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Structure, nomenclature. Monobasic unsaturated (unsaturated) acids most often have historical names.
The simplest ethylene acid is called acrylic acid:
The next representative of this series, containing 4 carbon atoms, may already exist in the track of isomeric
The position of the double bond in unsaturated acids ax in relation to the carboxyl group is denoted by the letters of the Greek alphabet, indicating the places of carbon atoms between which there is a double bond, with the addition of the words unsaturated acid. For example, vinyl acetic acid is - unsaturated acid, and acrylic, crotonic and methacrylic - unsaturated acids.
From structural formulas of these acids, it can be seen that the isomerism of unsaturated acids depends on the branching of the chain of carbon atoms and the position of the double bond.
Using the example of unsaturated organic acids, we will get acquainted with another bid of isomerism, which various unsaturated organic compounds possess - geometric isomerism (or, as it is often called, cis-trans isomerism).
Rice. 16. Model of the ethane molecule.
If we depict the spatial structure of the saturated hydrocarbon molecule ethane, then we can see that the valences of the carbon atoms of ethane are not located in the same plane, but at some angle to each other (Fig. 16).
In an ethane molecule, free rotation of carbon atoms around the direction of a single bond is possible without breaking it. It is clear that no matter how the hydrogen atoms move around the bond, we will always have the same structure.
Let us now consider the spatial structure of the crotonic acid molecule
In the molecule of crotonic acid, the free rotation of carbon atoms, as in the molecule of ethane, is no longer possible, since this would break the double bond between the carbon atoms.
If in the spatial model of crotonic acid (Fig. 17, a) we change the positions of the right hydrogen and the carboxyl group so that the hydrogen is under the plane of the double bond, and the carboxyl group is above the plane, then we get a different spatial model (Fig. 17, b).
These two spatial models differ from each other in that in the first of them, both hydrogen atoms are on the same side of the plane passing through the carbon atoms and the double bond, and in the second, on opposite sides of it. It may seem that there will be a third isomer of crotonic acid if the hydrogen atom and the methyl group are interchanged in the first spatial model (Fig. 17c). However, it is easy to see that this model is completely analogous to the second if the entire molecule depicted by the last spatial model is rotated around the plane of the double bond by 180°.
Rice. 17. Spatial models of the crotonic acid molecule.
For convenience, when depicting spatial models, we agreed to use the so-called projection formulas, which are obtained by projecting spatial models onto a plane. Then the formulas of crotonic acid will look like:
Such formulas are often depicted in a slightly different way, arranging the carbon atoms vertically:
Isomers that have the same atoms or atomic groups (in this case hydrogen atoms) are directed in one direction from the plane of the double bond, are called cis-isomers, if these substituents are directed in different hand, - trans-isomers.
Thus, geometric isomerism is one of the types of spatial isomerism and depends on the arrangement of atoms or groups of atoms with respect to the plane of the double bond.
Spatial isomers differ from each other in properties. For example, crotonic acid (trans-isomer) is a solid with a temp. isocrotonic acid (cis-isomer) - with normal conditions liquid with temp.
Usually one of the spatial isomers is stable (stable), and the other is unstable (labile), and the unstable isomer easily transforms into the stable isomer under the influence of heat, light or chemical influences. So, isocrotonic acid is very unstable and easily passes at elevated temperature and in action sunlight into stable crotonic acid.
Properties. The lower representatives of unsaturated acids are liquids with a pungent odor, readily soluble in water. Higher unsaturated acids - solids, odorless, insoluble in water.
Unsaturated acids are characterized by most of the reactions of acids of the limiting series (the formation of salts, esters, anhydrides, halogen derivatives, etc.) and, in addition, a number of reactions characteristic of unsaturated hydrocarbons.
When hydrogen is added in the presence of catalysts, an acid of the limiting series is formed from an unsaturated acid:
With vigorous oxidation, the carbon chain of an unsaturated acid is broken at the place of the double bond, and usually two acids are obtained - monobasic and dibasic:
When -unsaturated acids are heated with dilute mineral acids, so-called lactones are formed - internal cyclic esters of hydroxy acids (see p. 169).
For example, γ-butyrolactone is formed from vinylacetic acid under these conditions:
There are other ways to obtain lactones.
