SN1

SN1 is an acronym for an Nucleophilic substitution reaction where the rate of the reaction is dependent on the concentration of one of the reactants:

(S)substitution (N)Nucleophilic (1)reactant determines rate.

Counter intuitively, SN1 reactions proceed in two steps, the first of which is the rate determining step and the second of which occurs almost instantaneously. A good analogy is sitting down on a park bench where all the benches are occupied by sleeping vagrants. You wait for one to wake up and shamble off - then you sit down.

The more vagrants you have, the faster one will wake up and vacate a bench. In this scenario we call the vagrant a leaving group. The leaving group is an anion. When it leaves, the park bench becomes a carbocation, and is ready for you - it's Nucleophile.

Once the leaving group has shuffled off, the remaining carbocation's geometry changes. Where it had been tetrahedral (sp3), it becomes trigonal planar (sp2) and is flat. To either side, empty p orbitals invite attack.

[Note: SN1 SN2, E1 and E2 are important and retaining the information has been difficult for me. I am writing this out for my benefit and yours using a variety of references and my own POV.]

to be continued.