# Stability of carbocations: CF3+ vs CH3+

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?

• 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 2 '15 at 12:13
• Is there evidence that CF3(+) is more stable than CH3(+)? My intuition would be that methyl cation is more stable. – jerepierre Nov 2 '15 at 16:20
• 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 2 '15 at 17:03
• In the gas phase, the trifluoromethyl carbocation is more stable than the methyl carbocation, See p. 170 here – ron Nov 3 '15 at 17:10
• and p.54 here – 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.