Alkines: production, application, properties

Today, alkynes are of great importance in various fields of human activity. But a century ago, the preparation of most organic compounds began precisely with acetylene. This lasted until oil became the main source of raw materials for chemical synthesis.

All kinds of plastics, rubbers, synthetic fibers are obtained from this class of compounds in the modern world. Acetic acid is produced in large volumes from acetylene. Autogenous welding is an important stage in mechanical engineering, the construction of buildings and structures, laying communications. The well-known PVA glue is obtained from acetylene with an intermediate stage of vinyl acetate formation. It is also the starting point in the synthesis of ethanol, used as a solvent and for the perfume industry.

Alkines are hydrocarbons whose molecules contain a triple carbon-carbon bond. Their general chemical formula is C _n H _2n-2 . The simplest alkine, in accordance with the rules of systematic nomenclature, is called ethine, but its more common name is acetylene.

The nature of the bond and physical properties

Acetylene has a linear structure, and all bonds in it are much shorter than in ethylene. This is explained by the fact that sp hybrid orbitals are used to form the σ bond. A triple bond is formed from one σ-bond and two π-bonds. The space between carbon atoms has a high electron density, which pulls together their nuclei with a positive charge and increases the energy of the cleavage of the triple bond.

N ― S≡S ― N

In the homologous series of acetylene, the first two substances are gases, the next compounds containing from 4 to 16 carbon atoms are liquids, and then there are alkynes in the solid state of aggregation. As the molecular weight increases, the melting and boiling points of acetylene hydrocarbons increase.

Getting alkynes from carbide

This method is often used in industry. Acetylene is formed by mixing calcium carbide and water:

CaC ₂ + 2H ₂ 0 → ΗC≡CΗ + Ca (OΗ) ₂

In this case, the release of bubbles of the resulting gas is observed. During the reaction, you can smell a specific odor, but it is not related to acetylene. The reason for it is impurities Ca ₃ P ₂ and CaS in carbide. Acetylene is also produced by a similar reaction from barium and strontium carbides (SrC ₂ , BaC ₂ ). And from magnesium carbide you can get propylene:

MgC ₂ + 4 ₂ → ₃ ―≡ + 2Mg () ₂

Acetylene synthesis

These methods are not suitable for other alkynes. Obtaining acetylene from simple substances is possible at temperatures above 3000 ° C by the reaction:

2 + ₂ → ≡

In fact, the reaction is carried out in an electric arc between carbon electrodes in a hydrogen atmosphere.

However, this method has only scientific value. In industry, acetylene is often obtained by pyrolysis of methane or ethane:

2CH ₄ → ≡ + 3 ₂

Η ₃ ―Η ₃ → Η≡Η + 2 ₂

Pyrolysis is usually carried out at very high temperatures. So, methane is heated to 1500 ° C. The specificity of this method for the production of alkyne is the need for rapid cooling of the reaction products. This is due to the fact that at such temperatures, acetylene itself can decompose into hydrogen and carbon.

The production of alkynes by dehydrohalogenation

As a rule, a cleavage reaction of two HBr or HCl molecules from dihaloalkanes is carried out. A prerequisite is the bonding of the halogen with either adjacent carbon atoms, or with the same. If you do not reflect the intermediate products, the reaction will take the form:

Η ₃ ―CHBr ― Η ₂ Br → Η ₃ ―≡Η + 2HBr

Η ₃ ―Η ₂ ―CBr ₂ ―Η ₃ → Η ₃ ―≡ ― ₃ + 2

In this way, it is possible to obtain alkynes from alkenes, but first they are halogenated:

Η ₃ ―Η ₂ ―Η = Η ₂ + Br ₂ → Η ₃ ―Η ₂ ―CHBr ― Η ₂ Br → Η ₃ ―Η ₂ ―≡Η + 2HBr

Chain extension

This method can simultaneously demonstrate the preparation and use of alkynes, since the starting material and product of this reaction are acetylene homologs. It is carried out according to the scheme:

R ― ≡ ― Η → R ― ≡ ― Μ + R'― → R ― ≡ ― R '+ Μ

An intermediate stage is the synthesis of salts of alkynes - metal acetylenides. To obtain sodium acetylenide, ethine must be acted upon with sodium metal or its amide:

≡ + NaNH ₂ → = ― Na + NH ₃

In order for alkyne to form, the resulting salt must react with haloalkane:

≡ ― Na + Br ― Η ₂ ―Η ₃ → Η ₃ ―≡ ― Η ₂ ―Η ₃ + NaBr

≡ ― Na + Cl ― Η ₃ → Η ₃ ―≡ ― Η ₃ + NaCl

The methods for producing alkynes are not limited to this list, however, it is the above reactions that have the greatest production and theoretical significance.

