The general formula of alkenes. Properties and characteristics of alkenes

The simplest organic compounds are the limiting and unsaturated hydrocarbons. They include substances of the class alkanes, alkynes, alkenes.

Their formulas include hydrogen and carbon atoms in a certain sequence and quantity. They are often found in nature.

Definition of alkenes

Another name is olefins or ethylene hydrocarbons. This is the name for this class of compounds in the 18th century when an oily liquid, ethylene chloride, was discovered.

Alkenes include substances consisting of hydrogen and carbon elements. They belong to acyclic hydrocarbons. In their molecule there is a single double (unsaturated) bond connecting two carbon atoms to each other.

Formulas of alkenes

Each class of compounds has its chemical designation. In them, the symbols of the elements of the periodic system indicate the composition and structure of the connection of each substance.

The general formula of alkenes is denoted as follows: C _n H _2n , where the number n is greater than or equal to 2. When it is deciphered, it is clear that there are two hydrogen atoms per carbon atom.

Molecular formulas of alkenes from the homologous series are represented by the following structures: C ₂ H ₄ , C ₃ H ₆ , C ₄ H ₈ , C ₅ H ₁₀ , C ₆ H ₁₂ , C ₇ H ₁₄ , C ₈ H ₁₆ , C ₉ H ₁₈ , C ₁₀ H ₂₀ . It can be seen that each successive hydrocarbon contains one more carbon and 2 more hydrogen.

There is a graphic designation of the location and order of the chemical compounds between atoms in the molecule, which shows the structural alkenes formula. With the help of valence dashes the connection of carbons with hydrogen is indicated.

The alkene structural formula can be depicted in expanded form, when all the chemical elements and bonds are displayed. With a more concise expression of olefins, the carbon-hydrogen bond is not shown using valence dashes.

The skeletal formula denotes the simplest structure. The broken line represents the basis of a molecule in which the carbon atoms are represented by its tips and ends, and the links indicate hydrogen.

How are the names of olefins formed

Proceeding from the systematic nomenclature, the alkene formulas and their names are made up of the structure of alkanes belonging to the ultimate hydrocarbons. To do this, in the name of the last suffix -an is replaced by -ylene or -ene. An example is the formation of butylene from butane, and pentene from pentane.

To indicate the position of the double bond with respect to the carbon atoms, indicate the Arabic figure at the end of the name.

The name of alkenes is based on the designation of a hydrocarbon with the longest chain, in which a double bond is present. The end of the chain numbering is usually chosen, which is closest to the unsaturated compound of carbon atoms.

If the structural formula of alkenes has branches, then indicate the names of the radicals and their number, and before them put the numbers corresponding to the place in the carbon chain. Then the name of the hydrocarbon follows. After the numbers are usually put a hyphen.

There are unsaturated radical branches. Their names can be trivial or formed according to the rules of the nomenclature systematic.

For example, HNS = CH- is called ethenyl or vinyl.

Isomers

Molecular formulas of alkenes can not indicate isomerism. However, for this class of substances, with the exception of the ethylene molecule, spatial modification is inherent.

The isomers of ethylene hydrocarbons can be along the carbon skeleton, the position of unsaturated bonds, interclass or spatial.

The general formula of alkenes determines the number of carbon and hydrogen atoms in the chain, but it does not show the presence and location of the double bond. An example is cyclopropane as the interclass isomer C ₃ H ₆ (propylene). Other kinds of isomerism are manifested in C ₄ H ₈ or butene.

Different position of unsaturated bond is observed in butene-1 or butene-2, in the first case, the double compound is near the first carbon atom, and in the second - in the middle of the chain. The isomerism of the carbon skeleton can be considered by the example of methyl propene (CH ₃ -C (CH ₃ ) = CH ₂ ) and isobutylene ((CH ₃ ) 2 C = CH ₂ ).

Spatial modification is inherent in butene-2 in trans and cis position. In the first case, the side radicals are located at the top and bottom of the main carbon chain with a double bond, in the second isomer the substituents are on the same side.

