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

by James

in Uncategorized

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

Starting with an alkene, adding strong acid gets us to a carbocation (A–>B). 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. 

Alternatively the carbocation can be generated through loss of a leaving group (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. 

Finally, if the carbocation is generated through loss of a leaving group but there isn’t any reasonably good nucleophile present, elimination may occur to give the alkene. 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. 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. :- )

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{ 1 comment… read it below or add one }

Sheala

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!

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