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Rearrangements

By James Ashenhurst

Carbocation Rearrangement Reactions (2) – Alkyl Shifts

Last updated: October 3rd, 2019 |

Alkyl Shifts In Carbocation Rearrangement Reactions, Including Ring Expansion

In the last post we saw how certain carbocations can sometimes rearrange (through hydride shifts) to give more stable carbocations. However, sometimes there are situations where a hydride shift would not lead to a more stable carbocation, but an alkyl shift would!

Table of Contents

  1. Alkyl Shifts Can Lead To More Stable Carbocations, Too
  2. The Mechanism Of Alkyl Shift Reactions
  3. Example of An SN1 With Alkyl Shift
  4. Ring-Expansion Reactions Also Involve Alkyl Shifts

1. Alkyl Shifts Can Lead To More Stable Carbocations, Too

Take a look at this carbocation. If a hydride shift occurred here, we’d be going to a less stable (primary) carbocation! Not going to happen.

You might note something with this example, however: it is possible for a more stable tertiary carbocation to be formed.

How? If an alkyl  group migrates instead! (Look at that red methyl group! )

The most common situation where alkyl shifts can occur is when a quaternary carbon (that’s a carbon attached to 4 carbons) is adjacent to a secondary carbocation.

2. The Mechanism Of Alkyl Shift Reactions In Carbocation Rearrangements

How does this work? First, the pair of electrons from the C-C bond must align with the empty p orbital on the carbocation (side note: this means they have to be aligned in the same plane). Then, as the pair of electrons from the C–C bond is donated into the empty p-orbital, one C–C bond begins to break and the new C–C bond begins to form.

In the transition state, there are partial bonds between the carbon being transferred and each of the two adjacent carbon atoms. Then, as one bond shortens and the other lengthens, we end up with a (more stable) tertiary carbocation.


Rearrangements can potentially occur any time a carbocation is formed. That includes SN1 reactions (and as we’ll later see, elimination and addition reactions).

3. Example of An SN1 With Alkyl Shift

Here’s an example of an SN1 with an alkyl shift (note that the CH3 groups here are just shown as lines).

4. Ring-Expansion Reactions Also Involve Migration of Carbon

It doesn’t always have to be a methyl group that moves!  One interesting example is when a carbocation is formed adjacent to a strained ring, such as a cyclobutane. Even though the CH3 could potentially migrate in this case, it’s more favorable to shift one of the alkyl groups in the ring, which leads to ring expansion and the formation of a less strained, five-membered ring.

Here’s an example of an SN1 where migration of a carbon-carbon bond leads to ring expansion.

Having gone through two types of rearrangements in substitution reactions, the next series of posts will cover a different class of reactions: elimination reactions.

 Next Post: Walkthrough of Elimination Reactions (1) 

Some further reading… E1 With Rearrangement

Comments

Comment section

26 thoughts on “Carbocation Rearrangement Reactions (2) – Alkyl Shifts

    1. no, i think it’s correct. When you expand you realize that the C that shifted is satisfied with 4 bonds C-H, C-CH2, C-CH3, C-C+. Whereas the C to which OH later gets attached has only 3 bonds C-CH2, C-CH3 and C-CH. Moreover if the OH had attached to the C you think it should’ve, it wouldn’t have enough free valence e- to another atom.
      I hope I haven’t confused you.

  1. The 1,2-hydride shift occurs because of the hydrogen atom moving to the adjacent carbon with both of the electrons in the bond, according to what my teacher explained. He said that even alkyl groups can shift (though alkyls are much larger than hydrogen) since only the electron cloud is actually shifting.

    Is it then possible to have any other shift that stabilizes the carbocation intermediate in a reaction, e.g. a -OH shift? You mentioned “allowed” rearrangement reactions, so what reactions are disallowed and why?

    Thanks in advance!

  2. Hi James,

    Greetings.
    How would you explain the methyl and ethyl carboxylate groups attached to the same carbon and only the ester group migrates as the 1,2 shift?
    Is the migration via a radical or to the carbanion? I do not think that to the carbocation center. You said there are no 1,2-shifts in radicals.

    Best,
    Helena

    1. It would help to see the specific example you’re thinking of, but it wouldn’t be a radical shift. There are two lone pairs on oxygen that could come down to help stabilize any positive charge buildup on the carbonyl carbon during the shift.

  3. Please explain why the reaction of neopentane yields 1- chloro- 2,2-dimethyl propane when treated with chlorine molecule during free radical mechanism ?
    why not 1,2 methyl shift to give more stable tertiary free radical as compared to less stable primary free radicle????
    please reply.

  4. Hi! Why did another water molecule appear in the first example of a alkyl shift? I thought you already attached one molecule to the carbocation. Shouldn’t the “leftovers” be H+Cl-? I understand the extra hydrogen makes the other water molecule turn into hydronium ion but where did the other water come in the first place? Is this a water characteristic in reactions?

    Thank you!

  5. Hey
    Can you tell about ethyl shifts …

    If there are two groups on carbon one methyl and second ethyl then which group will shift to carbonation

  6. Sir,
    If suppose during ring expansion, in the figure you have uploaded we have an oxygen atom at 4th position.Then which of the following bonds C3-C6 or C3-O4 will be broken for ring expansion?

    1. What you are describing is the ring expansion of an oxetane into a tetrahydrofuran derivative. I would expect the C-C bond should migrate with simultaneous formation of an intermediate C=O pi bond. This would be a variant of the pinacol rearrangement.

  7. Hello sir. If we have cyclohexyl methyl bromide and we react it to water, will it go as per Sn1 reaction and cause a rearrangement( a hydride shift) or will it proceed directly via Sn2 mechanism?? In general, if a compound is 1° but it can go rearrangement, will it proceed via Sn1 or Sn2 mechanism??

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