In tasks of the C3 category of the Unified State Examination, the reactions of oxidation of organic substances with potassium permanganate KMnO 4 in an acidic environment, occurring with a break in the carbon chain, cause particular difficulties. For example, the propene oxidation reaction proceeding according to the equation:
CH 3 – CH = CH 2 + KMnO4 + H 2 SO 4 → CH 3 COOH + CO 2 + MnSO 4 + K 2 SO 4 + H 2 Oh
To factor in complex redox equations like this one, the standard technique suggests an electronic balance, but after another attempt, it becomes obvious that this is not enough. The root of the problem here lies in the fact that the coefficient in front of the oxidizer, taken from the electronic balance, must be replaced. This article offers two methods that allow you to choose the right factor in front of the oxidizer, in order to finally equalize all the elements. Substitution method to replace the coefficient in front of the oxidizing agent, it is more suitable for those who are able to count for a long time and painstakingly, since the arrangement of the coefficients in this way can be lengthy (in this example, it took 4 attempts). The substitution method is used in conjunction with the "TABLE" method, which is also discussed in detail in this article. Method "algebraic" allows you to replace the coefficient in front of the oxidizing agent no less simply and reliably, but much faster KMnO 4 compared to the substitution method, but has a narrower scope. The "algebraic" method can only be used to replace the coefficient in front of the oxidizer KMnO 4 in the equations of oxidation reactions of organic substances proceeding with a break in the carbon chain.
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On the topic: methodological developments, presentations and notes
Arrangement of coefficients in chemical equations
The teacher, being the main character in the organization of the cognitive activity of students, is constantly in search of ways to improve the effectiveness of learning. Organization of effective training...
Drawing up equations of redox reactions involving organic substances
IN In connection with the introduction of the Unified State Examination (USE) as the only form of final certification of secondary school graduates and the transition of high school to specialized education, the preparation of high school students for the most “expensive” tasks in terms of points of part “C” of the USE test in chemistry is becoming increasingly important. Despite the fact that the five tasks of part “C” are considered different: the chemical properties of inorganic substances, the chains of transformations of organic compounds, computational tasks, all of them are to some extent connected with redox reactions (ORD). If the basic knowledge of the OVR theory is mastered, then it is possible to correctly complete the first and second tasks in full, and the third - partially. In our opinion, a significant part of the success in the implementation of part "C" lies precisely in this. Experience shows that if, studying inorganic chemistry, students cope well enough with the tasks of writing OVR equations, then similar tasks in organic chemistry cause great difficulties for them. Therefore, throughout the study of the entire course of organic chemistry in specialized classes, we try to develop in high school students the skills of compiling OVR equations.
When studying the comparative characteristics of inorganic and organic compounds, we introduce students to the use of the oxidation state (s.o.) (in organic chemistry, primarily carbon) and methods for determining it:
1) calculation of the average s.d. carbon in a molecule of organic matter;
2) definition of s.d. every carbon atom.
We clarify in which cases it is better to use one or another method.
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P When studying the topic “Alkanes”, we show that the processes of oxidation, combustion, halogenation, nitration, dehydrogenation, and decomposition are redox processes. When writing the equations for the reactions of combustion and decomposition of organic substances, it is better to use the average value of s.d. carbon. For example:
We pay attention to the first half of the electronic balance: at the carbon atom in the fractional value of s.d. the denominator is 4, so we calculate the transfer of electrons using this coefficient.
In other cases, when studying the topic “Alkanes”, we determine the values of s.d. each carbon atom in the compound, while drawing students' attention to the sequence of substitution of hydrogen atoms at primary, secondary, tertiary carbon atoms:
Thus, we bring students to the conclusion that at the beginning the process of substitution occurs at the tertiary, then at the secondary, and, last of all, at the primary carbon atoms.
P When studying the topic “Alkenes”, we consider oxidation processes depending on the structure of the alkene and the reaction medium.
When alkenes are oxidized with a concentrated solution of potassium permanganate KMnO 4 in an acidic medium (hard oxidation), - and - bonds break with the formation of carboxylic acids, ketones and carbon monoxide (IV). This reaction is used to determine the position of the double bond.
