Open chained hydrocarbons containing at least one double bond are known as alkenes. The general formula for alkene containing one double bond is CnH2n. The first member of alkene is ethene (CH2=CH2). The double bond is made up of one strong σ bond and one weak π bond. The presence of weak π bond makes alkenes somewhat less stable compounds in comparison to alkanes.
Alkenes show both structural isomerism and geometrical isomerism.
Ethene and propene have only one structure. Alkenes higher than propene have different structures. Let us see how many structural isomers an alkene with formula C4H8 has.
Isomers of C4H8
The above three structural isomers contain position isomers and chain isomers. Can you identify them?
Doubly bonded carbon atoms
Each doubly bonded carbon has to form bonds with other atoms or groups to satisfy its tetravalent nature. If the doubly bonded carbon atoms form bonds in such a way that the structure can be represented by YXC=CXY, the compound will show geometrical isomerism. Let us take the example of but-2-ene.
The above structure can be represented in space in the following two ways :
In such cases, the isomer in which the similar atoms or groups lie on the same side of the double bond is called the cis-isomer whereas the isomer, in which the similar atoms or groups lie on the opposite sides of the double bond is called the trans-isomer. Therefore, the cis and trans isomers in but-2-ene are :
Note : Geometrical isomerism is also known as cis-trans isomerism.
Cis isomer of alkene is more polar than the trans isomer. This can be understood by taking the example of but-2-ene :
Clearly, in the trans-but-2-ene, the two methyl groups are in opposite directions, therefore, dipole moments of C−CH3 bonds cancel out.
1) Besides compounds having structure YXC=CXY, compounds having structures XYZ=CXZ and XYC=CZW also show geometrical isomerism.
If we assume that the atomic number of X is greater than that of Y and the atomic number of Z is greater than that of W, then the cis and trans isomers will be as given below :
2) Geometrical isomers are not possible if one or both the doubly bonded carbons carry two similar substituents. Because in such cases, both the structures actually refer to the same configuration.
Both the structures are identical