pp. 3 & 4
To summarize sp2 bonding, 1) An atom that is sp2-hybridized consists of 3-sigma bonds and 1-pi bond. The sigma and pi-bonds are two separate classes of bonding, and each will have different properties that we will be discussing. 2) Bonds containing s-character are lower in energy than bonds constructed of only p-orbitals. The more s-character in a bond, the closer the electrons are to the nucleus and the stronger, and more electronegative those bonds will be. The following movie summarizes sp2-hybridization and pi-bond formation for your viewing enjoyment:
(For a larger version of the movie, please, click here.)
Structure of Alkenes:
Now that we have studied the formation of alkene bonds, let us focus a bit on the structure of these bonds and how they differ from saturated bonds.
1. Bond Length: Carbon will almost always from 4 bonds but it may form those bonds to a differing number of atoms. In alkene systems there are 4 total bonds and 3 total groups attached to the carbon. As discussed above, 3 of the bonds are sigma (sp2) and one bond is a pi-bond. Since one group is attached by both an sp2-bond and a pi-bond, we call alkenes double bonds or olefins. Having 2-bonds to 1-atom results in a very shortened distance between the nuclei when compared to sp3-sp3 bonding discussed in chapter 1. As shown in the structure below, a double bond is only 1.34 angstroms in length, whereas an sp3-sp3 sigma bond is 1.54 angstroms in length.
2. Rotation: Not only are double bonds shorter, but they also cannot rotate in the same fashion as sigma bonds. Sigma bonds were able to rotate freely (conformers) without breaking any bonds. The ease of rotation was due to the fact that the electron density resided in orbitals located on a line in-between the two nuclei. Rotation does not alter the electronic configuration, and costs very little energy. Note that pi-bonds are formed from side-to-side overlap, which differs from sigma bonds. If a pi-bond is rotated, the p-orbitals no longer overlap, hence there is not a bond. For a double bond to rotate, the pi-bond must be broken, forming a diradical intermediate, which will be considerably higher in energy (Nature is lazy!). Rotation of a double bond requires approximately 64 kcal/mol (ethene) which is considerably higher than the 2.9 kcal/mole energy required to rotate around the sigma bond in ethane. Energy transformations that require more than 10 kcal/mole of energy will, generally, not occur at room temperature. Rotation of pi-bonds, costing 64 kcal/mol, do not occur spontaneously at room temperature, and isomers of these molecules may be isolated and studied.
(For a larger version of this movie, please click here.)
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