Stability of tropylium cation vs triphenyl methyl cation

[Vishesh Jain]

Homework Statement

The textbook "Organic Chemistry by O P Tandon" says that the tropylium cation (cycloheptatri-2,4,6-enyl cation) is more stable than the triphenylmethyl cation ( Ph₃C⁺) (phase/solvent not mentioned). Is this correct assuming it's in a non-polar medium ? On a related note, are compounds such as Ph₃C⁺ and the benzyl carbocation C6H5CH2+ aromatic ?

The Attempt at a Solution

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I have learned that the 3 phenyl rings in Ph₃C⁺ are not coplanar. (https://socratic.org/questions/what-is-the-most-stable-carbocation) So the 6 pi electrons of each ring are delocalized over the ring and the carbon attached to the ring (total 7 carbons). And in tropylium cation, the 6 pi electrons are delocalised over 7 C atoms, all part of the ring. Does that make a difference ? isn't Ph₃C⁺ still aromatic ? Repulsion among the 3 phenyl rings could be a factor

In compounds like Ph₃C⁺ & benzyl carbocation, the conjugation of 6 pi electrons forms a closed loop ... even though one carbon is outside the ring, so they should be aromatic (according to "requirements for aromaticity" defined in http://www.chem.ucla.edu/~harding/notes/notes_14C_intaroma02.pdf ). I hope this reasoning for the 2nd question is correct..

 

[TeethWhitener]

And in tropylium cation, the 6 pi electrons are delocalised over 7 C atoms, all part of the ring. Does that make a difference ?

If the nuclei are not coplanar, then the p-orbitals cannot align and their electrons do not delocalize as effectively (if at all). Do you think this should make a difference in tropylium vs Ph₃C?

isn't Ph3C+ still aromatic ?

The individual phenyl groups are aromatic, but they are tilted out of plane such that the p-orbitals of the phenyl groups cannot donate electrons into the empty p-orbital on the carbocation center.

Repulsion among the 3 phenyl rings could be a factor

Repulsion among the phenyl rings is what tilts them out of plane.

the conjugation of 6 pi electrons forms a closed loop ... even though one carbon is outside the ring, so they should be aromatic

I'm not sure what you mean by this. The phenyl rings are aromatic, sure, but this has nothing to do with what's attached to the phenyl rings. Toluene is aromatic, even though there are atoms outside the ring that are nonplanar.

Benzyl carbocation is quite stable, and the aromaticity of the phenyl group is important for this stability. It allows electron density to be donated into the empty p-orbital on the cation center, and it supports resonance structures which increase its stability.