Three And A Half Steps To Any Substitution or Elimination Reaction, Step 3: The Nature of The Solvent by @azmanam
Step 3: What is the nature of the solvent?
You might think the solvent shouldn’t have much influence on a reaction mechanism. Its whole job is to just dissolve the reagents, right? Well, yes, but solvents can also modulate the electron density within a reagent. And by now, we all know it’s all about the electron density. Some solvents have the ability to diffuse electron density over a larger volume, and some solvents can concentrate electron density. Can you see where we’re going here? More electron density will make nucleophiles and bases stronger than they otherwise would have been, and diffused electron density will make nucleophiles and bases weaker than they otherwise would have been.
There are three main classes of solvents for organic reactions: nonpolar solvents, polar protic solvents, and polar aprotic solvents. James already has a nice roundup of these classes of solvents, and you should read his post before reading on. Since most SN and E reactions utilize polar reagents, we typically don’t see nonpolar solvents for these reactions very often. So let’s focus on the polar solvents.
Polar solvents have some permanent net dipole. What separates a polar protic solvent from a polar aprotic solvent is the presence or absence of a hydrogen atom capable of hydrogen bonding; some hydrogen atom attached to an electronegative element (typically oxygen) which can engage in a hydrogen bond. Polar protic solvents have this hydrogen atom, and polar aprotic solvents lack this hydrogen atom.
Polar protic solvents are typically alcohols, water, or carboxylic acids. Polar aprotic solvents include ethers and carbonyl-containing molecules such as ketones (usually acetone), amides (usually dimethylformamide), and a few specific solvents like acetonitrile and dimethylsulfoxide.
How these solvents interact with nucleophiles and electrophiles (specifically carbocations) will influence the amount of electron density in a molecule, and this can sometimes have an impact on the mechanism.
Polar protic solvents have a hydrogen atom which can hydrogen bond with the lone pair in a nucleophile. That lone pair is now not as concentrated locally on the nucleophile. Now that electron density is spread out over a slightly larger volume as it shares some electron density with the hydrogen atom of the solvent. This makes the nucleophile slightly weaker than it otherwise would be.
At the same time that the polar protic solvent is stabilizing the nucleophile, it also has the ability to stabilize any carbocations formed during the reaction. The lone pair of electrons on the solvent can donate electron density to the carbocation, making the carbocation more stable. A weaker nucleophile and a stabilized carbocation mean that polar protic solvents are evidence for SN1 and E1 reactions.
Polar aprotic solvents, by contrast, can’t hydrogen bond with nucleophiles. For ionic nucleophiles, though, polar aprotic solvents can stabilize the counter cation to the nucleophile. So with no nucleophile stabilization other than some dipole-dipole interactions, the electron density on the nucleophile is not diffused to a great extent like the protic solvents, and polar aprotic solvents tend to be evidence for SN2 reactions.
A couple of notes about the evidence we gain from solvents. I don’t like to say that solvents are evidence against any mechanism. It is often possible to carry out, for instance, an SN2 reaction in a polar protic solvent, and other examples of ‘mismatched’ solvents can be found. Use solvents more to corroborate evidence you already have, or as a tie breaker if needed. Did you notice the E2 mechanism wasn’t listed above? Remember that nucleophilicity and basicity are closely related, but they are different concepts. It turns out that polar protic solvents diminish nucleophilicity a lot, but diminish basicity to a lesser extent. This information can be useful when trying to decide between an SN2 and E2 mechanism with a strong nuc/strong base. In general, polar protic solvents favor elimination, while polar aprotic solvents tend to favor substitution.
|Solvent classification||Evidence for||Evidence against|
|Polar protic||SN1, E1, E2||—|
One final note that fits best here, even though it’s not a solvent. In general, all else being equal, elevated temperatures tend to favor elimination reactions. The extra energy from the heat gives the reaction just enough boost to form the double bond product. So if all our evidence contradicts, or if the evidence points in two clear directions, the temperature (if given) can help us decide which will be the major organic product. Although a mixture of products will likely form if everything else really is equal.
That’s it! We now have all the evidence we need to determine the mechanism for our substitution/elimination reactions. Next time, we’ll see how to pull it all together and start predicting some products!