What alkenes react with table. General formula and types of isomerism of alkenes

Alkenes are unsaturated aliphatic hydrocarbons with one or more carbon-carbon double bonds. A double bond transforms two carbon atoms into a planar structure with bond angles between adjacent bonds of 120°C:

The homologous series of alkenes has a general formula; its first two members are ethene (ethylene) and propene (propylene):

Members of a number of alkenes with four or more carbon atoms exhibit bond position isomerism. For example, an alkene with the formula has three isomers, two of which are bond position isomers:

Note that the alkene chain is numbered from the end closest to the double bond. The position of the double bond is indicated by the lower of the two numbers, which correspond to the two carbon atoms connected by the double bond. The third isomer has a branched structure:

The number of isomers of any alkene increases with the number of carbon atoms. For example, hexene has three bond position isomers:

The diene is buta-1,3-diene, or simply butadiene:

Compounds containing three double bonds are called trienes. Compounds with multiple double bonds are collectively called polyenes.

Physical properties

Alkenes have slightly lower melting and boiling points than their corresponding alkanes. For example, pentane has a boiling point. Ethylene, propene and three isomers of butene are gaseous at room temperature and normal pressure. Alkenes with the number of carbon atoms from 5 to 15 are in a liquid state under normal conditions. Their volatility, like that of alkanes, increases in the presence of branching in the carbon chain. Alkenes with more than 15 carbon atoms are solids under normal conditions.

Obtained in laboratory conditions

The two main methods for producing alkenes in the laboratory are the dehydration of alcohols and the dehydrohalogenation of haloalkanes. For example, ethylene can be obtained by dehydration of ethanol under the action of an excess of concentrated sulfuric acid at a temperature of 170 ° C (see section 19.2):

Ethylene can also be produced from ethanol by passing ethanol vapor over the surface of heated alumina. For this purpose, you can use the installation schematically shown in Fig. 18.3.

The second common method for the preparation of alkenes is based on the dehydrohalogenation of halogenated alkanes under basic catalysis conditions

The mechanism of this type of elimination reaction is described in Section. 17.3.

Alkene reactions

Alkenes are much more reactive than alkanes. This is due to the ability of the -electrons of the double bond to attract electrophiles (see Section 17.3). Therefore, the characteristic reactions of alkenes are mainly electrophilic addition reactions at the double bond:

Many of these reactions have ionic mechanisms (see Section 17.3).

Hydrogenation

If any alkene, for example ethylene, is mixed with hydrogen and passed this mixture over the surface of a platinum catalyst at room temperature or a nickel catalyst at a temperature of about 150 ° C, then addition will occur

hydrogen at the double bond of the alkene. This produces the corresponding alkane:

This type of reaction is an example of heterogeneous catalysis. Its mechanism is described in Section. 9.2 and is shown schematically in Fig. 9.20.

Addition of halogens

Chlorine or bromine easily adds to the double bond of the alkene; this reaction occurs in non-polar solvents, such as tetrachloromethane or hexane. The reaction proceeds by an ionic mechanism, which involves the formation of a carbocation. The double bond polarizes the halogen molecule, turning it into a dipole:

Therefore, a solution of bromine in hexane or tetrachloromethane becomes colorless when shaken with an alkene. The same thing happens if you shake an alkene with bromine water. Bromine water is a solution of bromine in water. This solution contains hypobromous acid. A hypobromous acid molecule attaches to the double bond of the alkene, resulting in the formation of a bromo-substituted alcohol. For example

Addition of hydrogen halides

The mechanism of this type of reaction is described in Section. 18.3. As an example, consider the addition of hydrogen chloride to propene:

Note that the product of this reaction is 2-chloropropane, not 1-chloropropane:

In such addition reactions, the most electronegative atom or the most electronegative group always adds to the carbon atom bonded to

the smallest number of hydrogen atoms. This pattern is called Markovnikov's rule.

