Chemical properties of ethylene glycol, characteristic. The dihydric alcohol. Ethylene glycol ethers

The most known and used in the life of man and in 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 largely unique and unique, which makes them irreplaceable 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: the history of obtaining

In 1859, through the two-step process of the reaction of dibromoethane with silver acetate and subsequent treatment with caustic potassium prepared in the first reaction of ethylene glycol diacetate, Charles Wurz first synthesized ethylene glycol. Some time later, a method for the direct hydrolysis of dibromoethane was developed, but on an industrial scale in the early twentieth century, a dihydric alcohol 1,2-dihydroxyethane, also monoethylene glycol, or simply glycol, was produced in the United States by hydrolysis of ethylene chlorohydrin.

To date, both in industry and in the laboratory, a number of other methods are being used, new, more economical from raw materials and energy points of view, and environmentally friendly, since the use of reagents containing or emitting chlorine, toxins, carcinogens and other environmental and human hazards Substance, is reduced as the development of "green" chemistry.

Apothecary Carl Wilhelm Scheele in 1779 was discovered glycerol, and the particular composition of the compound was studied in 1836 by Theophilus Julius Peluz. Two decades later, the structure of the molecule of this triatomic alcohol was established and justified in the writings of Pierre Eugene Marcelia Vertello and Charles Wurz. Finally, twenty years later Charles Friedel performed a complete synthesis of glycerin. 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 glycerol, are widely used in various fields of chemical production.

Structure and structure of the connection

The molecule is based on the unsaturated hydrocarbon skeleton of ethylene, consisting of two carbon atoms, in which a double bond was broken. Two hydroxyl groups joined the liberated valence sites at the carbon atoms. The ethylene formula is C ₂ H ₄ , after rupture of the crane bond and the addition of 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 some kind of trans-configuration in the arrangement of hydroxyl groups with respect to the carbon core 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 the hydrogen from the functional groups, less energy, and hence - maximum stability of the system. Simply put, one ON-group "looks" up, and the other - down. At the same time, compounds with two hydroxyls are unstable: when one carbon atom is formed in the reaction mixture, they are immediately dehydrated, passing to aldehydes.

Classification affiliation

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 moieties at adjacent carbon atoms. A substance that also contains several OH substituents is also glycerin. It has three alcohol functional groups and is the most common representative of its subclass.

Many compounds of this class are also produced and used in chemical production for various syntheses and other purposes, but the use of ethylene glycol is of a more serious scale and is used in virtually all industries. This question 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 the distinguishing features characteristic only of a given class of organic compounds.

Most important of the properties is the unlimited ability to mix with H2O. Water + ethylene glycol gives a solution that has a unique characteristic: its freezing point, 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 nonlinear, and upon reaching a certain quantitative content of glycol, the reverse effect begins - the freezing point increases with increasing percentage of solute. This feature has found application in the field of production of various antifreezes, non-frost-free liquids, which 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. In contrast to its aliphatic parent - a gaseous substance such as ethylene, ethylene glycol - is a syrup-like, transparent, with a slight yellow hue, a liquid that is sweet to taste, with an uncharacteristic smell, almost 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 / cm ³ .

Methods of obtaining

Ethylene glycol can be obtained in several ways, some of them today have only historical or preparative value, while others are actively used by man on an industrial scale and not only. Following in chronological order, consider the most important.

The first method for the preparation of ethylene glycol from dibromoethane has already been described above. The formula of ethylene, whose double bond is broken, and the free valences are occupied by halogens, - the main starting material in this reaction - in addition to carbon and hydrogen has in its composition two bromine atoms. The formation of an intermediate in the first stage of the process is possible precisely because of their cleavage, i.e., replacement by acetate groups, which upon further hydrolysis are converted to alcoholic ones.

In the course of further development of science, it became possible to obtain ethylene glycol by direct hydrolysis of any ethanes substituted by two halogens from neighboring carbon atoms, using aqueous solutions of metal carbonates from the alkali group or (less environmentally friendly) H ₂ O and lead dioxide. The reaction is rather "time-consuming" and proceeds only at greatly 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.