If the double bond is at the end of the molecule (for example, in butene-1), then one of the oxidation products is formic acid, which is easily oxidized to carbon dioxide and water:
We emphasize that if in the alkene molecule the carbon atom at the double bond contains two carbon substituents (for example, in the molecule of 2-methylbutene-2), then during its oxidation a ketone is formed, since the transformation of such an atom into an atom of the carboxyl group is impossible without breaking C–C bond, relatively stable under these conditions:
We clarify that if the alkene molecule is symmetrical and the double bond is contained in the middle of the molecule, then only one acid is formed during oxidation:
We report that a feature of the oxidation of alkenes, in which the carbon atoms in the double bond contain two carbon radicals, is the formation of two ketones:
Considering the oxidation of alkenes in neutral or slightly alkaline media, we focus the attention of high school students on the fact that under such conditions, oxidation is accompanied by the formation of diols (dihydric alcohols), and hydroxyl groups are attached to those carbon atoms between which there was a double bond:
IN In a similar way, we consider the oxidation of acetylene and its homologues, depending on the medium in which the process takes place. So, we clarify that in an acidic environment, the oxidation process is accompanied by the formation of carboxylic acids:
The reaction is used to determine the structure of alkynes by oxidation products:
In neutral and slightly alkaline media, the oxidation of acetylene is accompanied by the formation of the corresponding oxalates (salts of oxalic acid), and the oxidation of homologues is accompanied by the breaking of the triple bond and the formation of salts of carboxylic acids:
IN All rules are worked out with students on specific examples, which leads to a better assimilation of theoretical material. Therefore, when studying the oxidation of arenes in various media, students can independently make assumptions that in an acidic medium one should expect the formation of acids, and in an alkaline medium, salts. The teacher will only have to clarify which reaction products are formed depending on the structure of the corresponding arena.
We show by examples that benzene homologues with one side chain (regardless of its length) are oxidized by a strong oxidizing agent to benzoic acid at the -carbon atom. Benzene homologues, when heated, are oxidized by potassium permanganate in a neutral medium to form potassium salts of aromatic acids.
5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 \u003d 5C 6 H 5 COOH + 6MnSO 4 + 3K 2 SO 4 + 14H 2 O,
5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 \u003d 5C 6 H 5 COOH + 5CO 2 + 12MnSO 4 + 6K 2 SO 4 + 28H 2 O,
C 6 H 5 -CH 3 + 2KMnO 4 \u003d C 6 H 5 COOK + 2MnO 2 + KOH + H 2 O.
We emphasize that if there are several side chains in an arene molecule, then in an acidic medium each of them is oxidized at an a-carbon atom to a carboxyl group, resulting in the formation of polybasic aromatic acids:
P The acquired skills in compiling OVR equations for hydrocarbons allow them to be used in the study of the “Oxygen-containing compounds” section.
So, when studying the topic “Alcohols”, students independently compose the equations for the oxidation of alcohols, using the following rules:
1) primary alcohols are oxidized to aldehydes
3CH 3 -CH 2 OH + K 2 Cr 2 O 7 + 4H 2 SO 4 \u003d 3CH 3 -CHO + K 2 SO 4 + Cr 2 (SO 4) 3 + 7H 2 O;
2) secondary alcohols are oxidized to ketones
3) for tertiary alcohols, the oxidation reaction is not typical.
In order to prepare for the exam, it is advisable for the teacher to give additional information to these properties, which will undoubtedly be useful for students.
When methanol is oxidized with an acidified solution of potassium permanganate or potassium dichromate, CO 2 is formed, primary alcohols during oxidation, depending on the reaction conditions, can form not only aldehydes, but also acids. For example, the oxidation of ethanol with potassium dichromate in the cold ends with the formation of acetic acid, and when heated, acetaldehyde:
3CH 3 -CH 2 OH + 2K 2 Cr 2 O 7 + 8H 2 SO 4 \u003d 3CH 3 -COOH + 2K 2 SO 4 + 2Cr 2 (SO 4) 3 + 11H 2 O,
3CH 3 -CH 2 OH + K 2 Cr 2 O 7 + 4H 2 SO 4 3CH 3 -CHO + K 2 SO 4 + Cr 2 (SO 4) 3 + 7H 2 O.
