Organic solvent classification

Organic solvent

Ⅰ. Hydrocarbon solvents


Organic compounds that contain only two elements, hydrocarbons and carbon, are called hydrocarbons. Hydrocarbons are classified according to their structure into aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons include aliphatic chain hydrocarbons and alicyclic hydrocarbons. Open-chain aliphatic hydrocarbons are classified into saturated-chain hydrocarbons (alkanes) and unsaturated-chain hydrocarbons (olefins and alkynes) according to the degree of saturation. Aromatic hydrocarbons are hydrocarbons with a special structure containing a benzene ring. According to the specific structure, they are classified into monocyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons and thick ring aromatic hydrocarbons.

Hydrocarbon solvents are divided into two types according to their sources: petroleum solvent oil (PSO), which is a mixture of hydrocarbons obtained from petroleum fractionation; and pure hydrocarbon solvents, which are synthesized or refined from chemical raw materials and consist of a single hydrocarbon component. Pure solvents are more expensive and are usually used for special purposes only. 2.

Solvent oil

Petroleum is a mixture of a variety of hydrocarbons, which are fractionated to obtain products with different boiling point ranges. According to the boiling, sip range is usually divided into petroleum products such as petroleum ether, gasoline, kerosene, diesel, lubricating oil, paraffin and asphalt. Among them, the boiling point range of 30 ~ 90 ℃ pentane and hexane as the main components of the petroleum ether and boiling point range of 40 ~ 200 ℃ hydrocarbon molecules in the number of carbon molecules in 4 ~ 12 of gasoline, has a very good dissolution performance. It is often used as a solvent in industrial production and is called solvent oil or solvent gasoline. In recent years, it has also developed kerosene and even light diesel fraction as high boiling point solvent oil, which broadens the concept of solvent oil. Kerosene is the fraction of petroleum fractionation, boiling point in the range of 175-325 ℃, due to the long distillation range contains complex hydrocarbon composition. It can also be used as a solvent under certain circumstances, such as the dry-cleaning solvent gasoline (stoddard) used by the American dry-cleaning industry.
solvent) is actually a non-flammable kerosene solvent. Therefore, broadly speaking, solvent oil includes hydrocarbon mixtures of various boiling ranges and pure hydrocarbon solvents such as hexane, benzene, toluene, and xylene. For the convenience of description, the solvent oil introduced in this book refers to the hydrocarbon mixture solvent obtained from petroleum fractionation.

(1) Classification of solvent oils according to boiling range

According to the boiling range of the fractional distillation process, solvent oil is roughly divided into three categories: those with boiling range below 100℃ are called low boiling solvent oil, such as industrial No.6 extraction solvent oil with boiling range of 60-90℃; those with boiling range of 100-150℃ are called medium boiling solvent oil, such as rubber solvent oil with boiling range of 80-120℃; those with boiling range above 150℃ are called high boiling solvent oil, such as paint solvent oil; and those with boiling range above 150℃ are called high boiling solvent oil. The boiling range is 140-200℃, and the dry point of ink solvent oil up to 360℃ are high boiling solvent oils. From the boiling range, most of them belong to gasoline fraction.

(2) Chemical composition of solvent oil

Solvent oil is a mixture of various hydrocarbons, mainly consisting of open-chain alkanes, olefins, naphthenes and aromatics. Since olefins are chemically active and poorly stable, they are not suitable for use as solvents, so solvent oil contains few olefins and the other three hydrocarbons are the main components.

Low-boiling solvent oils, such as No. 6 extraction solvent oil, No. 120 rubber solvent oil, No. 200 paint solvent oil, mainly contain alkanes and naphthenes. Sometimes called aliphatic hydrocarbon solvents, aliphatic hydrocarbon solvent oil components are straight-chain alkanes, branched-chain alkanes, naphthenes. For example, the solvent oil with branched-chain alkane as the main component is called isomeric alkane solvent oil, and its solubility is better than ordinary aliphatic solvent oil, while the high boiling solvent oil with large content of aromatic hydrocarbons such as toluene and xylene is called aromatic solvent oil, such as the high boiling point aromatic solvent oil whose main component is molecules containing toluene and xylene. Aromatic hydrocarbons with 9 carbon atoms.