The preferential attachment of an electronegative atom or group to the carbon atom associated with the smallest number of hydrogen atoms is due to an increase in the stability of the carbocation as the number of alkyl substituents on the carbon atom increases. This increase in stability is in turn explained by the inductive effect that occurs in alkyl groups, since they are electron donors:

In the presence of any organic peroxide, propene reacts with hydrogen bromide, i.e., not according to Markovnikov’s rule. Such a product is called anti-Markovnikov. It is formed as a result of a reaction occurring by a radical rather than an ionic mechanism.

Hydration

Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogen sulfates. For example

This reaction is an addition because it involves the addition of an acid at a double bond. It is the reverse reaction to the dehydration of ethanol to form ethylene. The mechanism of this reaction is similar to the mechanism of addition of hydrogen halides at the double bond. It involves the formation of a carbocation intermediate. If the product of this reaction is diluted with water and heated gently, it hydrolyzes to form ethanol:

The reaction of addition of sulfuric acid to alkenes obeys Markovnikov’s rule:

Reaction with acidified solution of potassium permanganate

The violet color of an acidified solution of potassium permanganate disappears if this solution is shaken in a mixture with any alkene. Hydroxylation of the alkene occurs (the introduction of a hydroxy group formed as a result of oxidation), which as a result is converted into a diol. For example, when an excess amount of ethylene is shaken with an acidified solution, ethane-1,2-diol (ethylene glycol) is formed.

If an alkene is shaken with an excess amount of -ion solution, oxidative cleavage of the alkene occurs, leading to the formation of aldehydes and ketones:

The aldehydes formed in this case undergo further oxidation to form carboxylic acids.

Hydroxylation of alkenes to form diols can also be carried out using an alkaline solution of potassium permanganate.

Reaction with perbenzoic acid

Alkenes react with peroxyacids (peracids), such as perbenzoic acid, to form cyclic ethers (epoxy compounds). For example

When epoxyethane is gently heated with a dilute solution of an acid, ethane-1,2-diol is formed:

Reactions with oxygen

Like all other hydrocarbons, alkenes burn and, with plenty of air, form carbon dioxide and water:

With limited air access, combustion of alkenes leads to the formation of carbon monoxide and water:

Because alkenes have a higher relative carbon content than the corresponding alkanes, they burn with a smokier flame. This is due to the formation of carbon particles:

If you mix any alkene with oxygen and pass this mixture over the surface of a silver catalyst, epoxyethane is formed at a temperature of about 200 ° C:

Ozonolysis

When ozone gas is passed through a solution of an alkene in trichloromethane or tetrachloromethane at temperatures below 20 °C, the ozonide of the corresponding alkene (oxirane) is formed.

Ozonides are unstable compounds and can be explosive. They undergo hydrolysis to form aldehydes or ketones. For example

In this case, part of the methanal (formaldehyde) reacts with hydrogen peroxide, forming methane (formic) acid:

Polymerization

The simplest alkenes can polymerize to form high molecular weight compounds that have the same empirical formula as the parent alkene:

This reaction occurs at high pressure, a temperature of 120°C and in the presence of oxygen, which acts as a catalyst. However, ethylene polymerization can be carried out at lower pressure if a Ziegler catalyst is used. One of the most common Ziegler catalysts is a mixture of triethylaluminum and titanium tetrachloride.

The polymerization of alkenes is discussed in more detail in Section. 18.3.

General formula of alkenes: CnH2n(n 2)

The first representatives of the homologous series of alkenes:

The formulas of alkenes can be compiled from the corresponding formulas of alkanes (saturated hydrocarbons). The names of alkenes are formed by replacing the suffix -ane of the corresponding alkane with -ene or –ylene: butane - butylene, pentane - pentene, etc. The number of the carbon atom with a double bond is indicated by an Arabic numeral after the name.

The carbon atoms involved in the formation of the double bond are in a state of sp-hybridization. Three -bonds formed by hybrid orbitals and are located in the same plane at an angle of 120° to each other. An additional -bond is formed by lateral overlap of non-hybrid p-orbitals:

The length of the C=C double bond (0.133 nm) is shorter than the length of the single bond (0.154 nm). The energy of a double bond is less than twice the energy of a single bond because the energy of the -bond is less than the energy of the -bond.

Alkene isomers

All alkenes except ethylene have isomers. Alkenes are characterized by isomerism of the carbon skeleton, isomerism of the position of the double bond, interclass and spatial isomerism.

