Alkenes

General characteristics

Alkenes are a class of organic compounds having a double bond between carbon atoms, structural formula - C_nH_2n. The double bond in olefin molecules is one sigma- and one pi-bond. Thus, if we imagine two carbon atoms and place them on a plane, σ-the bond will be located on the plane, and π-the bond will be located above and below the plane (if you have a bad idea of what we are talking about, contact to the section chemical bonds).

Hybridization

In alkenes, there is sp²-hybridization, for which the angle H-C-H is 120 degrees, and the bond length is C=C is equal to 0.134 nm.

Structure

From the presence of a π-connection, it follows, and is confirmed experimentally, that:

• By its structure, the double bond in alkene molecules is more susceptible to external influences than the usual σ-bond
• The double bond makes it impossible to rotate around the σ-bond, which implies the presence of isomers, these isomers they are called cis- and trans-
• -the bond is less strong than the sigma-bond, since the electrons are farther from the centers of the atoms

Physical properties

The physical properties of alkenes are similar to the physical properties of alkanes. Alkenes having up to five carbon atoms, they are in a gaseous state under normal conditions. Molecules containing from six to 16 carbon atoms are in the liquid state and from 17 carbon atoms, alkenes are in the solid state under normal conditions.

The boiling point of alkenes increases on average by 30 degrees for each CH₂-group, as with alkanes, branches reduce the boiling point of the substance.

The presence of a π-bond makes olefins slightly soluble in water, which causes their small polarity. Alkenes - non-polar substances and dissolve in non-polar solvents and weakly polar solvents.

The density of alkenes is higher than that of alkanes, but lower than that of water

Isomerism

• Carbon skeleton isomerism: 1-butene and 2-methylpropene
• Isomerism of the double bond position: 1-butene and 2-butene
• Interclass isomerism: 1-butene and cyclobutane

Reactions

Characteristic reactions of alkenes are addition reactions, π-the bond breaks and the electrons formed willingly accept the new element. The presence of a π-connection means more energy, therefore, as a rule, the addition reactions are exothermic in nature, i.e. they proceed with the release of heat.

Attachment reactions

Connection of hydrogen halides

Hydrogen halides easily attach to the double bond of alkenes, forming halidesls. Halogenated hydrocarbons mixed with acetic acid, or directly, in a gaseous state, mixed with an alkene. For consideration the reaction mechanism, it is necessary to know the Markovnikov rule.

Markovnikov's rule

When ethylene homologues interact with acids, hydrogen attaches to a more hydrogenated carbon atom.
An exception to the rule, hydroboration of alkynes, will be discussed in the article about alkynes.

The reaction mechanism of addition of hydrogen halides to alkenes is as follows: a homolytic bond break occurs in the hydrogen halide molecule, a proton and a halogen anion are formed. The proton attaches to the alkene forming carbcation, such a reaction is endothermic and has a high level of activation energy, so the reaction it happens slowly. The formed carbcation is very reactive, therefore it easily binds to the halogen, energy activation is low, so this stage does not slow down the reaction.

Halogenation

At room temperature, alkenes react with chlorine and bromine in the presence of carbon tetrachloride. Reaction mechanism The addition of halogens looks like this: electrons with π-bonds act on the halogen molecule X₂. As the halogen approaches the olefin, the electrons in the halogen molecule shift to a more distant atom, thus the halogen molecule is polarized, the nearest atom has a positive charge, the more distant one has a negative charge. A heterolytic bond break occurs in the halogen molecule, a cation and anion are formed. The halogen cation attaches to two atoms carbon by means of an electron pair π-bond and a free electron pair of a cation. The remaining halogen anion it acts on one of the carbon atoms in the haloalken molecule, breaking the C-C-X cycle and forming a dihaloalken.

Alkene addition reactions have two main applications, the first is quantitative analysis, determination of the amount of double bonds by the number of absorbed molecules X₂. The second is in industry. Plastic production is based on on vinyl chloride. Trichloroethylene and tetrachloroethylene are excellent solvents of acetylene fats and rubbers.

Hydrogenation

The addition of hydrogen gas to the alkene occurs with Pt, Pd or Ni catalysts. As a result of the reaction alkanes are formed. The main application of the reaction of catalytic addition of hydrogen is, firstly, quantitative analysis. By the residue of H₂ molecules, it is possible to determine the number of double bonds in a substance. Secondly, vegetable fats and fish fats are unsaturated carbons and such hydrogenation leads to an increase in the melting point, converting into solid fats. Margarine production is based on this process.

