[betterbesafehero]

I would assume that everyone is well aware of term 'migratory aptitude'.

Which group has a greater migratory aptitude and why ? Does migratory aptitude depends on 'how stable the carbocation is after the transfer' or on stabilization by pi electrons is considered ?

I think this is the general migratory aptitude trend : Ar>>>> hydride > Ph^- > tertiary > secondary > methyl

Is there any transition state involved in it ?....As far as I know 'yes, transition state is involved'..!

Main confusion is for phenyl and hydride, which one would have greater migratory aptitude ?

[betterbesafehero]

Any help would be appreciated..!

[opsomath]

What do you mean by Ar? Ph should be a subset of Ar, yes?

[betterbesafehero]

What do you mean by Ar? Ph should be a subset of Ar, yes?

Yes the same I mean, Ar means any substituent of phenyl.

If you have read the question, my main doubt is regarding hydride and phenyl.

Thankyou

[orgopete]

I would assume that everyone is well aware of term 'migratory aptitude'.

Which group has a greater migratory aptitude and why ? Does migratory aptitude depends on 'how stable the carbocation is after the transfer' or on stabilization by pi electrons is considered ?

I think this is the general migratory aptitude trend : Ar>>>> hydride > Ph^- > tertiary > secondary > methyl

Is there any transition state involved in it ?....As far as I know 'yes, transition state is involved'..!

Main confusion is for phenyl and hydride, which one would have greater migratory aptitude ?

I don't find this a good question because we are being asked to generalize over all reactions. In some reactions, the migrating group is determined by stereoelectronic control that overrides aptitude. Perhaps we could answer a which group should migrate for a specific reaction.

[betterbesafehero]

^^^Yes I know that, but's what's the general trend, I am saying leaving aside the factors which sometimes overrides the aptitude, and just considering the conditions which would not affect the migrating group. Please help me with my questions.

[betterbesafehero]

Let's take a simple example of  pinacol-pinacolone rearrangement..!

[souro10]

There is lack of understanding and measurements for the conflicting migratory aptitudes of Hydride in Pinacol-Pinacolone rearrangement.

In an authentic chapter written by Bruce Rickborn of University of California, I remember reading that migratory aptitude of hydride is same as that of phenyl in Pinacol-Pinacolone rearrangement.

If not equal, it is believed that migratory aptitudes of Ph and H are very close. This leads to concept of reversibility and complications relating to kinetic v/s thermodynamic control in case of di and tri-substituted vicinal diols. Often aldehydes are formed as kinetic product, but more stronger conditions ( concentrated acid, higher temperature ) produces a phenyl-ketone conjugated system as the thermodynamic product.

[orgopete]

Let's take a simple example of  pinacol-pinacolone rearrangement..!

I generally think the group that is most electron rich migrates to the electron poor carbon. If one had two phenyl rings, one with a methoxy and the other a nitro, the ring with the methoxy will migrate.

There can be other factors also, relief of ring strain and I think inertia may play a role. The electrons attached to a hydrogen may be easier to move. I would have to look carefully at hydride migrations as some may involve loss of a proton though that is probably not the case for a pinacol rearrangement.