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

Polar aprotic solvents

Polar aprotic solvents are solvents that will dissolve many salts, but lack an acidic hydrogen. It can also be defined as all those species which are not sufficiently polar to form hydrogen bond. 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.

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	24.55	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.