Characteristics of olefins

The general formula of alkenes determines the physical state of all representatives of a given class. Starting with ethylene and ending with butylene (from C ₂ to C ₄ ), the substances exist in a gaseous form. So the colorless ethene has a sweetish smell, a small solubility in water, a molecular mass less than that of air.

In the liquid form, hydrocarbons of the homologous gap from C ₅ to C _{17 are} represented. Starting with an alkene having 18 carbon atoms in the main chain, the physical state changes into a solid form.

All olefins are considered to be poor in aqueous media, but good in solvents of organic nature, such as benzene or gasoline. Their molecular mass is less than that of water. The increase in the carbon chain leads to an increase in the temperature characteristics during the melting and boiling of these compounds.

Properties of olefins

The structural formula of alkenes shows the presence in the skeleton of a double bond from the π- and σ- compounds of two carbon atoms. This structure of the molecule determines its chemical properties. The π-bond is considered not very strong, which makes it possible to destroy it with the formation of new two bonds-σ, which are obtained as a result of the addition of a pair of atoms. Unsaturated hydrocarbons are electron donors. They take part in electrophilic joining processes.

An important chemical property of all alkenes is the halogenation process, with the isolation of compounds similar to dihalogen derivatives. Halogen atoms can be attached via a double bond to carbon. An example is bromination of propylene with the formation of 1,2-dibromopropane:

H ₂ C = CH-CH ₃ + Br ₂ → BrCH ₂ -CHBr-CH ₃ .

This process of color neutralization in bromine water with alkenes is considered a qualitative proof of the presence of a double bond.

Important reactions include the hydrogenation of oligines with the addition of a hydrogen molecule under the action of catalytic metals such as platinum, palladium or nickel. As a result, hydrocarbons with a saturated bond are obtained. Formulas of alkanes, alkenes are given below in the hydrogenation reaction of butene:

CH ₃ -CH ₂ -CH═CH ₂ + H ₂ ^Ni → CH ₃ -CH ₂ -CH ₂ -CH ₃ .

The process of attaching a hydrogen halide molecule to olefins is called
Hydrohalogenation, passing according to the rule discovered by Markovnikov. An example is the hydrobromination of propylene to form 2-bromopropane. In it, hydrogen combines with a double bond with carbon, which is considered to be the most hydrogenated:

CH ₃ -CH = CH ₂ + HBr → CH ₃ -BrCH-CH ₃ .

The reaction of alkene addition of water under the action of acids is called hydration. As a result, a propanol-2 alcohol molecule is obtained:

CH ₃ -HC = CH ₂ + H ₂ O → CH ₃ -OHCH-CH ₃ .

When exposed to alkenes with sulfuric acid, the process of sulfonation takes place:

CH ₃ -HC = CH ₂ + HO-OSO-OH → CH ₃ -CH ₃ CH-O-SO ₂ -OH.

The reaction proceeds to form acid esters, for example, isopropylsulfuric acid.

Alkenes are susceptible to oxidation during their combustion with the action of oxygen with the formation of water and gas carbon dioxide:

2CH3-HC = CH2 + 9O2 → 6CO2 + 6H2O.

The interaction of olefinic compounds and diluted potassium permanganate in the form of a solution leads to the formation of glycols or diols of a diatomic structure. This reaction is also oxidative with the formation of ethylene glycol and the discoloration of the solution:

3H ₂ C = CH ₂ + 4H ₂ O + 2KMnO ₄ → 3OHCH-CHOH + 2MnO ₂ + 2KOH.

Alken molecules can be involved in the polymerization process with a free radical or cation-anion mechanism. In the first case, under the influence of peroxides, a polymer of the polyethylene type is obtained.

According to the second mechanism, cationic catalysts are acids, and anionic substances are organometallic substances with the isolation of a stereoselective polymer.

What are alkanes

They are also called paraffins or limiting acyclic hydrocarbons. They have a linear or branched structure, which contains only saturated simple bonds. All representatives of the homologous series of this class have the general formula C _n H _{2n + 2} .

They contain only carbon and hydrogen atoms. The general formula of alkenes is formed from the designation of the ultimate hydrocarbons.

