Protic solvent

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In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group) or a nitrogen (as in an amine group). In general terms, any solvent that contains 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[edit]

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 :

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[edit]

Polar aprotic solvents are solvents that will dissolve many salts. They 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 DMF, HMPA, and DMSO.[1]

Common characteristics of aprotic solvents:

  • solvents that can accept hydrogen bonds
  • solvents do not have acidic hydrogen
  • solvents 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 Grignard reagents or n-butyllithium. These reagents require ethers, not nitriles, amides, sulfoxides, etc.

Properties of common solvents[edit]

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
dichloromethane (DCM) CH2Cl2 40 °C 9.1 1.3266 g/mL 1.60 D
Polar aprotic solvents
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 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
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 CH3NO2 100–103 °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) CH3-CO2H 118 °C 6.2 1.049 g/mL 1.74 D
Water H2O 100 °C 80 1.000 g/mL 1.85 D

See also[edit]

References[edit]

  • Loudon, G. Mark. Organic Chemistry 4th ed. New York: Oxford University Press. 2002. pg 317.