Alkaline oxidation of alkenes. Oxidation reactions of organic substances. What exactly will be discussed

As already mentioned, the oxidation of organic matter is the introduction of oxygen into its composition and (or) the elimination of hydrogen. Reduction is the reverse process (introduction of hydrogen and elimination of oxygen). Taking into account the composition of alkanes (СnH2n + 2), it can be concluded that they are incapable of entering into reduction reactions, but the possibility of participating in oxidation reactions.

Alkanes are compounds with low oxidation states of carbon, and depending on the reaction conditions, they can be oxidized to form various compounds.

At ordinary temperatures, alkanes do not react even with strong oxidants (H2Cr2O7, KMnO4, etc.). When introduced into an open flame, alkanes burn. In this case, in an excess of oxygen, they are completely oxidized to CO2, where carbon has the highest oxidation state of +4, and water. Combustion of hydrocarbons leads to the rupture 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 involves a radical chain process, since oxygen itself is not very reactive; in order to remove a hydrogen atom from an alkane, a particle is needed that will initiate the formation of an alkyl radical, which will react with oxygen, giving a peroxy radical. The peroxy radical can then remove a hydrogen atom from another alkane molecule to form an alkyl hydroperoxide and a radical.

Possible oxidation of 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 a stream of air is blown through molten paraffin wax containing a manganese salt.

Because As a result of the reaction, a mixture of acids is formed, then they are separated from 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:

Alkenes oxidation

Alkenes 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 at the double bond.

Alkenes oxidation reactions with preservation of the carbon skeleton

1. Epoxidation (Prilezhaev reaction)

Acyclic and cyclic alkenes interact with peracids in a non-polar medium to form epoxides (oxiranes).

Oxiranes can also be obtained by oxidizing alkenes with hydroperoxides in the presence of molybdenum-, tungsten-, vanadium-containing catalysts:

The simplest oxirane, ethylene oxide, is produced industrially by oxidizing 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 ring with the formation of transdiols.

In the first stage, the epoxide oxygen atom is protonated to form a cyclic oxonium cation, which opens as a result of a nucleophilic attack of a 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 ester of manganic acid is formed, which is hydrolyzed to a vicinal diol:

In addition to the Wagner reaction, there is another method of syn-hydroxylation of alkenes by the action of osmium (VIII) oxide, which was proposed by Kriege. When osmium tetroxide acts on an alkene in ether or dioxane, a black precipitate of a cyclic ester of osmium acid is formed - osmate. However, the addition of OsO4 to the multiple bond is noticeably accelerated in pyridine. The resulting black osmate sediment is easily decomposed by action aqueous solution sodium hydrosulfite:

Potassium permanganate or osmium (VIII) oxide oxidizes the alkene to cis-1,2-diol.

Oxidative cleavage of alkenes

The oxidative cleavage of alkenes includes their reaction 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 cleavage 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 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 the tetrasubstituted alkene at the double bond is a ketone:

The reaction of ozonolysis of alkenes is of much greater preparative importance. 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 Kriege:

Ozonides are unstable compounds that decompose with an explosion. There are two ways to decompose 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 essential has a reductive decomposition of ozonides. The products of ozonolysis are aldehydes or ketones, depending on the structure of the initial alkene:

In addition to the above methods, there is another method proposed in 1955 by Lemieux:

In the Lemieux method, there are no laborious procedures for the separation of manganese dioxide, because dioxide and manganate are again oxidized by periodate to permanganate ion. This allows only catalytic amounts of potassium permanganate to be used.

Saint Petersburg State Technological Institute

(Technical University)

Department of Organic Chemistry Faculty 4

Group 476

Course work

Alkenes oxidation

Student ……………………………………… Rytina A.I.

Teacher ……………………………… ... Piterskaya Yu.L.

St. 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 leading to the depletion of a compound in hydrogen or its enrichment with oxygen. In this case, the removal of electrons from the molecule occurs. Accordingly, restoration is understood as a separation from organic molecule oxygen or the addition of hydrogen to it.