Electrophilic addition reactions

The chemical properties of acetylene hydrocarbons are explained by the presence of the π-electron density of the triple bond, which is exposed to electrophilic particles. Due to the fact that the ≡ bond is very short, it is more difficult for these particles to interact with alkynes than in similar alkenes reactions. This also explains the lower joining speed.

Halogenation. The addition of halogens occurs in two stages. At the first stage, a dihalogen substituted alkene is formed, and then a tetrahalogen substituted alkane. So, with bromination of acetylene, 1,1,2,2-tetrabromoethane is obtained:

Η≡Η + Br ₂ → CHBr = CHBr

CHBr = CHBr + Br ₂ → CHBr _{2 ―} CHBr ₂

Hydrohalogenation The course of these reactions obeys the Markovnikov rule. Most often, the final reaction product has two halogen atoms connected to the same carbon:

Η ₃ ―≡Η + HBr → Η ₃ ―CBr = Η ₂

Η ₃ ―CBr = Η ₂ + HBr → Η ₃ ―CBr ₂ ―Η ₃

The same applies to alkenes with a non-terminal triple bond:

Η ₃ ―Η ₂ ―≡ ― Η ₃ + HBr → Η ₃ ―Η ₂ ―CBr = Η ― Η ₃

Η ₃ ―Η ₂ ―CBr = Η ― Η ₃ + HBr → Η ₃ ―Η ₂ ―CBr ₂ ―Η ₂ ―Η ₃

In fact, in the reactions of such alkynes, the production of pure substances is not always possible, since in parallel there is a reaction in which the halogen is attached to another carbon atom with a triple bond:

Η ₃ ―Η ₂ ―≡ ― Η ₃ + HBr → ₃ ―Η ₂ ―Η ₂ ―CBr ₂ ―Η ₃

In this example, a mixture of 2.2-dibromopentane and 3,3-dibromopentane is obtained.

Hydration. This is a very important chemical property of alkynes. And obtaining in its course various carbonyl compounds is of great importance in the chemical industry. The reaction is named after its discoverer, the Russian chemist M. G. Kucherov. Water addition is possible in the presence of H2SO4 and HgSO4.

Acetic aldehyde is obtained from acetylene:

Η≡Η + Η ₂ → Η ₃ ―Η

Acetylene homologs participate in the reaction with the formation of ketones, since the addition of water is subject to the Markovnikov rule:

Η ₃ ―≡Η + Η ₂ → Η ₃ ― ― Η ₃

Acidic properties of alkynes

Acetylene hydrocarbons with a triple bond at the end of the chain are capable of cleaving a proton under the influence of strong oxidizing agents, for example, alkalis. The preparation of sodium salts of alkynes has already been discussed above.

Silver and copper acetylenides are widely used to isolate alkynes from mixtures with other hydrocarbons. The basis of this process is their ability to precipitate during the passage of alkyne through an ammonia solution of silver oxide or copper chloride:

CH≡CH + 2Ag (NH ₃ ) ₂ OH → Ag ― ≡ ― Ag + NH ₃ + 2 ₂

R ― ≡ + Cu (NH ₃ ) ₂ → R ― ≡ ― Cu + 2NH ₃ + ₂

The oxidation and reduction reaction. Combustion

Alkines are easily oxidized by potassium permanganate solution, and its discoloration occurs. Along with the destruction of the triple bond is the formation of carboxylic acids:

R ― C≡C ― R '→ R ― COOH + R'― COOH

Alkynes are reduced by sequential addition of two hydrogen molecules in the presence of platinum, palladium or nickel:

Η ₃ ―≡Η + Η ₂ → Η ₃ ―Η = Η ₂

Η ₃ ―Η ― Η ₂ + Η ₂ → Η ₃ ―Η ₂ ―Η ₃

The use of acetylene is also associated with its ability to release a huge amount of heat during combustion:

2 ₂ Η ₂ + 5 ₂ → 4 ₂ + 2Η ₂ + 1309.6 kJ / mol

The resulting temperature is enough for the melting of metals, which is used in acetylene welding and metal cutting.

Polymerization

Equally important is the ability of acetylene to form di-, tri- and polymers under special conditions. So, in an aqueous solution of copper and ammonium chlorides a dimer is formed - vinylacetylene:

Η≡Η + Η≡Η → Η ₂ = Η ― ≡Η

Which, in turn, entering into the hydrochlorination reaction, forms chloroprene - a raw material for artificial rubber.

At a temperature of 600 ° C over activated carbon, acetylene is trimerized with the formation of an equally valuable compound - benzene:

3 _{2 2} → _{6 6}

According to recent results, the use of alkynes has slightly decreased due to their replacement with oil products, but in many industries they also continue to occupy leading positions. Thus, acetylene and other alkynes, the properties, application and preparation of which are discussed in detail above, will be an important link not only in scientific research, but also in the life of ordinary people.