Dehydrogenation of ethanol. Oxidation of primary alcohols. Oxidation of ethyl alcohol to acetic acid
Depending on the type of hydrocarbon radical, and also, in some cases, the features of attaching the -OH group to this hydrocarbon radical, compounds with a hydroxyl functional group are divided into alcohols and phenols.
alcohols refers to compounds in which the hydroxyl group is attached to the hydrocarbon radical, but is not attached directly to the aromatic nucleus, if any, in the structure of the radical.
Examples of alcohols:
If the structure of the hydrocarbon radical contains an aromatic nucleus and a hydroxyl group, and is connected directly to the aromatic nucleus, such compounds are called phenols .
Examples of phenols:
Why are phenols classified in a separate class from alcohols? After all, for example, formulas
very similar and give the impression of substances of the same class of organic compounds.
However, the direct connection of the hydroxyl group with the aromatic nucleus significantly affects the properties of the compound, since the conjugated system of π-bonds of the aromatic nucleus is also conjugated with one of the lone electron pairs of the oxygen atom. Because of this, the O-H bond in phenols is more polar than in alcohols, which significantly increases the mobility of the hydrogen atom in the hydroxyl group. In other words, phenols have much more pronounced acidic properties than alcohols.
Chemical properties of alcohols
Monohydric alcohols
Substitution reactions
Substitution of a hydrogen atom in the hydroxyl group
1) Alcohols react with alkali, alkaline earth metals and aluminum (purified from the protective film of Al 2 O 3), while metal alcoholates are formed and hydrogen is released:
The formation of alcoholates is possible only when using alcohols that do not contain water dissolved in them, since alcoholates are easily hydrolyzed in the presence of water:
CH 3 OK + H 2 O \u003d CH 3 OH + KOH
2) Esterification reaction
The esterification reaction is the interaction of alcohols with organic and oxygen-containing inorganic acids, leading to the formation of esters.
This type of reaction is reversible, therefore, in order to shift the equilibrium towards the formation of an ester, it is desirable to carry out the reaction under heating, as well as in the presence of concentrated sulfuric acid as a water-removing agent:
Substitution of the hydroxyl group
1) When alcohols are treated with halogen acids, the hydroxyl group is replaced by a halogen atom. As a result of this reaction, haloalkanes and water are formed:
2) By passing a mixture of alcohol vapors with ammonia through heated oxides of some metals (most often Al 2 O 3), primary, secondary or tertiary amines can be obtained:
The type of amine (primary, secondary, tertiary) will depend to some extent on the ratio of the starting alcohol and ammonia.
Elimination reactions (cleavage)
Dehydration
Dehydration, which actually involves the splitting off of water molecules, in the case of alcohols differs by intermolecular dehydration And intramolecular dehydration.
At intermolecular dehydration alcohols, one water molecule is formed as a result of the elimination of a hydrogen atom from one alcohol molecule and a hydroxyl group from another molecule.
As a result of this reaction, compounds belonging to the class of ethers (R-O-R) are formed:
intramolecular dehydration alcohols proceeds in such a way that one molecule of water is split off from one molecule of alcohol. This type of dehydration requires somewhat more stringent conditions, consisting in the need to use a markedly higher heating compared to intermolecular dehydration. In this case, one alkene molecule and one water molecule are formed from one alcohol molecule:
Since the methanol molecule contains only one carbon atom, intramolecular dehydration is impossible for it. When methanol is dehydrated, only an ether (CH 3 -O-CH 3) can be formed.
It is necessary to clearly understand the fact that in the case of dehydration of unsymmetrical alcohols, intramolecular elimination of water will proceed in accordance with the Zaitsev rule, i.e. hydrogen will be split off from the least hydrogenated carbon atom:
Dehydrogenation of alcohols
a) Dehydrogenation of primary alcohols when heated in the presence of metallic copper leads to the formation aldehydes:
b) In the case of secondary alcohols, similar conditions will lead to the formation ketones:
c) Tertiary alcohols do not enter into a similar reaction, i.e. are not dehydrated.