The performance of solvent oil is closely related to its chemical composition, because the order of solubility of hydrocarbons is: aromatics > naphthenes > chain alkanes. Therefore, solvent oil with the same boiling range containing more strepta and naphthenes has a higher aniline point, lower shellac butanol value and poorer solubility than solvent oil containing more aromatics.

Although pure aromatic solvent oils have high solvency power, they are also highly toxic. Therefore, there is a trend in the industry to use high aromatic solvent oils and low aromatic solvent oils instead of pure aromatic solvents such as benzene, toluene and xylene. Therefore, there is a trend in the industry to use high aromatic solvent oil and low aromatic solvent oil instead of pure aromatic solvent such as benzene, toluene and xylene. For example, the content of aromatic hydrocarbons in solvent oil for paints is required to be less than 15%. 3.

Hydrocarbon solvents of vegetable origin

Hydrocarbon solvents can be obtained from petroleum fractions as well as from plants, such as turpentine and lemon oil.

(1) Turpentine

This oil is a typical representative of the plant-derived hydrocarbon solvents that belong to the rosin group present in a natural resin called turpentine, and when the turpentine is distilled, the distillate is turpentine and the solid residue is rosin. When turpentine is distilled, the distillate is turpentine and the solid residue is rosin. Turpentine is a mixture of hydrocarbons, the main components are o- and household pinene. It is an excellent solvent for oils, waxes and resins, and its solvency capacity is between that of the petroleum solvent oil and benzene. It has a high boiling point and ignition point, so it is safe to use, but it has the disadvantage of being in contact with oxygen in the air or being easily oxidized to resinous substances and discolored under light conditions.

(2) Lemon oil (orange oil)

It is a hydrocarbon solvent derived from the distillation of orange peel, orange peel, and lemon peel. Its main chemical component is ramie; a monocyclic terpene (methacryloylcyclohexene), and other components include citral.

The boiling point of ramie is 175.5-176°C. Its physical properties are similar to turpentine, and it has a lemon fruit flavor.

Because of the abundance of oranges and citrus fruits in the United States, lemon oil is a hydrocarbon solvent that can be obtained relatively cheaply. At present, lemon oil has been used in the United States to replace CFC-113 (trichlorotrifluoroethane) as the solvent for cleaning semiconductor PCBs.

Hydrocarbon pure solvents

Hydrocarbon pure solvents obtained from the synthesis of chemical raw materials mainly include hexane, benzene, toluene, xylene and so on.

(1)Hexane (C6H14)

A low-boiling hydrocarbon pure solvent, usually obtained from the further refining and processing of natural gasoline, straight-run gasoline and other petroleum light fractions. It is a typical non-polar solvent that can dissolve all kinds of hydrocarbons and halogenated hydrocarbon B. It is usually used as a solvent for extracting all kinds of oils and essential oils, and also as a cleaning agent for precision instruments. Because of its low boiling point and flammability, it should be used with special attention to fire prevention. Although it has low toxicity, it is narcotic even when inhaled in large quantities.

(2) Benzene, toluene, xylene

They are three important aromatic solvents. They have high KB value, low aniline point, and strong solubility for oil-soluble dirt, making them excellent performance solvents. Their disadvantage is their high toxicity, which is also an important factor in causing photochemical smog and air pollution. Benzene, in particular, is more toxic, so special attention should be paid when using it. For example, the flash point of benzene is -11.1℃, and its content in the air may cause an explosion in the range of 1.5%-8%.

Due to the high price of pure aromatic hydrocarbon solvents, aromatic solvent oils have been used instead in many departments. High-boiling point aromatic solvent oil is a product of catalytic reforming in oil refining and chemical plants, with C9 and C10 aromatic hydrocarbons as the main components, such as isopropylbenzene, n-propylbenzene, p-methyl ethylbenzene and so on.

Fractions with initial distillation points of about 120°C, 120°C to 150°C, and 150 to 200°C can be used instead of toluene, xylene, and No. 200 paint solvent oil.

The most important thing to note is that xylene is the general name of o-, inter- and para-xylene. Because of its high boiling point, it is difficult to dry when used for cleaning.

Due to flammability, toxicity and high price, aromatic hydrocarbon solvents are used only in special cases of necessity.

