Carbocation Rearrangements | Chemistry Net

Carbocation Rearrangements

Carbocation rearrangements

Carbocation Rearrangements: 1,2-H and 1,2-alkyl Shifts

In a rearrangement a group moves from one atom to another in the same molecule. Most are migrations between adjacent atoms and are called 1,2-shifts. Carbocation rearrangements occur more frequently on secondary carbocations to form tertiary which are more stable and energetically more favorable. In general, the bonding electrons of a carbocation may shift between adjacent atoms to form a more stable carbocation.

What types of carbocation rearrangements are possible? When does a carbocation rearrangement occur?
Two types of carbocation rearrangements are possible: 1,2-H shift and 1,2-alkyl shift.
1,2-H shift (called 1,2 hydride shift, hydride ion = H:-): If a carbocation is vicinal to a tertiary carbon bearing a H, a 1,2-H shift should occur (Fig. 1).
Fig. 1: Two possible types of rearrangement. The carbocation desires electron pair to complete the octet at the C+ atom. An 1,2-H shift provides these electrons and gives a more stable tertiary carbocation. In principle, a 1,2-methyl shift can occur but is less favorable since it gives a secondary carbocation.
Fig. 1: Two possible types of rearrangement. The carbocation desires electron pair to complete the octet at the C+ atom. An 1,2-H shift provides these electrons and gives a more stable tertiary carbocation. In principle, a 1,2-methyl shift can occur but is less favorable since it gives a secondary carbocation.

1,2-alkyl shift: If a carbocation is vicinal to a tertiary carbon, a 1,2-alkyl shift should occur (Fig. 1).
Of these two examples shown in Fig. 1, hydride shift leads to a tertiary carbocation whereas alkyl (methyl) shift leads to a secondary carbocation. Because a tertiary carbocation is more stable than a secondary the hydride shift would occur more readily than the alkyl shift.
The following “rules” hold for carbocation rearrangements:
  • Carbocation rearrangements are equilibrium processes
  • Usually lead to more stable carbocations 
  • Sometimes lead to carbocations of equal stability (not so common)
  • Sometimes lead to less stable carbocations (very unusual but does happen)
  • Hydride shift is more common, favorable, than alkyl shift
  • The least bulky alkyl substituent shifts (usually CH3-)
  • Only groups adjacent to C+ can migrate
  • Only carbon groups and H atoms can shift (1,2-OH shift is forbidden)

Carbocations are intermediates in SN1 reactions and quite often yield rearranged products. A few examples are shown below:
The SN1 reaction above yields a rearranged reaction product. The intermediate secondary carbocation formed rearranges to a tertiary carbocation – a 1,2 hydride shift occurs – which is more stable. The carbocation reacts with CH3OH and an  SN1 reaction occurs.
Fig. 2: The SN1 reaction above yields a rearranged reaction product. The intermediate secondary carbocation formed rearranges to a tertiary carbocation – a 1,2 hydride shift occurs – which is more stable. The carbocation reacts with CH3OH and an SN1 reaction occurs.

An example of 1,2-methyl shift is shown below leading to rearranged racemic products:
Fig. 1: Two possible types of rearrangement. The carbocation desires electron pair to complete the octet at the C+ atom. An 1,2-H shift provides these electrons and gives a more stable tertiary carbocation. In principle, a 1,2-methyl shift can occur but is less favorable since it gives a secondary carbocation.
Fig. 3: A 1,2-alkyl shift gives a more stable tertiary carbocation. The carbocation reacts with CH3NH2 and an SN1 reaction occurs.
Another example of carbocation rearrangement is shown in Fig. 4.  The reaction is known as the pinacol rearrangement and the carbocation formed is already stable since it is a tertiary carbocation. However it rearranges – a CH3 shift occurs – because it is stabilized even more by the lone pairs of the oxygen atom.
Fig. 4: The 1,2-diol pinacol is treated with acid and a rearranged product pinacolone is produced. The reaction intermediate carbocation rearranges – a CH3 shift occurs – even though it is a tertiary carbocation. The carbocation is stabilized by the electron lone pairs of the oxygen atom.
Fig. 4: The 1,2-diol pinacol is treated with acid and a rearranged product pinacolone is produced. The reaction intermediate carbocation rearranges – a CH3 shift occurs – even though it is a tertiary carbocation. The carbocation is stabilized by the electron lone pairs of the oxygen atom.
Carbocation rearrangements often give products where there is change in ring size. In the post entitled “Carbocation rearrangements and change in ring size” examples of such rearrangements are shown.

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References
  1. R. Bruckner, “Advanced Organic Chemistry – Reaction Mechanisms”, 2nd Edition, Elsevier, 2002
  2. M.B. Smith & J. March “March’s Advanced Organic Chemistry”, 6th Edition, Wiley-Interscience, 2007
  3. P. Muller, J. Mareda in G.A. Olah “Cage Hydrocarbons”, Wiley, 1990


Key Terms
rearrangement, rearrangement of carbocation, secondary carbocation rearrangement, 1,2 shifts, carbocation rearrangements, secondary carbocations, 1,2 H-shift, 1,2 alkyl-shift, rearranged products, pinacol rearrangement

1 comment:

  1. Wow. Very interesting and knowledgeful post. Very simply explained. Thanks for sharing.

    -tatvachintan

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