Oxidation reactions
Combustion
Alcohols readily react with combustion. This creates a large number of heat:
2CH 3 -OH + 3O 2 \u003d 2CO 2 + 4H 2 O + Q
incomplete oxidation
Incomplete oxidation of primary alcohols can lead to the formation of aldehydes and carboxylic acids.
In the case of incomplete oxidation of secondary alcohols, the formation of only ketones is possible.
Incomplete oxidation of alcohols is possible when they are exposed to various oxidizing agents, such as air oxygen in the presence of catalysts (copper metal), potassium permanganate, potassium dichromate, etc.
In this case, aldehydes can be obtained from primary alcohols. As you can see, the oxidation of alcohols to aldehydes, in fact, leads to the same organic products, which is dehydrogenation:
It should be noted that when using such oxidizing agents as potassium permanganate and potassium dichromate in an acidic medium, deeper oxidation of alcohols, namely to carboxylic acids, is possible. In particular, this manifests itself when using an excess of an oxidizing agent during heating. Secondary alcohols can only oxidize to ketones under these conditions.
LIMITED POLYTOMIC ALCOHOLS
Substitution of hydrogen atoms of hydroxyl groups
Polyhydric alcohols as well as monohydric react with alkali, alkaline earth metals and aluminum (cleaned from the filmAl 2 O 3 ); in this case, a different number of hydrogen atoms of hydroxyl groups in an alcohol molecule can be replaced:
2. Since the molecules of polyhydric alcohols contain several hydroxyl groups, they influence each other due to the negative inductive effect. In particular, this leads to a weakening O-N connections and increase the acidic properties of hydroxyl groups.
B O The greater acidity of polyhydric alcohols is manifested in the fact that polyhydric alcohols, unlike monohydric ones, react with some hydroxides heavy metals. For example, one must remember the fact that freshly precipitated copper hydroxide reacts with polyhydric alcohols to form a bright blue solution of the complex compound.
Thus, the interaction of glycerol with freshly precipitated copper hydroxide leads to the formation of a bright blue solution of copper glycerate:
This reaction is qualitative for polyhydric alcohols. For passing the exam it is enough to know the signs of this reaction, and it is not necessary to be able to write the interaction equation itself.
3. Just like monohydric alcohols, polyhydric ones can enter into an esterification reaction, i.e. react with organic and oxygen-containing inorganic acids to form esters. This reaction is catalyzed by strong inorganic acids and is reversible. In this regard, during the esterification reaction, the resulting ester is distilled off from the reaction mixture in order to shift the equilibrium to the right according to the Le Chatelier principle:
If carboxylic acids with a large number of carbon atoms in the hydrocarbon radical react with glycerol, resulting from such a reaction, esters are called fats.
In the case of esterification of alcohols with nitric acid, the so-called nitrating mixture is used, which is a mixture of concentrated nitric and sulfuric acids. The reaction is carried out under constant cooling:
An ester of glycerol and nitric acid, called trinitroglycerin, is an explosive. In addition, a 1% solution of this substance in alcohol has a powerful vasodilating effect, which is used for medical indications to prevent a stroke or heart attack.
Substitution of hydroxyl groups
Reactions of this type proceed by the mechanism of nucleophilic substitution. Interactions of this kind include the reaction of glycols with hydrogen halides.
So, for example, the reaction of ethylene glycol with hydrogen bromide proceeds with the successive replacement of hydroxyl groups by halogen atoms:
Chemical properties of phenols
As mentioned at the very beginning of this chapter, the chemical properties of phenols differ markedly from those of alcohols. This is due to the fact that one of the lone electron pairs of the oxygen atom in the hydroxyl group is conjugated with the π-system of conjugated bonds of the aromatic ring.
