Acetic acid dehydration reaction. Dehydration of carboxylic acids
Get alkenes and alkadienes. Dehydration of alcohols can occur in two directions: intramolecular and intermolecular.
Intramolecular dehydration of alcohols belongs to elimination (cleavage) reactions ($E$). Depending on the structure of the alcohol, elimination can occur via the $E1$ and $E2$ mechanisms. In this case, primary alcohols react mainly according to the $E2$ mechanism, while secondary and tertiary alcohols react according to the $E1$ mechanism. As in the case of nucleoprofile substitution, the elimination of alcohols occurs with the formation of an oxonium cation.
Like haloalkanes, primary alcohols react and intermolecular dehydration usually by the $S_N2$ mechanism, tertiary ones - by the $S_N1$ mechanism, secondary ones can react with both the $S_N2$ and the $S_N1$ mechanism.
intramolecular dehydration
It is easier to dehydrate tertiary alcohols, then secondary ones, and then primary ones, according to the $E1$ or $E2$ mechanism, similarly to dehydrohalogenation reactions. The process of dehydration of alcohols obeys the rule of A. Zaitsev with the formation of the most branched alkenes. Thus, the dehydration of a tertiary alcohol proceeds according to the $E1$ mechanism and is often accompanied by a nucleophilic substitution reaction according to the $Sn1$ mechanism:
Picture 1.
The slowest step in this mechanism is the conversion of alkoxonium cations into carbocations:
Figure 2.
Obtaining one or another alkene during dehydration is determined by the lability of intermediate carbocations and the thermodynamic stability of branched alkenes. For example, for isoamyl alcohol, in accordance with the Zaitsev rule, only 3-methyl-1-butene should be formed, but actually three alkenes $C_5H_(10)$ are obtained:
Figure 3
The resulting primary carbocation is the least stable and, in addition to the elimination of a proton, is also prone to isomerize due to 1,2-hydride displacements into a stable secondary carbocation, from which alkenes are obtained:
Figure 4
The secondary carbocation, in turn, can also isomerize into a tertiary one, which is as stable as possible:
Figure 5
Thus, during the dehydration of isoamyl alcohol, a mixture of 3-methyl-1-butene, 2-methyl-2-butene and 2-methyl-1-butene is formed, and most of all in the reaction products there will be 2-methyl-2-butene as the most branched product.
The mechanism of $E1$ rather than $E2$ is typical for alcohols in elimination reactions. This is also related to the acidity of the reaction medium, in which the strong base, the $RO-$ alkoxide anion, does not exist, since it rapidly interacts with the proton.
Figure 6
Intermolecular dehydration
The reactions considered are examples of intramolecular dehydration, next to which there is also intermolecular dehydration, an example of which, as mentioned above, is the formation of an ether:
Figure 7
Intermolecular dehydration of alcohols in the presence of concentrated acids, depending on the temperature, the ratio of the volumes of alcohol and acid, can occur with the formation of various products. For example, ethyl alcohol at 105$^\circ$C forms an acid ester with sulfuric acid - ethylsulfuric acid (reaction 1). With an excess of alcohol and high temperature (130-140$^\circ$C), intermolecular dehydration occurs, the main product of which is diethyl ether (ether; reaction 3). At temperatures above 160$^\circ$C, ethylsulfuric acid decomposes to form ethylene (reaction 2):
Figure 8
Substitutes for acids in the process of acid dehydration
For processes (both intra- and intermolecular) dehydration of alcohols, especially on an industrial scale, instead of conventional acids, it is more convenient to use anhydrous Lewis acids or other oxidizing agents, such as aluminum oxide, as dehydrating agents. The process of heterogeneous catalytic dehydration of alcohols over $Al_2O_3$ at 350-450$^\circ$С leads to alkenes:
Figure 9
abstract
Dehydration processes
Introduction 3
1. Dehydration processes 4
2. Technology of dehydrogenation processes 9
References 11
Introduction
The processes of hydrolysis, hydration, dehydration, esterification and amidation are very important in the industry of basic organic and petrochemical synthesis. Soap, glycerin, ethanol and other valuable products have long been obtained by hydrolysis of fats, cellulose and carbohydrates. In the field of organic synthesis, the processes in question are used mainly for the production of C 2 -C 5 alcohols, phenols, ethers,
-oxides, many unsaturated compounds, carboxylic acids and their derivatives (esters, anhydrides, nitriles, amides) and other compounds.The listed substances have a very important application as intermediate products of organic synthesis (alcohols, acids and their derivatives, aldehydes,
-oxides), monomers and starting materials for the synthesis of polymeric materials (phenol, esters of acrylic and methacrylic acids, melamine, chloroolefins), plasticizers and lubricants (esters), solvents (alcohols, ethers and esters, chloroolefins), pesticides (esters carbamic and thiocarbamic acids). Very often, the reactions considered are an intermediate step in multistage synthesis of other target products.The production of these substances is on a large scale. Thus, in the USA, 500 thousand tons of ethanol and isopropanol, 900 thousand tons of propylene oxide, 200 thousand tons of epichlorohydrin, over 4 million tons of esters, and about 300 thousand tons of isocyanates are synthesized.
