Alkenes names. General formula of alkenes

Alkenes undergo various reactions in which compounds of other classes are formed. Therefore, alkenes are important intermediates in organic synthesis. In the synthesis of many types of substances, it is useful to first obtain an alkene and already convert it into the required compound.

All reactions of alkenes can be roughly divided into two groups. One of them is formed by electrophilic addition reactions proceeding in two stages, the other - by all other reactions. We will begin below with the second group of reactions.

Hydrogenation

Alkenes react with hydrogen gas in the presence of catalysts (usually precious metals). In this case, two hydrogen atoms are attached to the double bond of the alkene and an alkane is formed. This reaction was discussed in detail in Ch. 3. Let's give two more examples:

Ozonolysis

This reaction is unusual in terms of what happens in it. complete break double carbon-carbon bond and splitting the carbon skeleton of the molecule into two parts. Alkenes are treated with ozone and then zinc dust. As a result, the alkene molecule is cleaved at a double bond and two molecules of an aldehyde and / or a ketone are formed. Acyclic compounds with two aldehyde (or ketone) groups are formed from cycloalkenes:

For instance:

Note that in the last two examples, ring opening of a cycloalkene produces one acyclic molecule, rather than two as from acyclic alkenes.

The ozonolysis reaction is used both for the synthesis of aldehydes and ketones, and for establishing the structure of alkenes. For example, suppose that the ozonolysis of an unknown alkene forms a mixture of two aldehydes:

In this case, the structure of the alkene can be logically established as follows. Carbon atoms bound in aldehyde molecules by double bonds with oxygen atoms were linked by a double bond in the initial alkene molecule:

Another example:

The alkene structure must be cyclic, since we must connect the two ends of the same molecule:

Oxidation

Diluted water solution potassium permanganate converts alkenes to diols (glycols). As a result of this reaction, two hydroxyl groups are attached to one side of the double bond (cis- or syn-addition).

Therefore, cis-diols are formed from cycloalkenes. V general view the reaction equation looks like this:

For instance:

The synthesis of diols proceeds best in a weakly alkaline medium and soft conditions(low temperature and dilute potassium permanganate solution). Under more severe conditions (acid catalysis, heating), the molecule is split at the double bond and carboxylic acids are formed.

The reaction with potassium permanganate is used not only for the preparation of diols, but also serves as a simple test for easy determination of alkenes. Permanganate solution has an intense violet color. If the test sample contains alkene, then when a few drops of permanganate solution are added to it, the violet color of the latter immediately turns brown. Only alkynes and aldehydes cause the same color change. Compounds of most other classes do not react under these conditions. The procedure described above is called Bayer breakdown. The ratio of compounds of various classes to the Bayer test is shown below: positive test (purple color disappears), negative test (purple color remains).

Allyl halogenation

If alkenes are subjected to free radical halogenation, the hydrogen atoms on the carbon atom adjacent to the double bond are most easily replaced by halogen. This position in the alkene molecule is called allyl:

A specific reagent for allylic bromination is β-bromosuccinimide It is a solid,

which is convenient to work with in the laboratory, while molecular bromine is a volatile, highly toxic and hazardous liquid to handle. When heated (sometimes catalysis with peroxides is required), N-bromosuccinimide becomes a source of bromine atoms.

Halogenation proceeds to the allyl position, since the intermediate allyl radical formed in this case is more stable than any other free radical that can be obtained from an alkene molecule. Therefore, it is this radical that is formed more easily than others. The increased stability of the allyl radical is explained by its resonance stabilization, as a result of which an unpaired electron is delocalized over two carbon atoms. The mechanism of allylic chlorination is shown below:

Alkenes are cleaved by ozone to form aldehydes and ketones, which makes it possible to establish the structure of alkenes. Alkenes undergo hydrogenation to form alkanes and oxidation to form diols. In addition to these reactions involving a double bond, alkenes are characterized by selective halogenation to a position adjacent to the double bond. The double bond itself is not affected.