Reactions involving the hydroxyl group
Acid properties
Phenols are stronger acids than alcohols and dissociate to a very small extent in aqueous solution:
B O The greater acidity of phenols compared to alcohols in terms of chemical properties is expressed in the fact that phenols, unlike alcohols, are able to react with alkalis:
However, the acidic properties of phenol are less pronounced than even one of the weakest inorganic acids - carbonic. So, in particular, carbon dioxide, when passed through an aqueous solution of alkali metal phenolates, displaces free phenol from the latter as an acid even weaker than carbonic acid:
Obviously, any other stronger acid will also displace phenol from phenolates:
3) Phenols are stronger acids than alcohols, while alcohols react with alkali and alkaline earth metals. In this regard, it is obvious that phenols will also react with these metals. The only thing is that, unlike alcohols, the reaction of phenols with active metals requires heating, since both phenols and metals are solids:
Substitution reactions in the aromatic nucleus
The hydroxyl group is a substituent of the first kind, which means that it facilitates substitution reactions in ortho- And pair- positions in relation to oneself. Reactions with phenol proceed under much milder conditions than with benzene.
Halogenation
The reaction with bromine does not require any special conditions. When bromine water is mixed with a solution of phenol, a white precipitate of 2,4,6-tribromophenol is instantly formed:
Nitration
When phenol is exposed to a mixture of concentrated nitric and sulfuric acids (nitrating mixture), 2,4,6-trinitrophenol is formed - a crystalline explosive yellow color:
Addition reactions
Since phenols are unsaturated compounds, they can be hydrogenated in the presence of catalysts to the corresponding alcohols.
Specialty: chemical technology
Department: inorganic chemistry and chemical technology
APPROVE
Department head
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COURSE WORK
By discipline: Industrial catalysis
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On the topic: Catalytic dehydrogenation
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Designation of work KR - 02068108 - 240100 - 2015
Student Fazylova L.A.
Login 435
Head _______________ Kuznetsova I.V.
Voronezh - 2015
Introduction
Production of catalysts for dehydrogenation of alkylaromatic hydrocarbons.
Catalytic dehydrogenation of alkanes
Equipment for catalytic dehydrogenation of alkanes
Regeneration of catalysts.
List of used literary sources
Introduction
Dehydrogenation - the reaction of splitting off hydrogen from a molecule of an organic compound; is reversible, the reverse reaction is hydrogenation. The equilibrium shift towards dehydrogenation is promoted by an increase in temperature and a decrease in pressure, including dilution of the reaction mixture. Hydrogenation-dehydrogenation reaction catalysts are metals 8B and 1B subgroups (nickel, platinum, palladium, copper, silver) and semiconductor oxides (Fe 2 O 3 , Cr 2 O 3 , ZnO, MoO 3).
Dehydrogenation processes are widely used in industrial organic synthesis:
1) by dehydrogenation of alcohols, formaldehyde, acetone, methyl ethyl ketone, cyclohexanone are obtained.
2) by dehydrogenation of alkylaromatic compounds, styrene, α-methylstyrene, vinyltoluene, divinylbenzene are obtained.
3) paraffin dehydrogenation produces: olefins (propylene, butylene and isobutylene, isopentene, higher olefins) and dienes (butadiene and isoprene)
Catalytic dehydrogenation of alcohols
Alcohol dehydrogenation reactions are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Copper, silver, copper chromites, zinc oxide, etc. serve as catalysts in the processes. It should be noted that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, however, it can provoke a dehydration reaction. IN general view alcohol dehydrogenation reactions can be represented as follows:
In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone, and cyclohexanone. The processes proceed in a stream of water vapor. The most common processes are:
Ethanol dehydrogenation carried out on a copper or silver catalyst at a temperature of 200 - 400 ° C and atmospheric pressure. The catalyst is some kind of Al 2 O 3 , SnO 2 or carbon fiber supported with silver or copper components. This reaction is one of the components of the Wacker process, which is an industrial method for obtaining acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.
