Deciding SN1/SN2/E1/E2 (3) – The Solvent

in Organic Chemistry 1, Organic Reactions

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.

After having examined the substrate and the base/nucleophile in an SN1/SN2/E1/E2 reaction, this post is about the next question to ask:

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.

Beatlemania!

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.
That’s especially the case in example #2, where we really can’t make the call as to whether it’s SN2 or E2 until we’ve looked at the solvent. The fact that we are using a polar protic solvent (EtOH) is the crucial piece of information that clinches the reaction as E2.
In the last post in this series we’ll look at the impact of temperature on these reactions.
[Edit Dec 13]: What’s the weakness of the Quick N’ Dirty approach? Well, it doesn’t really capture the true situation in these reactions, that is, that they don’t always give one product exclusively. So keep in mind that when the Quick N’ Dirty approach says that E2 is the major product, you may also have a small amount of competing SN2 occurring as well, or if SN1 is favored, you might have small amounts of other reaction products also.  The key points of this exercise is to 1) remember the major factors that affect each type of reaction, and 2) once you know the type of reaction, be able to apply it.

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Tagged as: e1, e2, elimination, nucleophilicity, polar aprotic, polar protic, sn1, SN2, solvents, substitution

{ 2 comments… read them below or add one }

azmanam December 5, 2012 at 9:48 am

2 comments :)

-I think it may be important to say that in the truly ‘toss up’ cases (like #2 above), if we ran the reaction and analyzed the results by, say, GC/MS (something we do in our ugrad OChem labs… tho not for this reaction), we’d probably see BOTH E2 and SN2 products… but with protic solvents the E2 is the major organic product, and with aprotic solvents the SN2 is the major organic product. It’s not as if one solvent gives 100% of elimination, and another solvent gives 100% of substitution.

-I also have my students analyze the reactions in this way. What does the nucleophile tell us? What does the electrophile tell us? What does the solvent tell us? I use these to generate ‘evidence for’ a mechanism, or ‘evidence against’ a mechanism.

That’s my comment. I would hesitate to use a phrase like ‘rule out’ since psychologically that’s exclusionary thinking. I could conjure up an example where the evidence from the nucleophile ‘rules out’ an SN2 mechanism… but based on the other evidence SN2 is exactly the pathway (off the top of my head, KOtBu + MeBr). But if I tell myself a Big Bulky Base ‘rules out’ SN2, I exclude that possibility mentally. ‘Evidence against’ mentally tells me this is not my first choice… but it’s not impossible.

(Also, I tell my students solvent can sometimes lean us in one direction or another, but solvent should never be used as ‘evidence against’ a particular mechanism… especially because our SN2 lab uses ethanol as the solvent!)

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Hyunsook December 12, 2012 at 12:57 am

Please post the next installment soon!!

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