1. Dehydration processes
1. Dehydration with the formation of unsaturated compounds
The process is used to recover isobutene from C 4 -fractions of cracking and pyrolysis gases, when one of the stages consists in the dehydrogenation of tert-butanol catalyzed by sulfuric acid or sulfonic cation. Or dehydration to obtain isobutene is carried out with tert-butanol formed during the hydroxide method for producing propylene oxide:
(CH 3) 3 COH → (CH 3) 2 C \u003d CH 2 + H 2 O
In this and other cases, dehydration with the formation of unsaturated substances is most often one of the stages in the production of many monomers. So, in one of the new processes, styrene is produced by dehydration of methylphenylcarbinol:
C 6 H 5 -CHOH-CH 3 → C 6 H 5 -CH \u003d CH 2 + H 2 O
A well-known method for the synthesis of isoprene from isobutene and formaldehyde is also associated with the final dehydration of the diol and unsaturated alcohol:
(CH 3) 2 C (OH) -CH 2 CH 2 (OH)
(CH 3) 2 \u003d CHCH 2 OH(CH 3) 2 \u003d CHCH 2 OH
CH 2 \u003d C (CH 3) CH \u003d CH 2When the first water molecule is split off from the diol, a mixture of unsaturated alcohols of various structures is obtained, but all of them give isoprene upon further dehydration, and the reaction is accompanied by the displacement of double bonds:
Another preparation option for unsaturated compounds, consisting in the introduction of a vinyl group by reactions such as aldol condensation followed by dehydration, examples of the synthesis of nitroethylene, vinyl methyl ketone and 2-vinylpyridine can be given:
CH 3 NO 2 + HCHO
HOCH 2 -CH 2 NO 2 CH 2 \u003d CHNO 2CH 3 COCH 3 + HCHO
CH 3 COCH 2 CH 2 OH CH 3 COCH \u003d CH 2Dehydration is also one of the stages in the production of methacrylic acid esters CH 2 \u003d C (CH 3) COOR, some primary alcohols, for example n-butanol:
2CH 3 CHO → CH 3 CH(OH)CH 2 CHO
CH 3 CH \u003d CHCHO CH 3 (CH 2) 2 -CH 2 OH2-ethylhexanol, methylisobutylketone and many other substances.
