The alkynes are the third homologous series of organic compounds of hydrogen and carbon, where there is at least one triple-bond between the atoms in the molecules.

The alkenes are said to be unsaturated because of the existence of a multiple bond in the molecule. The general structure of the alkene series of hydrocarbons is CnH2n-2. The first member of the ethene series is ethyne (previously called acetylene). The names of all alkynes end in "-yne". Rules for the systematic naming of alkynes are similar to those for alkenes. In the case of higher members of the alkene series, the triple bond may be between the terminal carbon atoms of the chain, or may be between internal carbon atoms in the chain.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The bond formed between the hydrogen atom and the unsaturated carbon atom, and first bond between the unsaturated carbon atoms in the ethynes are s bonds (sigma bonds) and these bonds are formed by the end-on overlap of sp hybrid orbitals of the carbon atoms and the bonds are arranged as far apart in space as possible (i.e. at 180 degree) to form a linear molecule. The second and third bonds that makes up the triple bond of the unsaturated carbon atoms in alkenes are p-bonds (pi-bonds), formed by the side-on overlap of the two p-orbitals on each of the carbon atoms. The p-bonds (pi-bonds) are much more reactive than the s bonds (sigma bonds), and react readily in addition reactions.

Acetylene is a linear molecule, all four atoms lying along a straight line. This linear structure can only be explained by the existence of sp hybridisation of the orbitals of the carbon atoms of ethyne.

The carbon-carbon triple bond is thus made up of one strong bond and two weaker (bonds; it has a total strength 123 kcal. It is stronger than the carbon-carbon double bond of ethylene 100 kcal or the single carbon-carbon bond of ethane 83 kcal, and therefore is shorter than either.

The C-C distance is 1.2 A, as compared with 1.34 A in ethylene and 1.54 A in ethane and is a more electronegative grouping than that formed by carbon atoms joined by either a double or a single bond.

The hydrogen attached to the carbon-carbon triple bond in ethyne or in any alkyne where the carbon-carbon triple bond is situated at the end of a carbon chain is able to separate from the rest of the molecule as a hydrogen ion; the electronegative carbon is able to retain both electrons from the broken covalent bond.

A significant result of this bonding is that ethyne can unite with metals and so be distinguished from alkenes by chemical means.

The linear structure does not permit geometric isomerism of ethyne.

 

 

 

 

 

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