Protic solvent
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
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 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
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
References
- Loudon, G. Mark. Organic Chemistry 4th ed. New York: Oxford University Press. 2002. pg 317.