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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?

Aditya Dev
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    So what is the question? – orthocresol Nov 02 '15 at 11:07
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    Is CF3+ more stable that CH3+ – Aditya Dev Nov 02 '15 at 11:10
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    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. – orthocresol Nov 02 '15 at 12:13
  • @orthocresol: Is CF3+ more stable than propyl carbocation (+ charge at secod carbon of propane) and tert-butyl carbocation? – Aditya Dev Nov 02 '15 at 14:20
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    Alright, I don't know the answer to that. You'll have to edit it into the post and hopefully someone else will answer. – orthocresol Nov 02 '15 at 14:26
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    Is there evidence that CF3(+) is more stable than CH3(+)? My intuition would be that methyl cation is more stable. – jerepierre Nov 02 '15 at 16:20
  • @jerepierre : at DMSO, pka of methane is about 56 while pka of CF3H is 25-30 – Aditya Dev Nov 02 '15 at 16:31
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    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 $\mathrm{p}K_\mathrm{a}$. – orthocresol Nov 02 '15 at 17:03
  • So the pka values only explain the carbanion stability.... – Aditya Dev Nov 02 '15 at 21:39
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    In the gas phase, the trifluoromethyl carbocation is more stable than the methyl carbocation, See p. 170 here – ron Nov 03 '15 at 17:10
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    and p.54 here – ron Nov 03 '15 at 17:10

2 Answers2

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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.

TAR86
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Rakshit
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  • So should I consider that net electron donating tendency of halogens to carbocations is more than the net electron withdrawing tendency? – user1825567 Mar 20 '16 at 18:56
  • @user1825567: I guess the extent of effect follows, Mesomeric > Resonance > Inductive. No? – Rahul Verma Dec 31 '19 at 13:39
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    @Rahul Verma Sorry, I am no longer fluent in this topic. BTW...All the best for JEE ! – user1825567 Dec 31 '19 at 17:20
  • @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'? – Vishnu Jul 02 '20 at 03:45
  • @GuruVishnu It seems like a mistake, mesomeric effect and resonance are essentially the same thing, and this effect is stronger than inductive – Yusuf Hasan Jul 02 '20 at 05:37
  • @GuruVishnu, the mesomeric effect is included / subsumed in resonance effect category. It is a special case of resonance, where $\ce{\pi}$ orbitals are in conjugation. Other cases of resonance can include $\ce{C^+}$ or $\ce{C^-}$ – Rahul Verma Jul 05 '20 at 06:11
  • @RahulVerma Is there any explanation for the trend? I would like to know why resonance > Inductive in determining organic cation stability. Thanks – 234ff Jul 21 '20 at 10:39
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    @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 – Rahul Verma Jul 21 '20 at 11:00
  • @234ff, or if you need a very detailed insight, you can look up this chapter. – Rahul Verma Jul 21 '20 at 11:09
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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).
Mathew Mahindaratne
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