Ace Your Next Organic Chemistry Exam.

With these Downloadable PDF Study Guides

Our Study Guides

Rearrangements

By James Ashenhurst

The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

Last updated: March 26th, 2019

Was going to include this in my last post but it was getting too big.

Note how the SN1, E1, and alkene addition reactions all pass through a carbocation intermediate. Here’s how they’re related.
carbocat 2

Alkene Addition

Adding strong acid to an alkene gets us to a carbocation (A–>C).

If the counterion to that acid is a decent nucleophile (think Cl-, Br-, or I-) then it will then add to it, giving us the addition product D. Alternatively if we use an acid like H2SO4 (which has a poorly nucleophilic counterion) in the presence of water or another nucleophilic solvent, we can also get addition products.

The pathway A –> C –> D is an example of alkene addition. 

The SN1 Reaction

Alternatively the carbocation can be generated through loss of a leaving group from an alkyl halide (B–> C) . Attack of that carbocation by a nucleophile (e.g. a nucleophilic solvent, again, like H2O or CH3OH) will give us a new product.

The pathway B –> C –> D is what we call the SN1 reaction. 

The E1 Reaction

Finally, if the carbocation is generated through loss of a leaving group from an alkyl halide but there isn’t any reasonably good nucleophile present, elimination may occur to give the alkene. This is particularly favored by heat.

The pathway B –> C –> A is what we call the E1 reaction. 

So there you have three very important reactions all intersecting through a common intermediate.

[Remember that leaving groups (LG) are just nucleophiles (Nu) acting in reverse. That’s why there aren’t double [equilibrium] arrows going between B, C and D]

One complication that’s left out here is carbocation rearrangements, which can arise when a less stable carbocation (often secondary) can rearrange to a more stable carbocation (often tetiary) through a hydride or alkyl shift. [See post: Rearrangement Reactions – Hydride Shifts]

Important to remember that they can occur but I couldn’t think of a way to put them in while keeping the diagram neat and tidy. :- )

Related Posts:

Comments

Comment section

1 thought on “The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate

  1. I just want to say thank you for being able to actually make sense out of all of this. My professor has had me so confused and I actually understand you!!!! Go be a professor seriously, you make sense!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.