In addition to forming single and double bonds by sharing two and four elec- trons, respectively, carbon also can form a triple bond by sharing six electrons. To account for the triple bond in a molecule such as acetylene, H O C q C O H, we need a third kind of hybrid orbital, an sp hybrid. Imagine that, instead of combining with two or three p orbitals, a carbon 2s orbital hybridizes with only a single p orbital. Two sp hybrid orbitals result, and two p orbitals remain unchanged. The two sp orbitals are oriented 180° apart on the x-axis, while the remaining two p orbitals are perpendicular on the y-axis and the z-axis.
When two sp carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp–sp s bond. At the same time, the pz orbitals from each carbon form a pz–pz p bond by sideways overlap, and the py orbitals overlap similarly to form a py–py p bond. The net effect is the sharing of six electrons and formation of a carbon–carbon triple bond. The two remain- ing sp hybrid orbitals each form a s bond with hydrogen to complete the acety- lene molecule.
As suggested by sp hybridization, acetylene is a linear molecule with H]C]C bond angles of 180°. The C]H bonds have a length of 106 pm and a strength of 558 kJ/mol (133 kcal/mol). The C-C bond length in acetylene is 120 pm, and its strength is about 965 kJ/mol (231 kcal/mol), making it the shortest and strongest of any carbon–carbon bond.
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