What is a Polar Aprotic Solvent? 

Solvents that are chemically polar in nature and are not capable of hydrogen bonding (implying that a hydrogen atom directly linked with an electronegative atom is not found) are referred to as polar aprotic solvents. Some commonly used polar aprotic solvents are acetone, DMF, acetonitrile, DMSO, etc.  

Types of Solvents 

Solvents are classified into polar and non-polar solvents.  

• Solvents that possess one or more electronegative atoms are identified as polar solvents and these have a high dielectric constant. Examples of polar solvents are acetone, DMSO (dimethyl sulfoxide), ammonia, methanol, water, etc.  
• Solvents that do not possess any electronegative atoms but rather contain atoms with comparable electronegativities (like hydrocarbons), and have very little dielectric constant value are identified as non-polar solvents. Common examples are benzene, toluene, chloroform, pentane, hexane, etc. 

Types of Polar Solvents

We have already established the fact that polar solvents have presence of partial charges which impart a dipole moment and as a result of which possess a large dielectric constant value.

These polar solvents are further classified into two major subtypes, namely polar protic and polar aprotic solvents.

Polar Protic Solvents

Protic refers to proton which is hydrogen and can be related to hydrogen bonding. In protic solvents, hydrogen bonding is possible because there is the presence of an electronegative atom with which hydrogen is linked with. There are strong intermolecular forces acting between these molecules because of the intermolecular hydrogen bonding between them.

Protic refers to proton which is hydrogen and can be related with hydrogen bonding. In protic solvents, hydrogen bonding is possible because there is presence of electronegative atom to which hydrogen is linked with. There are strong intermolecular forces acting between these molecules because of the intermolecular hydrogen bonding between them. Protic solvents typically have the presence of $\text{O}-\text{H}$ or $\text{N}-\text{H}$bonds which act as a source of proton (${\text{H}}^{+}$ ) which implies that polar protic solvents behave like Bronsted Lowry acids as they are capable of donating a proton.

Note- In nucleophilic substitution reactions, polar protic solvents tend to decrease the reactivity of the nucleophile (this can be avoided by using polar aprotic solvents).

Examples are ammonia, water, acetic acid and methanol.

We can see here that all these molecules have a hydrogen atom which is attached to another electronegative atom which is going to engage in intermolecular hydrogen bonding.

Polar aprotic solvents 

‘Aprotic’ refers to a lack of proton (implying no hydrogen atom is present attached to an electronegative atom), but however, these solvents have a high to intermediate values of the dielectric constant and possess a dipole moment. These solvents are not involved in hydrogen bonding.  

Due to their very high polarity, they facilitate dissolution of charged species in them (which are anions and will hence act as nucleophiles, like cyanide ion, hydroxide ion, etc.) 

This is due to the fact that they are not involved in hydrogen bonding and will therefore be more readily available for a reaction. They are reactive in solution and will dissolve charged species.  

These solvents when used as a medium for certain reactions may or may not participate in reactions. But mostly, we can say that they do not participate in reactions unlike protic solvents because those are believed to participate in reactions. 

A reaction wherein polar aprotic solvents are used as a medium would be ${\text{S}}_{\text{N}}\text{2}$ reactions.

Examples for aprotic solvents are acetone, acetonitrile, DMF (N,N-Dimethylformamide), DMSO (Dimethyl sulfoxide), DCM, (Dichloromethane) , THF (Tetrahydrofuran) etc.

Remember that all these are aprotic solvents because they do not possess any dissociable hydrogen atoms in the molecule.

Ethyl acetate, dicloromethane and tetrahydrofuran do not have really high dielectric constants but rather have intermediate values of dielectric contants.

(NOTE-  If the dielectric constant is less than 5, the molecule is considered non-polar).

Since these are polar and lack any $\text{O}-\text{H}$ or $\text{N}-\text{H}$bonds, they are classified under polar aprotic solvents.

Both protic and aprotic solvents are capable of dissolving salts in them.

While, the protic solvents are favored for ${\text{S}}_{\text{N}}1$and $\text{E1}$reactions, aprotic solvents are favored for ${\text{S}}_{\text{N}}\text{2}$ and $\text{E2}$ reactions.

Effect on Solvent on Reactions

In ${\text{S}}_{\text{N}}\text{2}$ and ${\text{E}}_{\text{2}}$reactions, the reactant species are ions and hence, use of aprotic solvents is more favored. In ${\text{S}}_{\text{N}}\text{2}$reactions, usage of protic solvents will weaken the nucleophile and render them less reactive. When aprotic solvents are used, the solvent does not make the nucleophile less nucleophilic, rather, the reaction proceeds at a faster rate.

Ions are often stabilized upon usage of protic solvents. In ${\text{S}}_{\text{N}}1$ reactions, the reactant species are not ions but ions are formed as products which are stabilized by protic solvents.

Polar Aprotic Solvents and S_N2 Reactions

We have now established the fact that ${\text{S}}_{\text{N}}\text{2}$reactions are favored with the use of polar aprotic solvents because they are adequately polar to facilitate dissolution of the substrate and the nucleophile but they do so without participating in hydrogen bonding with the nucleophile.

The mechanism of action is that the charge separation involved in the transition state of a ${\text{S}}_{\text{N}}\text{2}$reaction gets stabilized by the solvent. The transition state is an intermediate and not a carbocation like in the case of ${\text{S}}_{\text{N}}1$. The aprotic solvent molecules have a partial negative charge on them owing to the fact that they are electron rich species. This allows the solvent molecules to solvate any positively charged species very easily but they do not interact with the nucleophile at all.

$\text{H}-\text{Nu}$, where $\text{H}$is hydrogen and $\text{Nu}$ is nucleophile, the aprotic solvent molecules only solvate ${\text{H}}^{+}$ions leaving behind ${\text{Nu}}^{-}$for the reaction. The solvent molecules do not solvate the ${\text{Nu}}^{-}$ ions. As a result of this, concentration of ${\text{Nu}}^{-}$ ions increases and ${\text{Nu}}^{-}$ ions are free to attack the substrate.

When concentration of${\text{Nu}}^{-}$ ions is higher, the reaction rate is going to be faster. Hence, aprotic solvents boost reaction rates of a${\text{S}}_{\text{N}}\text{2}$reaction.

Had a protic solvent been used instead of an aprotic solvent, the reaction would not have proceeded or proceeded very slowly because protic solvent molecules will participate in hydrogen bonding with the nucleophile and prevent the nucleophile from attacking the substrate. As a result, we can say that nucleophiles are less nucleophilic in protic solvents.  

Importance 

• They can also be used in analytical techniques in chemistry, like chromatography.  
• Another distinctive application of these solvents which is less studied is how they can be used in the pharmaceutical industry. A mode of action of a drug depends on its solubility in a suitable solvent.  

Key Takeaways 

• Solvent classification into three types, namely polar protic, polar aprotic, and non-polar solvents.  
• Aprotic solvents in detail and their examples with structures. 
• Effect of solvent on reactions. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters of Biochemistry, Chemistry and Biology.