Remember what happens in the SN1 reaction?
- The leaving group leaves, forming a carbocation
- The nucleophile attacks.
Well, if you try running some SN1 reactions, you’ll see that they’re often accompanied by a little byproduct.
You also get alkenes.
As we know by now, alkenes are the product of elimination reactions.
So what could be happening that’s leading to elimination?
Here’s the most likely possibility:
- The leaving group leaves, forming a carbocation.
- Instead of a nucleophile attacking the carbocation, we remove a hydrogen from the carbon adjacent to the carbocation. This forms an alkene.
Just like the SN1, the rate of this reaction only depends on the concentration of the alkyl halide. So the rate is “unimolecular”.
We call this the E1 mechanism (elimination, unimolecular). Here’s an example.
Here’s some other facts about the E1 reaction
- like the SN1, the big barrier is carbocation stability.
- so the rate is tertiary > secondary >> primary
- Like the SN1, reactions with carbocations can undergo rearrangements if it’s possible to form a more stable carbocation.
- Like all elimination reactions the E1 follows Zaitsev’s rule
- It’s favored by heat (just like all elimination reactions are)
- The E1 can also be promoted through addition of acid to alcohols. An important acid for elimination reactions is H2SO4. It’s important because it leads to formation of the carbocation and the HSO4(-) anion, but HSO4(-) is a poor nucleophile. So instead of a nucleophile adding to the carbocation, water can remove a proton to form the alkene.
Thanks for reading! James
P.S. Sometimes you’ll also see silver salts like AgNO3 used to form carbocations from alkyl halides.
P.P.S. The E1 is often accompanied by the SN1, and vice versa.
P.P.P.S. Reaction guide example