Chemistry - Migratory aptitude in pinacol-pinacolone rearrangement

Solution 1:

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.

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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.

Solution 2:

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.