Structural Integrity and Isomerism of Alkenes
The carbon atoms which contain the double bonds in alkenes are sp2 hybridized. Each carbon-carbon double bond is made of one sigma and one pi bond.
IsomerismAny compound with a carbon-carbon double bond can exhibit cis-trans isomerism, provided that the carbons involved in the double bond do not have two of the same groups or atoms attached to each of them. 2-Butene is the simplest alkene that can have cis-trans isomerism. Trans isomers are generally more stable than their cis counterparts. Alkyl groups are mildly electron-donating toward the double bond. This can lead to polarity. For instance, cis-2-butene has net dipole moment as shown in the diagram given below. On the other hand, in trans-2-butene the net dipole is zero since the dipole moments cancel out (the vector sum of the dipole moments is zero).
Both cis and trans-2-butene have van der Waals attractive forces. But, only the cis-isomer can have dipole-dipole interactions because it has a net dipole moment. Hence, cis-2-butene has a higher boiling point than trans-2-butene.
The E-Z System of Naming Alkenes
Sometimes we are not able to categorize alkenes into trans and cis isomers. The following example will reveal why that is the case.
For compounds such as the ones shown above, we have to prioritize the substituents, and that is the only way to name and identify these compounds. A new system was proposed by scientists, which categorizes these compounds under E (entgegen), and Z (zusammen) configurations. E configuration describes opposite, whereas Z configuration describes same side. Study the following examples to get familiarized with this system of naming alkenes. In order to recognize which substituent is higher or lower, you have to compare their atomic numbers. That means an atom with a higher atomic number takes precedence over an atom with a lower atomic number.
One key aspect to remember is that when we compare which substituent is higher or lower, we should compare the substitutions in the same carbon. In the above example, it is easy to see that on the left substitution, obviously bromine is higher (higher in the atomic number sense) than hydrogen. On the right side, chlorine is higher (atomic number:17) than carbon (atomic number:6). So the compound shown in Fig. (a) has an E configuration, and the one shown in Fig.(b) has a Z configuration.