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 molecular solvent that contains dissociable H+ is called a protic solvent. The molecules of such solvents can donate an H+ (proton). However, aprotic solvents cannot donate hydrogen.
Polar protic solvents are solvents that share ion dissolving power with aprotic solvents but have an acidic hydrogen. In general, these solvents have high dielectric constants and high polarity.
Common characteristics of protic solvents :
Examples are water, methanol, ethanol, formic acid, hydrogen fluoride, and ammonia.
Polar aprotic solvents are solvents that share ion dissolving power with protic solvents but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity.
Common characteristics of aprotic solvents:
Examples are dimethyl sulfoxide, dimethylformamide, dioxane and hexamethylphosphorotriamide, tetrahydrofuran.
Polar protic solvents are favorable for SN1 reactions, while polar aprotic solvents are favorable for SN2 reactions.
Apart from solvent effects, polar aprotic solvents may be essential for reactions that use strong bases, such as reactions involving Grignard reagents or n-butyllithium. If a protic solvent were to be used, the reagent would be consumed by a side-reaction with the solvent.
An example of a dipolar aprotic solvent is methylpyrrolidone.
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 | CH3CH2-O-CH2-CH3 | 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 |
| 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 |
| n-Propanol | CH3-CH2-CH2-OH | 97 °C | 20 | 0.803 g/ml | 1.68 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 |