Electrophilic connection to alkenes

Electrophilic addition reactions, differing from each other in the nature of the groups attached at the double bond, have the same two-stage mechanism. At its first stage, an electrophilic (having an affinity for an electron) particle (for example, a cation) is attracted by an electron cloud and joins at a double bond:

In most cases, Markovnikov's rule is fulfilled - the electrophile is attached to the most hydrogenated end of the double bond, and the nucleophile to the opposite one. These reactions are discussed in more detail in those chapters where the formation of the corresponding functional groups is considered. For example, the addition of hydrogen bromide is discussed in Ch. 5 (where we are talking about the synthesis of haloalkanes) the addition of water is considered in Ch. 7 (synthesis of alcohols). Here we will only once again emphasize the role of positively charged particles, which have an unfilled outer electron shell, and their interaction with electrons. Here are some examples as well:

Alkenes react with electrophilic reagents, which are attached at a double bond. The reaction takes place in two stages. In this way, compounds of various classes are obtained, for example haloalkanes and alcohols.

Figure 6-1. Electrophilic addition reactions to alkenes

UNSATURATED OR UNSATURATED HYDROCARBONS OF THE ETHYLENE SERIES (ALKENES, OR OLEFINS)

Alkenes, or olefins(from Latin olefiant - oil - an old name, but widely used in chemical literature. The reason for this name was ethylene chloride obtained in XVIII century, Is a liquid oily substance.) - aliphatic unsaturated hydrocarbons, in the molecules of which there is one double bond between the carbon atoms.

Alkenes form a homologous series with the general formula CnH2n

1. Homological series of alkenes

Homologues:

WITHH2 = CH2 ethen

WITHH2 = CH- CH3 propene

WITHH2 = CH-CH2-CH3butene-1

WITHH2 = CH-CH2-CH2-CH3 pentene-1

2. Physical properties

Ethylene (ethene) is a colorless gas with a very weak sweetish odor, slightly lighter than air, slightly soluble in water.

C2 - C4 (gases)

C5 - C17 (liquid)

C18 - (solid)

Alkenes are insoluble in water, soluble in organic solvents (gasoline, benzene, etc.)

Lighter than water

With an increase in Mr, the melting and boiling points increase

3. The simplest alkene is ethylene - C2H4

The structural and electronic formulas of ethylene are:

In the ethylene molecule, one s- and two p-orbitals of C ( sp 2-hybridization).

Thus, each C atom has three hybrid orbitals and one non-hybrid p-orbital. Two of the hybrid orbitals of the C atoms overlap and form between the C atoms

σ - bond. The other four hybrid orbitals of C atoms overlap in the same plane with four s-orbitals of H atoms and also form four σ-bonds. Two non-hybrid p-orbitals of C atoms mutually overlap in the plane, which is located perpendicular to the plane σ - bond, i.e. one P- connection.

By it's nature P- bond differs sharply from σ - bond; P- the bond is less strong due to the overlapping of electron clouds outside the plane of the molecule. Under the influence of reagents P- the connection is easily broken.

The ethylene molecule is symmetrical; the nuclei of all atoms are located in the same plane and the bond angles are close to 120 °; the distance between the centers of the C atoms is 0.134 nm.

If the atoms are connected by a double bond, then their rotation is impossible without the electron clouds P- the connection was not opened.

4. Isomerism of alkenes

Along with structural isomerism of the carbon skeleton for alkenes, firstly, other types of structural isomerism are characteristic - multiple bond position isomerism and interclass isomerism.

Secondly, in the series of alkenes, spatial isomerism associated with different positions of substituents relative to the double bond, around which intramolecular rotation is impossible.

Structural isomerism of alkenes

1. Isomerism of the carbon skeleton (starting from C4H8):

2. Isomerism of the position of the double bond (starting with C4H8):

3. Interclass isomerism with cycloalkanes, starting with C3H6:

Spatial isomerism of alkenes

Rotation of atoms around the double bond is impossible without breaking it. This is due to the structural features of the p-bond (the p-electron cloud is concentrated above and below the plane of the molecule). Due to the rigid attachment of atoms, rotational isomerism with respect to the double bond does not appear. But it becomes possible cis-trance-isomerism.

