Whether an alkyl halide will undergo an S N1 or an S N2 reaction depends upon a number of factors. Some of the more common factors include the natures of the carbon skeleton, the solvent, the leaving group, and the nature of the nucleophile.
Nature of the carbon skeleton
Only those molecules that form extremely stable cations undergo S N1 mechanisms. Normally, only compounds that yield 3° (tertiary) carbonications (or resonance‐stabilized carbocations) undergo S N1 mechanisms rather than S N2 mechanisms. Carbocations of tertiary alkyl halides not only exhibit stability due to the inductive effect, but the original molecules exhibit steric hindrance of the rear lobe of the bonding orbital, which inhibits S N2 mechanisms from occurring. Primary alkyl halides, which have little inductive stability of their cations and exhibit no steric hindrance of the rear lobe of the bonding orbital, generally undergo S N2 mechanisms. Figure 1 illustrates the tendencies of alkyl halides toward the two types of substitution mechanisms.
Polar protic solvents such as water favor S N1 reactions, which produce both a cation and an anion during reaction. These solvents are capable of stabilizing the charges on the ions formed during solvation. Because S N2 reactions occur via a concerted mechanism (a mechanism which takes place in one step, with bonds breaking and forming at the same time) and no ions form, polar protic solvents would have little effect upon them. Solvents with low dielectric constants tend not to stabilize ions and thus favor S N2 reactions. Conversely, solvents of high dielectric constants stabilize ions, favoring S N1 reactions.
In general, good leaving groups are those capable of forming stable ions or molecules upon displacement from the original molecule. Conversely, poor leaving groups form ions of poor to moderate stability. Strong bases, such as OH −, NH 2 −, and RO −, make poor leaving groups. Water, which is less basic than a hydroxide ion, is a better leaving group. Poor bases usually make good leaving groups. A poor base is an ion or group in which the electrons are tightly bound to the molecule due to high electronegativity or resonance. Some good leaving groups are the sulfate ion and the p‐toluenesulfonate (tosylate ion).
The following list ranks atoms and molecules in order of their stability as leaving groups, from most to least stable.