# Precedence of 1,2 carbocation rearrangement

How can one choose which group has more shifting tendency in 1,2 carbocation rearrangement? The obvious order is via the stability of the carbocation of the group. But, phenylic groups have high shifting tendency--and a phenylic carbocation is unstable.

Is there a way of predicting these? I was thinking that it will be related to the delocalization in the triangular intermediate formed.

Update: I'm talking about comparing "migratory aptitude" of the $R$ groups in a system similar to $>C(+)-C(R_1R_2R_3)$

• @CHM: The first case.. Eg if we have $C(+)-C(R_1,R_2,R_3)$, which $R$ will shift? – ManishEarth Apr 26 '12 at 4:22
• "migratory aptitude" is the more established term, IIRC. – user95 Apr 26 '12 at 6:36

Reactions are ruled by the $ΔG$ of the activated complex. This may be close to educts, or to products, or some halfway state or a real intermediate minimum is on the reaction path. So, in some cases the model of a separate carbenium ion (in a ionic pair maybe) is useful, sometimes you need to think of a three-center bond state.