Learn the differences between protic and aprotic solvents in the context of their definition, molecular characteristics, properties and applications.
What Is Protic Solvent?
Protic solvents are a class of solvents characterized by the presence of hydrogen atoms (H) bonded to electronegative atoms, such as oxygen (O) or nitrogen (N). These hydrogen atoms have a tendency to form strong hydrogen bonds with other molecules. The molecules of such solvents readily donate protons to reagents.
Examples of Protic solvents are:
- Water (H2O)
- Alcohol (C2H5OH)
- Formic acid (CH2O2)
- Hydrogen fluoride (HF)
- Ethanol (C2H5OH)
- Methanol (CH3OH)
- Ammonia (NH3)
- Acetic acid (CH3COOH)
Properties of Protic Solvent
- Protic solvents are capable of hydrogen bonding. In these solvents, the hydrogen atoms attached to electronegative atoms (usually O-H or N-H bonds) act as both hydrogen bond donors and acceptors. This results in a strong ability to solvate polar and ionic compounds.
- Protic solvents are highly polar due to the electronegative nature of the atoms involved in hydrogen bonding. This polarity makes them effective at dissolving other polar or ionic substances.
- Examples of Protic solvents are water, alcohol, formic acid, hydrogen fluoride, Ethanol, methanol, ammonia, acetic acid etc.
- Protic solvents display hydrogen bonding.
- Protic solvents have an acidic hydrogen (although they may be very weak acids such as ethanol).
- Polar protic solvents are often used to dissolve salts.
- SN1 reactions are significantly faster in polar Protic solvents than in polar Aprotic solvents.
- SN1 reaction works well for tertiary alkyl halide because the loss of the leaving group forms a tertiary carbonation which is the most stable form of carbocations. A polar protic solvent will stabilize this carbocation.
- Protic solvents tend to have higher boiling points compared to aprotic solvents of similar molecular weight. This is because the presence of hydrogen bonds requires more energy to break during the phase transition from liquid to gas.
- Polar Protic solvents have high dielectric constants and high polarity.
- All protic solvents are prone to proton reduction to yield hydrogen gas and are only used for reductive electrochemistry.
Uses of Protic Solvents
- They are often used in reactions where hydrogen bonding plays a crucial role, such as SN1 and SN2 reactions.
- Protic solvents like water are essential in biological and biochemical research, as they mimic the hydrogen bonding environment found in living systems.
- Protic solvents are excellent at dissolving polar and ionic substances; and thus suitable for preparing solutions and conducting experiments involving such compounds.
- Researchers use protic solvents to investigate solubility characteristics, dissolution kinetics, and thermodynamics of various solutes.
What Is Aprotic Solvent?
Aprotic solvents are a class of solvents that do not contain hydrogen atoms bonded directly to electronegative atoms like oxygen or nitrogen. Unlike protic solvents, which can readily form strong hydrogen bonds, aprotic solvents have limited hydrogen bonding capabilities. These solvents generally have intermediate dielectric constants and polarity.
Examples of Aprotic solvents are:
- Dimethyl sulfoxide
- DMF (N,N-dimethylformamide)
- HMF (hydroxymethylfurfural)
- Crown ethers
Properties of Aprotic Solvents
- Aprotic solvents lack O-H or N-H bonds that are capable of forming strong hydrogen bonds. Instead, they typically have functional groups like carbonyl (C=O) or nitro (NO2) that can participate in weaker dipole-dipole interactions.
- Examples of Aprotic solvents are acetone, dimethyl sulfoxide, DMF (N,N-dimethylformamide), acetonitrile, HMF (hydroxymethylfurfural), crown ethers etc.
- Aprotic solvents are solvents that can accept hydrogen bonds.
- Aprotic solvents can vary in polarity, depending on the specific functional groups and molecular structures present in the solvent molecules. Some aprotic solvents are highly polar, while others may be less polar.
- Polar Aprotic solvents can dissolve salt. Their ability to dissolve salts depends strongly on the nature of the salt.
- SN2 reactions are faster in polar Aprotic solvents than in protic solvents.
- Aprotic solvents often have lower boiling points compared to protic solvents of similar molecular weight. This is because they lack the strong hydrogen bonds that require additional energy to break during vaporization.
- Polar Aprotic solvents work well for SN2 reactions because they do not solvate nucleophiles. For example, if we have potassium fluoride and crown ether as a solvent, this ether will solvate potassium, not fluoride. Then, the fluoride will be free to participate in the reaction with a substrate.
- They have intermediate dielectric constants and high dipole moments.
- Aprotic solvents lack O-H and N-H bonds.
- Polar Aprotic solvents are generally incompatible with strong bases such as Grignard reagents or t-butyllithium. Aprotic solvents find applications in various industries such as oil and gas, paints and coatings, electronics and pharmaceuticals.
Uses of Aprotic Solvents
- Aprotic solvents are often preferred in organic synthesis reactions, such as Grignard reactions and nucleophilic substitutions, where strong hydrogen bonding could interfere with the desired chemical processes.
