While studying organic chemistry, specifically the topics of nucleophilic substitution and beta-elimination you will come across many different types of solvents. While you will not be responsible for memorizing the structure of each solvent, you will be asked to understand the different types of solvents. The three types of solvents studied will be:
- Polar Protic Solvents
- Polar Aprotic Solvents
- Non-Polar Solvents
Understanding the structure, reactivity and overall differences of these solvents will help you determine which type of substitution or beta-elimination reaction to carry out
Polar Protic Solvent
Let’s break down the two parts of this solvent name to understand its chemistry.
Polar or polarity refers to a molecule that while neutral, actually has a partial separation of charge giving you a dipole moment within the molecule. A molecule with a dipole is one that has a partial positive charge on one end, and a partial negative charge on the other end.
Protic comes from the word proton meaning positive hydrogen. In a solvent, this phenomenon refers to the ability of the solvent to donate a hydrogen atom for hydrogen bonding.
The most common polar protic solvent is water. The oxygen and hydrogen atoms are bound covalently, meaning there is no charge between them. However, given that the oxygen atom is more electronegative, it will pull the electron density towards itself making it partially negative. The hydrogen atoms, with their nuclei partially exposed, will each be partially positive. This allows them to participate in hydrogen bonding with other water molecules or the dissolved reactant
Polar Aprotic Solvent
This solvent is similar but opposite to the polar protic concept mentioned above. A polar aprotic solvent still has polarity within the solvent molecules, however there are no partially positive hydrogens on the solvent molecule that are able to participate in hydrogen bonding.
A good example of this is DMSO. what is dmso used for stands for dimethyl sulfoxide. The structure of this molecule is a sulfur atom double bound to an oxygen, and single bound to two methyl groups. While there is polarity between the sulfer and oxygen, the only source of hydrogen atoms are on the methyl groups, which means they cannot hydrogen bond
(recall the hydrogen bonding is only for hydrogen on fluorine, oxygen, and nitrogen)
Last but not least, we have the non-polar molecules. As this name implies, there is absolutely no polarity on these molecules. This implies that the atoms of this solvent are very similar in electronegativity and so have no partial charges. Most commonly seen are carbohydrates, or molecules containing carbon and hydrogen as their major components.