The most famous and used substances in human life and in the industry that belong to the category of polyhydric alcohols are ethylene glycol and glycerin. Their research and use began several centuries ago, but the properties of these organic compounds are largely unique and unique, which makes them indispensable to this day. Polyhydric alcohols are used in many chemical syntheses, industries, and spheres of human life.
The first "acquaintance" with ethylene glycol and glycerin: a history of production
In 1859, through a two-stage process of the interaction of dibromethane with silver acetate and subsequent treatment with potassium hydroxide obtained in the first reaction of ethylene glycol diacetate, Charles Wurz first synthesized ethylene glycol. Some time later, a method for the direct hydrolysis of dibromethane was developed, but on an industrial scale at the beginning of the twentieth century, the 1,2-dioxoethane diatomic alcohol, also known as monoethylene glycol, or simply glycol, was obtained in the USA by hydrolysis of ethylene chlorohydrin.
Today, both in industry and in the laboratory, a number of other methods are used, new, more economical from the raw materials and energy points of view, and environmentally friendly, since the use of reagents containing or releasing chlorine, toxins, carcinogens and other hazardous to the environment and humans substances, decreases with the development of "green" chemistry.
Glycerin was discovered by pharmacist Karl Wilhelm Scheele in 1779, and Teofil Jules Peluse studied the composition of the compound in 1836. Two decades later, the structure of the molecule of this trihydric alcohol was established and justified in the works of Pierre Eugene Marseille Vertelo and Charles Wurz. Finally, twenty years later, Charles Friedel conducted a complete synthesis of glycerin. Currently, the industry uses two methods for its production: through allyl chloride from propylene, as well as through acrolein. The chemical properties of ethylene glycol, like glycerol, are widely used in various fields of chemical production.
The structure and structure of the compound
The molecule is based on the unsaturated hydrocarbon skeleton of ethylene, consisting of two carbon atoms, in which a double bond has been broken. Two hydroxyl groups joined the vacant valencies of the carbon atoms. The ethylene formula is C 2 H 4 , after breaking the crane link and attaching the hydroxyl groups (after several stages), it looks like C 2 H 4 (OH) 2 . This is ethylene glycol.
The ethylene molecule has a linear structure, while the dihydric alcohol has a kind of trans configuration in the arrangement of hydroxyl groups with respect to the carbon backbone and to each other (this term is fully applicable to the position with respect to multiple bonds). Such a dislocation corresponds to the most remote arrangement of hydrogens from functional groups, less energy, and therefore - maximum system stability. Simply put, one OH group "looks up" and the other down. At the same time, compounds with two hydroxyls are unstable: at one carbon atom, forming in the reaction mixture, they immediately dehydrate, turning into aldehydes.
Classification
The chemical properties of ethylene glycol are determined by its origin from the group of polyhydric alcohols, namely a subgroup of diols, that is, compounds with two hydroxyl fragments at adjacent carbon atoms. A substance also containing several OH substituents is glycerol. It has three alcohol functional groups and is the most common representative of its subclass.
Many compounds of this class are also obtained and used in chemical production for various syntheses and other purposes, but the use of ethylene glycol has a more serious scale and is involved in almost all industries. This issue will be considered in more detail below.
physical characteristics
The use of ethylene glycol is explained by the presence of a number of properties that are inherent in polyhydric alcohols. These are distinctive features that are characteristic only for this class of organic compounds.
The most important property is the unlimited ability to mix with H 2 O. Water + ethylene glycol gives a solution with a unique characteristic: its freezing temperature, depending on the concentration of the diol, is 70 degrees lower than that of pure distillate. It is important to note that this dependence is non-linear, and upon reaching a certain quantitative glycol content, the opposite effect begins - the freezing temperature rises with an increase in the percentage of soluble substance. This feature has found application in the production of various antifreezes, non-freezing liquids, which crystallize at extremely low thermal characteristics of the environment.
In addition to water, the dissolution process proceeds well in alcohol and acetone, but is not observed in paraffins, benzenes, ethers and carbon tetrachloride. Unlike its aliphatic ancestor - a gaseous substance such as ethylene, ethylene glycol is a syrup-like, transparent, with a slight yellow tinge liquid, sweet in taste, with an uncharacteristic smell, practically non-volatile. Freezing 100% ethylene glycol occurs at -12.6 degrees Celsius, and boiling at +197.8. Under normal conditions, the density is 1.11 g / cm 3 .
Production Methods
Ethylene glycol can be obtained in several ways, some of them today have only historical or preparatory value, while others are actively used by humans on an industrial scale and not only. Following in chronological order, we consider the most important.
The first method for producing ethylene glycol from dibromoethane has already been described above. The formula of ethylene, the double bond of which is broken and the free valencies are occupied by halogens - the main starting material in this reaction - in addition to carbon and hydrogen, has two bromine atoms. The formation of an intermediate compound at the first stage of the process is possible precisely due to their cleavage, i.e., substitution with acetate groups, which, upon further hydrolysis, turn into alcohol.
In the process of further development of science, it became possible to obtain ethylene glycol by direct hydrolysis of any ethanes substituted with two halogens at adjacent carbon atoms, using aqueous solutions of metal carbonates from the alkali group or (less environmentally friendly reagent) H 2 O and lead dioxide. The reaction is rather "laborious" and proceeds only at significantly elevated temperatures and pressures, but this did not prevent the Germans from using the method for the production of ethylene glycol on an industrial scale during the world wars.
