The most famous and used in human life and industry substances belonging 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 in many ways inimitable and unique, which makes them indispensable to this day. Polyhydric alcohols are used in many chemical synthesis, industries and areas of human life.
The first "acquaintance" with ethylene glycol and glycerin: the history of obtaining
In 1859, through a two-stage process of reacting dibromoethane with silver acetate and then treating the ethylene glycol diacetate obtained in the first reaction with caustic potash, Charles Wurtz first synthesized ethylene glycol. Some time later, a method of direct hydrolysis of dibromoethane was developed, but on an industrial scale at the beginning of the twentieth century, dihydric alcohol 1, 2-dioxyethane, also known as monoethylene glycol, or simply glycol, in the USAobtained by hydrolysis of ethylene chlorohydrin.
Today, both in industry and in the laboratory, a number of other methods are used, new, more economical from a raw material and energy point of view, and environmentally friendly, since the use of reagents containing or releasing chlorine, toxins, carcinogens and other dangerous substances for the environment and humans, is declining with the development of "green" chemistry.
Glycerine was discovered by pharmacist Carl Wilhelm Scheele in 1779, and Theophile Jules Pelouze studied the composition of the compound in 1836. Two decades later, the structure of the molecule of this trihydric alcohol was established and substantiated in the works of Pierre Eugene Marseille Vertelot and Charles Wurtz. Finally, twenty years later, Charles Friedel carried out the complete synthesis of glycerol. Currently, the industry uses two methods for its production: through allyl chloride from propylene, and also through acrolein. The chemical properties of ethylene glycol, like glycerin, are widely used in various fields of chemical production.
The structure and structure of the connection
The molecule is based on an unsaturated hydrocarbon skeleton of ethylene, consisting of two carbon atoms, in which a double bond has been broken. Two hydroxyl groups were added to the vacated valence sites at the carbon atoms. The formula of ethylene is C2H4, after breaking the crane bond and adding hydroxyl groups (after several stages) it looks like C 2N4(OH)2. That's what it isethylene glycol.
The ethylene molecule has a linear structure, while the dihydric alcohol has a kind of trans configuration in the placement of hydroxyl groups in relation to the carbon backbone and to each other (this term is fully applicable to the position relative to the multiple bond). Such a dislocation corresponds to the most remote location of hydrogens from functional groups, lower energy, and hence the maximum stability of the system. Simply put, one OH group looks up and the other looks down. At the same time, compounds with two hydroxyls are unstable: at one carbon atom, being formed in the reaction mixture, they are immediately dehydrated, turning into aldehydes.
Classification
The chemical properties of ethylene glycol are determined by its origin from the group of polyhydric alcohols, namely the subgroup of diols, that is, compounds with two hydroxyl fragments at neighboring carbon atoms. A substance that also contains several OH substituents is glycerol. It has three alcohol functional groups and is the most common member of its subclass.
Many compounds of this class are also obtained and used in chemical production for various synthesis and other purposes, but the use of ethylene glycol is on a more serious scale and is involved in almost all industries. This issue will be discussed in more detail below.
Physical characteristics
The use of ethylene glycol is due to the presence of a number ofproperties that are inherent in polyhydric alcohols. These are distinctive features that are characteristic only for this class of organic compounds.
The most important of the properties is the unlimited ability to mix with H2O. Water + ethylene glycol gives a solution with a unique characteristic: its freezing point, depending on the diol concentration, 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 content of glycol, the opposite effect begins - the freezing point rises with an increase in the percentage of the dissolved substance. This feature has found application in the production of various antifreezes, anti-freeze liquids that crystallize at extremely low thermal characteristics of the environment.
Except in 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 - such a gaseous substance as ethylene, ethylene glycol is a syrup-like, transparent liquid with a slight yellow tint, sweetish in taste, with an uncharacteristic odor, practically non-volatile. Freezing of 100% ethylene glycol occurs at -12.6 degrees Celsius, and boiling at +197.8. Under normal conditions, the density is 1.11 g/cm3.
Getting Methods
Ethylene glycol can be obtained in several ways, some of which today have only historical or preparative significance, while othersactively used by man on an industrial scale and not only. In chronological order, let's take a look at the most important ones.
The first method for obtaining 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 in its composition. The formation of an intermediate compound at the first stage of the process is possible precisely due to their elimination, i.e., replacement by acetate groups, which, upon further hydrolysis, turn into alcohol ones.
In the process of further development of science, it became possible to obtain ethylene glycol by direct hydrolysis of any ethanes substituted by two halogens at neighboring carbon atoms, using aqueous solutions of metal carbonates from the alkaline group or (less environmentally friendly reagent) H2 Oh and lead dioxide. The reaction is quite "labor-intensive" and proceeds only at significantly elevated temperatures and pressures, but this did not prevent the Germans from using this method during the world wars to produce ethylene glycol on an industrial scale.