Let us remind students again about the influence of the environment on the products of alcohol oxidation reactions, namely: a hot neutral solution of KMnO 4 oxidizes methanol to potassium carbonate, and the remaining alcohols to salts of the corresponding carboxylic acids:
When studying the topic “Aldehydes and ketones”, we focus students' attention on the fact that aldehydes are more easily oxidized than alcohols into the corresponding carboxylic acids not only under the action of strong oxidizing agents (air oxygen, acidified solutions of KMnO 4 and K 2 Cr 2 O 7), but and under the influence of weak (ammonia solution of silver oxide or copper(II) hydroxide):
5CH 3 -CHO + 2KMnO 4 + 3H 2 SO 4 \u003d 5CH 3 -COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O,
3CH 3 -CHO + K 2 Cr 2 O 7 + 4H 2 SO 4 \u003d 3CH 3 -COOH + Cr 2 (SO 4) 3 + K 2 SO 4 + 4H 2 O,
CH 3 -CHO + 2OH CH 3 -COONH 4 + 2Ag + 3NH 3 + H 2 O.
We pay special attention to the oxidation of methanal with an ammonia solution of silver oxide, since in this case, ammonium carbonate is formed, and not formic acid:
HCHO + 4OH \u003d (NH 4) 2 CO 3 + 4Ag + 6NH 3 + 2H 2 O.
As our long-term experience shows, the proposed method of teaching high school students how to write OVR equations with the participation of organic substances increases their final USE result in chemistry by several points.
4.5. Alkene oxidation
It is advisable to divide the reactions of alkene oxidation into two large groups: reactions in which the carbon skeleton is preserved and reactions of oxidative destruction of the carbon skeleton of the molecule along the double bond. The first group of reactions includes epoxidation, as well as hydroxylation, leading to the formation of vicinal diols (glycols). In the case of cyclic alkenes, hydroxylation forms vicinal trance- or cis-diols. Another group includes ozonolysis and reactions of exhaustive oxidation of alkenes, leading to the formation of various kinds of carbonyl compounds and carboxylic acids.
4.5.a. Oxidation reactions of alkenes with preservation of the carbon skeleton
1. Epoxidation (reaction by N.A. Prilezhaev, 1909)
Acyclic and cyclic alkenes, when interacting with peracids (peracids) RCOOOH in a non-polar, indifferent medium, form epoxides (oxiranes), therefore the reaction itself is called the epoxidation reaction.
According to modern nomenclature IUPAC- a three-membered ring with one oxygen atom is called oxirane.
Epoxidation of alkenes should be considered as a synchronous, coordinated process, which does not involve ionic intermediates such as the OH+ hydroxyl cation. In other words, the epoxidation of alkenes is a process syn- addition of one oxygen atom to the double bond with the complete preservation of the configuration of the substituents at the double bond.
For epoxidation, a mechanism has been proposed that is characteristic of concerted processes.
Since the attack of the double bond by the oxygen atom of the peracid is equally probable on both sides of the plane of the double bond, the resulting oxiranes are either meso-forms, or mixtures of enantiomers. The following peracids are used as epoxidizing agents: perbenzoic, m-chloroperbenzoic, monoperphthalic, peracetic, trifluoroperacetic and performic. Aromatic peracids are used as individual reagents, while aliphatic peracids - CH 3 CO 3 H, CF 3 CO 3 H and HCO 3 H are not isolated individually, but are used after their formation in the interaction of 30% or 90% hydrogen peroxide and the corresponding carboxylic acid. Perbenzoic and m-chloroperbenzoic acid is obtained by oxidation of benzoic and m-chlorobenzoic acid with 70% hydrogen peroxide in a solution of methanesulfonic acid or from acid chlorides of these acids and hydrogen peroxide.
Monoperphthalic acid is obtained by a similar method from phthalic anhydride and 30% hydrogen peroxide.
