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Solvent

From Academic Kids

A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. The most common solvent in everyday life is water. The term organic solvent refers to most other solvents that are organic compounds and contain carbon atoms. Solvents usually have a low boiling point and evaporate easily or can be removed by distillation, thereby leaving the dissolved substance behind. Solvents should therefore not react chemically with the dissolved compounds - they have to be inert. Solvents can also be used to extract soluble compounds from a mixture, the most common example is the brewing of coffee or tea with hot water. Solvents are usually clear and colorless liquids and most of them have a characteristic smell. The concentration of a solution is the amount of compound that is dissolved in a certain volume of solvent. The solubility is the maximal amount of compound that is soluble in a certain volume of solvent at a specified temperature.

Common uses for organic solvents are in dry cleaning (e.g.tetrachloroethylene), as paint thinners (e.g. toluene, turpentine), as nail polish removers and glue solvents (acetone, methyl acetate, ethyl acetate), in spot removers (e.g. hexane, petrol ether), in detergents (citrus terpenes), in perfumes (ethanol), and in chemical syntheses.


Contents

Polarity, solubility, and miscibility

Solvents and solutes can be broadly classified into polar (hydrophilic) and non-polar (lipophilic). The polarity can be measured as the dielectric constant or the dipole moment of a compound. The polarity of a solvent determines what type of compounds it is able to dissolve and with what other solvents or liquid compounds it is miscible with. As a rule of thumb, polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds best: "like dissolves like". Strongly polar compounds like inorganic salts (e.g. table salt) or sugars (e.g. sucrose) dissolve only in very polar solvents like water, while strongly non-polar compounds like oils or waxes dissolve only in very non-polar organic solvents like hexane. Similarly, water and hexane (or vinegar and salad oil) are not miscible with each other and will quickly separate into two layers even after being shaken well.

Protic and aprotic solvents

Polar solvents can be further subdivided into polar protic solvents and polar aprotic solvents. Water (H-O-H), ethanol (CH3-CH2-OH), or acetic acid (CH3-C(=O)OH) are representative polar protic solvents. A Polar aprotic solvent is acetone (CH3-C(=O)-CH3). In chemical reactions the use of polar protic solvents favors the SN1 reaction mechanism, while polar aprotic solvents favor the SN2 reaction mechanism.

Boiling point

Another important property of solvents is their boiling point that also determines the speed of evaporation. Small amounts of low-boiling solvents like diethyl ether, methylene chloride, or acetone will evaporate in seconds at room temperature, while high-boiling solvents like water or dimethylsulfoxide need higher temperatures, an air flow, or the application of vacuum for fast evaporation.

Density

Most organic solvents have a lower density than water. Therefore they are lighter and will separate on top of water. An important exception are many halogenated solvents like methylene chloride or chloroform that will sink to the bottom. This is important to remember when partitioning compounds between solvents and water in a separatory funnel during chemical syntheses.

Chemical interactions

A solvent will create various weak chemical interactions with the solute in order to solubilize it. The most common of these interactions are the relatively weak van der Waals interactions (induced dipole interactions), the stronger dipole-dipole interactions, and the even stronger hydrogen bonds (interaction between O-H or N-H hydrogens with O or N atoms).

Safety

Most organic solvents are flammable or highly flammable, depending on their volatility. Exceptions are some chlorinated solvents like methylene chloride and chloroform. Mixtures of solvent vapors and air can explode. Solvent vapors are heavier than air, they will sink to the bottom and can travel large distances nearly undiluted. Solvent vapors can also form in supposedly empty drums and cans.

Ethers like diethyl ether and tetrahydrofuran (THF) can form highly explosive peroxides upon exposure to oxygen and light. These peroxides will concentrate during distillation due to their higher boiling point. Ethers have to be stored in the dark in closed canisters in the presence of stabilizers like BHT or over sodium hydroxide.

Many solvents can lead to a sudden loss of consciousness if inhaled in larger amounts. Solvents like diethyl ether and chloroform have been used in medicine as anesthetics and narcotics for a long time. Ethanol is a widely used and abused psychoactive drug. Diethyl ether, chloroform, and many other solvents (e.g. from gasoline or glues) are used recreationally in glue sniffing, often with harmful long term health effects like neurotoxicity or cancer.

Some solvents including chloroform and benzene (an ingredient of gasoline) are carcinogenic. Many others can damage internal organs like the liver, the kidneys, or the brain. Methanol can cause internal damage to the eyes including permanent blindness.

General precautions

  • Avoid the generation of solvent vapors by working in a fume hood or a well ventilated area
  • Keep the storage containers tightly closed.
  • Never use open flames near flammable solvents, use electrical heating instead.
  • Never flush flammable solvents down the drain to avoid explosions and fires.
  • Avoid the inhalation of solvent vapors.
  • Avoid contact of the solvent with the skin - many solvents are easily absorbed through the skin.

Properties table of common solvents

The solvents are grouped into non-polar, polar aprotic, and polar protic solvents and ordered by increasing polarity. The polarity is given as the dielectric constant. The density of unpolar solvents that are heavier than water is bolded.


Solvent Chemical Formula Boiling point Polarity Density
Non-Polar Solvents
Hexane CH3-CH2-CH2-CH2-CH2-CH3 69 C 2.0 0.655 g/ml
Benzene C6H6 80 C 2.3 0.879 g/ml
Toluene C6H5-CH3 111 C 2.4 0.867 g/ml
Diethyl ether CH3CH2-O-CH2-CH3 35 C 4.3 0.713 g/ml
Chloroform CHCl3 61 C 4.8 1.498 g/ml
Ethyl acetate CH3-C(=O)-O-CH2-CH3 77 C 6.0 0.894 g/ml
Tetrahydrofuran (THF) /CH2-CH2-O-CH2-CH2\ 66 C 7.5 0.886 g/ml
Methylene chloride CH2Cl2 40 C 9.1 1.326 g/ml
Polar Aprotic Solvents
Acetone CH3-C(=O)-CH3 56 C 21 0.786 g/ml
Acetonitrile (MeCN) CH3-C≡N 82 C 37 0.786 g/ml
Dimethylformamide (DMF) H-C(=O)N(CH3)2 153 C 38 0.944 g/ml
Dimethyl sulfoxide (DMSO) CH3-S(=O)-CH3 189 C 47 1.092 g/ml
Polar Protic Solvents
Acetic acid CH3-C(=O)OH 118 C 6.2 1.049 g/ml
n-Butanol CH3-CH2-CH2-CH2-OH</sub> 118 C 18 0.810 g/ml
Isopropanol CH3-CH(-OH)-CH3 82 C 18 0.785 g/ml
n-Propanol CH3-CH2-CH2-OH</sub> 97 C 20 0.803 g/ml
Ethanol CH3-CH2-OH</sub> 79 C 24 0.789 g/ml
Methanol CH3-OH 65 C 33 0.791 g/ml
Formic acid H-C(=O)OH 100 C 58 1.21 g/ml
Water H-O-H 100 C 80 0.998 g/ml

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