2. Dehydration with the formation of ethers
Through the secondary formation of ethers during the hydrolysis of chlorine derivatives and the hydration of olefins, all the required amount of ethers such as diisopropyl ether is obtained. But diethyl ether has a fairly wide application, and it is specially produced by intermolecular dehydration of ethanol at 250 0 C on a heterogeneous AI 2 O 3 catalyst:
2C 2 H 5 OH → (C 2 H 5) 2 O + H 2 O
The possibility of using the same method for the synthesis of ethers from isopropanol and higher alcohols is limited by the development of side formation of olefins. As a result, most esters are obtained in the liquid phase at a lower temperature using acid catalysts - sulfuric, phosphoric, arylsulfonic acids. The method is suitable mainly for the synthesis of symmetrical ethers having the same alkyl groups, since when a mixture of two alcohols is dehydrated, the yield of a mixed ester is low:
3ROH + 3R"OH → R 2 O + R" 2 O + ROR" + 3H 2 O
Of the symmetrical ethers with a straight chain of carbon atoms, of interest is
- dichlorodiethyl ether (chlorex), which is a valuable solvent and extractant, as well as the starting material for the production of polysulfide polymers. It is produced by dehydration of anhydrous ethylene chlorohydrin on an acid catalyst:2CICH 2 -CH 2 OH → (CICH 2 -CH 2) 2 + H 2 O
Dihydric alcohols under acid catalysis are capable of closing stable five- or six-membered rings. In this way, dioxane (1) is obtained from diethylene glycol, morpholine (2) from diethanolamine, and tetrahydrofuran (3) from butanediol-1,4. All of these substances are solvents:
3. Dehydration of carboxylic acids
The process of dehydration of carboxylic acids occupies a somewhat special position compared to other dehydration reactions. In this case, the products of intra- and intermolecular dehydration are ketene and acetic anhydride:
CH 2 \u003d C \u003d O2CH 3 COOHCH 3 -C=O 
(CH 3 -CO) 2 O
Ketene is a gas with a pungent odor that condenses into a liquid at -41 0 C. It is highly reactive, interacting with various substances to form acetic acid and its derivatives. In particular, with acetic acid it gives acetic anhydride:
CH 2 \u003d C \u003d O + CH 3 COOH → (CH 3 CO) 2 O
Acetic anhydride is a liquid with a pungent odor (bp 141 0 C). It is an important product of organic synthesis, widely used as an acetylating agent in the synthesis of acetic acid esters that are difficult to obtain in other ways - phenol acetates, tertiary alcohol acetates, and especially cellulose acetate and acetate fiber.
Acetic anhydride was previously obtained by the chlorine method - from sulfuryl chloride and sodium acetate:
SO 2 CI 2 + 4CH 3 COONa → 2(CH 3 CO) 2 O + Na 2 SO 4 + 2NaCI
This process occupies a somewhat special position compared to other dehydration reactions. In this case, the products of intra- and intermolecular dehydration are ketene and acetic anhydride:
These reactions are endothermic, and their equilibrium shifts to the right only at high temperatures: 500-600°C in the case of anhydride formation and 700°C in the case of ketene formation. Note that in the formation of ketene, the equilibrium transformation is also positively affected by reduced pressure. Both reactions proceed in the presence of heterogeneous acid-type catalysts (metal phosphates and borates) or vapors of phosphoric acid, which can be introduced into the initial mixture in the form of esters that readily hydrolyze into free acid. The reaction mechanism is generally similar to other dehydration processes:
Keten- a gas with a pungent odor, condensing into a liquid at -41°C. It is highly reactive, interacting with various substances to form acetic acid and its derivatives. In particular, with acetic acid it gives acetic anhydride:
Acetic anhydride is a liquid with a pungent odor (bp 141 ° C). It is an important product of organic synthesis, widely used as an acetylating agent in the synthesis of acetic acid esters that are difficult to obtain in other ways - phenol acetates, tertiary alcohol acetates, and especially cellulose acetate and acetate fiber.