A role in the formation of organic chemistry was also played by the method of producing ethylene glycol from ethylene chlorohydrin by its hydrolysis with coal salts of alkali metal metals. When the reaction temperature was increased to 170 degrees, the yield of the target product reached 90%. But there was a significant drawback - the glycol had to be somehow extracted from the salt solution, which was directly fraught with a number of difficulties. Scientists solved this problem by developing a method with the same starting material, but dividing the process into two stages.
The hydrolysis of ethylene glycol acetate, which was the final stage of the Würz method, became a separate method when they were able to obtain the starting reagent by oxidizing ethylene in acetic acid with oxygen, that is, without the use of expensive and completely non-ecological halogen compounds.
There are also many methods for the production of ethylene glycol by oxidation of ethylene with hydroperoxides, peroxides, organic acids in the presence of catalysts (osmium compounds), potassium chlorate , etc. There are also electrochemical and radiation-chemical methods.
General chemical characterization
The chemical properties of ethylene glycol are determined by its functional groups. One hydroxyl substituent or both may be involved in the reactions, depending on the process conditions. The main difference in reactivity is that due to the presence of several hydroxyls in the polyhydric alcohol and their mutual influence, stronger acidic properties are manifested than in the monatomic "counterparts". Therefore, in reactions with alkalis, the products are salts (for glycol - glycolates, for glycerol - glycerates).
The chemical properties of ethylene glycol, as well as glycerol, include all reactions of alcohols from the monoatomic category. Glycol gives full and partial esters in reactions with monobasic acids, glycolates, respectively, are formed with alkali metals, and in the chemical process with strong acids or their salts, acetic acid aldehyde is released due to the removal of a hydrogen atom from the molecule.
Active metal reactions
The interaction of ethylene glycol with active metals (standing after hydrogen in the chemical series of tension) at elevated temperatures gives ethylene glycolate of the corresponding metal, plus hydrogen is released.
C 2 H 4 (OH) 2 + X → C 2 H 4 O 2 X, where X is the active divalent metal.
Qualitative reaction to ethylene glycol
Polyhydric alcohol can be distinguished from any other liquid with the help of a visual reaction characteristic only for this class of compounds. For this, a freshly precipitated copper hydroxide (2) having a characteristic blue hue is poured into a colorless alcohol solution. During the interaction of mixed components, the precipitate dissolves and the solution stains in a deep blue color due to the formation of copper glycolate (2).
Polymerization
The chemical properties of ethylene glycol are of great importance for the production of solvents. The intermolecular dehydration of the aforementioned substance, that is, the removal of water from each of the two glycol molecules and their subsequent combination (one hydroxyl group is completely eliminated, and only hydrogen leaves the other), makes it possible to obtain a unique organic solvent - dioxane, which is often used in organic chemistry, despite its high toxicity.
Exchange of hydroxyl to halogen
When ethylene glycol reacts with hydrohalic acids, hydroxyl groups are replaced by the corresponding halogen. The degree of substitution depends on the molar concentration of hydrogen halide in the reaction mixture:
HO-CH 2 —CH 2 —OH + 2HX → X — CH 2 —CH 2 —X, where X is chloro or bromo.
Getting esters
In the reactions of ethylene glycol with nitric acid (of a certain concentration) and monobasic organic acids (formic, acetic, propionic, buttered, valerianic, etc.), complex and, accordingly, simple monoesters are formed. In others, the concentration of nitric acid is the glycol di- and trinitroesters. Sulfuric acid of a given concentration is used as a catalyst.
Essential Ethylene Glycol Derivatives
Valuable substances that can be obtained from polyhydric alcohols using simple chemical reactions (described above) are ethylene glycol esters. Namely: monomethyl and monoethyl, the formulas of which are HO — CH 2 —CH 2 —O — CH 3 and HO — CH 2 —CH 2 —O — C 2 H 5, respectively. In chemical properties, they are largely similar to glycols, but, like any other class of compounds, they have unique reaction features that are unique to them:
- Monomethylene glycol is a liquid without color, but with a characteristic disgusting odor, boiling at 124.6 degrees Celsius, perfectly soluble in ethanol, other organic solvents and water, much more volatile than glycol, and with a density lower than that of water (about 0.965 g / cm 3 ).
- Dimethylethylene glycol is also a liquid, but with a less characteristic odor, density 0.935 g / cm 3 , a boiling point of 134 degrees above zero and solubility, compared with the previous homologue.
The use of cellosolves - as ethylene glycol monoesters are generally called - is fairly common. They are used as reagents and solvents in organic synthesis. Their physical properties are also used for anticorrosion and anti-crystallization additives in antifreezes and motor oils.
Fields of application and pricing policy
The cost in factories and enterprises involved in the production and sale of such reagents varies on average about 100 rubles per kilogram of a chemical compound such as ethylene glycol. The price depends on the purity of the substance and the maximum percentage of the target product.
The use of ethylene glycol is not limited to any one area. So, as a raw material it is used in the production of organic solvents, artificial resins and fibers, liquids, freezing at low temperatures. He is involved in many industrial sectors, such as automotive, aviation, pharmaceutical, electrical, leather, tobacco. Its importance for organic synthesis is undeniably significant.
It is important to remember that glycol is a toxic compound that can cause irreparable harm to human health. Therefore, it is stored in sealed containers made of aluminum or steel with an obligatory inner layer that protects the tank from corrosion, only in vertical positions and rooms that are not equipped with heating systems, but with good ventilation. The term is not more than five years.