Methanol dehydrogenation. This process is not fully understood, but most researchers highlight it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are proposed: temperature 600 - 900 °C, active component of the catalyst zinc or copper, silicon oxide carrier, the possibility of initiating the reaction with hydrogen peroxide, etc. On this moment Most of the world's formaldehyde is produced by the oxidation of methanol.
2. Production of catalysts for alcohol dehydrogenation processes
Known catalyst for the dehydrogenation of alcohols containing oxides, 5 zinc and iron. The newest is a catalyst for the dehydrogenation of alcohols, which is an oxide of yttrium or a rare earth element 10 selected from the group including neodymium, praeodymium, ytterbium ..
The disadvantage of the known catalysts is their insufficiently high activity and selectivity.
The goal of science is to increase the activity and selectivity of the catalyst for the dehydrogenation of alcohols. This goal is achieved in that the catalyst based on oxides of yttrium or a rare earth element selected from the group including neodymium, praseodymium, ytterbium, additionally contains technetium.
The introduction of technetium into the catalyst makes it possible to increase the activity of the catalyst, which is expressed in an increase in the degree of alcohol conversion by 2-5 times and a decrease in the temperature of the onset of the dehydrogenation reaction by 80-120 0 C. In this case, the catalyst acquires purely dehydrogenating properties, which makes it possible to increase selectivity. In the reaction of dehydrogenation of alcohol, for example, isopropyl alcohol to acetone up to 100%.
Such a catalyst is obtained by impregnating the preformed catalyst particles with a technetium salt solution. The volume of the solution exceeds the bulk volume of the catalyst by 1.4–1.6 times. The amount of technetium in the catalyst is determined by specific radioactivity. The wet catalyst is dried. The dry product is heated for 1 hour in a stream of hydrogen, first at 280-300 0 C (to convert pertechnetate into technetium dioxide), then at 600-700 0 C for 11 hours (to reduce technetium dioxide to metal).
Example. The catalyst is prepared by impregnating yttrium oxide with a solution of ammonium pertechnetate, the volume of which is 1.5 times that of yttrium oxide. The impregnated catalyst particles are dried at 70-80 0 C for 2 hours. Then reduction is carried out in a hydrogen flow for 1 hour at 280 0 C at a temperature of 600 C.
The study of catalytic activity is carried out on the example of the decomposition of isopropyl alcohol in a flow type installation. Catalyst weight
0.5 g at a volume of 1 cm. The size of the catalyst particles is 1.5 - 2 mm. Specific surface area 48.5 m/g. The alcohol feed rate is 0.071 ml/min.
The decomposition of isoaropyl alcohol on the proposed catalyst occurs only in the direction of dehydrogenation with the formation of acetone and hydrogen; no other products were found. On yttrium oxide without the addition of technetium, the decomposition of isopropyl alcohol proceeds in two directions: dehydrogenation and dehydration. The increase in catalyst activity is the greater, the higher the amount of technetium introduced. Catalysts containing 0.03 - 0.05% technetium are selective, leading the process in only one direction towards dehydrogenation.
3. Dehydrogenation of alkylaromatic compounds
The dehydrogenation of alkylaromatic compounds is an important industrial process for the synthesis of styrene and its homologues. In most cases, the process catalysts are iron oxides promoted by potassium, calcium, chromium, cerium, magnesium, and zinc oxides. Their distinctive feature is the ability to self-regenerate under the influence of water vapor. Phosphate, copper-chromium and even catalysts based on a mixture of iron oxide and copper are also known.
The processes of dehydrogenation of alkylaromatic compounds proceed at atmospheric pressure and at a temperature of 550 - 620 ° C in a molar ratio of raw materials to water vapor of 1:20. Steam is necessary not only to reduce the partial pressure of ethylbenzene, but also to maintain the self-regeneration of iron oxide catalysts.