Initially, perbenzoic or monoperphthalic acids were used to obtain oxiranes (epoxides):
Currently, epoxidation is most often used m-chloroperbenzoic acid. Unlike other peracids, it is stable during storage for a long time (up to 1 year) and is absolutely safe to handle. Yields of oxiranes obtained by oxidation of acyclic and cyclic alkenes m-chloroperbenzoic acid in a solution of methylene chloride, chloroform or dioxane are usually quite high.
Peracids are often generated directly from a reaction mixture of 90% hydrogen peroxide and carboxylic acid in methylene chloride.
Alkenes with a double bond conjugated with a carbonyl group or another acceptor substituent are inactive and it is better to use stronger oxidizing agents for their oxidation, such as trifluoroperacetic acid obtained from trifluoroacetic acid anhydride and 90% hydrogen peroxide in methylene chloride. The simplest oxirane, ethylene oxide, is produced industrially by the oxidation of ethylene with oxygen in the presence of silver as a catalyst.
2. anti-Hydroxylation
The three-membered ring of oxiranes is easily opened under the action of a wide variety of nucleophilic reagents. These reactions will be discussed in detail in the section on acyclic and cyclic ethers. Here, only the hydrolysis of oxiranes will be considered. The hydrolysis of oxiranes is catalyzed by both acids and bases. In both cases, vicinal diols, i.e., glycols, are formed. During acid catalysis, in the first stage, the protonation of the oxygen atom of oxirane occurs with the formation of a cyclic oxonium cation, which opens as a result of the nucleophilic attack of a water molecule:
The key step in ring opening, which determines the rate of the entire process, is the nucleophilic attack of water on the protonated form of oxirane. From the point of view of the mechanism, this process is similar to the opening of the bromonium ion during the nucleophilic attack of the bromide ion or another nucleophilic agent. From these positions, the stereochemical result should be the formation trance-glycols in the cleavage of cyclic epoxides. Indeed, during the acid-catalyzed hydrolysis of cyclohexene oxide or cyclopentene oxide, exclusively trance-1,2-diols.
Thus, the two-stage process of alkene epoxidation followed by acid hydrolysis of the epoxide corresponds in total to the reaction anti-hydroxylation of alkenes.
Both stages anti-hydroxylation of alkenes can be combined if the alkene is treated with aqueous 30-70% hydrogen peroxide in formic or trifluoroacetic acid. Both of these acids are strong enough to open the oxirane ring.
Opening of the oxirane ring, catalyzed by the base, also leads to the formation of cyclic trance-glycols.
Therefore, the two-stage process of epoxidation of alkenes followed by alkaline hydrolysis of epoxides is also a reaction anti-hydroxylation of alkenes.
3. syn-Hydroxylation
Some salts and oxides of transition metals in higher oxidation states are effective reagents. syn-hydroxylation of the double bond of an alkene, when both hydroxyl groups are attached to the same side of the double bond. Oxidation of alkenes with potassium permanganate is one of the oldest methods syn-double bond hydroxylation - continues to be widely used despite its inherent limitations. cis-1,2-cyclohexanediol was first obtained by V.V. Markovnikov in 1878 by hydroxylation of cyclohexene with an aqueous solution of potassium permanganate at 0 0 C.
This method was further developed in the works of the Russian scientist E.E. Wagner, therefore syn-hydroxylation of alkenes under the action of an aqueous solution of potassium permanganate is called the Wagner reaction. Potassium permanganate is a strong oxidizing agent that can not only hydroxylate the double bond, but also cleave the resulting vicinal diol. In order to avoid further degradation of the glycols as much as possible, the reaction conditions must be carefully controlled. Glycol yields are usually low (30-60%). The best results are achieved by hydroxylation of alkenes in a slightly alkaline medium (рН~8 9) at 0-5 0 С with a dilute 1% aqueous solution of KMnO 4 .
Initially, when alkenes are oxidized with potassium permanganate, a cyclic permanganic acid ester is formed, which is immediately hydrolyzed to a vicinal diol.
