I am confused about the migratory aptitude of various groups, as there are many different orders for the same given in different places, especially about -Ph and -H. I would like to know if someone could reliably tell the order.
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$\begingroup$ Partly resolved here $\endgroup$– stochastic13May 20, 2014 at 4:55
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3 Answers
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In the pinacol rearrangement, a 1,2-diol is treated with acid and rearranges to a carbonyl compound. Here is a reaction scheme showing a mechanism for the rearrangement.
In this case the molecule is symmetric and methyl migration is the only reaction pathway available. Methyl migration can 1) help stabilize the developing carbocation center and 2) once fully migrated, produces a resonance stabilized hydroxyl-carbocation.
If one or more of the methyl groups is replaced with some other substituent (for example, alkyl, hydrogen, aryl), then our reaction possibilities become more complex; which hydroxyl group will protonate, which group will migrate? Generally, both hydroxyls will protonate and lead to separate products. In terms of which group will migrate, the only thing agreed upon is that a tertiary carbon will migrate in preference to a secondary carbon which will migrate in preference to a primary carbon. Clearly, the ability of the migrating group to stabilize positive charge plays a role.
The reaction is reversible and product distribution can change as reaction conditions are changed (thermodynamic control vs. kinetic control). All of these complexities make it difficult to sort things out in detail, thereby making it difficult to determine the true relative migratory preference of other groups like H and phenyl. This is why the migratory aptitudes of these groups (H and phenyl) can change from textbook to textbook.
See section 2. Pinacol Rearrangement in this reference for a very nice, straightforward discussion of this reaction mechanism, and all its subtleties.
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$\begingroup$ @Aditya Can't say. It changes when you change the other substituents, or even with the same substituents it can change if you change the reaction conditions. $\endgroup$– ronSep 19, 2014 at 3:41
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According to my FIITJEE (a coaching institute in India) textbook, the relative order of migratory aptitude of groups in pinacol-pinacolone rearrangement is:
$$\ce{H > Ph > Me_3C > MeCH_2 > Me}$$
There does not seem to be a direct correlation between the mass and the migratory aptitude of these groups.
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16$\begingroup$ Your answer would have benefited if it also gave the reasoning of the stated order. $\endgroup$ May 21, 2018 at 1:53
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$\begingroup$ This is not a citable reference. Does your textbook have citations as to where it got the order from? $\endgroup$– TRCSep 21, 2021 at 16:07
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In general, the group whose migration stabilizes the carbocation formed will migrate first. Although there is an accepted trend to migratory aptitude, it is ultimately arbitrary. We know that Tertiary carbocations are more stable than Secondary Carbocations which in turn are more stable than Primary Carbocations. When a conflict arises, the nature of the carbocation formed by the migration of the group is considered. If a tertiary carbocation is formed, then said group will migrate. The ability of the group to stabilize the carbocation formed is key here.
