Understanding Polar and Non-polar Solvents: The Ultimate Guide to Solubility

When studying the solubility of substances, we often use the empirical rule of ‘like dissolves like,’ which can be simply explained as ‘polar solutes are easily soluble in polar solvents, and non-polar solutes are easily soluble in non-polar solvents.’

What are polar solvents and non-polar solvents?

• Polar solvents: These are solvents containing polar groups such as hydroxyl (-OH) or carbonyl (-C=O), meaning the solvent molecules are polar molecules. Due to the mismatch of positive and negative charge centres within the molecule, the molecule exhibits polarity. The physical quantities used to characterise molecular polarity are the dipole moment or dielectric constant, with a higher dielectric constant indicating greater polarity. Common polar solvents include water, formamide, ethanol, glycerol, and propylene glycol.

• Non-polar solvents: These are solvents composed of non-polar molecules. Non-polar molecules are primarily formed by covalent bonds and have little or no electron activity. They also refer to solvents with small dipole moments, i.e., solvents with low dielectric constants, also known as inert solvents. These solvents do not undergo proton self-transfer reactions nor do they undergo solventisation with solutes. Commonly used non-polar solvents include benzene, liquid paraffin, chloroform, diethyl ether, carbon tetrachloride, and gasoline.

PS: Chemical covalent bonds are divided into polar bonds and non-polar bonds. Non-polar bonds involve shared electron pairs without displacement, found in elements such as O2; polar bonds involve displaced shared electron pairs, such as in HCl. When displacement becomes extreme, one side appears to lose electrons completely while the other gains them, forming an ionic bond, such as in NaCl. The polarity of a compound is determined by the functional groups and molecular structure within the molecule.

Comparison of functional group polarity:

Alkanes (—CH₃, —CH₂—) < Alkenes (—CH=CH—) < Ethers (—O—CH₃, —O—CH₂—) < Nitro compounds (—NO₂) < Dimethylamine (CH₃—N—CH₃) < Esters (—COOR) < Ketones (—CO—) < Aldehydes (—CHO) < Thiols (—SH) < Amines (—NH₂) < Amides (—NHCO—CH₃) < Alcohols (—OH) < Phenols (< Ar—OH) < Carboxylic acids (—COOH)

How does molecular polarity affect things?

Due to the extremely weak intermolecular forces between carbon and hydrogen, carbon in alkanes does not carry a significant negative charge, and hydrogen does not carry a significant positive charge. This is fundamentally different from the significant polarity observed in hydroxyl and carbonyl groups. Therefore, in organic chemistry, carbon-hydrogen bonds are often considered non-polar bonds, which is based on the properties of the substance and differs from the definition of non-polar bonds in inorganic chemistry.

Molecular polarity primarily affects intermolecular forces, thereby exerting a decisive influence on processes such as dissolution, melting, vaporisation, sublimation, and their reverse processes. The principle of ‘like dissolves like’ essentially means that the more similar the polarities of the solute and solvent, the stronger the intermolecular attractive forces, which facilitates the dispersion of the solute in the solvent. At the molecular level, this dispersion is referred to as dissolution. The overall polarity of a molecule typically does not affect its inherent chemical properties, but it can significantly influence the reaction rate and mechanism in solution by stabilizing transition states and intermediates. However, the polarity of chemical bonds significantly influences a molecule's chemical properties, such as the ionisation of acidic or basic substances in water.

Why do highly polar solvents favour substitution reactions, while less polar solvents favour elimination reactions?

The magnitude of polarity can be determined by the size of the dipole moment. Generally, molecules are asymmetric, and the greater the difference in electronegativity between the two atoms, the greater the polarity! In other words, high polarity indicates a very high concentration of charged ions in the solution. According to the principle of reaction equilibrium, the higher the concentration, the stronger the reaction, and substitution reactions are no exception.

In many reactions in solution, polar solvents may affect the reaction rate or even alter the reaction mechanism!For example, highly polar solvents stabilize charged species such as carbocation intermediates, which facilitates SN1 substitution reactions. This is why SN1 reactions are favored in highly polar solvents, while SN2 reactions are not!