In redox reactions, oxidants are compounds with a high affinity for an electron (electrophiles), and reducing agents are compounds with a tendency to donate electrons (nucleophiles). The ease of oxidation of a compound increases with an increase in its nucleophilicity.

When organic compounds are oxidized, as a rule, complete transfer of electrons and, accordingly, no change in the valence of carbon atoms occurs. Therefore, the concept of the oxidation state - the conditional charge of an atom in a molecule, calculated on the basis of the assumption that a molecule consists only of ions - is only conditional, formal.

When drawing up the equations of redox reactions, it is necessary to determine the reducing agent, the oxidizing agent and the number of electrons given and received. 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 half-reactions of reduction and oxidation, one must proceed from the composition of the ions actually present in the solution. Substances that are poorly dissociating, poorly soluble or emitted as a gas should be written in molecular form.

As an example, consider the oxidation of ethylene 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. At the place of the double bond, two hydroxyls are attached:

3C 2 H 4 + 2KMnO 4 + 4H 2 O → 3C 2 H 6 O 2 + 2MnO 2 + 2KOH

Oxidation 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 such terms, we write the equation in molecular form:

3C 2 H 4 + 2KMnO 4 + 4 H 2 O = 3C 2 H 6 O 2 + 2MnO 2 + 2KOH.

Characterization of some oxidants

Oxygen

Air oxygen is widely used in technological processes, as it is the cheapest oxidizing agent. But oxidation with oxygen in the air is fraught with difficulties in controlling the process, which proceeds in different directions. 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 by other methods. Most often, ozone is used to establish the structure of unsaturated compounds. Ozone is obtained by the action of a quiet electric discharge on oxygen. One of the significant advantages of ozonation, in comparison 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 20 ° C), 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 medium, 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 medium is most often carried out in the presence of sulfuric acid. The alkaline medium during oxidation creates 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:

KMnO 4 + 3 e+ 2H 2 O = K + + MnO 2 + 4OH ¯

in an acidic environment:

KMnO 4 + 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, as well as for the oxidative cleavage of the carbon skeleton at multiple bonds.

In practice, a rather 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 organic substances are also explosive.

Peracids

Peracetic and performic acids are obtained by the reaction of 25-90% hydrogen peroxide with the corresponding carboxylic acid according to the following reaction:

RCOOH + H 2 O 2 = RCOOOH + H 2 O

In the case of acetic acid, this equilibrium is relatively slow, 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 rapid equilibration.

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.

Particularly popular is solid mβ-chloroperbenzoic acid, since it is relatively safe to handle, fairly 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 can react explosively with organic substances. Decomposes to oxygen and water during storage. The persistence of hydrogen peroxide increases with dilution. For oxidation, aqueous solutions of various concentrations (from 3 to 90%) in neutral, acidic or alkaline media are used.

H 2 O 2 = 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, and peracids are synthesized by the 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: for the production of organic and inorganic peroxides, perborate and Na percarbonate; as an oxidizing agent in rocket fuels; when receiving epoxides, hydroquinone, pyrocatechol, ethylene glycol, glycerin, accelerators of vulcanization of the thiuram group, etc .; for bleaching oils, fats, fur, leather, textile materials, paper; for cleaning germanium and silicon semiconductor materials; how disinfectant for neutralization of household and industrial Wastewater; in medicine; as a source of O 2 in submarines; H 2 O 2 is a 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, mp. 40.6 ° C; t. kip. 131.2 ° C. Sublimes already at room temperature, soluble in water (7.47 g in 100 ml at 25 ° C), CCl 4 (250 g in 100 g of solvent at 20 ° C). 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. 25.4 ° C, noticeably sublimes at room temperature. We will moderately dissolve in water (2.03 g in 100 ml at 20 ° С), we will very well dissolve in CCl 4. Stronger oxidizing agent than OsO 4. Above 100 ° C it explodes. Like tetroxide, osmium is highly toxic and expensive.

