bond in ethane is not completely free. The energy difference between the two extreme forms is of the order of . kJ mol – , which is very small. Even at ordinary temperatures, the ethane molecule gains thermal or kinetic energy sufficient enough to overcome this energy barrier of .
kJ mol – through intermolecular collisions. Thus, it can be said that rotation about carbon-carbon single bond in ethane is almost free for all practical purposes. It has not been possible to separate and isolate different conformational isomers of ethane. .
Alkenes Alkenes are unsaturated hydrocarbons containing at least one double bond. What should be the general formula of alkenes? If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes. Hence, general formula for alkenes is C n H 2n .
Alkenes are also known as olefins (oil forming) since the first member, ethylene or ethene (C H ) was found to form an oily liquid on reaction with chlorine. . . Structure of Double Bond Carbon-carbon double bond in alkenes consists of one strong sigma ( σ ) bond (bond enthalpy about kJ mol – ) due to head-on overlapping of sp hybridised orbitals and one weak pi ( π ) bond (bond enthalpy about kJ mol – ) obtained by lateral or sideways overlapping of the two p orbitals of the two carbon atoms.
The double bond is shorter in bond length ( pm) than the C–C single bond ( pm). You have already read that the pi ( π ) bond is a weaker bond due to poor sideways overlapping between the two p 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