Carbon-carbon double bond in alkenes consists of one strong sigma (σ) bond (bond enthalpy 397 kJ mol–1) due to head-on overlapping of sp 2 hybridised orbitals and one weak pi (π) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms. The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm).
You have already read that the pi (π) bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals.
Thus, the presence of the pi (π) bond makes alkenes behave as sources of loosely held mobile electrons. Therefore, alkenes are easily attacked by reagents or compounds which are in search of electrons. Such reagents are called electrophilic reagents.
The presence of weaker π-bond makes alkenes unstable molecules in comparison to alkanes and thus, alkenes can be changed into single bond compounds by combining with the electrophilic reagents.
Strength of the double bond (bond enthalpy, 681 kJ mol–1) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, 348 kJ mol–1). Orbital diagrams of ethene molecule are shown in Figure.