For nucleophilic substitution, the pattern of bonds that form and break is pretty straightforward. You break C-(leaving group) and you form C-(nucleophile). A straight swap. But every once in awhile you might see a “weird” substitution reaction. If you look closely at the pattern of bonds formed and bonds broken in the second reaction below, there’s an extra set!
In other words it’s a substitution reaction where the hydrogen has moved. We call these movements “rearrangements”, for reasons that will become clear shortly.
The big question is, what’s going on? How did this happen?
As it turns out, reactions that go through carbocations can sometimes undergo rearrangements. And looking back at substitution reactions, recall that theSN1 reaction goes through a carbocation intermediate. In this post we’ll go through when you’ll expect to see a rearrangement reaction.
Let’s think back to carbocations. They’re carbon atoms with six electrons bearing a positive charge. In other words, they’re electron deficient – 2 electrons short of a full octet. So it would make sense that carbocations become more stable as you increase the number of electron donating groups attached to them. Alkyl groups are a perfect example. That’s why carbocation stability increases as you go from primary to secondary to tertiary.
So what does this have to do with rearrangements? As it turns out, if a situation exists where an unstable carbocation can be transformed into a more stable carbocation, a rearrangement is possible.
One rearrangement pathway where an unstable carbocation can be transformed into a more stable carbocation is called a hydride shift. Look at the diagram below.
In this reaction we have a secondary carbocation on the left hand side. In this rearrangement reaction, the pair of electrons in the C-H bond is transferred to the empty p orbital on the carbocation. In the transition state of this reaction, there’s a partial C-H bond on C3 and a partial C-H bond on C2. The transition state here is kind of like that split second in a relay race where one sprinter is passing the baton to another sprinter and they both have their hands on it. Then, as the C2-H bond shortens and the C3-H bond weakens, we end up with a carbocation on C3 (a tertiary carbocation) in the product which is more stable.
Note that we only need one arrow to show this occurring!
Here are some examples of “allowed” rearrangement reactions. Notice how we’re always going from a less substituted carbocation to a more substituted carbocation. One exception is at the very bottom; the rearrangement is favorable because the new carbocation is resonance stabilized.
Now we’re ready to show how the rearrangement reaction occurs with the SN1. Recall that the first step in the SN1 is that the leaving group leaves to give a carbocation. In the case below, the carbocation that is formed is secondary, and there’s a tertiary carbon next door. Therefore, a rearrangement can occur to give the more stable tertiary carbocation, which is then attacked by the nucleophile (water in this case). Finally, the water is deprotonated to give the neutral alcohol. So this is an example of an SN1 reaction with rearrangement.
I’ve given some more examples of SN1 reactions with rearrangements below. See if you can draw the mechanisms! In the next post we’ll talk about a slightly different rearrangement pathway with substitution reactions.
can we do hydride and alkyl shift multiple times in order to get a stable carbocation or is it allowed only once?
What happens if a chiral centre is generated?
What is R/S configuration of rearranged carbocation(inversion or retention or both)?
Might be a dumb question, but are multiple hydride and or methyl shifts allowed? If so what is the justification/if not why as well?
Multiple shifts are certainly possible, and they could happen, but generally will only happen if each shift generates a successively more stable carbocation.
For example, you probably wouldn’t see a shift if it involved turning a tertiary carbocation into a secondary carbocation.
Although hydride shifts seem like I good method to offer shortcuts in synthesis, I’ve often wondered why you don’t see them that often and also why I learned about them relatively late in my chemical education. I think it’s because they often occur from carbocations which only form when they are stabilized. And hydride shifts occur to increase the stability of the carbocations, so there’s not an awful lot of scope for them. For aliphatic compounds this only really permits the carbocation to change from secondary to tertiary.
That’s a really nice page, thanks!
why does the reaction between 2- methylbromopropane and NaOH form two products??
Is the hydride//methyl shift a characteristic of only SN1 reactions or could this also happen in E1?
It can happen anytime a carbocation is generated. That can happen via loss of a leaving group as the beginning step of E1/SN1 or it can also happen during addition of acids to alkenes (if you’ve covered that).
Why double bond of cyclohexene won’t break on addition of Br₂, UV light and heat? The product I expected was bromocyclohexane but it was 3-bromocyclohex-1-ene!
Out of methyl and hydride shift which will be preferred?
Thanks, thanks, thanks a million!!!!!
greetings james, is it general for 1,3 Hydride shifts to occur?