- They are used for extracting and purifying organic compounds, especially when a nonpolar solvent is needed to separate target compounds from a mixture.
- Aprotic solvents are frequently used in analytical chemistry techniques, such as chromatography and spectroscopy, due to their compatibility with a wide range of solutes and their low UV absorbance.
- Aprotic solvents can be used in the polymer industry for dissolving and processing various types of polymers and resins.
Protic vs Aprotic Solvents: Key Differences
Hydrogen Bonding Capability
- Protic Solvents: Can readily form strong hydrogen bonds due to the presence of hydrogen atoms bonded to electronegative atoms like oxygen or nitrogen.
- Aprotic Solvents: Have limited hydrogen bonding capability because they lack hydrogen atoms with proton donor abilities.
Presence of O-H or N-H Bonds
- Protic Solvents: Contain O-H or N-H bonds, which are involved in hydrogen bonding.
- Aprotic Solvents: Lack O-H or N-H bonds that can participate in strong hydrogen bonding.
- Protic Solvents: Tend to be highly polar due to the polar bonds in the molecule.
- Aprotic Solvents: Can be polar or less polar, depending on the specific functional groups present in the molecule.
- Protic Solvents: Have higher boiling points compared to aprotic solvents of similar molecular weight due to stronger intermolecular forces.
- Aprotic Solvents: Often have lower boiling points because of weaker intermolecular forces.
Hydrogen Bond Donor and Acceptor
- Protic Solvents: Can act as both hydrogen bond donors and acceptors.
- Aprotic Solvents: Can act as hydrogen bond acceptors but not donors.
Interaction with Ionic Solutes
- Protic Solvents: Effectively dissolve and stabilize ionic solutes due to strong hydrogen bonding with ions.
- Aprotic Solvents: May not dissolve ionic solutes as effectively due to weaker or no hydrogen bonding with ions.
- Protic Solvents: Examples include water (H2O), ethanol (CH3CH2OH), and methanol (CH3OH).
- Aprotic Solvents: Examples include acetone (CH3COCH3), dimethyl sulfoxide (DMSO), and acetonitrile (CH3CN).
Use in Organic Reactions
- Protic Solvents: Often used in reactions where hydrogen bonding is desirable, such as SN1 and SN2 reactions.
- Aprotic Solvents: Preferred for reactions where strong hydrogen bonding could interfere with the desired chemical processes, such as Grignard reactions.
Solvation of Anions
- Protic Solvents: Tend to solvate anions effectively due to their ability to form hydrogen bonds with them.
- Aprotic Solvents: May solvate anions less effectively due to weaker or no hydrogen bonding.
Use in Polar Protic and Aprotic Protocols
- Protic Solvents: Often used in polar protic solvent protocols in chemistry.
- Aprotic Solvents: Preferred in polar aprotic solvent protocols.
- Protic Solvents: Commonly used in biological and biochemical applications due to their ability to mimic the hydrogen bonding environment found in biological systems.
- Aprotic Solvents: Widely used in organic synthesis, chemical reactions, and as solvents in various industrial processes.
- Protic Solvents: Excellent solvents for polar and ionic compounds.
- Aprotic Solvents: Better suited for dissolving nonpolar or less polar substances.
Also Read: Difference Between SN1 And SN2 Reaction
Protic vs Aprotic Solvents: Key Takeaways
|Property/Characteristic||Protic Solvents||Aprotic Solvents|
|Hydrogen Bonding Capability||Can readily form strong hydrogen bonds due to O-H or N-H bonds.||Have limited hydrogen bonding capability due to the absence of O-H or N-H bonds.|
|Presence of O-H or N-H Bonds||Contain O-H or N-H bonds.||Lack O-H or N-H bonds.|
|Polarity||Tend to be highly polar.||Can be polar or less polar.|
|Boiling Points||Typically have higher boiling points.||Often have lower boiling points.|
|Hydrogen Bond Donor and Acceptor||Act as both hydrogen bond donors and acceptors.||Act as hydrogen bond acceptors but not donors.|
|Interaction with Ionic Solutes||Effectively dissolve and stabilize ionic solutes.||May not dissolve ionic solutes effectively.|
|Common Examples||Water (H2O), ethanol (CH3CH2OH), methanol (CH3OH), etc.||Acetone (CH3COCH3), dimethyl sulfoxide (DMSO), acetonitrile (CH3CN), etc.|
|Use in Organic Reactions||Used in reactions where hydrogen bonding is desirable (e.g., SN1, SN2).||Preferred for reactions where strong hydrogen bonding could interfere (e.g., Grignard reactions).|
|Solvation of Anions||Tend to solvate anions effectively.||May solvate anions less effectively.|
|Use in Polar Protic and Aprotic Protocols||Used in polar protic solvent protocols.||Preferred in polar aprotic solvent protocols.|
|Applications||Commonly used in biological and biochemical applications.||Widely used in organic synthesis, chemical reactions, and industrial processes.|
|Solubility||Excellent solvents for polar and ionic compounds.||Better suited for dissolving nonpolar or less polar substances.|
Also Read: Difference Between Cis And Trans Isomers