In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group), a nitrogen (as in an amine group) or a fluorine (as in hydrogen fluoride). In general terms, any solvent that contains a labile H+ is called a protic solvent. The molecules of such solvents readily donate protons (H+) to reagents. Conversely, aprotic solvents cannot donate hydrogen.
Polar protic solvents
Common characteristics of protic solvents :
- solvents display hydrogen bonding
- solvents have an acidic hydrogen (although they may be very weak acids such as ethanol)
- solvents dissolve salts
Examples include water, most alcohols, formic acid, hydrogen fluoride, and ammonia. Polar protic solvents are favorable for SN1 reactions, while polar aprotic solvents are favorable for SN2 reactions.
Polar aprotic solvents
Polar aprotic solvents are solvents that lack an acidic hydrogen. Consequently, they are not hydrogen bond donors. These solvents generally have intermediate dielectric constants and polarity. Although discouraging use of the term "polar aprotic", IUPAC describes such solvents as having both high dielectric constants and high dipole moments, an example being acetonitrile. Other solvents meeting IUPAC's criteria include pyridine, ethyl acetate, DMF, HMPA, and DMSO.
Common characteristics of aprotic solvents:
- solvents that can accept hydrogen bonds
- solvents that do not have acidic hydrogen
- solvents that can dissolve salts
The criteria are relative and very qualitative. A range of acidities are recognized for aprotic solvents. Their ability to dissolve salts depends strongly on the nature of the salt.
Polar aprotic solvents are generally incompatible with strong bases, such as Grignard reagents or t-butyllithium. These reagents require ethers, not nitriles, amides, sulfoxides, etc. The strong base may even deprotonate them ( such as methyl anion as a base having a pKaH of 50, and sulfoxides have pKa of approximately 35.
Properties of common solvents
The solvents are qualitatively grouped into non-polar, polar aprotic, and polar protic solvents, often ranked by dielectric constant.
|Solvent||Chemical formula||Boiling point||Dielectric constant||Density||Dipole moment (D)|
|Hexane||CH3-CH2-CH2-CH2-CH2-CH3||69 °C||2.0||0.655 g/mL||0.00 D|
|benzene||C6H6||80 °C||2.3||0.879 g/ml||0.00 D|
|toluene||C6H5CH3||111 °C||2.4||0.867 g/mL||0.36 D|
|1,4-dioxane||(CH2CH2O)2||101 °C||2.3||1.033 g/mL||0.45 D|
|chloroform||CHCl3||61 °C||4.8||1.498 g/mL||1.04 D|
|diethyl ether||(CH3CH2)2O||35 °C||4.3||0.713 g/mL||1.15 D|
|Polar aprotic solvents|
|dichloromethane (DCM)||CH2Cl2||40 °C||9.1||1.3266 g/mL||1.60 D|
|N-methylpyrrolidone||CH3NC(O)C3H6||202 °C||32.2||1.028 g/mL||4.1 D|
|tetrahydrofuran (THF)||C4H8O||66 °C||7.5||0.886 g/mL||1.75 D|
|ethyl acetate (EtOAc)||CH3CO2CH2CH3||77 °C||6.0||0.894 g/mL||1.78 D|
|acetone[note 1]||CH3C(O)CH3||56 °C||21||0.786 g/mL||2.88 D|
|dimethylformamide (DMF)||HC(O)N(CH3)2||153 °C||38||0.944 g/mL||3.82 D|
|acetonitrile (MeCN)||CH3CN||82 °C||37||0.786 g/mL||3.92 D|
|dimethyl sulfoxide (DMSO)||CH3S(O)CH3||189 °C||47||1.092 g/mL||3.96 D|
|propylene carbonate (PC)||C4H6O3||242 °C||64||1.205 g/mL||4.90 D|
|Polar protic solvents|
|formic acid||HCO2H||101 °C||58||1.21 g/mL||1.41 D|
|n-butanol||CH3CH2CH2CH2OH||118 °C||18||0.810 g/mL||1.63 D|
|isopropanol (IPA)||(CH3)2CH(OH)||82 °C||18||0.785 g/mL||1.66 D|
|nitromethane [note 2]||CH3NO2||101°C||35.87||1.1371 g/mL||3.56 D|
|ethanol (EtOH)||CH3CH2OH||79 °C||24.55||0.789 g/mL||1.69 D|
|methanol (MeOH)||CH3OH||65 °C||33||0.791 g/mL||1.70 D|
|Acetic acid (AcOH)||CH3CO2H||118 °C||6.2||1.049 g/mL||1.74 D|
|Water||H2O||100 °C||80||1.000 g/mL||1.85 D|
|note 1 Acetone is subject to keto-enol tautomerism to the enol form (propen-2-ol) and can therefore exhibit a protic behavior.|
|note 2 Although the hydrogen is bonded to the carbon, the carbon is next to a positively charged nitrogen and it is double bonded to an oxygen.|
- Loudon, G. Mark. Organic Chemistry 4th ed. New York: Oxford University Press. 2002. pg 317.