The Quick N’ Dirty Guide To SN1/SN2/E1/E2 Reactions, Part 3
Let’s continue with our Quick N Dirty guide to SN1/SN2/E1/E2 – a quick walkthrough of thinking through this reaction decision.
What’s The Solvent?
Recall that there are two important types of solvents to consider: polar protic solvents and polar aprotic solvents.
Let’s do a little review, looking at polar protic solvents first.
Polar protic solvents are capable of hydrogen bonding. Recall that hydrogen bonding occurs where we have a highly electronegative atom such as O or N directly bonded to hydrogen:
Quick N’ Dirty Tip: Solvents with OH or NH groups are polar protic solvents
Hydrogen bonding is directly responsible for the high boiling points of solvents such as water and ethanol; the partial positive charges on hydrogen are attracted to the partial negative charges on the electronegative atoms. This is also why water is such an excellent solvent for charged species such as halide ions; hydrogen bonding solvents surround negatively charged ions like a jacket.
This “jacket” of solvent molecules – much like a protective crowd of bobbies – means that these anions do not have the freedom of action they would normally have if they weren’t surrounded by clingy hordes of solvent molecules. That is, they’re less nucleophilic! The propensity to form hydrogen bonds is highest for small, highly electronegative ions such as fluorine and decreases as ions get larger (and the charge is more diffuse). This means that in polar protic solvents, the nucleophilicity of halide anions increases as we go down the periodic table.
Now let’s talk about polar aprotic solvents; polar aprotic solvents are polar enough to dissolve charged species (such as halide ions) but do not donate hydrogen bonds. This means that in solvents such as DMSO, DMF, acetone, or acetonitrile, nucleophilicity correlates much better with basicity – and therefore decreases as we go down the periodic table.
So what’s the bottom line when it comes to SN1/SN2/E1/E2?
The bottom line is this:
- Quick N’ Dirty Rule #5: Polar protic solvents tend to favor elimination (E2) over substitution (SN2). Polar aprotic solvents tend to favor substitution (SN2) relative to elimination (E2)
Let’s go back to the examples we were looking at in the first two posts. Practically, you’ll only need to consider the solvent in an SN1/SN2/E1/E2 decision when you’ve already analyzed the substrate and the nucleophile/base. This is usually the case when you have a secondary alkyl halide with a strongly basic nucleophile such as NaOCH3 or NaOH.