The simplest alkyne is ethyne. Both the carbon atoms in ethyne are sp-hybridised. The sp-hybrid orbital of one carbon atom overlaps axially with the similar orbital of the other carbon atom to form C-C sigma bond. The remaining sp-hybrid orbital of each carbon atom overlaps axially with Is-orbital of each of the two hydrogen atoms to form two C-H sigma bonds. H-C-C bond angle is 180°. Each carbon atom also has two unhybridised p-orbitals which are perpendicular to each other and also to the inter-nuclear axis. These p-orbitals undergo sideways overlap to form two pi-bonds (Fig. 42.1).
Thus, a carbon-carbon triple bond consists of one sigma bond and two pi-bonds. The strength of C = C bond is 823 kJ mol-1 C = C bond is stronger than C = C bond (605 kJ mol-1) and C-C bond (347 kJ mol- 1). The bond length of C = C bond is 120 pm. The C = C bond is shorter than C = C bond (134 pm) and C-C bond (154 pm).
Because of sp-hybridisation of the two carbon atoms, ethyne molecule is linear.