Rearrangements

by James

Today’s subject causes more confusion and difficulty than almost any other subject in Org 1.

Surprisingly, however – this reaction can be represented by just one arrow. That’s it. So if you master drawing this one arrow, you master the reaction. 

I’m talking here about rearrangements of carbocations.

Remember the order of carbocation stability: primary << secondary < tertiary ? Carbocations are much “happier” when they are attached to extra alkyl groups.

Imagine if a primary carbocation (really unstable) could “miraculously” transform itself into a more stable carbocation, such as a secondary or tertiary carbocation. That would be pretty strongly favored, right?

Here’s the thing: if we generate a primary carbocation adjacent to a secondary carbon, this “miracle” can occur: by breaking one C-H bond and forming one C-H bond. 

Well, look at this. Here comes the arrow!


The pair of electrons goes from the C2-H bond to the C-1 carbon. So we form C1-H and we break C-2 H. And look at the product we’ve made here. A secondary carbocation! More stable! And we’ve made this by moving a hydrogen and its electrons.

How can this happen? Because carbocations have empty p orbitals (they’re sp2 hybridized). And you can kind of imagine this empty p orbital “accepting” the pair of electrons from the C-H bond, while the initial C-H bond breaks.

It doesn’t just happen for primary carbocations going to secondary carbocations though.  It can happen any time there is the potential to form a more stable carbocation, a rearrangement can occur. 

So we can have

  1. primary carbocation -> secondary carbocation
  2. primary carbocation -> tertiary carbocation
  3. secondary carbocation -> tertiary carbocation

 

Like water flowing downhill, notice that we’re always going from less stable to more stable.

All of these transformations can occur through the breaking and forming of a C-H bond. These are called “hydride shifts” or “1,2-shifts”, or “Wagner-Meerwein shifts”, if you want to get really technical.

Tomorrow: let’s dig in a little more and talk about when carbons can move!

Thanks for reading! James
P.S. Relevant post: Introduction to Rearrangment Reactions