# 7.7: SN1 or SN2? Predicting the Mechanism


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

## Contributors

This page titled 7.7: SN1 or SN2? Predicting the Mechanism is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Tim Soderberg via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.