Its role in the development of organic chemistry was played by the method of obtaining ethylene glycol from ethylene chlorhydrin by its hydrolysis with coal salts of metals of alkaline group. With an increase in the reaction temperature to 170 degrees, the yield of the desired product reached 90%. But there was a significant drawback - the glycol needed to somehow be extracted from the salt solution, which directly involves a number of difficulties. Scientists solved this problem by developing a method with the same initial substance, but having broken the process into two stages.

The hydrolysis of ethylene glycol acetates, being the final stage of the Wurz method, became a separate method when they were able to prepare the initial 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 ways of producing ethylene glycol by oxidizing ethylene with hydroperoxides, peroxides, organic peracids in the presence of catalysts (osmium compounds), potassium chlorate , etc. Electrochemical and radiation-chemical methods also exist.

Characterization of general chemical properties

The chemical properties of ethylene glycol are determined by its functional groups. One hydroxyl substituent or both may participate in the reactions, depending on the process conditions. The main difference in reactivity lies in the fact that due to the presence of several hydroxyls in the polyhydric alcohol and their mutual influence, stronger acid properties appear than in monatomic "counterparts." Therefore, in reactions with alkali products are salts (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 monatomic. Glycol gives complete and incomplete ethers in reactions with monobasic acids, glycolates, respectively, are formed with alkali metals, and in the chemical process with strong acids or their salts, the aldehyde of acetic acid is liberated - by cleavage from the molecule of the hydrogen atom.

Reactions with active metals

The interaction of ethylene glycol with the active metals (standing after hydrogen in the chemical stress range) at elevated temperatures gives the ethylene glycolate of the corresponding metal, plus hydrogen is liberated.

C ₂ H ₄ (OH) ₂ + X → C ₂ H ₄ O ₂ X, where X is an active divalent metal.

Qualitative reaction to ethylene glycol

Distinguish polyhydric alcohol from any other liquid by using a visual reaction, characteristic only for this class of compounds. For this, freshly precipitated copper hydroxide (2), having a characteristic blue hue, is poured into a colorless alcohol solution. When the mixed components interact, dissolution of the precipitate is observed and the solution is colored in a saturated 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 said substance, i.e., the splitting of water from each of the two glycol molecules and their subsequent combination (one hydroxyl group is cleaved completely and the other is hydrogen only), 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, the 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 chlorine or bromine.

Preparation of ethers

In the reactions of ethylene glycol with nitric acid (a certain concentration) and monobasic organic acids (formic, acetic, propionic, oily, valerian, etc.), the formation of complex and correspondingly simple monoesters takes place. At others, the concentration of nitric acid is di- and trinitro-esters of glycol. As the catalyst, sulfuric acid of a given concentration is used.

The most important derivatives of ethylene glycol

Valuable substances that can be obtained from polyhydric alcohols with the help of simple chemical reactions (described above) are ethylene glycol ethers. 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. By chemical properties, they are similar in many respects to glycols, but, like any other class of compounds, they have unique reaction peculiarities that are unique to them:

• Monomethylethylene glycol is a liquid without color, but with a characteristic odor 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 (of the order 0.965 g / cm ³ ).
• Dimethylethylene glycol is also a liquid, but with a less characteristic odor, a density of 0.935 g / cm ³ , a boiling point of 134 degrees above zero and a solubility comparable to the previous homologue.

The use of cellosolves - so generally called monoesters of ethylene glycol - is quite common. They are used as reagents and solvents in organic synthesis. Their physical properties are also used for anticorrosive and anti-crystallization additives in antifreeze and motor oils.

Areas of application and price policy of the production line

The cost in factories and enterprises engaged in the production and sale of such reagents fluctuates an average of 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 that freeze at negative temperatures. It is involved in many industrial sectors, such as automobile, aviation, pharmaceutical, electrical, leather, tobacco. Its importance for organic synthesis is undeniably weighty.

It is important to remember that glycol is a toxic compound that can cause irreparable harm to human health. Therefore, it is stored in hermetic vessels made of aluminum or steel with an obligatory inner layer protecting the container from corrosion, only in vertical positions and in rooms not equipped with heating systems, but with good ventilation. Term - no more than five years.