Acetic anhydride was previously obtained by the chlorine method - from sulfuryl chloride and sodium acetate:
Due to the high consumption of reagents and the formation of waste salts, this method was replaced by the dehydration of acetic acid. The latter can be carried out in two ways: by intermolecular dehydration or through the intermediate formation of ketene. In both cases, the resulting gas mixture contains highly reactive acetic anhydride or ketene and water, which can easily be converted back into acetic acid upon cooling. Therefore, it is necessary to separate water from the reaction gases so that it does not have time to react with ketene or acetic anhydride. In the direct synthesis of acetic anhydride, this is achieved by rapid cooling of the reaction gas with the introduction of an azeotropic additive (ethyl acetate), which, together with water, is separated from the condensate, which is further separated into acetic anhydride and acetic acid. In the intermediate ketene process, the reaction gases are rapidly cooled to 0° C. and unconverted acetic acid and water are condensed from them. The residual gas is passed through a column refluxed with acetic acid, where acetic anhydride is formed. Side effects of these reactions are acetone and methane.
abstract
Dehydration processes
Introduction 3
1. Dehydration processes 4
2. Technology of dehydrogenation processes 9
References 11
Introduction
The processes of hydrolysis, hydration, dehydration, esterification and amidation are very important in the industry of basic organic and petrochemical synthesis. Soap, glycerin, ethanol and other valuable products have long been obtained by hydrolysis of fats, cellulose and carbohydrates. In the field of organic synthesis, the processes in question are used mainly for the production of C 2 -C 5 alcohols, phenols, ethers, α-oxides, many unsaturated compounds, carboxylic acids and their derivatives (esters, anhydrides, nitriles, amides) and other compounds.
The listed substances have a very important application as intermediate products of organic synthesis (alcohols, acids and their derivatives, aldehydes, -oxides), monomers and starting materials for the synthesis of polymeric materials (phenol, esters of acrylic and methacrylic acids, melamine, chloroolefins), plasticizers and lubricants (esters), solvents (alcohols, ethers and esters, chloroolefins), pesticides (esters of carbamic and thiocarbamic acids). Very often, the reactions considered are an intermediate step in multistage synthesis of other target products.
The production of these substances is on a large scale. Thus, in the USA, 500 thousand tons of ethanol and isopropanol, 900 thousand tons of propylene oxide, 200 thousand tons of epichlorohydrin, over 4 million tons of esters, and about 300 thousand tons of isocyanates are synthesized.
1. Dehydration processes
1. Dehydration with the formation of unsaturated compounds
The process is used to recover isobutene from C 4 -fractions of cracking and pyrolysis gases, when one of the stages consists in the dehydrogenation of tert-butanol catalyzed by sulfuric acid or sulfonic cation. Or dehydration to obtain isobutene is carried out with tert-butanol formed during the hydroxide method for producing propylene oxide:
(CH 3) 3 COH → (CH 3) 2 C \u003d CH 2 + H 2 O
In this and other cases, dehydration with the formation of unsaturated substances is most often one of the stages in the production of many monomers. So, in one of the new processes, styrene is produced by dehydration of methylphenylcarbinol:
C 6 H 5 -CHOH-CH 3 → C 6 H 5 -CH \u003d CH 2 + H 2 O
A well-known method for the synthesis of isoprene from isobutene and formaldehyde is also associated with the final dehydration of the diol and unsaturated alcohol:
(CH 3) 2 C (OH) -CH 2 CH 2 (OH) (CH 3) 2 \u003d CHCH 2 OH
(CH 3) 2 \u003d CHCH 2 OH CH 2 \u003d C (CH 3) CH \u003d CH 2
When the first water molecule is split off from the diol, a mixture of unsaturated alcohols of various structures is obtained, but all of them give isoprene upon further dehydration, and the reaction is accompanied by the displacement of double bonds:
Another preparation option for unsaturated compounds, consisting in the introduction of a vinyl group by reactions such as aldol condensation followed by dehydration, examples of the synthesis of nitroethylene, vinyl methyl ketone and 2-vinylpyridine can be given:
CH 3 NO 2 + HCHO HOCH 2 -CH 2 NO 2 CH 2 \u003d CHNO 2
CH 3 COCH 3 + HCHO CH 3 COCH 2 CH 2 OH CH 3 COCH \u003d CH 2
Dehydration is also one of the stages in the production of methacrylic acid esters CH 2 \u003d C (CH 3) COOR, some primary alcohols, for example n-butanol:
2CH 3 CHO → CH 3 CH(OH)CH 2 CHO CH 3 CH=CHCHO
CH 3 CH \u003d CHCHO CH 3 (CH 2) 2 -CH 2 OH
2-ethylhexanol, methylisobutylketone and many other substances.