Alkenes having different substituents on each of the two carbon atoms on the double bond can exist as two spatial isomers differing in the arrangement of the substituents relative to the p-bond plane. So, in the butene-2 ​​molecule CH3-CH = CH-CH3 CH3 groups can be located either on one side of the double bond in cis-isomer, or by different sides v trance-isomer.

ATTENTION! cis-trans- Isomerism does not appear if at least one of the C atoms at the double bond has 2 identical substituents.

For instance,

butene-1 CH2 = CH-CH2-CH3 does not have cis- and trance-isomers, because The 1st C atom is bonded to two identical H atoms.

Isomers cis- and trance- differ not only in physical

,

but also by chemical properties, since the convergence or removal of parts of a molecule from each other in space promotes or prevents chemical interaction.

Sometimes cis-trans-isomerism is not exactly called geometric isomerism... The imprecision is that all spatial isomers differ in their geometry, and not only cis- and trance-.

5. Nomenclature

Alkenes simple structure often called, replacing the suffix -an in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.

According to the systematic nomenclature, the names of ethylene hydrocarbons are produced by replacing the -ane suffix in the corresponding alkanes with the -ene suffix (alkane - alkene, ethane - ethene, propane - propene, etc.). Main chain selection and naming order are the same as for alkanes. However, the chain must include a double bond. The chain numbering begins from the end to which this link is located closer. For instance:

Unsaturated (alkene) radicals are called trivial names or according to systematic nomenclature:

(H2C = CH—) vinyl or ethenyl

(H2C = CH — CH2) allyl

The unsaturated hydrocarbons are those containing multiple bonds between carbon atoms in the molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the ring (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (three or four atoms) are also unsaturated. The property of "unsaturation" is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.

Alkenes structure

Acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula C n H 2n.

Its second name is olefins- alkenes were obtained by analogy with fatty unsaturated acids(oleic, linoleic), the remains of which are part of liquid fats - oils (from the English oil - oil).

Carbon atoms, between which there is a double bond, are in the state sp 2 -hybridization... This means that one s and two p orbitals are involved in hybridization, while one p orbital remains unhybridized.

Overlapping of hybrid orbitals leads to the formation of a σ-bond, and due to unhybridized p-orbitals of neighboring carbon atoms, a second, π-bond is formed. Thus, the double bond consists of one σ- and one π-bond.

The hybrid orbitals of the atoms forming the double bond are in the same plane, and the orbitals that form the π-bond are located perpendicular to the plane of the molecule.

A double bond (0.132 nm) is shorter than a single bond, and its energy is greater, since it is more durable. Nevertheless, the presence of a mobile, easily polarizable π-bond leads to the fact that alkenes are chemically more active than alkanes and are capable of entering into addition reactions.

Homologous series of alkenes

The first three members of the homologous series of alkenes are gases, from C 5 H 10 to C 17 H 34 - liquids, from C 18 H 36 - solids... Liquid and solid alkenes are practically insoluble in water, but readily soluble in organic solvents.

In accordance with the IUPAC rules, the suffix -ene is used in the names of homologues of a number of alkenes. The position of the double bond is indicated by a number indicating the location of the bond. The number is put after the name of the main chain, separated by a hyphen. The numbering of atoms in an alkene molecule begins from the end to which the bond is closer, for example, an alkene corresponding to the formula CH 3 - CH 2 - CH = CH - CH 3 should be called pentene-2, since the bond begins at the second carbon atom, starting from the end chains.

Unbranched alkenes form a homologous series of ethene (ethylene): C 2 H 4 - ethene, C 3 H 6 - propene, C 4 H 8 - butene, C 5 H 10 - pentene, C 6 H 12 - hexene, etc.

Isomerism and nomenclature of alkenes

Alkenes, as well as alkanes, are characterized by structural isomerism... Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene with structural isomers is butene.

A special type of structural isomerism is the isomerism of the position of the double bond:

Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans-isomerism.

Cis isomers differ from trans isomers in the spatial arrangement of the fragments of the molecule (in in this case methyl groups) with respect to the plane of the π-bond, and hence the properties.

Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:

The nomenclature of alkenes developed by IUPAC is similar to the nomenclature of alkanes.

1. Main circuit selection... The formation of a hydrocarbon name begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.

