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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)ubstitution (N)ucleophilic (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. (In SN2, you simply boot the bum off the bench and repose.)
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 it's Nucleophile.
Once the leaving group has shuffled off, the remaining carbocation's geometry changes. Where the target molecule had been tetrahedral with 4 sp3 bonds, the carbocation is trigonal planar with three sp2 orbitals and is flat. To either side, empty p orbitals invite attack.
Depending on the composition of the three entities surrounding the carbon at the center of the carbocation or substrate, the result of the reaction may be different for attack at the front from attack at the back. If all 3 are different, then the result of the reaction will be a racemic mixture of left and right handed enantiomers, which may not be equally useful.
Regardless of which side is attacked, a nucleophile clamps on creates an even larger carbocation. This promptly loses a proton to a base, usually the leaving group.
Halogens, particularly Bromine and Iodine, make great leaving groups. Their bond to carbon can be broken more easily. Chlorine is less desirable, and Fluorine, like unwanted in laws, almost never leaves.
Water is the simplest Nucleophile.
SN1 reactions work more favorably in a polar solvent. That carbocation will form more readily if the solvent can help the leaving group leave.
Water is also a polar solvent, as are alcohols.
If one visualizes a fat person on a unicycle, the situation is clear. When the unicycle leaves, the entity remaining becomes more stable, if bruised. The more encumbered the substrate, the more easily enough energy can be found to evict the leaving group. Smaller, tighter molecules are much less likely to lose the leaving group.
A tertiary halide is a much more desirable candidate for SN1 than a secondary halide, which is itself more likely a candidate than a primary halide.
[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.]