How to get unsaturated acids are similar to the methods for obtaining saturated acids. So, for example, unsaturated acids are obtained by careful oxidation of the corresponding unsaturated alcohols and aldehydes:
Acrylic acid. Liquid with a pungent odor, heavier than water; pace. Of the derivatives of acrylic acid, its nitrile (p. 148) and various esters are of great importance. It can be obtained from allyl alcohol.
At present, in industry, acrylic acid is obtained by heating ethylene cyanohydrin with dilute sulfuric acid:
Methacrylic acid obtained in a similar way from acetone cyanohydrin (p. 153). Great importance for the manufacture of organic glass (p. 326) has its methyl ester (methyl methacrylate).
Oleic acid. Its structure is expressed by the formula It is an oily liquid (odorless density; temp. Together with palmitic and stearic acids, it is part of fats. Oleic acid is especially large quantities is part of olive, almond and sunflower oils.
When reduced with hydrogen in the presence of catalysts, it turns into an acid of the limiting series - stearic acid. This process plays an important role in the production of margarine (page 139).
Under the action of small amounts of nitrous acid, oleic acid is converted into a solid isomer - elaidic acid.
Oleic and elaidic acids are cis-trans isomers:
Of the unsaturated acids with two double bonds, the largest practical value has sorbic acid. Due to the effective bactericidal properties and the absence of any undesirable side effects on the human and animal organisms, sorbic acid and its salts have found application as preservatives in food and other industries.
Sorbic acid is obtained by reacting ketene (p. 134) with crotonaldehyde in the presence of zinc butyrate. This reaction produces a polyester of 3-hydroxyhexanoic acid:
When the resulting polyester is treated with hydrochloric acid at 70 ° C, sorbic acid is obtained:
Acrylic (propenoic, ethylenecarboxylic) acid CH2=CH-COOH is a colorless liquid with a pungent odor; m.p. 285-286.5 K, bp 413.9-414.6 K, d420 = 1.0511. Soluble in water, alcohol, CHC13, benzene. It polymerizes during storage.
Acrylic acid and its salts are used for the manufacture of water-soluble polymers and copolymers, which are used as finishes, binders, and dispersants. Approximately half of the produced acrylic acid esters - acrylates - is spent on the production of paints for interior and exterior coatings. Coatings are resistant to abrasion, dry quickly and do not turn yellow. Acrylate-based varnishes are used for painting household appliances and car bodies by spraying. A significant part of the produced acrylates is used in textile industry. AT paper industry polyacrylates are used for coating paper and cardboard, as well as for coatings. The polymers ethyl-, butyl-, and 2-ethylhexyl acrylate, often in combination with styrene, vinyl acetate, or vinyl esters, are constituent parts many adhesives. Copolymers of ethyl acrylate and ethylene are valuable elastomers.
In industry, the following methods for producing acrylic acid are implemented:
- - hydrolysis of ethylene cyanohydrin;
- - hydrolysis of acrylonitrile;
- - hydrocarboxylation of acetylene;
- - oxidation of propylene in the vapor phase with the intermediate formation of acrolein;
- 1. Hydrolysis of ethylene cyanohydrin
One of the options for obtaining acrylic acid is based on the interaction of ethylene oxide with cyanohydrin to form ethylene cyanohydrin:
CH2--CH2 + HCN HOCH2 CH2CN.
The subsequent hydrolysis of ethylene cyanohydrin to acrylic acid is carried out in a sulfuric acid medium in accordance with the reactions:
HOCH2CH2CN + 2H2O HOCH2CH2COOH + NH4HSO4
CH2=CHCOOH + H2O.
The total yield of acrylic acid does not exceed 60-70%.
This method was developed by Union Carbide. However, it did not receive industrial development: the last operating installation using this method was stopped in 1971.
2. Hydrolysis of acrylonitrile
The hydrolysis of nitriles is one of the most common methods for the synthesis of carboxylic acids. The process is catalyzed by acids or alkalis and proceeds through an intermediate stage of amide formation:
CONH2 + H2O RCOOH + NH3
The reaction is carried out in aquatic environment at a temperature of 323–353 K. The ratio of the rates of both reactions depends on the structure of the nitriles, the nature of the catalyst used, and the hydrolysis conditions. If k1>>k2, then, despite the excess of water, the reaction can be stopped at the stage of amide formation. During hydrolysis with sulfuric acid, the ratio k1:k2 depends on the concentration of the acid. For example, in the hydrolysis of propionitrile with sulfuric acid, only propionic acid is obtained (k1:k2>100). As the acid concentration increases, the rates of both reactions become comparable. When treating many nitriles with 50% or more dilute sulfuric acid, as a rule, one obtains carboxylic acids. When nitriles interact with more concentrated acids, the reaction often stops at the stage of amide formation.