2. Main chain atom numbering... The numbering of the atoms of the main chain begins from the end to which the double bond is closer. For example, the correct connection name is:

5-methylhexene-2, not 2-methylhexene-4, as one might expect.

If by the position of the double bond it is impossible to determine the beginning of the numbering of atoms in the chain, then it is determined by the position of the substituents in the same way as for saturated hydrocarbons.

3. Name formation... Alkenes are named in the same way as alkane names. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and the suffix -en, indicating that the compound belongs to the class of alkenes. For instance:

Physical properties of alkenes

The first three representatives of the homologous series of alkenes- gases; substances of composition C 5 H 10 - C 16 H 32 - liquids; higher alkenes are solids.

The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.

Chemical properties of alkenes

Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism electrophilic connection.

1. Alkenes hydrogenation... Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction takes place at atmospheric and elevated pressure and does not require a high temperature, since it is exothermic. When the temperature rises on the same catalysts, the reverse reaction can take place - dehydrogenation.

2. Halogenation(addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (CCl 4) leads to rapid discoloration of these solutions as a result of the addition of a halogen molecule to an alkene and the formation of dihaloalkanes:

3. Hydrohalogenation(addition of hydrogen halide).

This reaction obeys the Markovnikov rule:

When a hydrogen halide is attached to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom with more hydrogen atoms, and halogen - to a less hydrogenated one.

4. Hydration(water connection). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:

Note that the primary alcohol (with a hydroxyl group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.

This reaction also proceeds in accordance with the Markovnikov rule- the hydrogen cation is attached to the more hydrogenated carbon atom, and the hydroxy group to the less hydrogenated one.

5. Polymerization... A special case of addition is the reaction of polymerization of alkenes:

This addition reaction proceeds through a free radical mechanism.

Oxidation reactions.

1. Combustion... Like any organic compounds, alkenes burn in oxygen with the formation of CO 2 and H 2 O:

2. Oxidation in solutions... Unlike alkanes, alkenes are readily oxidized under the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:

The unsaturated hydrocarbons are those containing multiple bonds between carbon atoms in the molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes)... Cyclic hydrocarbons containing a double bond in the cycle ( cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (three or four atoms). The property of "unsaturation" is associated with the ability of these substances to enter into addition reactions, primarily of hydrogen, with the formation of saturated, or saturated hydrocarbons - alkanes.

Alkenes structure

Acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula СnН2n. Its second name is olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils.
Carbon atoms, between which there is a double bond, are in the sp 2 -hybridization state. This means that one s and two p orbitals are involved in hybridization, while one p orbital remains unhybridized. The overlapping of hybrid orbitals leads to the formation of a σ-bond, and due to unhybridized p-orbitals
adjacent carbon atoms, a second, π-bond is formed. Thus, a double bond consists of one σ- and one π - bond. The hybrid orbitals of the atoms forming the double bond are in the same plane, and the orbitals forming the π-bond are located perpendicular to the plane of the molecule. A double bond (0.132 them) is shorter than a single bond, and its energy is greater, since it is more durable. Nevertheless, the presence of a mobile, easily polarizable π-bond leads to the fact that alkenes are chemically more active than alkanes and are capable of entering into addition reactions.

Ethylene structure

Double bond formation in alkenes

Homologous series of ethene

Unbranched alkenes make up the homologous series of ethene ( ethylene): C 2 H 4 - ethene, C 3 H 6 - propene, C 4 H 8 - butene, C 5 H 10 - pentene, C 6 H 12 - hexene, C 7 H 14 - heptene, etc.

Alkenes isomerism

Alkenes are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene with structural isomers is butene:

A special type of structural isomerism is the isomerism of the position of the double bond:

Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:

Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis and transisomerism.

Cis isomers differ from trans isomers the spatial arrangement of the fragments of the molecule (in this case, the methyl groups) relative to the plane of the π-bond, and hence the properties.

Alkenes nomenclature

1. Selection of the main circuit. The formation of a hydrocarbon name begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.
2. Numbering of the atoms of the main chain. The numbering of the atoms of the main chain begins from the end to which the double bond is closer.
For example, the correct connection name is:

If by the position of the double bond it is impossible to determine the beginning of the numbering of atoms in the chain, then it is determined by the position of the substituents in the same way as for saturated hydrocarbons.

