Chemists have classified solvents into three categories according to their polarity.
- polar protic
- dipolar aprotic
Polar Protic Solvents
Let's start with the meaning of the adjective protic. In the context used here, protic refers to a hydrogen atom attached to an electronegative atom. For our purposes that electronegative atom is almost exclusively oxygen. In other words, polar protic solvents are compounds that can be represented by the general formula ROH. The polarity of the polar protic solvents stems from the bond dipole of the O-H bond. The large difference in electronegativities of the oxygen and the hydrogen atom, combined with the small size of the hydrogen atom, warrant separating molecules that contain an OH group from those polar compounds that do not. Examples of polar protic solvents are water (HOH), methanol (CH3OH), and acetic acid (CH3CO2H).
Dipolar Aprotic Solvents
Here the key word is aprotic. In the context used here, aprotic describes a molecule that does not contain an O-H bond. Solvents in this class all contain a bond that has a large bond dipole. Typically this bond is a multiple bond between carbon and either either oxygen or nitrogen. Most dipolar aprotic solvents contain a C-O double bond. Examples are acetone [(CH3)2C=O] and ethyl acetate (CH3CO2CH2CH3).
Non-polar solvents are compounds that have low dielecrtic constants and are not miscible with water. Examples include benzene (C6H6), carbon tetrachloride (CCl4), and diethyl ether ( CH3CH2OCH2CH3).