The method of obtaining ethylene glycol from ethylene chlorohydrin by its hydrolysis with carbon s alts of alkali group metals also played its role in the development of organic chemistry. With an increase in the reaction temperature 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 s alt solution, which is directly associated witha number of difficulties. Scientists solved this issue by developing a method with the same starting material, but breaking the process into two stages.
Ethylene glycol acetate hydrolysis, being the earlier final stage of the Wurtz method, became a separate method when they managed to obtain the starting reagent by oxidizing ethylene in acetic acid with oxygen, that is, without the use of expensive and completely unenvironmental halogen compounds.
There are also many ways to produce ethylene glycol by oxidizing ethylene with hydroperoxides, peroxides, organic peracids in the presence of catalysts (osmium compounds), potassium chlorate, etc. There are also electrochemical and radiation-chemical methods.
Characterization of general chemical properties
The chemical properties of ethylene glycol are determined by its functional groups. The reactions may involve one hydroxyl substituent or both, depending on the process conditions. The main difference in reactivity lies in the fact that due to the presence of several hydroxyls in a polyhydric alcohol and their mutual influence, stronger acidic properties are manifested than those of monohydric "brothers". Therefore, in reactions with alkalis, the products are s alts (for glycol - glycolates, for glycerol - glycerates).
The chemical properties of ethylene glycol, as well as glycerin, include all reactions of alcohols from the category of monohydric. Glycol gives full and partial esters in reactions with monobasic acids, glycolates, respectively, are formed with alkali metals, and whenin a chemical process with strong acids or their s alts, acetic acid aldehyde is released - due to the elimination of a hydrogen atom from a molecule.
Reactions with active metals
Reaction of ethylene glycol with active metals (after hydrogen in the chemical strength series) at elevated temperatures gives ethylene glycolate of the corresponding metal, plus hydrogen is released.
C2N4(OH)2 + X → C 2H4O2X, where X is the active divalent metal.
Qualitative reaction to ethylene glycol
Distinguish polyhydric alcohol from any other liquid using a visual reaction that is characteristic only for this class of compounds. To do this, freshly precipitated copper hydroxide (2), which has a characteristic blue tint, is poured into a colorless solution of alcohol. When the mixed components interact, the precipitate dissolves and the solution turns into a deep blue color - as a result of 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 mentioned substance, that is, the elimination of water from each of the two molecules of glycol and their subsequent combination (one hydroxyl group is completely eliminated, and only hydrogen is removed from the other), makes it possible to obtain a unique organic solvent - dioxane, which is often used in organic chemistry, despite its high toxicity.
Hydroxy exchangeto halogen
When ethylene glycol interacts with hydrohalic acids, the replacement of hydroxyl groups by the corresponding halogen is observed. The degree of substitution depends on the molar concentration of hydrogen halide in the reaction mixture:
HO-CH2-CH2-OH + 2HX → X-CH2 -CH2-X, where X is chlorine or bromine.
Get Ether
In the reactions of ethylene glycol with nitric acid (of a certain concentration) and monobasic organic acids (formic, acetic, propionic, butyric, valeric, etc.), complex and, accordingly, simple monoesters are formed. At others, the concentration of nitric acid is glycol di- and trinitroesters. Sulfuric acid of a given concentration is used as a catalyst.
The most important derivatives of ethylene glycol
Valuable substances that can be obtained from polyhydric alcohols using simple chemical reactions (described above) are ethylene glycol ethers. Namely: monomethyl and monoethyl, the formulas of which are HO-CH2-CH2-O-CH3and HO-CH2-CH2-O-C2N 5 respectively. In terms of chemical properties, they are in many ways similar to glycols, but, like any other class of compounds, they have unique reactive features that are unique to them:
- Monomethylethylene glycol is a colorless liquid, but with a characteristic disgusting odor, boiling at 124.6 degrees Celsius, highly soluble in ethanol, otherorganic solvents and water, much more volatile than glycol, and with a density lower than that of water (on the order of 0.965 g/cm3).
- Dimethylethylene glycol is also a liquid, but with a less characteristic odor, a density of 0.935 g/cm3, a boiling point of 134 degrees above zero and a solubility comparable to the previous homologue.
The use of cellosolves - as ethylene glycol monoethers are generally called - is quite common. They are used as reagents and solvents in organic synthesis. Their physical properties are also used for anti-corrosion and anti-crystallization additives in antifreeze and motor oils.
Fields of application and pricing of the product range
The cost at factories and enterprises involved in the production and sale of such reagents fluctuates on average about 100 rubles per kilogram of such a chemical compound 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 that freeze at low temperatures. It is involved in many industries such as automotive, aviation, pharmaceutical, electrical, leather, tobacco. Its importance for organic synthesis is undeniably weighty.
It is important to remember that glycol istoxic compound that can cause irreparable harm to human he alth. Therefore, it is stored in sealed vessels made of aluminum or steel with a mandatory inner layer that protects the container from corrosion, only in vertical positions and in rooms that are not equipped with heating systems, but with good ventilation. Term - no more than five years.
