A lot of students I talk to have questions about solvents, so I’ve decided to put together a reference post on them.
Solvents can cause considerable confusion in reactions, because they’re listed along with the reagents of a reaction but often don’t actually participate in the reaction itself. And to be honest, a lot of instructors (myself included) are less than consistent about when to include solvents and when not to. So the whole exercise can come across as somewhat arbitrary: when do you know when to include the solvent?
Let’s back up. What’s a solvent, anyway?
A solvent is a liquid that serves as the medium for a reaction. It can serve two major purposes:
- (Non-participatory) to dissolve the reactants. Remember “like dissolves like” ? Polar solvents are best for dissolving polar reactants (such as ions); nonpolar solvents are best for dissolving nonpolar reactants (such as hydrocarbons).
- Participatory: as a source of acid (proton), base (removing protons), or as a nucleophile (donating a lone pair of electrons). The only class of solvents for which this is something you generally need to worry about are polar protic solvents (see below).
OK. So what does “polar” and “non-polar” mean?
- Polar solvents have large dipole moments (aka “partial charges”); they contain bonds between atoms with very different electronegativities, such as oxygen and hydrogen.
- Non polar solvents contain bonds between atoms with similar electronegativities, such as carbon and hydrogen (think hydrocarbons, such as gasoline). Bonds between atoms with similar electronegativities will lack partial charges; it’s this absence of charge which makes these molecules “non-polar”.
There are two common ways of measuring this polarity. One is through measuring a constant called “dielectric constant” or permitivity. The greater the dielectric constant, the greater the polarity (water = high, gasoline = low). A second comes from directly measuring the dipole moment.
There’s a final distinction to be made and this causes confusion. Some solvents are called “protic” and some are called “aprotic”. What makes a solvent a “protic” solvent, anyway?
- Protic solvents have O-H or N-H bonds. Why is this important? Because protic solvents can participate in hydrogen bonding, which is a powerful intermolecular force. Additionally, these O-H or N-H bonds can serve as a source of protons (H+).
- Aprotic solvents may have hydrogens on them somewhere, but they lack O-H or N-H bonds, and therefore cannot hydrogen bond with themselves.
These solvents have low dielectric constants (<5) and are not good solvents for charged species such as anions. However diethyl ether (Et2O) is a common solvent for Grignard reactions; its lone pairs are Lewis basic and can help to solvate the Mg cation.
“Borderline” Polar aprotic solvents
These solvents have moderately higher dielectric constants than the nonpolar solvents (between 5 and 20). Since they have intermediate polarity they are good “general purpose” solvents for a wide range of reactions. They are “aprotic” because they lack O-H or N-H bonds. For our purposes they don’t participate in reactions: they serve only as the medium.
Polar aprotic solvents
These solvents all have large dielectric constants (>20) and large dipole moments, but they do not participate in hydrogen bonding (no O-H or N-H bonds). Their high polarity allows them to dissolve charged species such as various anions used as nucleophiles (e.g. CN(-), HO(-), etc.). The lack of hydrogen bonding in the solvent means that these nucleophiles are relatively “free” in solution, making them more reactive. For our purposes these solvents do not participate in the reaction.
Polar protic solvents
Polar protic solvents tend to have high dielectric constants and high dipole moments. Furthermore, since they possess O-H or N-H bonds, they can also participate in hydrogen bonding. These solvents can also serve as acids (sources of protons) and weak nucleophiles (forming bonds with strong electrophiles).
They are most commonly used as the solvent for their conjugate bases. (e.g. H2O is used as the solvent for HO(-); EtOH is used as the solvent for EtO(-). )
These types of solvents are by far the most likely to participate in reactions. There are many examples (too many to list) where a polar protic solvent such as water, methanol, or ethanol can serve as the nucleophile in a reaction, often when a strong electrophile (such as an acid) is present. So if you see this type of solvent, be on the lookout.
I’m sure I missed something or something wasn’t clear. If you’d like something expanded on, please leave a comment!
Source for data: Wikipedia