The interclass isomer of propene (C 3 H 6) is cyclopropane. Starting with butene (C 4 H 8), isomerism appears by the position of the double bond (butene-1 and butene-2), isomerism of the carbon skeleton (methylpropene or isobutylene), as well as spatial isomerism (cis-butene-2 ​​and trans-butene-2 ). In cis isomers, the substituents are located on one side, and in trans isomers, they are located on opposite sides of the double bond.

The chemical properties and chemical activity of alkenes are determined by the presence of a double bond in their molecules. The most common reactions for alkenes are electrophilic addition: hydrohalogenation, hydration, halogenation, hydrogenation, polymerization.

Qualitative reaction to a double bond – discoloration of bromine water:

Examples of solving problems on the topic “formula of alkenes”

EXAMPLE 1

EXAMPLE 2

Alkenes are characterized primarily by reactions accession through a double bond. Basically, these reactions proceed by an ionic mechanism. The pi bond is broken and two new sigma bonds are formed. Let me remind you that substitution reactions were typical for alkanes and they followed a radical mechanism. Hydrogen molecules can attach to alkenes; these reactions are called hydrogenation, water molecules, hydration, halogens halogenation, hydrogen halides hydrohalogenation. But first things first.

Double bond addition reactions

So, first chemical property ability to add hydrogen halides, hydrohalogenation.

Propene and other alkenes react with hydrogen halides according to Markovnikov's rule.

A hydrogen atom attaches to the most hydrogenated, or more correctly hydrogenated, carbon atom.

Second number on our list of properties would be hydration, the addition of water.

The reaction takes place when heated in the presence of an acid, usually sulfuric or phosphoric. The addition of water also occurs according to Markovnikov’s rule, that is, primary alcohol can only be obtained by hydration of ethylene, the remaining unbranched alkenes give secondary alcohols.

There are exceptions to Markovnikov's rule for both hydrohalogenation and hydration. Firstly, contrary to this rule, the addition occurs in the presence of peroxides.

Secondly, for derivatives of alkenes in which electron-withdrawing groups are present. For example, for 3,3,3-trifluoropropene-1.

Fluorine atoms, due to their high electronegativity, attract electron density to themselves along a chain of sigma bonds. This phenomenon is called a negative inductive effect.

Because of this, the mobile pi electrons of the double bond are displaced and the outermost carbon atom ends up with a partial positive charge, which is usually designated as delta plus. It is to this that the negatively charged bromine ion will go, and the hydrogen cation will attach to the least hydrogenated carbon atom.

In addition to the trifluoromethyl group, for example, the trichloromethyl group, nitro group, carboxyl group and some others have a negative inductive effect.

This second case of violation of the Markovnikov rule in the Unified State Exam is very rare, but it is still advisable to keep it in mind if you plan to pass the exam with the maximum score.

Third chemical property attachment of halogen molecules.

This primarily concerns bromine, since this reaction is qualitative for a multiple bond. When, for example, ethylene is passed through bromine water, that is, a solution of bromine in water that is brown in color, it becomes discolored. If you pass a mixture of gases, for example, ethane and ethene, through bromine water, you can get pure ethane without ethene impurities, since it will remain in the reaction flask in the form of dibromoethane, which is a liquid.

Of particular note is the reaction of alkenes in the gas phase with strong heating, for example, with chlorine.

Under such conditions, it is not an addition reaction that occurs, but a substitution reaction. Moreover, exclusively at the alpha carbon atom, that is, the atom adjacent to the double bond. In this case, 3-chloropropene-1 is obtained. These reactions are infrequent in the exam, so most students do not remember them and, as a rule, make mistakes.

Fourth number is the hydrogenation reaction, and with it the dehydrogenation reaction. That is, the addition or removal of hydrogen.

Hydrogenation occurs at a not very high temperature on a nickel catalyst. At higher temperatures, dehydrogenation is possible to produce alkynes.

Fifth A property of alkenes is the ability to polymerize, when hundreds and thousands of alkene molecules form very long and strong chains due to the breaking of the pi bond and the formation of sigma bonds with each other.

