Geometrical Isomerism | Chemistry Class 11 - NEET PDF Download

What is Geometrical Isomerism?

• Isomers that possess the same molecular and structural formula but differ in the arrangement of atoms or groups in space due to restricted rotation are known as geometrical isomers and the phenomenon is known as geometrical isomerism.

Geometrical Isomers

Conditions for Geometrical Isomerism

(I) Geometrical isomerism arises due to the presence of a double bond or a ring structure

(i.e. , , - N = N - or ring structure).

Due to the rigidity of the double bond or the ring structure to rotate at room temperature, the molecules exist in two or more orientations. This rigidity to rotation is described as restricted rotation l hindered rotation l no rotation.

e.g.

(A)  (Restricted Rotation)

(B)  (Restricted Rotation)

(II) Different groups should be attached to each doubly bonded atom. For example

 and  are identical but not geometrical isomers.

On the other hand, the following types of compounds can exist as geometrical isomers :

,  or

Examples of Geometrical Isomers 

(I) Along  bond

   and  

   and

(II) Along

 and  
    

  and
 

  and  

  and
 

(III) Along - N = N - bond

Geometrical Isomers of N₂H₂

    and  

 and

(IV) Along the bond of cycloalkane

 and 

   and 

   and

  and 

(V) Along  in ring structures :

Usually in cycloalkenes double bond has its configuration. Their trans isomers do not exist due to large angle strain. But if the ring is large enough a trans stereoisomer is also possible. The smallest trans-cycloalkene that is stable enough to be isolated & stored is trans-cyclooctene.

Examples of Cis-Trans Cycloalkenes

Trans-Cyclohexene and Trans-Cyclooctene

Configurational Nomenclature in Geometrical Isomerism:

For deciding the seniority of groups in E-Z configuration, the Cahn - Ingold - Prelog sequence rules are applied :

Rue I: The group with the first atom having a higher atomic number is senior. According to this rule, the seniority of an atom is :

I > Br > Cl > S > F > O > N > C > H

Rule II: The higher mass isotope is senior.

Thus, 

(A) - T > - D > - H. 

(B) - C¹⁴H₃ > - C¹²H₃

Rule III: If the first atom of the group is identical then the second atom is observed for seniority.

e.g.

(A) - CH₂Cl > - CH₂OH > - CH₂NH₂ > - CH₂ CH₃ > - CH₃

(B)  >

Rule IV: Groups containing double or triple bonds are assigned seniority as if both atoms were duplicated or triplicated that

> C = Y as if it were  &  - C º Y as if it were

e.g. for deciding seniority among - C º CH, - CH = CH₂, their hypothetical equivalents are compared.

 >

Rule V: Bond pair gets priority over lone pair.

Rule VI: Z > E & R > S.

Some Examples of E-Z isomers

Number of Geometrical Isomers:

The number of geometrical isomers can be found by calculating the number of stereocentres in the compound. (stereocentre is defined as an atom with three or more different attachments, and interchanging of two of these attachments leads to another stereoisomer. ).

Physical Properties of Geometrical Isomers :