Introduction to Rearrangement Reactions

Introduction to Rearrangement Reactions

• Reactions that involve a carbocation intermediate may be accompanied by rearrangements where a pair of electrons from a C-H or C-C bond migrates toward the carbocation, resulting in breakage and formation of a C-H or C-C bond, and formation of a new carbocation.
• The new carbocation (generally more stable) then undergoes a second reaction to give the final product.
• Rearrangements can accompany addition, substitution, and elimination reactions if they progress through a carbocation intermediate.

Rearrangement Reactions

Rearrangement reactions can accompany many of the reactions we’ve previously covered such as substitution, addition, and elimination reactions.

• Substitution (SN1) (See post: Alkyl Halides From Alcohols) 
• Elimination (E1) (See post: Elimination (E1) With Rearrangement)
• Alkene addition reactions  (See post: Rearrangements in Alkene Addition Reactions)

The point of this article is not to understand why just yet, but to be able to see from the diagrams what bonds are broken and formed.

You need to understand how to read line diagrams.  But other than that no further skills are required. The point here is to be able to follow the plot – to see what is happening.

Subsequent posts will go into more detail as to why things happen, but it takes time to build up that knowledge.

Nucleophilic Substitution Accompanied By Rearrangement

In fact, if you don’t look closely, sometimes you can miss the fact that a rearrangement reaction has occurred. Let’s look at a substitution reaction first.

On the top is a “typical” substitution reaction: we’re taking an alkyl halide and adding water. The C-Br bond is broken and a C-OH bond is formed. If you look at the table on the right you’ll see this follows the typical pattern of substitution reactions.

However if we change one thing about this alkyl halide – move the bromine to C-3 instead of C-2 – now when we run this reaction we see a different product emerge.

It is also a substitution reaction (we’re replacing Br with OH) but it’s on a different carbon. That’s because if you look closely, you can see there are actually 3 bonds broken and 3 bonds formed. The C2-H bond broke and the C3-H bond formed.

Very strange!

This represents a rearrangement reaction – when you see a group “move” from one carbon to another.

Alkene Addition Accompanied By Rearrangement

Let’s look at another example, but involving an addition reaction (the addition of HCl to alkenes).

Here we have an addition reaction. On top, nothing special – as with all additions, we break a C-C double bond (π bond )and form two new single bonds to the adjoining carbons (H and Cl). But look at the bottom example. If we use that alkene instead, we find that the Cl ends up on C3, not C-2.

Again, examining the bonds broken/formed, we see that there’s an extra pair of events: the C3-H bond was broken and the C-2H bond was formed. In other words, the hydrogen “migrated” from one carbon to another. Weird!

Elimination (E1) Accompanied By Rearrangement

Finally, let’s look at an elimination reaction. If you take an alcohol like the one below and add an acid (like H₂SO₄, pictured) and help the reaction along with some heat, you break the C1-OH and C2-H bonds, and form a new double bond between C1-C2. This is, in other words, a typical elimination reaction.

But if you take a slightly modified alcohol like the bottom example (with an extra methyl group on C1) and try the same reaction, something strange happens again. Analyzing the bonds broken and formed,  there’s an “extra” bond being broken and an “extra” bond being formed here. If you look closely you can see that one of the methyl groups on C1 (we’ll call it C8) moved over to C2.

So what can we conclude about rearrangement reactions?

• They can accompany many of the reactions we’re already familiar with, such as substitution, addition, and elimination reactions.
• They involve the “movement” of an atom (H in the top two examples, C in the third) from one carbon to another.

What other insight might we glean from these examples? Here’s two questions.

• look up, if you don’t know already, what “primary, secondary and tertiary alcohols and alkyl halides are (See post: Primary, Secondary, Tertiary and Quaternary in Organic Chemistry)
• In the substitution reactions and the elimination reactions, classify every alcohol (or alkyl halide) according to whether it is primary, secondary or tertiary. Notice any difference between the “normal” cases and the “rearrangement” cases?

For more detail on rearrangement reactions, start here:

Rearrangement Reactions (1) – Hydride Shifts

Notes