In this case, the result was polyethylene. Please note that the resulting molecule contains no multiple bonds. Such substances are called polymers, the original molecules are called monomers, the repeating fragment is the elementary unit of the polymer, and the number n is the degree of polymerization.

Reactions to produce other important polymeric materials, such as polypropylene, are also possible.

Another important polymer is polyvinyl chloride.

The starting material for the production of this polymer is chloroethene, whose common name is vinyl chloride. Because this unsaturated substituent is called vinyl. The frequently encountered abbreviation on plastic products, PVC, stands for polyvinyl chloride.

We discussed five properties that represented double bond addition reactions. Now let's look at the reactions oxidation.

Alkene oxidation reactions

Sixth chemical property in our general list is mild oxidation or Wagner reaction. It occurs when an alkene is exposed to an aqueous solution of potassium permanganate in the cold, which is why the temperature of zero degrees is often indicated in exam tasks.

The result is a dihydric alcohol. In this case, ethylene glycol, and in general such alcohols are collectively called glycols. During the reaction, the purple-pink permanganate solution becomes discolored, so this reaction is also qualitative for a double bond. Manganese in a neutral environment is reduced from oxidation state +7 to oxidation state +4. Let's look at a few more examples. THE EQUATION

Here we get propanediol-1,2. However, cyclic alkenes will react in the same way. THE EQUATION

Another option is when the double bond is located, for example, in the side chain of aromatic hydrocarbons. The Wagner reaction involving styrene, its other name is vinylbenzene, is regularly encountered in exam assignments.

I hope that I have provided enough examples for you to understand that the mild oxidation of a double bond always follows a fairly simple rule: the pi bond is broken and a hydroxy group is added to each carbon atom.

Now, regarding hard oxidation. It will be ours seventh property. This oxidation occurs when an alkene reacts with an acidic solution of potassium permanganate when heated.

The destruction of the molecule occurs, that is, its destruction at the double bond. In the case of butene-2, two molecules of acetic acid were obtained. In general, the position of the multiple bond in the carbon chain can be judged from the oxidation products.

The oxidation of butene-1 produces a molecule of propionic (propanoic) acid and carbon dioxide.

In the case of ethylene, you get two molecules of carbon dioxide. In all cases, in an acidic environment, manganese is reduced from oxidation state +7 to +2.

And finally eighth property complete oxidation or combustion.

Alkenes burn, like other hydrocarbons, to carbon dioxide and water. Let us write the equation for the combustion of alkenes in general form.

There will be as many carbon dioxide molecules as there are carbon atoms in the alkene molecule, since the CO 2 molecule contains one carbon atom. That is, n CO 2 molecules. There will be two times fewer water molecules than hydrogen atoms, that is, 2n/2, which means just n.

There are the same number of oxygen atoms on the left and right. On the right there are 2n of carbon dioxide plus n of water, for a total of 3n. On the left there are the same number of oxygen atoms, which means there are two times fewer molecules, because the molecule contains two atoms. That is, 3n/2 oxygen molecules. You can write 1.5n.

We have reviewed eight chemical properties of 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 the 18th 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. Homologous series of alkenes

Homologs:

WITHH2 = CH2 ethene

WITHH2 = CH- CH3 propene

WITHH2=CH-CH2-CH3butene-1

WITHH2=CH-CH2-CH2-CH3 penten-1

2. Physical properties

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

C2 - C4 (gases)

C5 - C17 (liquids)

C18 - (solid)

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

Lighter than water

With increasing 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 undergoes hybridization s- and two p-orbitals of C atoms ( sp 2-hybridization).

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

σ - bond. The remaining four hybrid orbitals of the 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 a plane that is located perpendicular to the σ-bond plane, i.e. one is formed P- connection.

By it's nature P- connection is sharply different from σ - connection; P- the bond is less strong due to the overlap 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 bond angles are close to 120°; the distance between the centers of C atoms is 0.134 nm.

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

4. Isomerism of alkenes

Along with structural isomerism of the carbon skeleton Alkenes are characterized, firstly, by other types of structural isomerism - multiple bond position isomerism And interclass isomerism.

