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306 CHEMISTRY Relative stability of conformations: As the first member, ethylene or ethene (C H2) 4 mentioned earlier, in staggered form of ethane, was found to form an oily liquid on reaction the electron clouds of carbon-hydrogen bonds with chlorine. are as far apart as possible. Thus, there are 9.3.1 Structure of Double Bond minimum repulsive forces, minimum energy and maximum stability of the molecule. On the Carbon-carbon double bond in alkenes other hand, when the staggered form changes consists of one strong sigma ( ) bond (bond enthalpy about 397 kJ mol ) due to head-on into the eclipsed form, the electron clouds of theoverlapping of sp hybridised orbitals and carbon – h drogen bonds come closer to each other resulting in increase in electron cloud one weak pi–(π) bond (bond enthalpy about repulsions. To check the increased repulsive 284 kJ mol ) obtained by lateral or sideways overlapping of the two 2p orbitals of the two forces, molecule will have to possess more carbon atoms. The double bond is shorter in energy and thus has lesser stability. As already mentioned, the repulsive interaction between bond length ( m than the C–C single the electron clouds, which affects stability of bond (154 pm). You have already read that the pi (π) bond is a weaker bond due to poor a conformation, is called torsional strain. sideways overlapping between the two 2p Magnitude of torsional strain depends upon the angle of rotation about C–C bond. his orbitals. Thus, the presence of the pi (π) bond angle is also calleddihedral angle ortorsional makes alkenes behave as sources of loosely held mobile electrons. Therefore, alkenes are angle. Of all the conformations of ethane, the easily attacked by reagents or compounds staggered form has the least torsional strain and the eclipsed form, the maximum torsional which are in search of electrons. Such strain. Therefore, staggered conformation is reagents are called electrophilic reagents. The presence of weakerπ-bond makes alkenes more stable than the eclipsed conformation. unstable molecules in comparison to alkanes Hence, molecule largely remains in staggered and thus, alkenes can be changed into single conformation or we can say that it is preferred conformation. Thus it may be inferred that bond compounds by combining with the electrophilic reagents. Strength of the double rotation around C–C bond in ethane is not bond (bond enthalpy, 681 kJ mol ) is greater completely free. The energy difference between than that of a carbon-carbon single bond in the two extreme forms is of the order of 12.5 – kJ mol –, which is very small. Even at ordinary ethane (bond enthalpy, 348 kJ mol ). Orbital diagrams of ethene molecule are shown in temperatures, the ethane molecule gains Figs. 9.4 and 9.5. thermal or kinetic energy sufficient enough –o overcome this energy barrier of 12.5 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. 9.3 ALKENES Alkenes are unsaturated hydrocarbons containing at least one double bond. What Fig. 9.4 Orbital picture of ethene depicting bonds only should be the general formula of alkenes? If there is one double bond between two carbon 9.3.2 N omenclature atoms in alkenes, they must possess two For nomenclature of alkenes in IUPAC system, hydrogen atoms less than alkanes. Hence, the longest chain of carbon atoms containing general formula for alkenes is CnH 2nAlkenes the double bond is selected. Numbering of the are also known as olefins (oil forming) since chain is done from the end which is nearer to Reprint 2026-27