The cyclic ester of permanganic acid was not isolated as an intermediate, but its formation follows from experiments with labeled 18 O potassium permanganate: both oxygen atoms in glycol turn out to be labeled upon oxidation of the alkene KMn 18 O 4 . This means that both oxygen atoms are transferred from the oxidizing agent and not from the solvent - water, which is in good agreement with the proposed mechanism.
Another method syn-hydroxylation of alkenes under the action of osmium (VIII) oxide OsO 4 was proposed by R. Krige in 1936. Osmium tetroxide is a colorless, volatile, crystalline substance, readily soluble in ether, dioxane, pyridine, and other organic solvents. When osmium tetroxide reacts with alkenes in ether or dioxane, a black precipitate of the osmic acid cyclic ester is formed - osmate, which can be easily isolated individually. The addition of OsO 4 to the double bond is markedly accelerated in pyridine solution. The decomposition of osmates to vicinal glycols is achieved by the action of an aqueous solution of sodium hydrosulfite or hydrogen sulfide.
Product Outputs syn-hydroxylation of alkenes in this method is much higher than when using permanganate as an oxidizing agent. An important advantage of the Krige method is the absence of products of oxidative cleavage of alkenes, which is characteristic of permanganate oxidation.
Osmium tetroxide is a very expensive and hard-to-get reagent, besides it is toxic. Therefore, osmium(VIII) oxide is used in the synthesis of small amounts of hard-to-reach substances in order to obtain the highest diol yield. In order to simplify syn-hydroxylation of alkenes under the action of OsO 4 a technique was developed that allows using only catalytic amounts of this reagent. Hydroxylation of alkenes is carried out using hydrogen peroxide in the presence of OsO 4, for example:
To conclude this section, we present the stereochemical relationships between the alkene cis- or trance-configuration and configuration of the resulting vicinal diol, which can be cis- or trance-isomer, erythro- or treo-form, meso- or D,L-form depending on the substituents in the alkene:
Similar stereochemical relationships are observed in other reactions syn- or anti- multiple bond additions of hydrogen, hydrogen halides, water, halogens, boron hydrides, and other reagents.
As already mentioned, the oxidation of organic matter is the introduction of oxygen into its composition and (or) the elimination of hydrogen. Recovery is the reverse process (the introduction of hydrogen and the elimination of oxygen). Given the composition of alkanes (СnH2n+2), we can conclude that they are incapable of participating in reduction reactions, but they can participate in oxidation reactions.
Alkanes are compounds with low degrees of carbon oxidation, and depending on the reaction conditions, they can be oxidized to form various compounds.
At ordinary temperatures, alkanes do not react even with strong oxidizing agents (H2Cr2O7, KMnO4, etc.). When introduced into an open flame, alkanes burn. At the same time, in an excess of oxygen, they are completely oxidized to CO2, where carbon has the highest oxidation state of +4, and water. The combustion of hydrocarbons leads to the breaking of all C-C and C-H bonds and is accompanied by the release of a large amount of heat (exothermic reaction).
It is generally accepted that the mechanism of alkane oxidation includes a radical chain process, since oxygen itself is not very reactive, in order to abstract a hydrogen atom from an alkane, a particle is needed that will initiate the formation of an alkyl radical that will react with oxygen, giving a peroxy radical. The peroxy radical can then abstract a hydrogen atom from another alkane molecule to form an alkyl hydroperoxide and a radical.
It is possible to oxidize alkanes with atmospheric oxygen at 100-150 ° C in the presence of a catalyst - manganese acetate, this reaction is used in industry. Oxidation occurs when an air current is blown through molten paraffin containing a manganese salt.
Because as a result of the reaction, a mixture of acids is formed, then they are separated from the unreacted paraffin by dissolving in aqueous alkali, and then neutralized with mineral acid.
Directly in industry, this method is used to obtain acetic acid from n-butane:
Alkene oxidation
Alkene oxidation reactions are divided into two groups: 1) reactions in which the carbon skeleton is preserved, 2) reactions of oxidative destruction of the carbon skeleton of the molecule along the double bond.