Ⅱ. Halogenated solvents

Two types of halogenated hydrocarbons used for cleaning are chlorinated hydrocarbon solvents and hydrochlorofluorocarbon (HCFC) solvents. A chlorinated hydrocarbon is obtained if some or all of the hydrogen atoms in the hydrocarbon molecule are replaced by chlorine.i If fluorine atoms are introduced into the chlorinated hydrocarbon molecule, HCFCs are obtained. Halogen substitutes are commonly used.

Hydrocarbon solvents are obtained by halogenation of small molecule hydrocarbons such as methane, ethane, and ethylene. For example, dichloromethane, chloroform (CHCl3), carbon tetrachloride, fluorine dichloromethane, fluorotrichloromethane, ethane 1,1,1-trichloroethane, and ethylene trichloroethylene and tetrachloroethylene, etc. Halogenated hydrocarbons are more dense than water and are not soluble in water.

Halogenated hydrocarbons are denser than water, and all are insoluble in water. They are characterized by their nonflammable and flame-retardant properties, with those that do not contain hydrogen atoms in the molecule being nonflammable and those that do contain hydrogen atoms in the molecule being flame-retardant. This is an advantage compared to hydrocarbon solvents. Halogenated hydrocarbon solvents have strong solvency power for oily dirt, such as degreasing ability, which is about 10 times stronger than petroleum solvent oil. Generally, halogenated hydrocarbons have a low boiling point and are easy to volatilize, so they consume less energy for distillation and recovery. At present, it has replaced hydrocarbon solvents in many sectors in the field of industrial cleaning and plays an important role.

1.Chlorinated hydrocarbon solvent


It is a low boiling point (boiling point 39.8℃), volatile solvent with excellent degreasing ability and is often used as a washing solvent for precision machinery parts and as a stripper for plastic films and dichloromethane coatings.

It is a low boiling point (boiling point 39.8℃) and volatile solvent. It is currently used in many places to replace FC-113 (trichlorotrifluoroethane). Its disadvantage is the serious pollution to the atmosphere, in order to reduce the vapor escape to a minimum, it is necessary to take cooling measures when cleaning to control the maximum allowable content in the air within 500mg/kg.

(2)1,l,1,trichloroethane – (C13C-CH3)

It is the least toxic of the chlorinated hydrocarbon solvents and has good solvency capacity, but its greatest drawback is its poor chemical stability, and it is facing a ban because of its damaging effect on the ozone layer of the atmosphere.

(3) Trichloroethylene (C12C == CHCl)

It is a colorless toxic liquid with a chloroform-like odor, good chemical stability, non-flammable and an excellent solvent for degreasing. At room temperature, its KB value is 130, which is 4 times that of gasoline, and at 50℃, its KB value is 7 times that of gasoline. Its biggest drawback is its strong toxicity. When it meets water vapor under light conditions, it will slowly decompose and produce a highly toxic phosgene. Therefore, it is not suitable for use in factories.
There are strict limits on its content in industrial environments. It used to be used as a dry cleaning solvent for clothes, but has been replaced by perchloroethylene due to its high toxicity.

(4) Tetrachloroethylene (C12C == CCl2)

Also called perchloroethylene, it is a colorless and transparent flowing liquid with a special smell similar to ether. In order to prevent its decomposition, 0.5% ethanol is often added as stabilizer and stored in a sealed container away from light, and it is now widely used as a dry cleaning solvent.

(5) Chloroform (CHCl3)

Also known as trichloromethane, it is a colorless, transparent and volatile liquid, not easy to burn, slightly soluble in water, and a solvent with good lipolytic properties. However, under the action of light, it can be oxidized by oxygen in the air to produce hydrogen chloride and phosgene, so it is highly toxic, so it is usually added 1% to 2% ethanol to make the generation of dagger gas and ethanol to generate ethyl carbonate to eliminate the toxicity.

(6)Carbon tetrachloride (CCl)

It is a denser (1.515g/cm3), non-flammable five-colored liquid, good degreasing performance, but the biggest drawback as a cleaning agent is toxic to people, the use of caution, its maximum permissible concentration in the air is 25mg/kg.

chlorofluorocarbon solvent

This refers to CFC-113 as the main body of the series of cleaning agent, also known as fluorocarbon cleaning agent.

CFC-113, also known as F-133, R-133, its chemical name is trifluoroethane -, molecular formula for C2ClsF boiling point 47.6 ℃, the liquid density of i1.565g/cm3, 25 ℃ viscosity of 6.8X10 -No explosion in air.

