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Rearrangements

By James Ashenhurst

Rearrangement Reactions (2) – Alkyl Shifts

Last updated: March 26th, 2019

Rearrangement Reactions: Alkyl Shifts

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, such as in the case below.

If a hydride shift occurred here, we’d be going to a less stable (primary) carbocation!

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.

The Mechanism Of Alkyl Migration Reactions

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).

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

Ring-Expansion Reactions

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 an alkyl shift 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

Related Posts:

00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous

Comments

Comment section

24 thoughts on “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.

      Reply

  4. 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!

    Reply

  5. 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

    Reply

    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.

      Reply

  6. 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.

    Reply

  11. 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!

    Reply

  12. 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

    Reply

  13. 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?

    Reply

    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.

      Reply

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