These oxidants are used for the oxidation of alkenes to α-glycols under mild conditions.

Alkynes with non-terminal triple bonds serve as a potential source for the synthesis of 1,2-diketones under the action of a suitable oxidizing agent. However, until now, no universal reagent has been found that causes the oxidation of the triple carbon – carbon bond to the 1,2-dicarbonyl group. Proposed for this purpose RuO 4 - ruthenium (VIII) oxide - is too expensive and often causes further oxidative degradation of 1,2-diketones to carboxylic acids. When disubstituted acetylenes interact with such strong oxidants as potassium permanganate, oxidation can be stopped only in a completely neutral medium at pH 7–8 at 0 ° C at the stage of -diketone formation. For example, stearolic acid at pH 7.5 is oxidized to -diketone. In most cases, oxidation is accompanied by the cleavage of the triple bond with the formation of carboxylic acids:

The yield of the products of oxidative destruction of alkynes is low, and this reaction does not play a significant role in organic synthesis. It is used exclusively to prove the structure of the naturally occurring acetylenic acid found in the leaves of tropical plants in Central America. During its oxidative destruction, two acids were isolated - lauric and adipic. This means that the parent acid is 6-octadecic acid with a normal carbon skeleton of seventeen carbon atoms:

Much more important is the oxidative combination of alkynes-1, catalyzed by copper salts (Glaser – Eglinton reaction). In 1870, Glaser discovered that a suspension of copper (I) acetylide in alcohol is oxidized by atmospheric oxygen to form 1,3-diines:

For the oxidation of copper (I) acetylides, potassium hexacyanoferrate (III) K 3 in DME or DMF is more effective as an oxidizing agent. In 1959, Eglinton proposed a much more convenient modification of the oxidative condensation of alkynes. Alkyne is oxidized with copper (II) acetate in pyridine solution at 60–70 С. Eglinton's modification turned out to be extremely useful for the synthesis of macrocyclic polyyns from , -diyns. As an illustration, we present the synthesis of two cyclopolyines during the oxidative condensation of hexadiine-1,5 (F. Sondheimer, 1960):

One of the polyyns is a cyclotrimerization product, the other is a cyclotetramerization product of the starting hesadin-1.5. The trimer serves as a starting reagent for the synthesis of aromatic β-annulene (for more details on annulenes, see Chapter 12). Similarly, under the same conditions of nonadiine-1,8, its dimer is obtained - 1,3,10,12-cyclooctadecatetraene along with a trimer, tetramer and pentamer:

To obtain asymmetric diines, the condensation of haloacetylenes with alkyne-1 (terminal alkyne) in the presence of copper (I) salts and a primary amine (combination according to Kadio-Hodkevich, 1957) is used:

The starting bromoalkynes are obtained by the action of sodium hypobromite on alkynes-1 or from lithium and bromine acetylenides:

The organocopper derivative of teminal alkyne is generated directly in the reaction mixture from Cu 2 Cl 2 and alkyne-1.

6.3.4. Electrophilic triple bond reactions

The reactions of electrophilic addition to the triple bond are among the most typical and important reactions of alkynes. In contrast to the electrophilic addition to alkenes, the synthetic application of this large group of reactions far outstripped the development of theoretical concepts of its mechanism. However, over the past twenty years, the situation has changed significantly, and at present it is one of the rapidly developing areas of physical organic chemistry. The HOMO of an alkyne is located lower than the HOMO of an alkene (Chapter 2), and this circumstance predetermines in the overwhelming majority of cases a lower rate of addition of an electrophilic agent to an alkyne as compared to an alkene. Another factor determining the difference in the reactivity of alkynes and alkenes in electrophilic addition reactions is the relative stability of the intermediates arising upon the addition of an electrophilic particle to triple and double bonds. When an electrophilic particle H + or E + is attached to a double bond, a cyclic or open carbocation is formed (Chapter 5). The addition of H + or E + to a triple bond leads to the formation of an open or cyclic vinyl cation. In a linear open vinyl cation, the central carbon atom is located in sp-hybrid state, while vacant R-orbital is orthogonal to the-bond. Insofar as sp-hybrid carbon atom of vinyl cation has a higher electronegativity compared to sp 2 -hybrid atom of the alkyl cation, the vinyl cation should be less stable than the alkyl cation:

