The characteristic feature of alkenes is the presence of a carbon-carbon double bond. In order to understand the structure of alkenes let us study the structure of etbene CH2 = CH2, the simplest member of this family. Each of the two carbon atoms of ethene is SP2 hybridised and forms three sigma bonds, two with two hydrogens and the third with one another. They are all in the same plane. The pi bond between the two carbon atoms is perpendicular to this plane, as shown in Fig. 41.1, and is present both above and below the plane.
C-C bond length in ethene is 134 pm which is much shorter than the C-C bond length in ethane (154 pm). C-H bond length in ethene is 108 pro.
Thus a carbon-carbon double bond comprises of a sigma bond and a pi bond. A pi bond is not as strong as a sigma bond. For example, the bond energies of a carbon-carbon single bond and a carbon-carbon double bond are respectively 347 and 605 kJ/mole. However, the pi bond is sufficiently strong to prevent rotation of the two carbon atoms with respect to one another. This restricted rotation about pi bond gives rise to geometric isomerism in alkenes.