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Alkene and Its Isomers

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Alkenes

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.

Isomerism in Alkenes

Alkenes show both structural isomerism and geometrical isomerism.

Structural 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

Isomers of C4H8
C4H8 isomers
isomers of alkene : C4H8

The above three structural isomers contain position isomers and chain isomers. Can you identify them?

Find position and chain isomers in C4H8 :

Chain isomers and position isomers in C4H8

Geometrical Isomerism

Doubly bonded carbon atoms

Geometrical isomerism : doubly bonded carbon

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.

Geometrical Isomerism : structure of but-2-ene

The above structure can be represented in space in the following two ways :

Geometrical isomerism : cis isomer
Geometrical isomerism : trans isomer

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 :

cis-isomer

Geometrical isomerism : cis isomer

trans-isomer

Geometrical isomerism : trans isomer

Note : Geometrical isomerism is also known as cis-trans isomerism.

Polarity of cis-trans isomers

Cis isomer of alkene is more polar than the trans isomer. This can be understood by taking the example of but-2-ene :

cis-isomer

Geometrical isomerism : Polarity of cis-trans isomers

trans-isomer

Geometrical isomerism : Polarity of cis-trans isomers

Clearly, in the trans-but-2-ene, the two methyl groups are in opposite directions, therefore, dipole moments of C−CH3 bonds cancel out.

Important Points Regarding Geometrical Isomers

1) Besides compounds having structure YXC=CXY, compounds having structures XYZ=CXZ and XYC=CZW also show geometrical isomerism.

XYC=CXZ

cis-isomer

Geometrical isomerism YXC=CXY : cis isomer

trans-isomer

Geometrical isomerism YXC=CXY : trans isomer

XYC=CZW

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 :

cis-isomer

Geometrical isomerism XYC=CZW : cis isomer

trans-isomer

Geometrical isomerism XYC=CZW : trans isomer

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

Geometrical isomerism
Geometrical isomerism

Question : Which one of the following shows geometrical isomerism : CHCl=CHCl, CH2=CCl2, and CCl2=CHCl.

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