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 molecular solvent that contains dissociable H⁺ is called a protic solvent. The molecules of such solvents can donate an H⁺ (proton). Conversely, 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 :

• solvents display hydrogen bonding
• solvents have an acidic hydrogen (although they may be very weak acids)
• solvents are able to stabilize ions
  • cations by unshared free electron pairs
  • anions by hydrogen bonding

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:

• solvents do not display hydrogen bonding
• solvents do not have an acidic hydrogen
• solvents are able to stabilize ions

Examples are dimethyl sulfoxide, dimethylformamide, dioxane and hexamethylphosphorotriamide, tetrahydrofuran.

Polar protic solvents are favorable for S_N1 reactions, while polar aprotic solvents are favorable for S_N2 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.

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	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

See also

Autoprotolysis

References

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