Oxidation reactions of alkenes with preservation of the carbon skeleton
1. Epoxidation (Prilezhaev reaction)
Acyclic and cyclic alkenes, when interacting with peracids in a non-polar medium, form epoxides (oxiranes).
Also, oxiranes can be obtained by oxidation of alkenes with hydroperoxides in the presence of molybdenum-, tungsten-, vanadium-containing catalysts:
The simplest oxirane, ethylene oxide, is produced industrially by the oxidation of ethylene with oxygen in the presence of silver or silver oxide as a catalyst.
2. anti-hydroxylation (hydrolysis of epoxides)
Acid (or alkaline) hydrolysis of epoxides leads to the opening of the oxide cycle with the formation of transdiols.
In the first stage, the protonation of the oxygen atom of the epoxide occurs with the formation of a cyclic oxonium cation, which opens as a result of the nucleophilic attack of the water molecule.
Base-catalyzed epoxy ring opening also leads to the formation of trans-glycols.
3. syn-hydroxylation
One of the oldest methods for the oxidation of alkenes is the Wagner reaction (oxidation with potassium permanganate). Initially, during oxidation, a cyclic permanganate ester is formed, which is hydrolyzed to a vicinal diol:
In addition to the Wagner reaction, there is another method for the syn-hydroxylation of alkenes under the action of osmium (VIII) oxide, which was proposed by Krige. Under the action of osmium tetroxide on an alkene in ether or dioxane, a black precipitate of the cyclic ester of osmic acid is formed - osmate. However, the addition of OsO4 to the multiple bond is markedly accelerated in pyridine. The resulting black precipitate of osmate is easily decomposed by the action of an aqueous solution of sodium hydrosulfite:
Potassium permanganate or osmium(VIII) oxide oxidize the alkene to cis-1,2-diol.
Oxidative cleavage of alkenes
The oxidative cleavage of alkenes includes reactions of their interaction with potassium permanganate in alkaline or sulfuric acid, as well as oxidation with a solution of chromium trioxide in acetic acid or potassium dichromate and sulfuric acid. The end result of such transformations is the splitting of the carbon skeleton at the site of the double bond and the formation of carboxylic acids or ketones.
Monosubstituted alkenes with a terminal double bond are cleaved to a carboxylic acid and carbon dioxide:
If both carbon atoms in the double bond contain only one alkyl group, then a mixture of carboxylic acids is formed:
But if an alkene tetrasubstituted with a double bond is a ketone:
The reaction of ozonolysis of alkenes has acquired a much greater preparative significance. For many decades, this reaction served as the main method for determining the structure of the starting alkene. This reaction is carried out by passing a current of an ozone solution in oxygen, an alkene solution in methylene chloride or ethyl acetate at -80 ... -100 ° C. The mechanism of this reaction was established by Krige:
Ozonides are unstable compounds that decompose with an explosion. There are two ways of decomposition of ozonides - oxidative and reductive.
During hydrolysis, ozonides are split into carbonyl compounds and hydrogen peroxide. Hydrogen peroxide oxidizes aldehydes to carboxylic acids - this is oxidative decomposition:
Much more important is the reductive splitting of ozonides. Ozonolysis products are aldehydes or ketones, depending on the structure of the starting alkene:
In addition to the above methods, there is another method proposed in 1955 by Lemieux:
In the Lemieux method, there are no time-consuming procedures for separating manganese dioxide, since dioxide and manganate are again oxidized by periodate to the permanganate ion. This allows only catalytic amounts of potassium permanganate to be used.
St. Petersburg State Technological Institute
(Technical University)
Department of Organic Chemistry Faculty 4
Group 476
Course work
Alkene oxidation
Student……………………………………… Rytina A.I.
Lecturer………………………………... Piterskaya Yu.L.
Saint Petersburg
Introduction
1. Epoxidation (reaction of N.A. Prilezhaev, 1909)
2. Hydroxylation
2.1anti-Hydroxylation
2.2syn-Hydroxylation
3. Oxidative cleavage of alkenes
4.Ozonolysis
5. Oxidation of alkenes in the presence of palladium salts
Conclusion
List of sources used
Introduction
Oxidation is one of the most important and widespread transformations of organic compounds.