3. Formation of the name. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and the suffix -en, indicating that the compound belongs to the class of alkenes. For instance:

Physical properties of alkenes

The first three representatives of the homologous series of alkenes are gases; substances of composition С5Н10 - С16Н32 - liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.

Chemical properties of alkenes

Addition reactions... Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism electrophilic connection.
1. Hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction takes place at atmospheric and high blood pressure and does not require a high temperature, since it is exothermic. When the temperature rises on the same catalysts, the reverse reaction can take place - dehydrogenation.

2. Halogenation (addition of halogens)... The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (CC14) leads to rapid discoloration of these solutions as a result of the addition of a halogen molecule to an alkene and the formation of dihaloalkanes.
3. Hydrohalogenation (addition of hydrogen halide).

This reaction obeys
When a hydrogen halide is attached to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom with more hydrogen atoms, and halogen - to a less hydrogenated one.

4. Hydration (water addition). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene forms the basis of one of the industrial methods for producing ethyl alcohol.

Note that the primary alcohol (with a hydroxyl group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols.

This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is attached to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.
5. Polymerization. A special case of addition is the reaction of polymerization of alkenes:

This addition reaction proceeds through a free radical mechanism.
Oxidation reactions.
1. Combustion. Like any organic compounds, alkenes burn in oxygen with the formation of CO2 and H2O:

2. Oxidation in solutions. Unlike alkanes, alkenes are readily oxidized under the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:

Alkenic hydrocarbons (olefins) are one of the classes organic matter, which have their own. The types of isomerism of alkenes in representatives of this class are not repeated with the isomerism of other organic substances.

In contact with

Characteristic features of the class

Ethylene olefins are called one of the classes of unsaturated hydrocarbons containing one double bond.

By physical properties representatives of this category of unsaturated compounds are:

• gases
• liquids,
• solid compounds.

In the composition of the molecules, there is not only a "sigma" bond, but also a "pi" bond. The reason for this is the presence in structural formula hybridization " sp2», Which is characterized by the arrangement of the atoms of the compound in one plane.

In this case, an angle of at least one hundred and twenty degrees is formed between them. Non-hybridized orbitals " R»Is inherent to be located both above the molecular plane and below it.

This structural feature leads to the formation of additional bonds - "pi" or " π ».

The described bond is less strong in comparison with the "sigma" -links, since the overlapping sideways has a weak adhesion. The total distribution of the electron densities of the bonds formed is characterized by inhomogeneity. When rotating around the carbon-carbon bond, there is a violation of the overlap of "p" -orbitals. For each alkene (olefin), this pattern is a distinctive feature.

Almost all ethylene compounds have high boiling and melting points, which are not typical for all organic substances. Representatives of this class of unsaturated carbohydrates quickly dissolve in other organic solvents.

Attention! Acyclic unsaturated compounds ethylene hydrocarbons have the general formula - C n H 2n.

Homology

Proceeding from the fact that the general formula of alkenes is C n H 2n, a certain homology is inherent in them. The homologous series of alkenes begins with the first representative ethylene or ethene. This substance v normal conditions is a gas and contains two carbon atoms and four hydrogen atoms -C 2 H 4. Behind ethene, the homologous series of alkenes continues with propene and butene. Their formulas are as follows: "C 3 H 6" and "C 4 H 8". Under normal conditions, they are also gases that are heavier, which means that they must be collected with a test tube turned upside down.

General formula alkenes allows you to calculate the next representative of this class, which has at least five carbon atoms in the structural chain. It is pentene with the formula "C 5 H 10".

By physical characteristics the specified substance refers to liquids, as well as the twelve following compounds of the homologous line.

Among the alkenes with the indicated characteristics, there are also solids that begin with the formula C 18 H 36. Liquid and solid ethylene hydrocarbons do not dissolve in water, but when they get into organic solvents they react with them.

The described general formula for alkenes implies the replacement of the previously existing suffix "an" by "en". This is enshrined in the IUPAC rules. Whichever representative of this category of compounds we take, they all have the described suffix.

