{CF3+}$ has been stabilized relative to that of $\ce{CH3+}$. For example, the reaction: $$\ce{CH3+ + CHF3 -> CF3+ + CH4}$$ is exothermic by $\pu{13.2 kcal mol-1}$ (Ref.1). It was also found that hydrogen abstraction in following reaction is even more exothermic than previous one: $$\ce{CH3+ + CH2F2 -> CHF2+ + CH4} \qquad \Delta H = \pu{-28.3 kcal mol-1}$$

According to authors, the only order which can be constructed that is consistent with all observations gives the relative hydride affinities as $\ce{CH3+ \gt CF3+ \gt CH2F+ \gt CHF2+}$. It is also noteworthy that within the references in Ref.1 has given that $\Delta H_f (\ce{CH3+}) = \pu{276.1 kcal mol-1}$ while that of $\Delta H_f (\ce{CF3+}) = \pu{99.3 kcal mol-1}$.

References:

  1. R. J. Blint, T. B. McMahon, and J. L. Beauchamp, "Gas-phase ion chemistry of fluoromethanes by ion cyclotron resonance spectroscopy. New techniques for the determination of carbonium ion stabilities," J. Am. Chem. Soc. 1974, 96(5), 1269–1278 (DOI: https://doi.org/10.1021/ja00812a001).
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