In organic chemistry, oxidation is understood as processes that lead to the depletion of a compound in hydrogen or its enrichment in oxygen. In this case, electrons are removed from the molecule. Accordingly, reduction is understood as the detachment from an organic oxygen molecule or the addition of hydrogen to it.
In redox reactions, oxidizing agents are compounds with a high electron affinity (electrophiles), and reducing agents are compounds that have a tendency to donate electrons (nucleophiles). The ease of oxidation of the compound increases with the growth of its nucleophilicity.
During the oxidation of organic compounds, as a rule, complete transfer of electrons and, accordingly, a change in the valence of carbon atoms does not occur. Therefore, the concept of the degree of oxidation - the conditional charge of an atom in a molecule, calculated on the basis of the assumption that the molecule consists only of ions - is only conditional, formal.
When compiling the equations of redox reactions, it is necessary to determine the reducing agent, oxidizing agent and the number of given and received electrons. As a rule, the coefficients are selected using the electron-ion balance method (half-reaction method).
This method considers the transition of electrons from one atom or ion to another, taking into account the nature of the medium (acidic, alkaline or neutral) in which the reaction takes place. To equalize the number of oxygen and hydrogen atoms, either water molecules and protons (if the medium is acidic) or water molecules and hydroxide ions (if the medium is alkaline) are introduced.
Thus, when writing the reduction and oxidation half-reactions, one must proceed from the composition of the ions actually present in the solution. Substances that are poorly dissociated, poorly soluble or evolved as a gas should be written in molecular form.
As an example, consider the process of ethylene oxidation with a dilute aqueous solution of potassium permanganate (Wagner reaction). During this reaction, ethylene is oxidized to ethylene glycol, and potassium permanganate is reduced to manganese dioxide. Two hydroxyls are added at the site of the double bond:
3C 2 H 4 + 2KMnO 4 + 4H 2 O → 3C 2 H 6 O 2 + 2MnO 2 + 2KOH
Reduction half-reaction: MnO 4 ¯ + 2H 2 O + 3 e→ MnO 2 + 4OH ¯ 2Oxidation half-reaction: C 2 H 4 + 2OH − − 2 e → C 2 H 6 O 2 3
Finally, we have in ionic form:
2MnO 4 ¯ + 4H 2 O + 3C 2 H 4 + 6OH ¯ → 2MnO 2 + 8OH ¯ + 3C 2 H 6 O 2
After carrying out the necessary reductions of similar terms, we write the equation in molecular form:
3C 2 H 4 + 2KMnO 4 + 4 H 2 O \u003d 3C 2 H 6 O 2 + 2MnO 2 + 2KOH.
Characteristics of some oxidizing agents
Oxygen
Air oxygen is widely used in technological processes, as it is the cheapest oxidizing agent. But oxidation with air oxygen is fraught with difficulties associated with the control of the process, which proceeds in various directions. The oxidation is usually carried out at high temperature in the presence of catalysts.
Ozone
Ozone O 3 is used to obtain aldehydes and ketones, if it is difficult to obtain them in other ways. Most often, ozone is used to establish the structure of unsaturated compounds. Ozone is produced by the action of a quiet electrical discharge on oxygen. One of the significant advantages of ozonation, compared with chlorination, is the absence of toxins after treatment.
Potassium permanganate
Potassium permanganate is the most commonly used oxidizing agent. The reagent is soluble in water (6.0% at 20ºC), as well as in methanol, acetone and acetic acid. For oxidation, aqueous (sometimes acetone) solutions of KMnO 4 are used in a neutral, acidic or alkaline medium. When carrying out the process in a neutral environment, salts of magnesium, aluminum are added to the reaction mass or carbon dioxide is passed through to neutralize the potassium hydroxide released during the reaction. The oxidation reaction of KMnO 4 in an acidic environment is most often carried out in the presence of sulfuric acid. The alkaline environment during oxidation is created by the KOH formed during the reaction, or it is initially added to the reaction mass. In slightly alkaline and neutral media, KMnO 4 oxidizes according to the equation:
KMnO4+ 3 e+ 2H 2 O \u003d K + + MnO 2 + 4OH ¯
in an acidic environment:
KMnO4+ 5 e+ 8H + = K + + Mn 2+ + 4H 2 O
Potassium permanganate is used to obtain 1,2-diols from alkenes, in the oxidation of primary alcohols, aldehydes and alkylarenes to carboxylic acids, and also for the oxidative cleavage of the carbon skeleton at multiple bonds.