The data of quantum mechanical calculations, as well as thermodynamic data for the gas phase, obtained using high pressure mass spectrometry and cyclotron resonance spectroscopy, are in full agreement with these considerations. Table 6.3 shows the thermodynamic data for the formation of a number of carbocations and hydrocarbons, related to the gas phase at 25 С.

From the data presented in tal. 6.3, it follows that the vinyl cation is 47 kcal / mol less stable than the ethyl cation containing the same number of atoms. The same conclusion can be drawn from the enthalpy of ionization in the gas phase CH 3 CH 2 Cl and CH 2 = CHCl:

It is easy to see that the combination of both factors - the higher energy of the vinyl cation and the low-lying HOMO of the alkyne - represents a lower reactivity of alkynes compared to alkenes in electrophilic addition reactions. Table 6.4 compares comparative data on the addition of halogens, sulfene and selenyl chlorides, trifluoroacetic acid and water to various alkenes and alkynes that do not contain any activating or deactivating functional group.

Table 6.4

Comparative characteristics of alkynes and alkenes

in electrophilic addition reactions

It follows from these data that only the addition of acidic agents and water to triple and double bonds occurs at similar rates. The addition of halogens, sulfenchlorides and a number of other reagents to alkenes proceeds 10 2  10 5 times faster than to alkynes. This means that hydrocarbons containing non-conjugated triple and double bonds selectively bind these reagents at a double bond, for example:

The data on the comparative hydration of alkynes and alkenes should be treated with caution, since the hydration of alkynes requires catalysis by mercury (II) ions, which is ineffective for the addition of water to the double bond. Therefore, the data on the hydration of the triple and double bonds, strictly speaking, are not comparable.

The addition of halogens, hydrogen halides, sulfenchlorides and other electrophilic agents can be carried out stepwise, which can be easily illustrated using the following examples:

Alkenes - these are hydrocarbons, in the molecules of which there is ONE double C = C bond.

Alkenes nomenclature: suffix appears in the name -EN.

The first member of the homologous series is C2H4 (ethene).

For the simplest alkenes, historically established names are also used:

Ethylene (ethene),

Propylene (propene),

The nomenclature often uses the following monovalent alkene radicals:

Types of isomerism of alkenes:

1. Carbon skeleton isomerism:(starting with C4H8 - butene and 2-methylpropene)

2. Isomerism of the position of the multiple bond:(starting from C4H8): butene-1 and butene-2.

3. Interclass isomerism: with cycloalkanes(starting with propene):

C4H8 - butene and cyclobutane.

4. Spatial isomerism of alkenes:

Due to the fact that free rotation around the double bond is impossible, it becomes possible cis-trans isomerism.

Alkenes having two carbon atoms in a double bond various alternates, can exist in the form of two isomers, differing in the arrangement of substituents relative to the plane of the π-bond:

Chemical properties of alkenes.

Alkenes are characterized by:

· double bond addition reactions,

· oxidation reactions,

· substitution reactions in the "side chain".

OBTAINING ALKENS

ALCADIENES.

These are hydrocarbons containing two double bonds. The first member of the series is C3H4 (propadiene or allene). The suffix appears in the name - DIEN .

Types of double bonds in dienes:

Isomerism of dienes

Electronic structure of conjugated dienes.

Molecule of butadiene-1,3 CH2 = CH-CH = CH2 contains four carbon atoms in sp2 - hybridized and has a flat structure.

π-Electrons of double bonds form a single π-electron cloud (conjugate system ) and are delocalized between all carbon atoms.

