Structure of Ethyne:
sp Hybridization in Ethyne ( Acetylene):
The hybridization in which one s-orbital and one p-orbital are mixed to form two identical hybrid orbitals is called sp hybridization. The two sp hybrid orbitals degenerate (have equal energy) and they are at an angle of 180o. The remaining two un-hybridized p-orbitals are perpendicular to each other as well as to the sp hybridized orbitals. Each sp hybrid orbital has 50% s-character and 50% p-character.
Acetylene or Ethyne (C2H2):
In ethyne, the triply bonded carbon atoms are sp hybridized. The electronic configuration of carbon is 1s2, 2s2, 2p2. Its valence shell has 2 electrons in the s-orbital and one electron in each of two p-orbitals while the third p-orbital is lying vacant.
By absorption of energy one electron of 2s promotes to vacant p-orbital and the atom becomes in an excited state, which is not a stable state. Thus to gain a state of stability mixing of atomic orbitals take place and energy is lowered. One s-orbital and one p-orbital are mixed together and give two sp hybridized orbitals. These two sp hybrid orbitals are identical in shape and energy and each has a single electron in it. Also, two un-hybridized p-orbitals each have single electrons in it. So carbon is tetravalent and not divalent.
The half-filled s-orbitals of one hydrogen atom overlap with the half-filled sp hybrid orbital of one carbon while half-filled s-orbitals of another hydrogen atom overlap with the half-filled sp hybrid orbital of the second carbon forming a total of two C-H sigma bonds, one with each carbon. As overlapping has taken place on the bond axis so two C-H sigma bonds are produced. The two C-H sigma bonds are formed due to sp-s overlapping.
The remaining one sp hybrid orbital of each carbon overlaps among them on the bond axis thus forming a C-C sigma bond. This bond is formed due to sp-sp overlapping.
The two unhybridized p-orbitals (one of each carbon) are parallel to each other so they overlap side wisely (i.e. laterally) and form a pi-bond. Similarly, the other two un-hybridized p-orbitals (one of each carbon) are also parallel to each other so they also overlap side wisely (i.e. laterally) and form another pi-bond. The two pi-bonds are formed due to p-p overlapping. Pi-bonds are weak bonds and are easy to break hence ethyne is reactive.
The bond angle between C≡C and C-H bond is 180o. The C-H bond length is 1.08×10 –10m or 1.08 Ao. While C≡C bond length is 1.20×10 –10m or 1.20 Ao.