In practice, a fairly large excess (more than 100%) of KMnO 4 is usually used. This is due to the fact that under normal conditions KMnO 4 partially decomposes into manganese dioxide with the release of O 2 . Explosively decomposes with concentrated H 2 SO 4 when heated in the presence of reducing agents; mixtures of potassium permanganate with organics are also explosive.
Peracids
Peracetic and performic acids are obtained by reacting 25-90% hydrogen peroxide with the corresponding carboxylic acid according to the following reaction:
RCOOH + H 2 O 2 \u003d RCOOOH + H 2 O
In the case of acetic acid, this equilibrium is established relatively slowly, and sulfuric acid is usually added as a catalyst to accelerate the formation of peracid. Formic acid is strong enough on its own to provide a quick equilibrium.
Pertrifluoroacetic acid, obtained in a mixture with trifluoroacetic acid by the reaction of trifluoroacetic anhydride with 90% hydrogen peroxide, is an even stronger oxidizing agent. Similarly, peracetic acid can be obtained from acetic anhydride and hydrogen peroxide.
Solid m-chloroperbenzoic acid, because it is relatively safe to handle, quite stable and can be stored for a long time.
Oxidation occurs due to the released oxygen atom:
RCOOOH = RCOOH + [O]
Peracids are used to obtain epoxides from alkenes, as well as lactones from alicyclic ketones.
Hydrogen peroxide
Hydrogen peroxide is a colorless liquid, miscible with water, ethanol and diethyl ether. A 30% solution of H 2 O 2 is called perhydrol. A highly concentrated preparation may react explosively with organic substances. On storage, it decomposes into oxygen and water. The persistence of hydrogen peroxide increases with dilution. For oxidation, aqueous solutions of various concentrations (from 3 to 90%) are used in neutral, acidic or alkaline media.
H 2 O 2 \u003d H 2 O + [O]
By the action of this reagent on α,β-unsaturated carbonyl compounds in an alkaline medium, the corresponding epoxyaldehydes and ketones are obtained, peracids are synthesized by oxidation of carboxylic acids in an acidic medium. A 30% solution of H 2 O 2 in acetic acid oxidizes alkenes to 1,2-diols. Hydrogen peroxide is used: to obtain organic and inorganic peroxides, Na perborate and percarbonate; as an oxidizing agent in rocket fuels; upon receipt of epoxides, hydroquinone, pyrocatechol, ethylene glycol, glycerin, vulcanization accelerators of the thiuram group, etc.; for bleaching oils, fats, fur, leather, textile materials, paper; for cleaning germanium and silicon semiconductor materials; as a disinfectant for the neutralization of domestic and industrial wastewater; in medicine; as a source of O 2 in submarines; H 2 O 2 is part of Fenton's reagent (Fe 2 + + H 2 O 2), which is used as a source of OH free radicals in organic synthesis.
Ruthenium and osmium tetroxides
Osmium tetroxide OsO 4 is a white to pale yellow powder with mp. 40.6ºС; t. kip. 131.2ºС. Sublimates already at room temperature, soluble in water (7.47 g in 100 ml at 25ºС), СCl 4 (250 g in 100 g of solvent at 20ºС). In the presence of organic compounds, it turns black due to reduction to OsO 2 .
RuO 4 is a golden yellow prism with so pl. 25.4ºС, noticeably sublimates at room temperature. Sparingly soluble in water (2.03 g in 100 ml at 20ºС), very soluble in CCl 4 . A stronger oxidizing agent than OsO 4 . Above 100ºС explodes. Like osmium tetroxide, it has high toxicity and high cost.
These oxidizing agents are used for the oxidation of alkenes to α-glycols under mild conditions.
