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Topic: Carbocation shift  (Read 4782 times)

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Offline betterbesafehero

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Carbocation shift
« on: May 05, 2013, 06:42:49 PM »
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 ?


« Last Edit: May 05, 2013, 07:06:40 PM by betterbesafehero »
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Offline betterbesafehero

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Re: Carbocation shift
« Reply #1 on: May 06, 2013, 12:30:23 PM »
Any help would be appreciated..!
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Offline opsomath

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Re: Carbocation shift
« Reply #2 on: May 06, 2013, 02:13:02 PM »
What do you mean by Ar? Ph should be a subset of Ar, yes?

Offline betterbesafehero

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Re: Carbocation shift
« Reply #3 on: May 06, 2013, 02:33:33 PM »
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
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Offline orgopete

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Re: Carbocation shift
« Reply #4 on: May 07, 2013, 01:17:17 AM »
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.
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Offline betterbesafehero

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Re: Carbocation shift
« Reply #5 on: May 07, 2013, 04:32:35 AM »
^^^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.
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Offline betterbesafehero

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Re: Carbocation shift
« Reply #6 on: May 07, 2013, 04:34:53 AM »
Let's take a simple example of  pinacol-pinacolone rearrangement..!
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Offline souro10

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Re: Carbocation shift
« Reply #7 on: May 07, 2013, 03:43:47 PM »
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.

Offline orgopete

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Re: Carbocation shift
« Reply #8 on: May 08, 2013, 01:49:47 PM »
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.
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