Alkynes

General characteristics

Structure

Alkynes have sp hybridization because carbon atoms bound by a triple bond share three electrons with a neighboring atom. One of the sp orbitals overlaps with the orbitals of the hydrogen atom, the other with the neighboring sp orbital carbon, forming one sigma bond and two perpendicular pi bonds.

Physical properties

Due to the low polarity, alkenes are insoluble in water, but highly soluble in organic solvents such as ether, benzene and carbon tetrachloride. Otherwise, the physical properties are similar to the physical properties of alkanes and alkenes: the boiling point corresponds to the boiling point of alkanes and alkenes with an equal amount of carbon. The density of alkynes is slightly higher than the density of the corresponding alkanes, but less than the density of water.

Reactions

Hydrogenation

The addition of hydrogen gas occurs with Pt, Pd or Ni catalysts. As a result of the reaction , alkanes. To obtain alkenes, a Lindlar catalyst is used. The Lindlar catalyst is palladium deposited on calcium carbonate and treated with lead or sulfur salt. Only cis-alkenes (Z) are formed with the Lindlar catalyst. Another method is sodium powder in liquid ammonia, as a result, trans alkenes are formed.

Hydrogenation into alkanes

In the presence of a Pt, Pd or Ni catalyst:
CH₃-C≡C-CH₃ (2-butin) + H₂ → CH₃-CH₂-CH₂-CH₃ (bhutan)

Hydrogenation into alkenes

In the presence of a Lindlar catalyst:
CH₃-C≡C-CH₃ (2-butin) + H₂ → CH₃-CH=CH-CH₃ (cis-2-butene)

Hydrogenation in ammonia with sodium

In the presence of liquid ammonia and sodium:
CH₃-C≡C-CH₃ → CH₃-C^•=C^••Na-CH₃ → CH₃-C^•=CH-CH₃ → CH₃-C^••Na=CH-CH₃ → CH₃-CH=CH-CH₃ (trans-2-butene)

Halogenation

The alkyne halogenation reaction occurs in two stages: first, a dihalogenalkene is formed, which, in turn, reacting with an excess of halogens forms tetrahalogenalkane.

CH≡CH + Br₂ (acetylene) → CHBr=CHBr + Br₂ (dibromoethene) → CHBr₂-CHBr₂ (tetrabromoethane)

Connection of hydrogen halides

Halohydrogens easily attach to the triple bond, forming halidesls. Hydrogen halides are mixed with acetic acid, or directly, in a gaseous state, mixed. For consideration the reaction mechanism, it is necessary to know the Markovnikov rule.

The reaction mechanism of addition of hydrogen halides is the same as that of alkenes: a homolytic bond break occurs in the hydrogen halide molecule, a proton and a halogen anion are formed. The proton attaches to the alkyne 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, so it easily binds to halogen, energy activation is low, so this stage does not slow down the reaction.

Polymerization

Polymerization of alkynes occurs in the presence of a catalyst, which can be an aqueous solution of CuCl₂ and NH₄Cl. As a result of the reaction, vinyl acetylene is formed, which has a high reactivity. When acetylene is passed over activated carbon at a temperature of 600 ° C, benzene is formed.

Getting

Alkynes are very rare in nature, but have been found in the atmosphere of Uranus, Jupiter and Saturn. In industry several methods are used to obtain, for example, calcination in furnaces of a mixture of calcium oxide with coke at a temperature of 1800-2000 ° C, followed by water treatment. Another method is methane pyrolysis. In the laboratory, as a rule, it is obtained by hydrolysis of calcium carbide.

The use of alkynes

Due to the low prevalence of alkynes on earth, only acetylene, from which synthesize:

• solvents tetrachloroethane and trichloroethylene
• chloroprene, butadiene and synthetic rubbers
• polyacrylamide, which is used for drinking water purification
• urethane polymers (based on tetrahydrofuran)
• PVC plastic
• PVA glue
• acetaldehyde, from which acetic acid, butadiene and other substances are synthesized

Nomenclature