Polar solvent

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 fluoride (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 solutes, often via hydrogen bonding. Water is the most common protic solvent. Conversely, aprotic solvents cannot donate hydrogen. Major uses of polar solvents are in paints, paint removers, inks, dry cleaning.[1]

Polar protic solvents

Polar protic solvents are often used to dissolve salts. In general, these solvents have high dielectric constants and high polarity.

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
    • cations by unshared free electron pairs
    • anions by hydrogen bonding

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.[2]

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 having pKas 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)
Non-polar solvents
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.
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See also

References
  1. Stoye, Dieter (2000). "Solvents". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_437.
  2. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "dipolar aprotic solvent". doi:10.1351/goldbook.D01751


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