The dehydrogenation of ethylbenzene is the second step in the process of obtaining styrene from benzene. At the first stage, benzene is alkylated with chloroethane (Friedel-Crafts reaction) on an aluminum-chromium catalyst, and at the second stage, the resulting ethylbenzene is dehydrogenated to styrene. The process is characterized by a high activation energy of 152 kJ/mol, due to which the reaction rate strongly depends on temperature. That is why the reaction is carried out at high temperatures.
In parallel, in the process of dehydrogenation of ethylbenzene, side reactions occur - coke formation, skeletal isomerization and cracking. Cracking and isomerization reduce the selectivity of the process, and coking affects the deactivation of the catalyst. In order for the catalyst to work longer, it is necessary to periodically carry out oxidative regeneration, which is based on the gasification reaction, which “burns out” most of the coke from the catalyst surface.
Hydration of alkenes The most important industrial value is the hydration of olefins. The addition of water to olefins can be carried out in the presence of sulfuric acid - sulfuric acid hydration or by passing a mixture of olefin with steam over a phosphate catalyst H3PO4 on aluminosilicate ...(ORGANIC CHEMISTRY)
Alcohol oxidation
During the combustion of alcohols, carbon dioxide and water are formed: Under the action of conventional oxidizing agents - a chromium mixture, potassium permangate, the carbon atom at which the hydroxyl group is located is primarily oxidized. Primary alcohols give aldehydes when oxidized, which easily pass ...(ORGANIC CHEMISTRY)
The oxidation of ethyl alcohol to acetic acid.
Ethyl alcohol is oxidized to acetic acid under the influence of acetic acid bacteria of the genera Gluconobacter and Acetobacter. They are Gram-negative chemoorganoheterotrophic, non-spore-forming, rod-shaped organisms, motile or immobile. Acetic acid bacteria of these genera differ from each other in ...(FUNDAMENTALS OF MICROBIOLOGY)
Catalytic dehydrogenation of alcohols
The transformation of alcohols into aldehydes and ketones can also be carried out by dehydrogenation - passing alcohol vapor over a heated catalyst - copper or silver at 300 ° C: The interaction of alcohols with organomagnesium compounds (Grignard reagents) leads to the formation of saturated hydrocarbons: This ...(ORGANIC CHEMISTRY)
Alcohol and alcohol-containing products
Excisable goods include only ethyl alcohol (raw and rectified alcohol), regardless of the type of raw material from which it is produced (food or non-food). Industrial alcohol (it is not ethyl alcohol) is not an excise product, it is obtained from wood or petroleum products. For the production of excise...(Taxation of commercial activities)
(ORGANIC CHEMISTRY)
Splitting of water from alcohols (dehydration):
Acid reagents are used as dehydration catalysts: sulfuric and phosphoric acids, alumina, etc. The order of splitting off is most often determined by Zaitsev's rule (1875): during the formation of water, hydrogen is most easily split off from the neighboring least hydrogenated carbon atom...(ORGANIC CHEMISTRY)
Alcohol oxidation
Alcohols are more easily oxidized than hydrocarbons, and the carbon at which the hydroxyl group is located is the first to be oxidized. The most suitable oxidizing agent in laboratory conditions is a chromium mixture. In industry - atmospheric oxygen in the presence of catalysts. Primary...(ORGANIC CHEMISTRY)
Oxidation of ethyl alcohol to acetic acid.