2. Dehydration with the formation of ethers
Through the secondary formation of ethers during the hydrolysis of chlorine derivatives and the hydration of olefins, all the required amount of ethers such as diisopropyl ether is obtained. But diethyl ether has a fairly wide application, and it is specially produced by intermolecular dehydration of ethanol at 250 0 C on a heterogeneous AI 2 O 3 catalyst:
2C 2 H 5 OH → (C 2 H 5) 2 O + H 2 O
The possibility of using the same method for the synthesis of ethers from isopropanol and higher alcohols is limited by the development of side formation of olefins. As a result, most esters are obtained in the liquid phase at a lower temperature using acid catalysts - sulfuric, phosphoric, arylsulfonic acids. The method is suitable mainly for the synthesis of symmetrical ethers having the same alkyl groups, since when a mixture of two alcohols is dehydrated, the yield of a mixed ester is low:
3ROH + 3R"OH → R 2 O + R" 2 O + ROR" + 3H 2 O
Of the symmetrical ethers with a straight chain of carbon atoms, of interest is dichlorodiethyl ether (chlorex), which is a valuable solvent and extractant, as well as the starting material for the production of polysulfide polymers. It is produced by dehydration of anhydrous ethylene chlorohydrin on an acid catalyst:
2CICH 2 -CH 2 OH → (CICH 2 -CH 2) 2 + H 2 O
Dihydric alcohols under acid catalysis are capable of closing stable five- or six-membered rings. In this way, dioxane (1) is obtained from diethylene glycol, morpholine (2) from diethanolamine, and tetrahydrofuran (3) from butanediol-1,4. All of these substances are solvents:
3. Dehydration of carboxylic acids
The process of dehydration of carboxylic acids occupies a somewhat special position compared to other dehydration reactions. In this case, the products of intra- and intermolecular dehydration are ketene and acetic anhydride:
CH 3 -COOH CH 2 \u003d C \u003d O
2CH 3 COOH (CH 3 CO) 2 O
These reactions are endothermic, and their equilibrium shifts to the right only at high temperatures: 500 - 600 0 C in the case of anhydride formation and 700 0 C in the case of ketene formation. In the formation of ketene, the equilibrium transformation is also positively affected by reduced pressure. Both reactions proceed in the presence of heterogeneous acid-type catalysts (metal phosphates and borates) or vapors of phosphoric acid, which can be introduced into the initial mixture in the form of esters that readily hydrolyze into free acid. The reaction mechanism is generally similar to other dehydration processes:
CH 3 -COOH CH 3 COOH 2 CH 3 -C \u003d O
CH 2 \u003d C \u003d O CH 3 -C \u003d O 
(CH 3 -CO) 2 O
Ketene is a gas with a pungent odor that condenses into a liquid at -41 0 C. It is highly reactive, interacting with various substances to form acetic acid and its derivatives. In particular, with acetic acid it gives acetic anhydride:
CH 2 \u003d C \u003d O + CH 3 COOH → (CH 3 CO) 2 O
Acetic anhydride is a liquid with a pungent odor (bp 141 0 C). It is an important product of organic synthesis, widely used as an acetylating agent in the synthesis of acetic acid esters that are difficult to obtain in other ways - phenol acetates, tertiary alcohol acetates, and especially cellulose acetate and acetate fiber.
