The classification and size order of polar solvents
Polar solvent
Polar solvent refers to the solvent containing polar groups such as hydroxyl or carbonyl, that is, the solvent molecule is a polar molecule. The polarity of the molecule is caused by the non coincidence of the centers of gravity of positive and negative charges in the molecule. The physical quantity used to characterize the polarity of molecules is dipole moment or dielectric constant. A large dielectric constant indicates a large polarity.
Chemical covalent bond can be divided into polar bond and nonpolar bond. Nonpolar bonds are common electron pairs that are not offset and appear in simple substances, such as O2; A polar bond is an offset of a common electron pair, such as HCl. When the offset is very strong, it seems that one side completely loses electrons and the other side gets electrons, which will become ionic bonds, such as NaCl.
The polarity of a compound depends on the functional groups and molecular structure contained in the molecule. The polarity of various compounds increases in the following order:
– CH3, – CH2 -, – CH =, – C III, – o-r, – S-R, – NO2, – n (R) 2, – ocor, – CHO, – COR, – NH2, – Oh, – COOH, – SO3H
Strong polar solvent
Methanol > ethanol > isopropanol
Medium polar solvent
Ethyl cyanide ＞ ethyl acetate ＞ chloroform ＞ dichloromethane ＞ ether ＞ toluene
Nonpolar solvent
Cyclohexane, petroleum ether, hexane, pentane
Polarity order 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)
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 concentration: 0 ~ 30%. However, it is sometimes difficult to completely remove the solvent on the rotary evaporator.
Ether / pentane system: it is commonly used when the concentration is 0 ~ 40%. It is very easy to remove on the rotary evaporator.
Ethanol / hexane or pentane: 5 ~ 30% is suitable for strong polar compounds.
Dichloromethane / hexane or pentane: 5 ~ 30%, which can be considered when other mixed solvents fail.
Comparison of functional group polarity
Alkanes (- CH3, – CH2 -) < olefins (- CH = ch -) < ethers (- O-CH3, – o-ch2 -) < nitro compounds (- NO2) < dimethylamine (ch3-n-ch3) < lipids (- coor) < ketones (- co -) < aldehydes (- CHO) < mercaptans (- SH) < amines (- NH2) < amides (- nhco-ch3) < alcohols (- OH) < phenols (- ar-oh) ＜ carboxylic acids (- COOH)
Common mobile phase polarity
Petroleum ether ＜ gasoline ＜ heptane ＜ hexane ＜ carbon disulfide ＜ xylene ＜ toluene ＜ chloropropane ＜ benzene ＜ bromoethane ＜ brominated benzene ＜ 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 ＜ nitrogen dimethylformamide (DMF) ＜ water
Indicates the polarity of an organic solvent, which is related to its physicochemical properties, such as dielectric constant, dipole moment or refractive index. This representation regards all solvents as continuous acting media rather than discontinuous unity composed of various molecules, and does not take into account the special interaction between solvents and solutes.
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