The name of ethylene compounds always contains a certain number, which indicates the location of the double bond in the formula. Examples of this are: "butene-1" or "pentene-2". Atomic numbering begins from the edge to which the double configuration is closer. This rule is "ironclad" in all cases.

Isomerism

Depending on the available type of hybridization of alkenes, some types of isomerism are inherent in them, each of which has its own characteristics and structure. Let's consider the main types of isomerism of alkenes.

Structural type

Structural isomerism is classified into isomers based on:

• carbon skeleton;
• the location of the double bond.

Structural isomers of the carbon skeleton arise in the case of the appearance of radicals (branches from the main chain).

Isomers of alkenes of the indicated isomerism will be:

CH 2 = CH — CH 2 — CH 3.

2-methylpropene-1:

CH 2 = C — CH 3

│

The presented compounds have the total number of carbon and hydrogen atoms (C 4 H 8), but different structure hydrocarbon skeleton. These are structural isomers, although their properties are not the same. Butene-1 (butylene) has a characteristic odor and narcotic properties that irritate Airways... 2-methylpropene-1 does not possess these features.

In this case, ethylene (C 2 H 4) has no isomers, since it consists of only two carbon atoms, where radicals cannot be substituted.

Advice! The radical is allowed to be placed near the middle and penultimate carbon atoms, but it is not allowed to place them near the extreme substituents. This rule works for all unsaturated hydrocarbons.

With respect to the location of the double bond, isomers are distinguished:

CH 2 = CH — CH 2 — CH 2 -CH 3.

CH 3 -CH = CH — CH 2 -CH 3.

The general formula of alkenes in the examples presented is:C 5 H 10,but the location of one double bond is different. The properties of the specified connections will vary. This is structural isomerism.

Isomerism

Spatial type

The spatial isomerism of alkenes is associated with the nature of the arrangement of hydrocarbon substituents.

Based on this, isomers are distinguished:

• "Cis";
• "Trance".

The general formula of alkenes allows the creation of "trans-isomers" and "cis-isomers" in the same compound. Take butylene (butene) for example. For it, you can create isomers of the spatial structure, in different ways relative to the double bond of the substituents. With examples of isomerism of alkenes it will look like this:

"Cis-isomer" "trans-isomer"

Butene-2 ​​Butene-2

It can be seen from this example that the "cis-isomers" have two identical radicals on one side of the double bond plane. For "trans-isomers" this rule does not work, since they have two dissimilar substituents relative to the "C = C" carbon chain. Taking into account this regularity, it is possible to construct "cis" and "trans" isomers for various acyclic ethylene hydrocarbons by ourselves.

The presented "cis-isomer" and "trans-isomer" for butene-2 ​​cannot be transformed into one another, since this requires rotation around the existing carbon double chain (C = C). To carry out this rotation, a certain amount of energy is needed to break the existing "p-bond".

Based on the foregoing, it can be concluded that the isomers "trans" and "cis" of the type are individual compounds with a certain set of chemical and physical properties.

Which alkene has no isomers. Ethylene has no spatial isomers due to the identical arrangement of hydrogen substituents relative to the double chain.

Interclass

Interclass isomerism in alkene hydrocarbons is widespread. The reason for this is the similarity of the general formula of representatives of this class with the formula of cycloparaffins (cycloalkanes). These categories of substances have the same number of carbon and hydrogen atoms, a multiple of the composition (C n H 2n).

Interclass isomers will look like this:

CH 2 = CH — CH 3.

Cyclopropane:

It turns out that the formulaC 3 H 6there are two compounds: propene-1 and cyclopropane. From structural structure it is seen different location carbon relative to each other. These compounds are also different in their properties. Propene-1 (propylene) is a gaseous compound with a low boiling point. Cyclopropane is characterized by a gaseous state with a pungent odor and a pungent taste. Chemical properties these substances also differ, but their composition is identical. Into organic given view isomers are called interclass.

Alkenes. Alkenes isomerism. Unified State Exam. Organic chemistry.

Alkenes: structure, nomenclature, isomerism

Conclusion

Alkenic isomerism is their important characteristic, due to which new compounds with other properties appear in nature, which are used in industry and everyday life.