Names of alkanes and their characteristics

The simplest representative of this class is methane. It is followed by substances like ethane, propane and butane. At the heart of their name is the root of the numerals in Greek, to which the suffix -an is added. The names of alkanes are listed in the IUPAC nomenclature.

The general formula of alkenes, alkynes, alkanes includes only two types of atoms. These include elements of carbon and hydrogen. The number of carbon atoms in all three classes is the same, the difference is observed only in the amount of hydrogen that can split off or join. Saturated hydrocarbons give unsaturated compounds. Paraffins in a molecule contain 2 more hydrogen atoms than olefins, which is confirmed by the general formula of alkanes, alkenes. Alkenes structure is considered unsaturated due to the presence of a double bond.

If we correlate the number of hydrogen and carbon atoms in alkanes, then the value will be the maximum in comparison with other classes of hydrocarbons.

Starting with methane and ending with butane (from C ₁ to C ₄ ), the substances exist in a gaseous form.

Hydrocarbons of the homologous gap from C ₅ to C _{16 are} represented in the liquid form. Starting with an alkane having 17 carbon atoms in the main chain, the physical state changes into a solid form.

They are characterized by isomerism along the carbon skeleton and optical modifications of the molecule.

In paraffins, carbon valences are assumed to be fully occupied by neighboring carbon or hydrogen to form a σ-type bond. From a chemical point of view, this is due to their weak properties, which is why alkanes are called saturated or saturated hydrocarbons, devoid of affinity.

They enter substitution reactions associated with radical halogenation, sulfochlorination or nitration of the molecule.

Paraffins undergo oxidation, combustion or decomposition at high temperatures. Under the action of reaction accelerators, hydrogen atoms are split off or alkanes are dehydrogenated.

What are alkynes

They are also called acetylene hydrocarbons, in which a triple bond is present in the carbon chain. The structure of alkynes is described by the general formula C _n H _{2 n-2} . It shows that unlike alkanes, acetylene hydrocarbons lack four hydrogen atoms. They are replaced by a triple bond formed by two π-connections.

This structure determines the chemical properties of this class. The structural formula of alkenes and alkynes clearly shows the unsaturation of their molecules, as well as the presence of a double (H ₂ C꞊CH ₂ ) and triple (HC≡CH) bonds.

Name of alkynes and their characteristics

The simplest representative is acetylene or HC≡CH. It is also called ethyne. It comes from the name of a saturated hydrocarbon, in which the suffix -an is removed and -in is added. In the names of long alkynes, the figure indicates the location of the triple bond.

Knowing the structure of hydrocarbons saturated and unsaturated, it is possible to determine under which letter the general formula of alkynes is designated: a) CnH2n; C) CnH2n + 2; C) CnH2n-2; D) CnH2n-6. The correct answer is the third option.

Starting with acetylene and ending with butane (from C ₂ to C ₄ ), the substances are gaseous in nature.

In the liquid form there are hydrocarbons of the homologous gap from C ₅ to C ₁₇ . Starting with an alkyne having 18 carbon atoms in the main chain, the physical state changes into a solid form.

They are characterized by isomerism along the carbon skeleton, by the position of the triple bond, and also by interclass modifications of the molecule.

According to their chemical characteristics, acetylene hydrocarbons are similar to alkenes.

If the alkyne has a terminal triple bond, then they act as an acid to form salts of alkynides, for example NaC≡CNa. The presence of two π-bonds makes the acetylidine molecule of sodium a strong nucleophile entering the substitution reaction.

Acetylene is subjected to chlorination in the presence of copper chloride to obtain dichloroacetylene, condensation under the action of haloalkynes with the isolation of diacetylene molecules.

Alkines participate in electrophilic addition reactions, the principle of which is the basis of halogenation, hydrohalogenation, hydrotreatment and carbonylation. However, such processes are weaker than in alkenes with a double bond.

For acetylenic hydrocarbons, reactions of addition by a nucleophilic type of an alcohol molecule, a primary amine or hydrogen sulphide are possible.