When we study the dissolution of substances, we often use the empirical rule of “similar phase dissolution”, which can be simply explained as “polar solute is easy to dissolve in polar solvent, non-polar solute is easy to dissolve in non-polar solvent”.

Recently, several customers have asked the salesman of our company whether the solvent is polar or nonpolar. What is polar solvent and what is nonpolar solvent? In this issue, let’s look for answers to the above questions.

What are polar and nonpolar solvents?

Polar solvent: refers to the solvent containing hydroxyl (- OH) or carbonyl (- C = O) and other polar groups, that is, the solvent molecule is a polar molecule, because the center of gravity of the positive and negative charges in the molecule does not coincide, resulting in the polarity of the molecule. The physical quantities used to characterize the molecular polarity are dipole moment or dielectric constant. Commonly used polar solvents are water, formamide, ethanol, glycerol, propylene glycol, etc.

Nonpolar solvent: it is a kind of solvent composed of nonpolar molecular solution. Nonpolar molecules are mostly composed of covalent bonds, and have no electrons or little electronic activity. It also refers to the solvent with small dipole moment. It refers to a kind of solvent with low dielectric constant, also known as inert solvent. This kind of solvent neither carries out proton self delivery reaction nor solvation with solute. The commonly used nonpolar solvents are benzene, liquid paraffin, chloroform, ether, carbon tetrahalide, gasoline, etc.

PS: chemical covalent bond is divided into polar bond and nonpolar bond. Nonpolar bond means that the shared electron pair is not shifted and appears in the simple substance, such as O2; polar bond means that the shared electron pair is shifted, such as HCl. And when the shift is very strong, it seems that one side completely loses electrons and the other side gets electrons, it will become an ionic bond, such as NaCl. The polarity of a compound depends on the functional groups and molecular structure of the molecule.

Comparison of polarity of functional groups:

Alkanes (- CH3, – CH2 -) < alkenes (- CH = ch -) < ethers (- O-CH3, – o-ch2 -) < nitro compounds (- NO2) < dimethylamine (ch3-n-ch3) < lipids (- coor) < ketones (- co -) < aldehydes (- CHO) < mercaptans (- SH) < amines (- nhco-ch3) < alcohols (- OH) < phenols (< nhco-ch3 -) Ar-oh) < carboxylic acid (- COOH)

What does the polarity of a molecule affect?

Because the interaction force between C and H is so small that carbon in alkanes is not significantly negatively charged, and hydrogen is not significantly positively charged, which is essentially different from the significant polarity in hydroxyl and carbonyl groups. Therefore, hydrocarbon bonds are often regarded as nonpolar bonds in organic chemistry, which is from the point of view of material properties, and has a certain difference with the definition of nonpolar bonds in inorganic chemistry difference.

The polarity of molecules mainly affects the intermolecular force, and then has a decisive impact on the dissolution, melting, gasification, sublimation and the corresponding reverse process. Similar compatibility is essentially that the more similar the polarity of solute and solvent, the stronger the relative attraction between molecules, which is more conducive to the dispersion of solute in the solvent. The (overall) polarity of a molecule usually does not affect its chemical properties, but the polarity of chemical bonds will significantly affect its chemical properties, such as the ionization of acid-base substances in water.

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Why is the solvent with high polarity good for substitution reaction and the solvent with low polarity good for elimination reaction?

The size of polarity can be kept constant by the size of dipole distance. Generally, molecules are asymmetric, and the greater the difference of electronegativity between two atoms, the greater the polarity! In other words, strong polarity means that the concentration of charged ions in the solution is very large. According to the equilibrium principle of reaction, the stronger the concentration is, the stronger the reaction is, and the substitution reaction is no exception.

Many reactions in solution, polar solvents may affect the reaction rate, and even change the reaction mechanism! For example, SN1 substitution reaction is advantageous in polar solvents, while SN2 substitution reaction is disadvantageous!

How to judge the polarity of solvent?

There is no generally accepted standard for judging the polarity of solvent in the industry. The more reliable one is to make a preliminary judgment according to the dielectric constant of solvent.

Polar solvents, generally with asymmetric molecular structure, have a large degree of electron cloud shift and concentrate near a certain functional group, so they show polarity. For example, methanol, ethanol, DMF, tetrahydrofuran, DMSO, etc

Nonpolar solvent, symmetrical molecular structure, uniform distribution of electron cloud. For example, petroleum ether, n-hexane, toluene, benzene, etc.

Why are hydrocarbon bonds in alkanes nonpolar?

Because the interaction force between C and H is so small that carbon in alkanes is not significantly negatively charged, and hydrogen is not significantly positively charged, which is essentially different from the significant polarity in hydroxyl and carbonyl groups. Therefore, hydrocarbon bonds are often regarded as nonpolar bonds in organic chemistry, which is from the point of view of material properties, and has a certain difference with the definition of nonpolar bonds in inorganic chemistry difference.

Comparison of polarity of common solvents

Strong polar solvent: methanol 〉 ethanol 〉 isopropanol

Medium polar solvents: acetonitrile 〉 ethyl acetate 〉 chloroform 〉 dichloromethane 〉 ether 〉 toluene

Nonpolar solvents: cyclohexane, petroleum ether, hexane, pentane

The order of polarity of single solvent: petroleum ether (small) → cyclohexane → carbon tetrachloride → trichloroethylene → benzene → toluene → dichloromethane → chloroform → ether → ethyl acetate → methyl acetate → acetone → n-propanol → methanol → pyridine → acetic acid (large)

Polar 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: ether (8 + 2) → chloroform: methanol (99 + 1) → benzene: methanol（ 9 + 1) → chloroform ∶ acetone (85 + 15) → benzene ∶ ether (4 + 6) → benzene ∶ ethyl acetate (1 + 1) → chloroform ∶ methanol (95 + 5) → chloroform ∶ acetone (7 + 3) → benzene ∶ ethyl acetate (3 + 7) → benzene ∶ ether (1 + 9) → ether ∶ methanol (99 + 1) → ethyl acetate ∶ methanol (99 + 1) → benzene ∶ acetone (1 + 1) → chloroform ∶ methanol (9 + 1)

PS: benzene: methanol (95 + 5) means 95 volumes of benzene mixed with 5 volumes of methanol to form a mixed solvent.

Common mixed solvents:

Ethyl acetate / hexane: common concentration 0 ~ 30%. But sometimes it is difficult to remove the solvent completely on the rotary evaporator.

Ether / pentane system: the concentration of 0 ~ 40% is commonly used. Very easy to remove on rotary evaporator.

Ethanol / hexane or pentane: 5 ~ 30% is suitable for strong polar compounds.

Dichloromethane / hexane or pentane: 5 ~ 30%, when other mixed solvents fail, it can be considered to use.

Polarity of common mobile phase:

Petroleum ether ＜ gasoline ＜ heptane ＜ hexane ＜ carbon disulfide ＜ xylene ＜ toluene ＜ chloropropane ＜ benzene ＜ bromoethane ＜ bromobenzene ＜ dichloroethane (DCM) ＜ chloroform ＜ isopropyl ether ＜ nitromethane ＜ butyl acetate ＜ ether ＜ ethyl acetate ＜ n-pentane ＜ n-butanol ＜ phenol ＜ methylethanol ＜ tert-butanol ＜ tetrahydrofuran ＜ dioxane ＜ acetone ＜ Ethanol ＜ acetonitrile ＜ methanol ＜ dimethylformamide (DMF) ＜ water