How to determine the polarity of a solvent?

There is no universally accepted standard for determining the polarity of a solvent. A relatively reliable method is to make an initial assessment based on the solvent's dielectric constant.

Polar solvents generally have an asymmetric molecular structure, with a significant shift in electron density concentrated around a specific functional group, thereby exhibiting polarity. Examples include methanol, ethanol, DMF, tetrahydrofuran, and DMSO.

Non-polar solvents have symmetrical molecular structures with uniformly distributed electron clouds. Examples include petroleum ether, n-hexane, toluene, and benzene.

Why are carbon-hydrogen bonds in alkanes non-polar bonds?

This is because the intermolecular forces between C and H are very weak, resulting in carbon in alkanes not carrying a significant negative charge and hydrogen not carrying a significant positive charge. This is fundamentally different from the significant polarity observed in hydroxyl and carbonyl groups. Therefore, in organic chemistry, carbon-hydrogen bonds are often considered non-polar bonds, which is based on the properties of the substance and differs from the definition of non-polar bonds in inorganic chemistry.

Comparison of the polarity of common solvents

• Strongly polar solvents: methanol > ethanol > isopropanol

• Moderately polar solvents: ethyl cyanate > ethyl acetate > chloroform > dichloromethane > diethyl ether > toluene

• Non-polar solvents: cyclohexane, petroleum ether, hexane, pentane

• Order of polarity for individual solvents: petroleum ether (low) → cyclohexane → carbon tetrachloride → trichloroethylene → benzene → toluene → dichloromethane → chloroform → diethyl ether → ethyl acetate → methyl acetate → acetone → n-propanol → methanol → pyridine → acetic acid (high)

Polarity order of mixed solvents

Benzene: chloroform (1+1) → cyclohexane: ethyl acetate (8+2) → chloroform: acetone (95+5) → benzene: acetone (9+1) → Benzene: Ethyl acetate (8+2) → Chloroform: Ether (9+1) → Benzene: Methanol (95+5) → Benzene: Ether (6+4) → Cyclohexane: Ethyl acetate (1+1) → Chloroform: Ethyl Ether (8+2) → Chloroform: Methanol (99+1) → Benzene: Methanol (9+1) → Chloroform: Acetone (85+15) → Benzene: Ethyl Ether (4+6) → Benzene: Ethyl acetate (1+1) → Chloroform: Methanol (95+5) → Chloroform: Acetone (7+3) → Benzene: Ethyl acetate (3+7) → Benzene: Diethyl ether (1+9) → Diethyl ether: Methanol (99+1) → Ethyl acetate: methanol (99+1) → Benzene: acetone (1+1) → Chloroform: methanol (9+1)

Note: Benzene: methanol (95+5) means that 95 volumes of benzene are mixed with 5 volumes of methanol to form a mixed solvent.

Common mixed solvents:

• Ethyl acetate/hexane: Common concentrations range from 0 to 30%. However, it can sometimes be difficult to completely remove the solvent using a rotary evaporator.

• Ether/pentane system: Concentrations of 0 to 40% are commonly used. These are very easy to remove using a rotary evaporator.

• Ethanol/hexane or pentane: 5–30% is suitable for strongly polar compounds.

• Dichloromethane/hexane or pentane: 5–30%. Consider using this when other mixed solvents fail.

Common mobile phase polarity:

Petroleum ether < gasoline < heptane < hexane < carbon disulphide < xylene < toluene < chloroform < benzene < bromoethane < bromobenzene < dichloromethane (DCM) < Trichloromethane < Isopropyl ether < Nitromethane < Butyl acetate < Ethyl ether < Ethyl acetate < n-Pentane < n-Butanol < Phenol < Methyl alcohol < tert-Butanol < Tetrahydrofuran < DIOXANE < Acetone < Ethanol < Acetonitrile < Methyl alcohol < Dimethylformamide (DMF) < Water