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, but lack an acidic hydrogen. 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 centers (acetone and esters fail this criterion)
  • solvents dissolve organic salts such as tetraethylammonium iodide

Polar aprotic solvents are often essential for reactions that involve strong bases, such as reactions involving Grignard reagents or n-butyllithium. These reagents react with protic solvents:

C4H9Li + HOCH3 → C4H10 + LiOCH3

An example of a dipolar aprotic solvent is methylpyrrolidone.

Properties of common solvents[edit]

The solvents are grouped into non-polar, polar aprotic, and polar protic solvents and ordered by increasing polarity. The polarity is given as the dielectric constant. The properties of solvents that exceed those of water are bolded.

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 C6H5-CH3 111 °C 2.4 0.867 g/ml 0.36 D
1,4-Dioxane /-CH2-CH2-O-CH2-CH2-O-\ 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 CH3-CH2-O-CH2-CH3 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
tetrahydrofuran (THF) /-CH2-CH2-O-CH2-CH2-\ 66 °C 7.5 0.886 g/ml 1.75 D
ethyl acetate (EtOAc) CH3-C(=O)-O-CH2-CH3 77 °C 6.0 0.894 g/ml 1.78 D
acetone CH3-C(=O)-CH3 56 °C 21 0.786 g/ml 2.88 D
dimethylformamide (DMF) H-C(=O)N(CH3)2 153 °C 38 0.944 g/ml 3.82 D
acetonitrile (MeCN) CH3-C≡N 82 °C 37 0.786 g/ml 3.92 D
dimethyl sulfoxide (DMSO) CH3-S(=O)-CH3 189 °C 47 1.092 g/ml 3.96 D
Polar Protic Solvents
formic acid H-C(=O)OH 101 °C 58 1.21 g/ml 1.41 D
n-butanol CH3-CH2-CH2-CH2-OH 118 °C 18 0.810 g/ml 1.63 D
isopropanol (IPA) CH3-CH(-OH)-CH3 82 °C 18 0.785 g/ml 1.66 D
nitromethane CH3-NO2 100–103 °C 35.87 1.1371 g/ml 3.56 D
ethanol (EtOH) CH3-CH2-OH 79 °C 30 0.789 g/ml 1.69 D
methanol (MeOH) CH3-OH 65 °C 33 0.791 g/ml 1.70 D
Acetic acid (AcOH) CH3-C(=O)OH 118 °C 6.2 1.049 g/ml 1.74 D
Water H-O-H 100 °C 80 1.000 g/ml 1.85 D

See also[edit]

Autoprotolysis

References[edit]

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