Solvents are broadly classified into two types, polar and nonpolar solvents. Polar solvents are further divided into protic and aprotic solvents.
Concisely, the difference between protic and aprotic solvents can be written as:
“Protic solvents contain at least one hydrogen atom connected directly to an electronegative atom (such as F, N, O) and thus, can make hydrogen bonds. Whereas aprotic solvents do not contain any hydrogen atom connected directly to highly electronegative atoms and so are not capable of forming hydrogen bonds.”
Protic vs aprotic solvents
|Protic solvents||Aprotic solvents|
|Protic solvents have at least one hydrogen atom directly connected to an electronegative atom (N-H, O-H, etc)||Aprotic solvents can have hydrogen atoms somewhere in their structures, but no hydrogen atom is directly connected to an electronegative atom|
|They are capable of hydrogen bonding||They cannot make hydrogen bonds|
|Protic solvents are acidic||Aprotic solvents are not acidic|
|They have high dielectric constants||They have intermediate and low dielectric constants|
|Polar protic solvents are rich in O-H and N-H bonds||They lack O-H and N-H bonds|
|They are preferably used to dissolve salts||Aprotic solvents can dissolve salts too. However, their ability to dissolve salts depends mostly on the nature of the salts|
|SN1 reactions are significantly faster in protic solvents||SN1 reactions are slow in aprotic solvents|
|SN2 reactions are very slow in protic solvents||SN2 reactions are significantly faster in aprotic solvents|
|Polar protic solvents stabilize carbocations and nucleophiles||Polar aprotic solvents cannot hydrogen bond with nucleophiles|
|Water, alcohol, formic acid, hydrogen fluoride, ammonia, acetic acid, ethanol, methanol, etc. are all protic solvents||Acetone, dimethyl sulfoxide, HMF (hydroxymethylfurfural), crown ethers, DMF (N,N-dimethylformamide), acetonitrile, etc. are all aprotic solvents|
|Polar solvents give hydrogen gas upon reduction and are therefore useful in reductive electrochemistry||Aprotic solvents are used in many industries including but not limited to oil and gas, paints and coatings, electronics, and pharmaceuticals|
Similarities between protic and aprotic solvents
- Both protic and aprotic solvents are polar solvents.
- Both can dissolve salts.
What are protic solvents?
Generally, any solvent with a labile H+ is the protic solvent. This proton is readily available to solutes, often donated via hydrogen bonding. Water is the most common protic solvent.
Hydrogen atom(s) is often bound to an oxygen atom, as in hydroxyl group, a nitrogen atom, as in amine group, and fluoride, as in hydrogen fluoride. These are the examples of polar protic solvents; Water (H2O), formic acid (CH2O2), hydrogen fluoride (HF), ethanol (C2H5OH), ammonia (NH3), methanol (CH3OH), acetic acid (CH3COOH), etc.
What are aprotic solvents?
Aprotic solvents lack acidic protons. Thus, they cannot act as proton donors in hydrogen bonding. Unlike polar solvents, they lack hydroxyl and amine groups.
Polar aprotic solvents are of particular interest because of their ability to dissolve salts, although they cannot donate protons. Many hydrocarbons and chlorocarbons are classified as aprotic solvents.
Some examples of aprotic solvents are acetone (H3C-(C=O)-CH3), Ethyl acetate, DMSO, dimethyl sulphoxide ((CH3)2S=O), DMF, N,N-dimethylformamide (C3H7NO), HMF (hydroxymethylfurfural), Oxolane/tetrahydrofuran (THF), crown ethers, and acetonitrile (CH3C≡N), etc.
Effect of protic and aprotic solvents on nucleophilic substitution
In protic solvents, anions are solvated due to the presence of hydrogen bonds, thus, their ability to serve as a nucleophile is inhabited. On the other hand, anions are not solvated and there will be no effect on the reaction rate in aprotic solvents.
In general, these conclusions can be drawn from the above statement:
- All nucleophiles tend to be more reactive in aprotic than protic solvents.
- The species that are strongly solvated in polar protic solvents will gain reactivity in polar aprotic e.g. F–, etc.
- Polar aprotic solvents are generally used when a polar protic solvent gives poor results due to weak nucleophiles such as F–, -CN, RCO2–, etc.
Key differences between protic and aprotic solvents
What does protic mean?
The word protic means, “of a solvent or other liquid medium, able to provide hydrogen atoms for the protonation”.
Why is acetone considered a polar aprotic solvent?
Acetone is a polar aprotic solvent since it does not have any hydrogen atom directly attached to an electronegative atom (O).
Is dimethyl sulfoxide protic or aprotic?
Dimethyl sulfoxide (DMSO), CH3SOCH3 is a polar aprotic solvent as it does not contain a proton directly bonded to an electronegative atom but still, it is polar.
Why are aprotic solvents better for SN2 reactions?
In aprotic solvents, nucleophiles are almost non-solvated so it is easier for them to attack the substrate. Hence, SN2 reactions are favored in an aprotic environment, as there is less hindrance to the nucleophiles. In protic environments, the nucleophiles are trapped in a cage of solvent molecules and so become inactive.
Is benzene a protic solvent?
Benzene is a nonpolar solvent used for manufacturing a vast number of chemicals.
Is methanol a protic solvent?
Yes. Methanol is a polar protic solvent due to (O-H) reactivity.
Is ethanol protic or aprotic?
Ethanol is a polar protic solvent because of the (O-H) bond presence.
How do I identify whether a solvent is polar aprotic or protic?
Protic solvents have O-H and N-H bonds and they can form hydrogen bonds. Aprotic solvents may have hydrogen atoms on them somewhere, but they are not directly attached to O or N (lack O-H or N-H bonds) and therefore cannot do hydrogen bonding.
Why does NaOH display the highest basic strength in a polar protic solvent?
NaOH itself is a good base. In protic solvents, O-H and N-H bonds are present which act as proton donors and form strong hydrogen bonds. This results in the immediate dissociation of NaOH into its constituent ions (Na+ and OH–). NaOH liberates OH– ions swiftly and thus, it exhibits its maximum strength in polar protic solvents.
- Ch 8: Solvent Effects (ucalgary.ca)