What is the most stable carbocation?

1 Answer
Feb 28, 2015

The tricyclopropropylcyclopropenium cation is the most stable carbocation.


1° < 2° < 3°

We know that the stability order of aliphatic cations is


Resonance Stabilization

Neighboring double bonds are better stabilizers, because they can donate electrons by resonance.

Hence, the triphenylmethyl cation, #("C"_6"H"_5)_3"C"^"+"# is stable because it has many resonance structures.

However, the resonance overlap of the π orbitals is not perfect because steric hindrance forces the ion to have a propeller shape.

(Adapted from Schoolbag.info)


More stable is the tricyclopropylmethyl cation.


The bent bonds of the cyclopropane ring can overlap with the empty #"p"# orbital without steric hindrance.

(Adapted from Wikipedia)

Arenium ions

Nothing beats the stability of aromatic cations like cyclopropenium and cycloheptatrienylium (tropylium) cations.

Aromatic ions
(Adapted from www1.biologie.uni-hamburg.de)

The most stable carbocation

Add the conjugation with cyclopropyl rings, and the most stable carbocation prepared to date is probably the tricyclopropylcyclopropenium cation.