Polar solvents are solvents with large dipole moments whereas nonpolar solvents have low or sometimes, no dipole moment at all. The dipole moment arises from polarity which basically comes from the difference in electronegativities of the bonded atoms in a molecule.
In other words, the solvents having charge separation (partial positive and negative charges) are polar solvents while the solvents which do not have charge separation are nonpolar solvents.
For example, water contains oxygen and hydrogen atoms. The electronegativity value of hydrogen is 2.1 while that of oxygen is 3.5. The oxygen atom, being more electronegative, attracts the electrons more towards itself. Hence, the poles are formed where oxygen gets a partial negative and hydrogen gets a partial positive charge.
This type of charge separation within a molecule is referred to as polarity. And polarity is what differs between polar and nonpolar solvents.
Let’s take benzene (C6H6) as another example. Carbon has an electronegativity of 2.5 while, for hydrogen, it is 2.1. The difference is 0.4, which is very small. The C–H bond is therefore considered nonpolar. So, benzene is a nonpolar solvent.
Refer to polar vs. nonpolar bonds to read more about bond polarity.
Polar vs Nonpolar Solvents
|Polar solvents||Nonpolar solvents|
|Polar solvents have a large dipole moment||Nonpolar solvents have zero or very small value of dipole moment|
|They dissolve polar compounds||They dissolve nonpolar compounds|
|Polar solvents have a high dielectric constant||They have a low dielectric constant|
|Charge separation in the form of partial positive and partial negative is observed in polar solvents||There is no charge separation in nonpolar solvents|
|They are composed of atoms that have a high difference in electronegativity||They are composed of atoms that have a negligible difference in electronegativity|
|Polar solvents have polar bonds||Nonpolar solvents have nonpolar bonds|
|Examples of polar solvents are water, acetone, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), isopropanol, and methanol, etc||The nonpolar solvents include alkanes, benzene, toluene, xylene, acetic acid, chloroform, diethyl ether, ethyl acetate, methylene chloride, and pyridine, etc|
Here it should be noted that neither dipole moment nor electronegativity difference is enough to explain the polarity of all compounds. The most suitable method is using the dielectric constants. The dielectric constant (ε) of a solvent is a measure of its polarity. The higher the dielectric constant of a solvent, the more polar it is.
Let’s look at chloroform, CHCl3, as an example to understand this:
Chloroform has tetrahedral molecular geometry. Chlorine atoms are more electronegative than carbon and hydrogen atoms and they are located at three of the pyramid’s vertices, pulling the negative charge in their direction, resulting in a polar molecule with a downward dipole. The dipole moment of CHCl3 is 1.15 D. Despite this, it is nonpolar. Why? The dielectric constant of chloroform is 4.81 F/m which, being a small value makes it nonpolar.
|Solvent||Dipole Moment "μ"||Dielectric Constant "ε"||Boiling Point ˚C||Catagory of Solvent|
What are Polar Solvents?
Polar solvents are the solvents that have a polar bond, the basis of which is the electronegativity difference between constituent atoms. Electronegativity is the tendency of an atom to attract a shared pair of electrons toward itself. The greater difference in electronegativity between the bonded atoms, the greater is the polarity. And thus, the more polar the solvent.
Polar solvents easily dissolve polar and ionic solutes because of the attraction of the opposite charges between solute-solvent particles. This does not happen when a nonpolar solvent is added to a polar solvent.
Polar solvents are further classified into polar protic and polar aprotic solvents. The key difference between the two is, “Protic solvents contain at least one hydrogen atom connected directly to a highly electronegative atom (such as F, N, O) and thus, can make hydrogen bonds. Whereas aprotic solvents do not contain any hydrogen atom connected directly to highly electronegative atoms and so are not capable of forming hydrogen bonds.”
Examples of polar solvents
- Water, H2O
- Acetone, C3H6O
- Acetonitrile, C2H3N
- N,N-Dimethylformamide (DMF), C3H7NO
- Dimelthylsulfoxide (DMSO), C2H6OS
- Isopropanol, C3H8O
- Methanol, CH3OH, etc
What are Nonpolar Solvents?
Nonpolar solvents have nonpolar bonds owning to the fact that there small or negligible difference of electronegativity (less than 0.4) between the bonded atoms. Nonpolar solvents dissolve only nonpolar solutes, as suggested by the general solubility principle, “Like dissolves like”, with some exceptions.
Examples of nonpolar solvents
- Alkanes (pentane, hexane, heptane)
- Aromatics (benzene, toluene, xylene)
- Diethyl ether, (C2H5)2O
- Tetrahydrofuran (THF)
- Pyrrole, C4H5N
- Ethyl acetate, C4H8O2
- 1,4-Dioxane, C4H8O2
- Pyridine, C5H5N
- Chloroform, CHCl3
- Carbon tetrachloride, CCl4
- Acetic acid, CH3COOH, (exits in the form of dimers and behaves as a nonpolar solvent).
Are all organic solvents non-polar?
No. Not all organic solvents are nonpolar. For example, acetone (C3H6O) is an organic solvent but is polar as well.
Is ethanol polar or nonpolar and why?
Ethanol is a very polar molecule due to its hydroxyl (OH) group (ΔEN ≈ 1.0), which forms hydrogen bonds with other molecules.
How do you tell if a molecule is polar or nonpolar without electronegativity?
Dipole moment is an alternative way to tell if a molecule is polar or nonpolar. Dipole moment is found by studying the symmetry of the molecule. However, an even better method is through the dielectric constants.
Can ionic compounds dissolve in nonpolar solvents?
Ionic compounds are soluble in polar solvents and insoluble in nonpolar solvents.
Why does it matter if a molecule is polar or nonpolar?
The polarity of a molecule is important because it gives an idea of the solubility of the molecules in solvents. The polar solvent dissolves polar solutes and nonpolar solvent dissolves nonpolar solutes.
Why is chloroform a non-polar solvent?
Chloroform is “nonpolar” because it has a low dielectric constant. The dielectric constant is the measure of the polarity of a solvent. The greater the dielectric constant, the more the polarity, and vice versa.
Why are carbohydrates polar molecules?
Monosaccharides and disaccharides are simple carbohydrates and are polar molecules, containing several hydroxyls (–OH) functional groups, which make them hydrophilic (water-loving). Polysaccharides are complex carbohydrates and are nonpolar molecules.
- Chemistry: The Practical Science (Media enhanced edition) by Paul Kelter (North Dakota State University), Michael Mosher (University of Tulsa), and Andrew Scott (Perth College UHI)