Secondly, in the series of alkenes there is 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 from C4H8):

3. Interclass isomerism with cycloalkanes, starting with C3H6:

Spatial isomerism of alkenes

Rotation of atoms around a 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 fixation of the atoms, rotational isomerism with respect to the double bond does not appear. But it becomes possible cis-trance-isomerism.

Alkenes, which have different substituents on each of the two carbon atoms at the double bond, can exist in the form of two spatial isomers, differing in the location of the substituents relative to the plane of the p-bond. 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 on opposite sides in 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 example,

butene-1 CH2=CH-CH2-CH3 doesn't have cis- And trance-isomers, because The 1st C atom is bonded to two identical H atoms.

Isomers cis- And trance- differ not only physically

,

but also chemical properties, because bringing parts of a molecule closer or further away from each other in space promotes or hinders chemical interaction.

Sometimes cis-trans-isomerism is not quite accurately called geometric isomerism. The inaccuracy is that All spatial isomers differ in their geometry, and not only cis- And trance-.

5. Nomenclature

Alkenes of simple structure are often named by replacing the suffix -ane in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.

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

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

(H2C=CH—) vinyl or ethenyl

(H2C=CH—CH2) allyl

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 the 18th century, is a liquid, oily substance.) - aliphatic unsaturated hydrocarbons, in the molecules of which there is one double bond between the carbon atoms.

Alkenes contain fewer hydrogen atoms in their molecule than their corresponding alkanes (with the same number of carbon atoms), therefore such hydrocarbons are called unlimited or unsaturated.

Alkenes form a homologous series with the general formula CnH2n

1. Homologous series of alkenes

Homologs:

WITHH 2 = CH 2 ethene

WITHH 2 = CH- CH 3 propene

WITHH 2 =CH-CH 2 -CH 3butene-1

WITHH 2 =CH-CH 2 -CH 2 -CH 3 penten-1

2. Physical properties

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

C 2 – C 4 (gases)

C 5 – C 17 (liquids)

C 18 – (solid)

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

Lighter than water

With increasing 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 undergoes hybridization s- and two p-orbitals of C atoms ( sp 2 -hybridization).

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

σ - bond. The remaining four hybrid orbitals of the 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 a plane that is located perpendicular to the σ-bond plane, i.e. one is formed P- connection.

By it's nature P- connection is sharply different from σ - connection; P- the bond is less strong due to the overlap 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 bond angles are close to 120°; the distance between the centers of C atoms is 0.134 nm.

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

4. Isomerism of alkenes

Along with structural isomerism of the carbon skeleton Alkenes are characterized, firstly, by other types of structural isomerism - multiple bond position isomerism And interclass isomerism.

Secondly, in the series of alkenes there is 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 C 4 H 8):

2. Isomerism of the position of the double bond (starting from C 4 H 8):

3. Interclass isomerism with cycloalkanes, starting with C 3 H 6:

Spatial isomerism of alkenes

Rotation of atoms around a 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 fixation of the atoms, rotational isomerism with respect to the double bond does not appear. But it becomes possible cis-trance-isomerism.

Alkenes, which have different substituents on each of the two carbon atoms at the double bond, can exist in the form of two spatial isomers, differing in the location of the substituents relative to the plane of the p-bond. So, in the butene-2 ​​molecule CH 3 –CH=CH–CH 3 CH 3 groups can be located either on one side of the double bond in cis-isomer, or on opposite sides in 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 example,

butene-1 CH 2 = CH – CH 2 – CH 3 doesn't have cis- And trance-isomers, because The 1st C atom is bonded to two identical H atoms.

Isomers cis- And trance- differ not only physically

but also chemical properties, because bringing parts of a molecule closer or further away from each other in space promotes or hinders chemical interaction.

Sometimes cis-trans-isomerism is not quite accurately called geometric isomerism. The inaccuracy is that All spatial isomers differ in their geometry, and not only cis- And trance-.

5. Nomenclature

Alkenes of simple structure are often named by replacing the suffix -ane in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.

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

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

(H 2 C=CH-)vinyl or ethenyl

(H 2 C=CH-CH 2) allyl