Thus, the use of highly concentrated mineral acids contributes to the production of amide, and in the region of low acid concentrations (k2>>k1), carboxylic acids are formed.
Upon receipt of acrylic acid by sulfuric acid hydrolysis, the process is carried out in two stages: first, acrylamide sulfate is synthesized, and then acrylamide sulfate is saponified with the release of acrylic acid.
After heat treatment of the mixture obtained by hydrolysis of acrylamide sulfate with water, acrylic acid is distilled off under reduced pressure. However, due to the polymerization of the acid in the vapor phase, a significant amount of it is lost. Recovery of the acid from the mixture after the hydrolysis of acrylamide sulfate can be carried out by distillation together with the organic solvent added to the hydrolyzed reaction mixture. In this case, the vapor mixture enters the condenser, into which an additional amount of water is supplied. The resulting mixture is separated into a layer organic solvent and layer aqueous solution acid, the concentration of which is controlled by the amount of water added. O-, m-, p-cresols, naphthol and oil fractions of kerosene can be used as solvents.
Side reactions during the hydrolysis of acrylonitrile. In the sulfuric acid hydrolysis of acrylonitrile, along with the main reaction of the formation of acrylamide sulfate, side reactions occur, leading to the formation of propionic acid amide sulfate, acrylic acid, etc. Etherification is carried out in a reactor with a stirrer made of anti-corrosion material - glass, ceramics, enameled materials, polytetrafluoroethylene . At the esterification stage, alkyl and alkoxyalkyl propionates, dialkyl ether, and ammonium sulfate are formed as by-products. At the stage of esterification of acrylamide sulfate in an acidic medium, the reaction of alcohol dehydration is possible with the formation of an ether, which, upon contact with air, easily turns into peroxide compounds, which are active polymerization initiators.
Acrylic acid polymerization inhibitors. When acrylic acid is purified by distillation, it polymerizes, and this occurs much faster in the gas phase than in the liquid, since the polymerization inhibitors commonly used in the synthesis - hydroquinone, methylhydroquinone, phenothiazine, methylene blue and others - are contained in the gas phase in a smaller amount than needed to stabilize the acid.
The resulting acrylic acid polymer, insoluble in acid and other solvents, quickly fills the distillation column, and a continuous process becomes impossible.
To prevent polymerization of the acid during distillation, various polymerization inhibitors are added, for example hydroquinone, phenol or derivatives thereof, and oxygen, diphenylamine or derivatives thereof.
Ammonium chloride can also be used as a polymerization inhibitor during the distillation of acrylic acid, a 1% solution of which is fed into upper part distillation column.
To avoid the formation of a polymer on the surface of steel vessels during the distillation of acrylic acid, they are coated with polytetrafluoroethylene, which is applied to the surface of the evaporator in the form of a film.
3. Hydrocarboxylation of acetylene
Acrylic acid or its esters can be obtained by reacting acetylene with nickel tetracarbonyl (a source of carbon monoxide) in the presence of water or another proton donor (alcohols, mercaptans, amines, organic acids):
4CH CH + 4H2O + Ni(CO)4 + 2HC1 4CH2=CH-COOH + NiC12 + H2
If a monohydric alcohol is used instead of water, an acrylic acid ester is formed:
4C2H2 + Ni(CO)4 + 4ROH + 2HC1 4CH2=CH-COOR + NiC12 + H2.
The reaction is carried out at a temperature of 313 K, atmospheric pressure and an acetylene:CO ratio of 1:1, in the presence of nickel tetracarbonyl as a catalyst.
The disadvantage of this method is the use of explosive acetylene.