The structure of commonly used trifluorochloroethane solvent is CCl2F-CClzF, a colorless, odorless liquid, insoluble in water; unlike other halogenated hydrocarbon solvents, it does not form an azeotropic mixture with water. Compared with other chlorinated hydrocarbon solvents, the biggest advantage of CCl2F-CClzF is its good chemical stability. It shows high stability to chemical reagents, water, and heat. It also does not react with lubricating oil and can be stored at room temperature for a long time without deterioration. Its KB value of 31 is lower than the KB value of commonly used chlorinated hydrocarbons, but its KB value and surface tension are similar to those of aliphatic chain hydrocarbon solvents, so it has good solubility to mineral oil. Because the fluorocarbon solvent surface tension and viscosity is small, strong penetration, evaporation rate, with its cleaning parts generally do not need to wipe or drying treatment: can be thousands of their own – dry, so it is conducive to the realization of mechanized automatic cleaning. And on the plastic and rubber swelling effect of small, can be very good at removing mineral oil dirt without harming the plastic and rubber surface and other chlorinated hydrocarbons have different properties to corrode metal materials. Therefore, it is one of the most compatible organic solvents, except for silicone rubber, polystyrene and metallic zinc, it has no effect on most metals, plastics, tapes, paint coatings and wire insulation, and it does not dissolve; moreover, it is non-flammable and safe to use, and it is non-toxic, with a maximum allowable concentration of 1000mg/L in the air environment of cleaning plants. CFC-113 has a very high dissolution and washing capacity, e.g., it can dissolve grease and oil up to 4.45kg/(m2-h), while trichloroethylene, which has a 90% degreasing efficiency, can dissolve grease and oil up to 3.1kg/(m2-h).

Due to the above advantages, TCE is widely used in metal material cleaning and precision cleaning in the electronic industry. In the electronics industry it is used to clean rosin flux contaminants. Since it does not damage electronic components that are sensitive to organic solvents, it is commonly used to clean computer disk drives, gyroscopes and miniature bearings in satellite and aerospace equipment, military communications equipment, and parts of optical instruments. But because it has a damaging effect on the ozone layer of the atmosphere, is currently facing a ban on the use of the situation, countries are pressing to develop a variety of its alternative solvents, the details will be described in detail later.

There are many types of fluorocarbon solvents used in industrial cleaning, including CFC-113 and CFC-113 mixed with anhydrous ethanol, propanol, methanol, acetone, dichloromethane, and aqueous surfactant solutions.

Ⅲ. Alcohol solvents

Organic compounds in which the fatty hydrocarbon group of the molecule is directly linked to the hydroxyl group belong to the alcohol group. They are classified into mono-alcohols and polyols based on the number of hydroxyl groups contained in the molecule. Aromatic alcohols that contain benzene or a benzene homologue in their structure, but the hydroxyl group is not directly connected to the benzene ring, are aromatic alcohols. Used in Cleaning
There are three main types of alcoholic organic solvents.

water-soluble mono alcohol solvents

The most widely used alcoholic solvents in cleaning, such as methanol, ethanol, isopropanol, etc.

Water-soluble monohydric alcohol solvents are flammable, strongly hydrophilic solvents that are miscible with water in any ratio. They can be anhydrous or aqueous solvents. Their highly concentrated aqueous solutions are highly soluble in oily substances and are therefore used to remove rosin-based solder fluxes from electronic printed circuit boards. They are also characterized by their high solubility in surfactants, so they are often used to remove residual adsorption films formed by surfactants on detergent surfaces, which is a special purpose of alcoholic solvents such as ethanol.

Due to the strong binding of water-soluble mono alcohol solvents with water, alcohol solvents are the most suitable when water needs to be replaced from the surface it is wetted on. However, alcohols and water form a constant-boiling mixture, so it is not possible to recover anhydrous ethanol from a water-containing ethanol solution using the usual distillation methods.

In addition, highly concentrated aqueous solutions of these alcohols have strong bactericidal and disinfectant properties.

Among the three kinds of mono-alcohols, methanol is highly toxic because of its poor solubility to oily dirt and strong toxicity. Ethanol is a low-toxic alcohol, easily biodegradable and less polluting to the environment, so it is the most widely used mono-alcohol solvent. Isopropyl alcohol has a better degreasing ability compared to methyl alcohol and ethanol.

