Nonpolar Solvents

Solvents can be protic or aprotic. A protic solvent is one that has an OH or NH bond. The importance of these is that they can participate in hydrogen bonding between molecules and can donate protons or H+ molecules. Aprotic solvents can have hydrogen in them but, because they do not have an OH or NH bond, they cannot participate in hydrogen bonding and cannot donate a H+ atom. Protic solvents will decrease the reactivity of nucleophiles in substitution reactions, while polar aprotic solvents do not.

There are three types of solvents based on what you have just learned. There are nonpolar aprotic solvents, polar aprotic solvents, and polar protic solvents. You should know that there is no such thing as a nonpolar protic solvent.

NONPOLAR SOLVENTS

A low dielectric constant is considered a constant of less than five. A solvent with this characteristic will not be good for charged species such as anions. The following is a list of nonpolar solvents; only chloroform and diethyl ether have dielectric constants approaching 5 and have a minimal dipole moment:

• Pentane—this has no dipole moment and a dielectric constant of 1.8

• Hexane—this has no dipole moment and a dielectric constant of 1.9

• Cyclohexane—this has no dipole moment and a dielectric constant of 2.0

• Benzene—this has no dipole moment and a dielectric constant of 2.4

• Toluene—this has a dipole moment of 0.36 and a dielectric constant of 2.3

• Chloroform (CHCl3)—this has a dipole moment of 1.04 and a dielectric constant of 4.8.

• Diethyl ether—this has a dipole moment of 1.15 and a dielectric constant of 4.3.

There is another classification of solvents, called “borderline” polar aprotic solvents. These have dielectric constants of between 5 and 20 and intermediate polarity. They do not have an NH or OH bond so they don’t participate in any reactions. These include the following:

• Dichloromethane (CH2Cl2)—this has a dipole moment of 1.6 and a dielectric constant of 9.1.

• Tetrahydrofuran or THF—this has a dipole moment of 1.75 and a dielectric constant of 7.5.

• Ethyl acetate—this has a dipole moment of 1.78 and a dielectric constant of 6.0.

Polar aprotic solvents (not those that are borderline) have large dielectric constants of greater than 20 but do not participate in hydrogen bonding because they have no OH or NH bonds. They can dissolve those solutes that have CN- or OH- bonds, which have more reactivity in these types of solvents because of their high polarity. Examples of these types of solvents include the following:

• Acetone (CH3COCH3)—this has a dipole moment of 2.88 and a dielectric constant of 21.

• DMF (N, N-dimethylformamide)—this has a dipole moment of 3.82 and a dielectric constant of 38.

• Acetonitrile (CH3CN)—this has a dipole moment of 3.92 and a dielectric constant of 37.

• DMSO (dimethyl sulfoxide)—this has a dipole moment of 3.96 and a dielectric constant of 47.

Polar protic solvents have high dielectric constants and high dipole moments. They possess OH and NH bonds and participate in hydrogen bonding. They act as weak acids because they donate protons and are weak nucleophiles that form bonds with strong electrophiles. They often become the solvent for their weak conjugate bases (such as ethanol or EtOH acting as the solvent for EtO-) and water acting as the solvent for OH.

Polar protic solvents include the following:

• Ammonia—this has a dielectric constant of about 25.

• t-Butanol—this has a dielectric constant of 12.

• n-Propanol—this has a dielectric constant of 20.

• Methanol—this has a dielectric constant of 33.

• Ethanol—this has a dielectric constant of 35.

• Acetic acid—this has a dielectric constant of 6.2.

• Water—this has a dielectric constant of 80.

These types of solvents are by far the most likely of all solvents to participate in reactions. They can act as nucleophiles when strong electrophiles are present.

For the purposes of this course, the solvent will be liquid but it can be a solid, gas, or supercritical fluid. The quantity of solute (which can also be a solid, liquid, or gas) that can dissolve most depends on the temperature of the solvent/solid conditions.

When a substance is dissolved into large quantities of another, this is called a solution. The ingredients in a solution are, by definition, evenly distributed in a single phase called a solvate (complexes of solvent and solute). The ability of one compound to be dissolved in another is called “solubility”. If it occurs in all proportions of solvents and solutes, this is called being “miscible”.

Molecules of the solvent will arrange around the molecules of the solute. There will be a transfer of heat and an increase in the entropy (or disorder) of the system. There will be the thermodynamic stability of the solution that is greater than the solvent and solute separately, made possible by things like the dipole moments of the molecules, their polarizability, and their hydrogen bonding. With solvation, there is a coordination complex formed that will allow for the stability of the solution.

The dielectric constant is a direct measure of polarity of a solvent. Less than 5 is nonpolar, while 5-15 is considered “borderline” polar. Those dielectric constants greater than 15 are definitely polar. The dielectric constant is a measure of the tendency of the solvent to partially cancel out the field strength of the electric field of a charged particle immersed in it. It is the ability of the solvent to dissolve ionic compounds, such as salts.

Interestingly, the dielectric constant is not the only measure of a substance’s polarity. There are a few others that are less commonly used, including these:

• The Grunwald-Winstein Y scale is a measure of a solvent’s ability to influence to buildup of positive charge of a solute in a chemical reaction.

• Kosower’s Z scale is a measure of the solvent’s influence on UV-absorption maxima of a salt.

• Hildebrand parameter is a measure of the cohesive energy density of a solvent, which is useful in measuring the impact of nonpolar compounds.

• Reichardt’s dye, which is a dye that changes color in response to the polarity of the solvent.

In general, the polarizability, dipole moment, polarity, and hydrogen bonding of a particular solvent will determine what type of solutes it can dissolve and which liquids are miscible with it. The rule of “like dissolves like” means that polar dissolves polar and nonpolar dissolves nonpolar. Things that are not miscible with one another include water and hexane, vinegar and vegetable oil, and octane and acetic acid. These represent polar and nonpolar substances.

Protic solvents can easily solvate anions because they have the strong ability to engage in hydrogen bonding. Water is the best-known protic solvent, while aprotic solvents like dichloromethane and acetone are polar but do not donate hydrogen ions.

In organic chemistry, there are pure solvents and many multicomponent solvents that are used and that have mixed characteristics because of their chemical differences. These must be miscible with one another so that they can be conceived of as a solvent. An example is called “solvent 645”, which contains differing amounts of toluene, butyl acetate, ethyl acetate, butanol, and ethanol. Another example is a “thinner” such as Thinner RKB-1, which is 50 percent butanol and 50 percent xylene.

Figure 32 shows some polar and nonpolar solvents, according to their chemical structure:

Figure 32.

Most organic solvents will have lower density than water, meaning that they will form a layer above water when mixed with it rather than actually mixing with it. The exceptions are the halogenated solvents, such as dichloromethane and chloroform, which sink to the bottom of the container. Knowing these things is crucial to separating substances in reactions. Those that sink to the bottom and are nonpolar will separate out those solutes first in a separatory funnel, while those that rise above water will collect in the nonpolar solvent and will be isolated after draining the water out first. Often specific gravity being greater or lesser than one will indicate whether the solvent will sit above or below the water in a mixture.

Most organic solvents will also be considerably more flammable than water, depending on how volatile they are. The major exceptions are things like chloroform and dichloromethane, which are both chlorinated compounds. Other organic compounds have the capacity to explode in air because they have dense vapors that can go many distances nearly undiluted. Flash fire hazards are possible so empty containers should be stored both open and upside down.

Solvents like diethyl ether and carbon disulfide are particularly dangerous because of low autoignition temperatures that greatly increase the fire risk. They can ignite and burn even in the presence of steam pipes, light bulbs, and hotplates because they are