First of all, it is important to understand that the \(S_N1\) and \(S_N2\) mechanism models are just that: models. While many nucleophilic substitution reactions can be described as proceeding through 'pure' \(S_N1\) or \(S_N2\) pathways, other reactions - in particular some important biochemical reactions we'll see later - lie somewhere in the continuum between the \(S_N1\) and the \(S_N2\) model (more on this later). With that being said, here are some guidelines to help you predict whether a reaction is likely to have more of an \(S_N1\) or \(S_N2\) character.
First, look at the electrophile: as stated above, an \(S_N1\) reaction requires that a relatively stable carbocation intermediate be able to form. An \(S_N2\) reaction requires a relatively unhindered electrophilic center. Therefore, methyl and primary carbon electrophiles will react by the \(S_N2\) pathway, and tertiary carbon electrophiles will react by the \(S_N1\) pathway.
Secondary carbon electrophiles, or primary carbon electrophiles adjacent to a potential carbocation-stabilizing group (double bond or heteroatom) can react by either or both pathways. The reasoning here is that these electrophiles are unhindered (favoring \(S_N2\)), but can also form stabilized carbocation intermediates (favoring \(S_N1\))
Next, look at the nucleophile. More powerful nucleophiles, particularly anionic nucleophiles such as hydroxides, alkoxides or thiolates, favor an \(S_N2\) pathway: picture the powerful nucleophile 'pushing' the leaving group off the electrophile. Weaker, uncharged nucleophiles like water, alcohols, and amines, favor the \(S_N1\) pathway: they are not nucleophilic enough to displace the leaving group, but will readily attack a carbocation intermediate.
Finally look at the solvent in the reaction. As a general rule, water and other protic solvents (for example methanol or ethanol) favor \(S_N1\) pathways, due to the ability of the solvents to stabilize carbocation intermediates, combined with their tendency to weaken the nucleophile by enclosing it in a 'solvent cage'. In laboratory reactions, the presence of a polar aprotic sovent such as acetone or dimethylformamide points to the probability of an \(S_N2\) reaction.
factors favoring the two pathways
Factors favoring the \(S_N1\) pathway:
- hindered electrophile
- potential for a tertiary, secondary, or resonance-stabilized carbocation intermediate
- uncharged nucleophile
- protic solvent such as water
Factors favoring the \(S_N2\) pathway:
- Unhindered (methyl or primary) electrophile
- powerful, anionic nucleophile
- polar aprotic solvent
Video tutorial: nucleophilic substitution reactions