As some countries (such as Japan) no longer classify the aqueous solution of 60% volumetric part of methyl alcohol, ethyl alcohol and isopropanol as flammable dangerous goods in fire regulations, which means that they can be used safely under certain conditions, the interest in using aqueous monohydric alcohol as a degreasing and cleaning solvent has increased greatly.

Low water-soluble monohydric alcohol solvents

As the number of carbon atoms in a molecule increases, its boiling point increases and its water solubility decreases. Therefore, low water-soluble meta-alcohols are often used to remove oil and dirt.

The main solvents used for cleaning such mono-alcohols are n-butanol, cyclohexanol and benzyl alcohol.

The characteristics of commonly used low water-soluble monohydric alcohol solvents are as follows.


The main component is n-butanol, which is both hydrophilic and lipophilic. It can dissolve 7.9 g of n-butanol in 100 g of water, which has a greater affinity for oily dirt than ethanol. It can be used as a solvent alone, or mixed with hydrophilic or oleophilic solvents for various cleaning applications.


It is also a solvent with a wide range of solubility for polar organic substances, it is both hydrophilic and lipophilic, and it is more lipophilic than butanol. Since it has certain emulsifying and solubilizing effects when mixed with water, it can also be mixed with water.

(3) Benzyl alcohol

It is an alcohol that is difficult to dissolve in water and has a high solubility for polar organic compounds.


A newly developed fluorinated, low-water reactive monohydric alcohol for cleaning that has attracted attention as an alternative solvent to HCFC synthetic solvents (e.g., CFC-113). It has the advantage of being non-flammable and safe to use. The introduction of fluorine atoms into the molecular structure of the alcohol makes it particularly low in surface tension. However, its disadvantages of poor water solubility, high corrosiveness, and high price limit its use to some extent.

(5) Terpineol (terpene alcohol)

Terpineol is a low water-soluble mono alcohol derived from plants with 10 carbon atoms in the molecule and has four isomers. Commercially available turpentine alcohol is a mixture of isomers of various terpene alcohols and other hydrocarbons, and is a five-colored viscous liquid or a five-colored transparent low-solubility solid. It has excellent foaming and emulsifying properties, and is often used as an emulsifying detergent for special purposes.

polyol solvent

Its solubility is similar to that of a monohydric alcohol, with the larger the percentage of hydroxyl groups in the molecule, the more hydrophilic it is. The most important and typical representative of polyol solvents is ethylene glycol, which is a five-colored viscous liquid with a little sweet smell that can be miscible with water in any ratio. The propylene glycol in polyols is oleophilic compared to ethylene glycol because of the reduced proportion of hydroxyl groups in the molecule.
It has enhanced properties and is often used as a rinse agent to remove aviation kerosene used on aircraft because it has little toxicity.

The various ethers derived from ethylene glycol, such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether and ethylene glycol monobutyl ether, are all good organic solvents. Among them, ethylene glycol monoethyl ether, commonly known as fibrinolytic agent, has strong solubility to polymer resins. Therefore, in addition to being a general cleaning solvent, they are often used as the main raw materials for paint strippers. However, due to their strong toxicity, special care should be taken when using them.

Ⅳ. Ketone solvents

The ketone solvents used in cleaning are mainly acetone and methyl ethyl ketone (2-butanone). Acetone, with molecular formula C3H60, is a lipophilic solvent soluble in water, a solvent with wide solubility range and low toxicity, so it is widely used as cleaning solvent. However, it has a low flash point and is a fusible solvent, so special attention should be paid to safety when using it.

Methyl ethyl ketone (2-butanone), with molecular formula C4H80, is a ketone solvent with great solvency power for oily organic substances. In addition to being a general washing solvent, it is mainly used as the main component of the solvent for stripping polymer resins from surfaces, and is more toxic than acetone.

Ⅴ. Ester solvents

Ester solvents are characterized by low toxicity, aromatic odor and insolubility in water. They are mostly used as solvents for oily organic substances, but lack of characteristics as cleaning solvents. Methyl acetate and ethyl acetate

Ⅵ. Phenolic solvents

Phenol solvents include phenol and methylphenol. They are slightly acidic organics with high melting points, have strong toxicity, and are mainly used as germicides and disinfectants. The main properties of phenol and o-cresol are shown in Table 6-27.