4. Vapor-phase oxidation of propylene
The process of vapor-phase oxidation of propylene is the main industrial method for the production of acrylic acid. The production of acrylic acid by the oxidation of propylene in the gas phase through the intermediate formation of acrolein is realized in two stages:
CH2=CHCH3 + O2 CH2=CHCHO + H2O DH298 = -340 kJ/mol,
CH2=CHCHO + 0.5O2 CH2=CHCOOH DH298 = -250 kJ/mol
At the first stage, propylene is oxidized, and at the second stage, acrolein is oxidized.
propylene oxidation. The oxidation of propylene proceeds by a radical chain mechanism and includes the following stages:
CH2=CH--CH3 + O CH2=CH--CH2 + H2O, (chain initiation)
CH2=CH--CH2 + O CH2=CH--CH + OH, (chain growth)
СH2=CH--CH + O CH2=CH--CHO, (chain open)
CH2=CH--CHO + OH CH2=CH--CO* + H2O,
СH2=CH--CO + OH CH2=CH--COOH.
During the oxidation process, side products are formed, which are the result of reactions of partial or complete oxidation of propylene (acetaldehyde, acetic acid, CO, CO2) and polymerization reactions. An increase in the yield of acrolein and acrylic acid and, accordingly, suppression of side reactions is favored by low temperatures: 673–773 K. Reducing the reaction temperature is possible when using highly selective catalysts.
The oxidation of propylene is carried out at 573-623, a pressure of 0.1-0.3 MPa and the addition of water vapor on catalysts containing oxides of bismuth, cobalt, nickel, iron, tin, etc. The molar ratio of water: propylene is maintained at the level of 4-5, and the molar ratio of oxygen: propylene is ~ 2. Steam and nitrogen reduce not only the possibility of overheating, but also the risk of creating explosive situations. These gases also contribute to an increase in the activity of the catalyst, facilitating the desorption of reaction products, and an increase in the duration of stable operation up to 24 months. The degree of conversion of propylene in one pass is 90-95% and the yield of acrolein and acrylic acid is 80-90%.
Acrolein oxidation. The oxidation of acrolein is carried out in a heterogeneous catalytic variant on catalysts obtained on the basis of mixed oxides of molybdenum and vanadium modified with oxides of tungsten, chromium, copper, tellurium, arsenic, etc.
The activity of various oxides during the catalytic oxidation of acrolein decreases in the following order:
MoO3 > V2O5 > WO3 > SeO2 > TeO2 > Nb2O5 > Ta2O5 > CrO3.
For catalytic oxidation, only catalysts with an electronegativity above 2.93 are used. The inactive oxides Co2O3 and PbO2 acquire activity as a result of the introduction of H3PO4. Strongly electronegative additives have an activating effect: H3PO4, H2SO4, MoO3, H3BO3, TeO2. The most effective catalyst for the oxidation of acrolein is MoO3.
The process is carried out at a temperature of 523-553 K and a pressure of 0.1-0.2 MPa in the presence of water vapor at a molar ratio of water: acrolein equal to 2: 1. The degree of conversion in one pass is 95-97%, the yield of acrylic acid is more 90% based on acrolein.
The technology for producing acrylic acid by propylene oxidation was first developed by Distillers, and later by BASF, Sohio, Toyota Soda, Union Carbide, and Japan Catalytic.
In industry, acrylic acid is obtained by a two-stage method of propylene oxidation through acrolein without separating and purifying the acrolein formed in the first stage.
Name Acrylic acid Synonyms propenoic acid; Registration number CAS 79-10-7 Molecular formula C 3 H 4 O 2 Molecular weight 72.06 InChI InChI=1S/C3H4O2/c1-2-3(4)5/h2H,1H2,(H,4,5) InChIKey NIXOWILDQLNWCW-UHFFFAOYSA- N SMILES C=CC(=O)O EINECS 201-177-9 HS Code 29161110
Chemical and physical properties
Density 1.051 Boiling point 139°C Melting point 13°C Flash point 48°C Storage temperature 15-25°C Refractive index 1.4192-1.4212 Solubility Miscible with water. Stability Unstable - may contain p-methoxyphenol as an inhibitor. Prone to dangerous polymerization. Fuel. Incompatible with strong oxidizers, strong bases, amines. Contact with oxidizing agents may cause fire. Sensitivity to light and air. Hygroscopic. Appearance Colorless liquid.