Ethyl alcohol is oxidized to acetic acid under the influence of acetic acid bacteria of the genera Gluconobacter and Acetobacter. They are Gram-negative chemoorganoheterotrophic, non-spore-forming, rod-shaped organisms, motile or immobile. Acetic acid bacteria of these genera differ from each other in ...(FUNDAMENTALS OF MICROBIOLOGY)
Catalytic dehydrogenation of paraffins
An important industrial process is also catalytic dehydrogenation paraffins over chromium oxide: Most laboratory methods for obtaining olefins are based on the reactions of elimination (elimination) of various reagents: water, halogens or hydrogen halides from the corresponding derivatives of saturated ...(ORGANIC CHEMISTRY)
The generally accepted mechanism for the dehydration of alcohols is as follows (for simplicity, ethyl alcohol is taken as an example):
The alcohol adds a hydrogen ion in step (1) to form a protonated alcohol, which dissociates in step (2) to give a water molecule and a carbonium ion; then the carbonium ion in step (3) loses a hydrogen ion and an alkene is formed.
Thus, the double bond is formed in two steps: loss of the hydroxyl group as [step (2)] and loss of hydrogen (step (3)). This is the difference between this reaction and the dehydrohalogenation reaction, where the elimination of hydrogen and halogen occurs simultaneously.
The first stage represents the Brønsted-Lowry acid-base balance (section 1.19). When sulfuric acid is dissolved in water, for example, the following reaction occurs:
The hydrogen ion went from a very weak base to a stronger base to form the oxonium ion. The main properties of both compounds are, of course, due to the lone pair of electrons that can bind the hydrogen ion. Alcohol also contains an oxygen atom with a lone pair of electrons and its basicity is comparable to that of water. The first stage of the proposed mechanism can most likely be represented as follows:
The hydrogen ion passed from the bisulfate ion to a stronger base (ethyl alcohol) to form the substituted oxonium ion of the protonated alcohol.
Similarly, step (3) is not the expulsion of a free hydrogen ion, but its transition to the strongest base available, namely to
For convenience, this process is often depicted as the addition or elimination of a hydrogen ion, but it should be understood that in all cases, in fact, there is a transfer of a proton from one base to another.
All three reactions are given as equilibrium because each step is reversible; as will be shown below, the reverse reaction is the formation of alcohols from alkenes (Sec. 6.10). Equilibrium (1) is shifted very strongly to the right; it is known that sulfuric acid is almost completely ionized in an alcoholic solution. Since the concentration of carbonium ions available at any moment is very small, equilibrium (2) is shifted strongly to the left. At some point, one of these few carbonium ions reacts according to equation (3) to form an alkene. During dehydration, the volatile alkene is usually distilled off from the reaction mixture, and thus the equilibrium (3) shifts to the right. As a result, the entire reaction comes to an end.
The carbonium ion is formed as a result of the dissociation of the protonated alcohol; the charged particle is separated from
neutral particle Obviously, this process requires much less energy than the formation of a carbonium ion from the alcohol itself, since in this case it is necessary to separate the positive particle from the negative one. In the first case, a weak base (water) is split off from a carbonium ion (Lewis acid) much more easily than a very strong base, the hydroxyl ion, i.e. water is a better leaving group than the hydroxyl ion. It has been shown that the hydroxyl ion almost never splits off from alcohol; bond cleavage reactions in alcohol in almost all cases require an acid catalyst, the role of which, as in the present case, is to protonate the alcohol.
Finally, it should be understood that the dissociation of the protonated alcohol becomes possible only due to the solvation of the carbonium ion (cf. Section 5.14). The energy to break the carbon-oxygen bond is taken from the formation of a large number of ion-dipole bonds between the carbonium ion and the polar solvent.
The carbonium ion can enter into various reactions; which one occurs depends on the experimental conditions. All reactions of carbonium ions end in the same way: they acquire a pair of electrons to fill an octet at a positively charged carbon atom. In this case, a hydrogen ion is split off from a carbon atom adjacent to a positively charged, electron-depleted carbon atom; a pair of electrons that previously bonded with this hydrogen can now form a -bond
This mechanism explains acid catalysis during dehydration. Does this mechanism also explain the fact that the ease of dehydration of alcohols decreases in the series tertiary secondary primary? Before answering this question, it is necessary to find out how the stability of carbonium ions changes.