1.4 Multiple bonds
1.4 Multiple bonds
Carbon atom is not always connected to four different atoms. Carbon atoms involved in double or triple bonds combine three or two another atoms, respectively. First of all, we will consider carbon atoms that form a double bond. A carbon atom with the electron configuration 2s12px12p y12pz1 can form three hybridized orbitals of equal energy from three atomic orbitals, 2s and two of 2p orbitals. One of the 2p orbital (e.g. 2pz orbital) is remained unaffected. This hybridized orbital is called sp2hybridized orbital from its constitution. All of the sp2-hybridized orbitals are on the same plane including the carbon atom, and the remaining 2p orbital is perpendicular to the plane. Fig 1.4 (b) shows the atomic orbital of sp2 hybridized carbon atom.
Skeleton of ethylene is formed when sp2 hybridized orbitals of each of two carbon atoms overlap to form a s bond. Overlap can take place when two p orbitals are placed parallel and the result is the formation of a p bond. Therefore, a double bond is made up from a s bond and a p bond. Ethylene C 2H4 is completed when hydrogen atoms attached to each of the residual sp2 hybridized orbitals.
Figure 1.4 Hybridized atomic orbitals of carbon (p orbitals are omitted).
If carbon atom forms triple bond, it can be connected to only two other atoms. In this case, the carbon in 2s12p x12py12p z1 electron configuration forms two equivalent sp hybridized orbitals (Fig 1.4(c)) from a 2s and one of 2p orbitals, and two 2p orbitals are left over. The sp hybridized orbitals are in the opposite direction. In other words, the angle between each of two sp hybridized orbitals is 180°. In acetylene C2H2, a s bond is formed by the overlap of sp hybridized orbitals from each sp hybridized carbon, and two p bonds are also formed by the overlap of two pairs of p orbitals, and hydrogen atoms are bonded to the remaining two sp hybridized orbitals.
Figure 1.5 The structure of basic organic compounds.