Alkynes

Alkynes (also known as acetylenes) are unsaturated acyclic hydrocarbons that contain in their molecule a triple bond between two carbon atoms. They correspond to the molecular formula CnH(2n-2). Although the name acetylenes can be used for the class, it is also the name of the simplest alkyne, ethyne. Similar to other hydrocarbons, alkynes are hydrophobic.

Nomenclature

Replace the suffix "-an" from alkanes with the same number of carbon atoms with the suffix "-ine".

CH≡CH ethyne (acetylene)
CH≡C-CH3 proline

For alkynes with more than three carbon atoms, the position of the triple bond is indicated by the carbon atom that has the lowest number in the main chain numbering for that bond.

CH≡C-CH2-CH3 1-butyn
CH≡C-CH2-CH2-CH3 1-pentyne

Structure

The two carbon atoms between which there is a triple bond are Sp hybridized, the other atoms in the chain are Sp3 hybridized. The nonpolar triple covalent bond-C≡C- consists of one σ bond and two π bonds. Presenting two weak π bonds that break relatively easily, acetylene and in general alkynes are reactive substances. Although the two π bonds block the free rotation of the two carbon atoms, alkynes do not show geometric isomerism because the two carbon atoms involved in the triple bond have only one substituent.

Method of obtaining

Obtaining from methane

Electric arc pyrolysis is achieved by passing methane through an electric arc between two metal electrodes connected to a direct current source.
it is carried out at a temperature of 1500 °C 2CH4→CH≡CH+3H2 CH4→C+2H2
The burning of a part of the methane is carried out in furnaces where a part of the methane burns, thus giving the necessary heat to the other part of the methane to pass into acetylene. 2CH4→CH≡CH+3H2 2CH4+O2→2CO+4H2

Dehydrohalogenation of geminal or vicinal dihalogenated derivatives

it is carried out by heating with strong bases KOH in alcoholic solution R-CHCl-CH2Cl→R-C≡CH+2HCL R-CCl2-CH3→R-C≡CH+2HCl

Chemical properties

Alkynes have a more unsaturated character than alkenes. Their characteristic reactions are polymerization and addition reactions due to the two weak π bonds, which break relatively easily.

Oxidation reaction

Mild oxidation in neutral or slightly basic medium R-C≡CH+[O](KMnO4/HOH)→R-CO-COOH

Addition reaction (catalytic hydrogenation)

Addition of hydrogen
in the presence of Pd poisoned with Pb salt: R-C≡CH+H2→R-CH=CH2
V-in the presence of Ni: R-C≡CH+2H2→R-CH2-CH3
Addition of halogens
the addition of Br - is a reaction identifying unsaturation by decolorizing bromine water R-C≡CH+Br2→Br-CH=CH-Br+Br2→R-CBr2-CHBr2
the addition of Cl
-in gas phase, the reaction is violent with explosion and results in coal and HCl CH≡CH+Cl2→2C+2HCl
with inert solvents (methyl tetrachloride CCl4) CH≡CH+Cl2→CHCl=CHCl+Cl2→CHCl2-CHCl2