Classification of Solvents

There are two broad solvent categories, and they are not mutually exclusive; that is, a solvent can be in more than one category.

1. A solvent can be protic or aprotic.

2. A solvent can be polar or apolar.

1. Protic or Aprotic Solvent.

A protic solvent consists of molecules that can act as hydrogen-bond donors.

Water, alcohols, and carboxylic acids are examples of protic solvents.

Solvents that cannot act as hydrogen-bond donors are called aprotic solvents.

Ether, methylene chloride, and hexane are examples of aprotic solvents.

2. Polar or Apolar (Nonpolar )Solvent.

A polar solvent has a high dielectric constant; 

An Apolar solvent has a low dielectric constant.

Dielectric constant 

The dielectric constant is defined by the electrostatic law, which gives the interaction energy E between two ions with respective charges q1 and q2 separated by a distance r:

In this equation, k is a proportionality constant and ε is the dielectric constant of the solvent in which the two ions are imbedded. This equation shows that when the dielectric constant ε is large, the magnitude of E, the energy of interaction between the ions, is small. This means that

both attractions between ions of opposite charge and repulsions between ions of like charge are weak in a polar solvent. 

Thus, a polar solvent effectively separates, or shields, ions from one another. This means, in turn, that the tendency of oppositely charged ions to associate is less in a polar solvent than it is in an apolar solvent. 

If a solvent has a dielectric constant of about 15 or greater, it is considered to be polar. Water (ε = 78), methanol (ε = 33), and formic acid (ε = 59) are polar solvents. 

Hexane (ε = 2), ether (ε = 4), and acetic acid (ε = 6) are apolar solvents

Unfortunately, the word polar has a double usage in organic chemistry. When we say that a molecule is polar, we mean that it has a significant dipole moment, μ . When we say that a solvent is polar, we mean that it has a high dielectric constant. 

In other words, solvent polarity, or dielectric constant, is a property of many molecules acting together, but molecular polarity, or dipole moment, is a property of individual molecules. 

Although it is true that all polar solvents consist of polar molecules, the converse is not true. 

The contrast between acetic acid and formic acid is particularly striking:

These two compounds contain identical functional groups and have very similar structures and dipole moments. Both are polar molecules. Yet they differ substantially in their dielectric constants and in their solvent properties! Formic acid is a polar solvent; acetic acid is not.