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