The phenol as soluble rj is a mixture of three isomers of o-, m-, and p-cresol. Phenols have a narrow range of applications as solvents, but are uniquely effective as solvents for removing carbon deposits from automobile and aircraft engines.

Commonly used non-polar solvents, such as chloroform, benzene, liquid paraffin, vegetable oil, ether, etc. Soluble substances such as iodine, bromine. Soluble substances such as iodine, bromine….

Commonly used polar solvents, such as water, formamide, methanol, ethanol, propanol, etc., can dissolve so many substances, so common that there is no need to give examples.

Solvents are usually divided into two categories: polar solvents and non-polar solvents.

The relationship between the type of solvents and the solubility of substances can be summarized as: “similarly soluble”. This means that polar solvents can dissolve ionic compounds and covalent compounds that can be dissociated, while non-polar solvents can only dissolve non-polar covalent compounds.

Non-polar solvents

The concept and definition of nonpolar solvents refers to a class of solvents with a low dielectric constant. They are also called inert solvents.

These solvents neither carry out proton self-delivery reactions nor solubilization with solutes.

Most of them are saturated hydrocarbons or compounds such as benzene, such as benzene, carbon tetrachloride and dichloroethane. For example, benzene, carbon tetrachloride, dichloroethane, etc.

Non-polar solvents are composed of non-polar molecules, meaning that the combined force of the chemical bonds of the atoms in the molecule is zero.

Non-polar solvents are composed of fatty oils (fattyoils), liquid paraffins (1iquidparaffin), ethyl acetate (ethyloleate), and isopropylmyristate. A nonpolar solvent is a solvent composed of a solution of nonpolar molecules, which are mostly composed of covalent bonds and have no or little electronic activity. It also refers to solvents with small dipole moments.

Non-polar solvents have a low dielectric constant and cannot weaken the gravitational force of electrolyte ions or form hydrogen bonds with other polar molecules. Non-polar solvents dissolve non-polar substances as a result of the van der Waals force between the solute and solvent molecules, and the instantaneous dipole generated inside the solvent molecules.

Polar solvents

Polar solvents are defined as solvents containing polar groups such as hydroxyl or carbonyl groups, which have a strong polarity and a large dielectric constant.

First of all, chemical covalent bonds are divided into polar bonds and non-polar bonds. A non-polar bond is a shared electron pair with no offset, which is found in monomeric substances such as O2; a polar bond is a shared electron pair with offset, such as HCl; and when the offset is very strong, it appears that one side has lost electrons completely and the other side has gained electrons, which will turn into an ionic bond, such as NaCl.

Let’s talk about polar and non-polar molecules. Due to the appearance of polar bonds, some molecules appear polar, but it does not mean that all molecules with polar bonds are polar molecules. For example, although CH4 contains four polar C-H bonds, because of its spatially symmetrical tetrahedral structure, the polarity of the bonds is eliminated, and the whole molecule is not polar.

H2O has the same type of molecular formula and polar covalent bond as CO2, but the polarity of the two molecules is different: CO2 is spatially symmetrical and linear, so the molecule is non-polar, while H2O is polar and asymmetrical, so it is a polar molecule and the solvent is called polar solvent.

The polarity of a compound depends on the functional groups and molecular structure contained in the molecule. The polarity of each compound increases in the following order: -CH3, -CH2-, -CH=, -CH3, -O-R. -S-R, -NO2, -N(R)2, -OCOR, -CHO, -COR,- NH2, -OH, -COOH, -SO3H

Polarity order of common solvents.

Water (maximum)>Formamide>acetonitrile>methanol>ethanol>propanol>acetone>dioxane>tetrahydrofuran>acetyl ketone>n-butanol>ethyl acetate>ethyl ether>isopropyl ether>dichloromethane>chloroform>bromoethane>benzene>carbon tetrachloride>carbon disulfide>cyclohexane>hexane>kerosene>kerosene (minimum)

Class I solvents are organic solvents that are human carcinogens, suspected human carcinogens, or environmental hazards. They should be avoided because of their unacceptable toxicity or environmental hazards. The Technical Guidelines for the Study of Chemical Drug Residual Solvents stipulates that five organic solvents such as benzene, carbon tetrachloride, 1,2-dichloroethane, 1,1-dichloroethylene, and 1,1,1-trichloroethane are Class I solvents.