The multiplicity of bonds (the number of common electron pairs) between carbon atoms has an intermediate value: there are no purely single and purely double bonds. The structure of butadiene more accurately reflects the formula with delocalized "one and a half" bonds.

CHEMICAL PROPERTIES OF CONJUGATED ALKADIENES.

OBTAINING ALCADIENES.

Select the main carbon chain in the molecule. First, it should be the longest. Secondly, if there are two or more chains of the same length, then the most branched one is selected. For example, a molecule has 2 chains with the same number (7) C atoms (highlighted in color):

In case (a), the chain has 1 substituent, and in (b) - 2. Therefore, option (b) should be chosen.

1. Number the carbon atoms in the main chain so that the C atoms linked to the substituents get the lowest possible number. Therefore, the numbering starts from the end of the chain closest to the branch. For example:

Name all radicals (substituents), leading numbers indicating their location in the main chain. If there are several identical substituents, then for each of them a number (location) is written separated by commas, and their number is indicated by prefixes di-, three-, tetra-, penta- etc. (for example, 2,2-dimethyl or 2,3,3,5-tetramethyl).

Arrange the names of all substitutes in alphabetical order (as established by the latest IUPAC rules).

Name the main chain of carbon atoms, i.e. the corresponding normal alkane.

Thus, in the name of a branched alkane, the root + suffix is ​​the name of a normal alkane (Greek numeral + suffix "an"), prefixes are numbers and names of hydrocarbon radicals. An example of building a name:

Chem. Sv-va alkanesCracking of alkanes. Cracking is a process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules to form compounds with a shorter chain. Isomerization of alkanes Alkanes of normal structure under the influence of catalysts and upon heating are capable of converting into branched alkanes without changing the molecular composition, i.e. enter into isomepization reactions. These reactions involve alkanes, the molecules of which contain at least 4 carbon atoms. For example, isomerization of n-pentane to isopentane (2-methylbutane) occurs at 100 ° C in the presence of an aluminum chloride catalyst:

The starting material and the product of the isomerization reaction have the same molecular formulas and are structural isomers (carbon skeleton isomerism).

Dehydrogenation of alkanes

When alkanes are heated in the presence of catalysts (Pt, Pd, Ni, Fe, Cr 2 O 3, Fe 2 O 3, ZnO), their catalytic dehydrogenation- the elimination of hydrogen atoms due to the rupture of C-H bonds.

The structure of the dehydrogenation products depends on the reaction conditions and the length of the main chain in the molecule of the starting alkane.

1.Lowest alkanes containing from 2 to 4 carbon atoms in the chain, when heated over a Ni catalyst, remove hydrogen from neighboring carbon atoms and turn into alkenes:

As well as butene-2 this reaction produces butene-1 CH 2 = CH-CH 2 -CH 3. In the presence of a Cr 2 O 3 / Al 2 O 3 catalyst at 450-650 С from n-butane is also obtained butadiene-1,3 CH 2 = CH-CH = CH 2.

2. Alkanes containing more than 4 carbon atoms in the main chain are used to obtain cyclical connections. When this happens dehydrocyclization- dehydrogenation reaction, which leads to the closure of the chain in a stable cycle.

If the main chain of an alkane molecule contains 5 (but not more) carbon atoms ( n-pentane and its alkyl derivatives), then when heated over a Pt catalyst, hydrogen atoms are split off from the terminal atoms of the carbon chain, and a five-membered cycle is formed (cyclopentane or its derivatives):

Alkanes with a main chain of 6 or more carbon atoms also undergo a dehydrocyclization reaction, but always form a 6-membered ring (cyclohexane and its derivatives). Under the reaction conditions, this cycle undergoes further dehydrogenation and turns into an energetically more stable benzene cycle of an aromatic hydrocarbon (arene). For example:

These reactions are at the heart of the process reforming- refining petroleum products in order to obtain arenas ( aromatization saturated hydrocarbons) and hydrogen. Transformation n- alkanes in arenas leads to improved detonation resistance of gasoline.