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Polar Protic and Aprotic Solvents

Solvents used in organic chemistry are characterized by their physical characteristics.  Among the most important are whether the solvents are polar or non-polar, and whether they are protic or aprotic.  Because non-polar solvents tend to be aprotic,the focus is upon polar solvents and their structures. 

Solvent Polarity

Solvents are generally classified by the polarity, and considered either polar or non-polar, as indicated by the dielectric constant.  However, as with many properties, the polarity is a continuous scale, and the correct question is not "is it polar or non-polar" but "how polar is it."  Nonetheless, guidelines have been created to make it easier.  Generally, solvents with dielectric constants greater than about 5 are considered "polar" and those with dielectric constants less than 5 are considered "non-polar." 

Table 1: Examples of a few common solvents used in organic chemistry
Solvent Boiling Point, Celsius Dielectric Constant
  NON-POLAR SOLVENTS  
Pentane, C5H12 36 1.8
Hexane, C6H14 69 1.9
Benzene, C6H6 80 2.3
Chloroform, CHCl3 61 4.8
Diethyl ether, (CH3CH2)2O 35 4.3
1,40-Dioxane, cyc-(CH2CH2OCH2CH2O) 101 2.3
  POLAR PROTIC SOLVENTS  
Water, H2O 100 78.5
methanol, CH3OH 65 32.6
ethanol, CH3CH2OH 78.5 24.3
isopropyl alcohol, CH3CH(OH)CH3 82 18
acetic acid, CH3COOH 118 6
  POLAR APROTIC SOLVENTS  
dichloromethane, CH2Cl2 40 9.1
tetrahydrofuran (THF), cyc-(CH2)4O 66 7.5
ethyl acetate, CH3C(O)OCH2CH3 77 6
acetonitrile, CH3CN 81.6 37.5
dimethylformamide (DMF), HCON(CH3)2 153 38
dimethyl sulfoxide (DMSO), CH3SOCH3 189 47
acetone, CH3COCH3 56.5 21
hexamethylphosphoric triamde (HMPT), [(CH3)2N]3PO 232 30

Protic vs Aprotic Solvents

The table above distinguishes between protic and aprotic solvents. For the solvents included in the table, the distinguishing feature is the presence of an -OH group, and that is the most common characteristic of a protic solvent.  However, there are exceptions, such as nitromethane, CH3NO2, which is also considered a protic solvent. That might suggest that Bronsted acidity is the most important feature, because nitromethane is very acidic, with a pKa of about 10. However, acetone is still considered a polar aprotic solvent, despite the fact that it is relatively acidic, and not significantly less acidic than alcohols. Then again, acetone (and other carbonyl containing solvents) are, indeed, poor solvents when using strong bases due to their relatively high acidity.

Significance

Solvent properties are in important consideration in many chemical reactions, including nucleophilic substitution reactions.  As strong hydrogen-bond donors, protic solvents are very effective at stabilizing ions.  Therefore, they favor reactions in which ions are formed, such as the SN1 reaction, and disfavor reactions where ions are reactants, such as the SN2 reaction.