Acetic anhydride was previously obtained by the chlorine method - from sulfuryl chloride and sodium acetate:
SO 2 CI 2 + 4CH 3 COONa → 2(CH 3 CO) 2 O + Na 2 SO 4 + 2NaCI
Due to the high consumption of reagents and the formation of waste salts, this method was replaced by the dehydration of acetic acid. The latter can be carried out in two ways: by intermolecular dehydration or through the intermediate formation of ketene. In both cases, the resulting gas mixture contains highly reactive acetic anhydride or ketene and water, which can easily be converted back into acetic acid upon cooling. Therefore, it is necessary to separate water from the reaction gases so that it does not have time to react with ketene or acetic anhydride. In the direct synthesis of acetic anhydride, this is achieved by rapid cooling of the reaction gas with the introduction of an azeotropic additive (ethyl acetate), which, together with water, is separated from the condensate, which is further separated into acetic anhydride and acetic acid. In the ketene-intermediate process, the reaction gases are rapidly cooled to 0 0 C, and unconverted acetic acid and water are condensed from them. The residual gas is passed through a column refluxed with acetic acid, where acetic anhydride is formed. Side effects of these reactions are acetone and methane:
2CH 3 COOH → CH 3 COCH 3 + CO 2 + H 2 O
CH 3 COOH → CH 4 + CO 2
But the yield of acetic anhydride is quite high and equal to 90%.
2. Technology of dehydrogenation processes
Dehydrogenation processes are carried out by two main methods: in the liquid and gas phases.
Liquid-phase dehydration is used in cases where the product or initial reagents are not sufficiently stable at elevated temperatures of the gas-phase process. This applies to the synthesis of chlorex, dioxane, and morpholine, but nitroalcohols, hydroxyaldehydes, and hydroxyketones are often dehydrated in the liquid phase, which can also be converted into the corresponding unsaturated substances in the gas phase. Sulfuric acid (concentration up to 70%), phosphoric acid, calcium or magnesium acid phosphates, sulfonic cations (the latter at temperatures up to 150 0 C) are used as catalysts. The process is carried out at a temperature of 100 to 160 - 200 0 C and normal pressure.
Liquid-phase dehydration (Fig. 1) is most often carried out continuously in two main ways. In the first of these, the process is carried out by continuously distilling off the more volatile products from the catalyst solution - the target unsaturated substance or simple ether and water, which often give low-boiling azeotropic mixtures. The reactor is heated with steam and the initial organic reagent is continuously fed into the apparatus. Above the reactor is a return condenser (sometimes a reflux column) with which the return of the condensate can be controlled, keeping the catalyst concentration constant.
Rice. 1 Reaction unit for liquid phase dehydration process
The second method is used to carry out practically irreversible and fairly fast H 2 O elimination reactions with the formation of nitroolefins, unsaturated aldehydes and ketones, and other substances. It consists in passing the acidified reagent through a serpentine or tubular reactor at the desired temperature.
Gas phase dehydration is used to produce styrene (from methylphenylcarbinol), isoprene (from tert-butanol), diethyl ether (from ethanol), tetrahydrofuran (from 1,4-butanediol), acetic anhydride (directly from acetic acid or via ketene), and other products. . The most commonly used catalysts are phosphoric acid on porous media, alumina, acidic and medium calcium or magnesium phosphates. The temperature ranges from 225 - 250 0 С (obtaining diethyl ether) to 700 - 720 0 С (dehydration of acetic acid to ketene). The pressure is most often normal, but when receiving diethyl ether, it can be 0.5 - 1.0 MPa, and when dehydrated to ketene, 0.02 - 0.03 MPa.
Gas-phase dehydration is also carried out by two main methods. The first is used to carry out endothermic processes of intramolecular dehydration. The reactor is a tubular apparatus heated by a coolant (Fig. 2a), in the pipes of which a heterogeneous catalyst is placed.
Rice. 2 Reaction units of the gas-phase dehydration process
In view of the high metal content of these apparatuses, adiabatic reactors with a continuous layer of a heterogeneous catalyst (Fig. 2b), which do not have heat exchange surfaces, are most widely used. They are especially suitable for carrying out weakly exothermic reactions of the formation of unsaturated compounds, in order to maintain the required temperature regime, they often dilute the initial mixture with superheated water vapor, which prevents the mixture from cooling excessively and at the same time promotes an increase in the selectivity of the reaction. Finally, there are installations with two adiabatic type reactors in series: the gas cooled in the first apparatus is heated to the desired temperature in a heat exchanger using a suitable heat carrier before being fed into the second apparatus.
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