Hydration

When alkenes are mixed with sulfuric acid, alkyl hydrosulfates are formed. When alkyl hydrosulfates are diluted with water and concomitant heating, alcohol is formed. An example of a reaction is the mixing of ethene (ethylene) with sulfuric acid, subsequent mixing with water and heating, the result is ethanol.

Oxidation

Alkenes are easily oxidized by various substances, such as, for example, KMnO₄, O₃, OsO₄, etc. There are two types of alkene oxidation: breaking π-bonds without breaking σ-bonds and breaking σ- and π-bonds. Oxidation without breaking the sigma bond is called soft oxidation, with breaking the sigma bond - hard oxidation.

Oxidation of ethene without breaking the sigma bond forms epoxides (epoxides are cyclic C-C-O compounds) or diatomic alcohols. Oxidation with the breaking of the sigma bond forms acetones, aldehydes and carboxylic acids.

Oxidation with potassium permanganate

The oxidation reactions of alkenes under the influence of potassium permanganate are called were discovered by Egor Wagner and bears his name. In the Wagner reaction, oxidation occurs in an organic solvent (acetone or ethanol) at a temperature of 0-10 °C, in a weak solution of potassium permanganate. As a result of the reaction, diatomic alcohols are formed and potassium permanganate is discolored.

Polymerization

Most simple alkenes can undergo self-coupling reactions, thus forming large molecules from structural units. Such large molecules are called polymers, the reaction that makes it possible to obtain a polymer is called polymerization. Simple structural units forming polymers are called monomers. The polymer is indicated by the conclusion of a repeating group in parentheses indicating the index "n", which means a large number of repetitions, for example: "-(CH₂-CH₂)_n-" - polyethylene. Polymerization processes are the basis of plastic production and fibers.

Radical polymerization

Radical polymerization is initiated using a catalyst - oxygen or peroxide. The reaction consists of three stages:

Initiation
ROOR → 2RO^•
CH₂=CH-C₆H₅ → RO-CH₂C^•H-C₆H₅

Chain Growth
RO-CH₂C^•H-C₆H₅ + CH₂=CH-C₆H₅ → RO-CH₂-CH(C₆H₅)-CH₂-C^•-C₅H₆

Chain breakage by recombination
CH₂-C^•H-C₆H₅ + CH₂-C^•H-C₆H₅ → CH₂-CH-C₆H₅-CH₂-CH-C₆H₅
Chain breakage by disproportionation
CH₂-C^•H-C₆H₅ + CH₂-C^•H-C₆H₅ → CH=CH-C₆H₅ + CH₂-CH₂-C₆H₅

Ionic polymerization

Another way of polymerization of alkenes is ionic polymerization. The reaction proceeds with the formation of intermediate products - carbations and carbanions. The formation of the first carbation, as a rule, is carried out with the help of Lewis acid, carbanion formation occurs, respectively, by reaction with the Lewis base.

A + CH₂=CH-X → A-CH₂-C⁺H-X → ... → A-CH₂-CHX-CH₂-CHX-CH₂C⁺HX ...

B + CH₂=CH-X → B-CH₂-C^-H-X → ... → B-CH₂-CHX-CH₂-CHX-CH₂C^-HX ...

Common polymers

The most common polymers are:

Nomenclature

The name of alkenes, similar to alkanes, consists of the first part - a prefix denoting the number of atoms carbon in the main chain, and the suffix -en. An alkene is a double-bonded compound, so alkene molecules start with two carbon atoms. The first in the list is ethene, et- - two carbon atoms, -en - the presence of a double bond.

Ethene: CH₂—CH₂

If there are more than three carbon atoms in the molecule, then it is necessary to specify the position of the double bond, for example, butene there can be two types:

CH₂=CH—CH₂—CH₃
CH₃—CH=CH—CH₃

To indicate the position of the double bond, it is necessary to add a digit, for the example above it is there will be 1-butene and 2-butene, respectively (the names 1-butene and 2-butene also apply, but they are not systematic).

The presence of a double bond entails isomerism, when molecules can be located on different sides of the double bond, for example:

_Cl/=_\Cl and _Cl/=^/Cl

This isomerism is called cis- (Z-zusammen, from German together) and trans- (E-entgegen, from German opposite), in the first case cis-1,2-dichloroethene (or (Z)-1,2-dichloroethene), in the second - trans-1,2-dichloroethene (or (E)-1,2-dichloroethene).