Risks, safety and conditions of use
Safety instructions S26; S36/37/39; S45; S61 Risk Statement R10; R20/21/22; R35; R50 Hazard category 8 Hazard symbols
Classification of chemical reagents
Pure ("pure") Acrylic acid P. The content of the main component is 98% or more (no impurities). The color of the strip on the packaging is green. Pure for analysis (“analytical grade”, “analytical grade”) Acrylic acid of analytical grade. The content of the main component is higher or significantly higher than 98%. Impurities do not exceed the allowable limit for accurate analytical studies. The color of the strip on the package is blue. Chemically pure ("chemically pure", "chemically pure") Acrylic acid chemically pure. The content of the main component is more than 99%. The color of the strip on the packaging is red. Extra pure (“high purity”) Acrylic acid, high purity. The content of impurities in such a small amount that they do not affect basic properties. The color of the strip on the packaging is yellow.
acrylic acid formula
- this is one of the simplest representatives of carboxylic unsaturated monobasic acids. Its formula is as follows: CH 2 \u003d CH-COOH. It is a colorless liquid with a sharp and bad smell. Soluble in water, chloroform, diethyl alcohol and ethanol, easily polymerizes to form polyacrylic acid. Acrylic acid has other names: ethenecarboxylic acid and propenoic acid.
Getting Acrylic Acid
How is acrylic acid obtained (or synthesized)?
- Acrylic acid is currently produced by vapor-phase oxidation of propylene with oxygen (O 2 ) on molybdenum, cobalt or bismuth catalysts. An example would be the following reaction: CH 2 \u003d CH-CH 3 (propylene) + O 2 (oxygen) \u003d CH 2 \u003d CH-COOH (acrylic acid)
- In the past, a reaction was used in which carbon monoxide II (CO), acetylene (CH≡CH) and water (H 2 O) interacted. Chemical reaction it will be like this: CH≡CH (acetylene) + CO (carbon monoxide II) + H 2 O (water) → CH 2 = CH-COOH (acrylic acid).
- E We also used the reaction of formaldehyde with ketene: CH 2 \u003d C \u003d O (ketene) + H 2 C \u003d O (formaldehyde) → CH 2 \u003d CH-COOH (propenoic acid).
- Rohm and Haas are now developing a special technology for the synthesis of ethenecarboxylic acid from propane.
Chemical properties of acrylic acid
The acid we are considering can form salts, esters, anhydrides, amides, acid chlorides and other compounds. It can also enter into addition reactions, which are characteristic of ethylene carbons. The addition of water, protic acids and NH3 does not occur according to Markovnikov's rule. In this case, substituted derivatives are formed. Acrylic acid is involved in the synthesis of dienes. It also condenses with various aryldiazonium salts. Under ultraviolet irradiation, it forms polyacrylic acid.
Application of acrylic acid:
- used as a raw material in the production of a wide range of polymer products with different chemical and physical properties (eg plastics and coatings);
- used in the production of dispersions for acrylic water-based paints and varnishes; while the scope of such paints will depend on chemical properties copolymer - from the final color Vehicle and before painting the ceilings;
- acrylic acid and its derivatives are used to create impregnations for leather and fabrics, emulsions for paintwork materials, as a raw material for acrylate rubbers and polyacrylonitrile fibers, building adhesives and mixtures; esters of metaacrylic and acrylic acids (in most cases, methyl esters of methyl methacrylate and methyl acrylate are used) are used in the production of polymers;
- often acrylic acid is used in the creation of superabsorbents.
Proper storage of acrylic acid:
During storage of this substance in order to avoid polymerization, an inhibitor, hydroquinone, is added. The acid must be distilled with care before use, as explosive polymerization may develop.
Safety when using acrylic acid
When working with acrylic acid, it should be noted that given substance has an irritating effect on the skin and mucous membranes. The acid irritant threshold is 0.04 mg/litre. When it comes into contact with the mucous membrane of the eyeballs, as a rule, it causes severe burns of the cornea, which can lead to irreversible changes (damages that cannot be treated). Inhalation of acrylic acid vapors may cause headache, irritation respiratory tract, and in excessive doses - the development of pulmonary edema. In rooms where work with acrylic acid is carried out, constant air control is necessary. MPC for this acid is 5 mg/m³. Security measures must also be observed when working with other derivatives. An example is acrylic acid nitrile.
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