In the hybrid orbital picture of acetylene, both carbons are sp-hybridized. The 2 p y and 2 p z orbitals remain unhybridized, and are oriented perpendicularly along the y and z axes, respectively. The C-C sigma bond, then, is formed by the overlap of one sp orbital from each of the carbons, while the two C-H sigma bonds are formed by the overlap of the second sp orbital on each carbon with a 1 s orbital on a hydrogen. Each carbon atom still has two half-filled 2 p y and 2 p z orbitals, which are perpendicular both to each other and to the line formed by the sigma bonds.
These two perpendicular pairs of p orbitals form two pi bonds between the carbons, resulting in a triple bond overall one sigma bond plus two pi bonds. The carbon-carbon bond in ethane structure A below results from the overlap of two sp 3 orbitals. In alkene B, however, the carbon-carbon single bond is the result of overlap between an sp 2 orbital and an sp 3 orbital, while in alkyne C the carbon-carbon single bond is the result of overlap between an sp orbital and an sp 3 orbital.
These are all single bonds, but the bond in molecule C is shorter and stronger than the one in B, which is in turn shorter and stronger than the one in A. New York: Van Nostrand Reinhold, Marc Loudon, Organic Chemistry , 4th ed. New York: Oxford University Press, , p. Englewood Cliffs: Prentice Hall, , p. Sharp, The Penguin Dictionary of Chemistry , 2nd ed. London: Penguin Books, Wade, Jr. Martha Windholz ed.
Navigation Bar. Molecule Gallery. Download 3D. Increasing the length of the carbon chain by one brings us to 1-propyne also known as methylacetylene ; all three carbon atoms lie in a straight line.
An alkyne which contains four carbon atoms in the chain has two structural isomers: 1-butyne and 2-butyne. Bonding in acetylene Finally, the hybrid orbital concept applies well to triple-bonded groups, such as alkynes and nitriles. A B C In propene B , however, the carbon-carbon single bond is the result of overlap between an sp 2 orbital and an sp 3 orbital, while in propyne C the carbon-carbon single bond is the result of overlap between an sp orbital and an sp 3 orbital.
Hybridization Summary A single bond is a sigma bond. A double bond is made up of a sigma bond and a pi bond. A triple bond is made up of a sigma bond and two pi bonds. Sigma bonds are made by the overlap of two hybrid orbitals or the overlap of a hybrid orbital and a s orbital from hydrogen.
Pi bonds are made by the overlap of two unhybridized p orbitals. Lone pair electrons are usually contained in hybrid orbitals. Exercises 1 For the molecule acetonitrile: a How many sigma and pi bonds does it have? Solutions 1 a 5 sigma and 2 pi b An sp 3 hybrid orbital from carbon and an a s orbital from hydrogen.
Questions Q1. Solutions S1.
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