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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 IsomersGeometrical Isomers

Question for Geometrical Isomerism
Try yourself:
Which of the following best defines geometrical isomerism?
View Solution

Conditions for Geometrical Isomerism

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

(i.e. Geometrical Isomerism | Chemistry Class 11 - NEET, Geometrical Isomerism | Chemistry Class 11 - NEET, - 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) Geometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEET (Restricted Rotation)

 

(B) Geometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEET (Restricted Rotation)

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

Geometrical Isomerism | Chemistry Class 11 - NEET and Geometrical Isomerism | Chemistry Class 11 - NEET are identical but not geometrical isomers.

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

Geometrical Isomerism | Chemistry Class 11 - NEET, Geometrical Isomerism | Chemistry Class 11 - NEET or Geometrical Isomerism | Chemistry Class 11 - NEET

Examples of Geometrical Isomers 

(I) Along Geometrical Isomerism | Chemistry Class 11 - NEET bond

Geometrical Isomerism | Chemistry Class 11 - NEET   and  
Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET   and
Geometrical Isomerism | Chemistry Class 11 - NEET

(II) Along Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET and  
Geometrical Isomerism | Chemistry Class 11 - NEET    

Geometrical Isomerism | Chemistry Class 11 - NEET  and
 Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET  and  
Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET  and
 Geometrical Isomerism | Chemistry Class 11 - NEET

(III) Along - N = N - bond

Geometrical Isomers of N2H2Geometrical Isomers of N2H2

Geometrical Isomerism | Chemistry Class 11 - NEET    and  
Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET and
Geometrical Isomerism | Chemistry Class 11 - NEET

(IV) Along the bond of cycloalkane

Geometrical Isomerism | Chemistry Class 11 - NEET and 

  Geometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEET and 

  Geometrical Isomerism | Chemistry Class 11 - NEET

Geometrical Isomerism | Chemistry Class 11 - NEET   and

Geometrical Isomerism | Chemistry Class 11 - NEETGeometrical Isomerism | Chemistry Class 11 - NEET  and 

  Geometrical Isomerism | Chemistry Class 11 - NEET

(V) Along Geometrical Isomerism | Chemistry Class 11 - NEET 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 CycloalkenesExamples of Cis-Trans Cycloalkenes

Trans-Cyclohexene and Trans-CycloocteneTrans-Cyclohexene and Trans-Cyclooctene

Configurational Nomenclature in Geometrical Isomerism:

 

Geometrical Isomerism | Chemistry Class 11 - NEET

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) Geometrical Isomerism | Chemistry Class 11 - NEET > Geometrical Isomerism | Chemistry Class 11 - NEET

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 Geometrical Isomerism | Chemistry Class 11 - NEET &  - C º Y as if it were Geometrical Isomerism | Chemistry Class 11 - NEET

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

Geometrical Isomerism | Chemistry Class 11 - NEET > Geometrical Isomerism | Chemistry Class 11 - NEET

Rule V: Bond pair gets priority over lone pair.

Rule VI: Z > E & R > S.

Geometrical Isomerism | Chemistry Class 11 - NEET

Some Examples of E-Z isomersSome 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. ).

Nature of compound

No. of G.I. (n = no. of stereocentres)

Example

No. of Isomers

Isomers

(I) Compound with dissimilar ends

2n

CH3 - CH = CH - CH = CH - C2H5

4

I: (cis, cis)

II: (trans, trans)

III: (cis, trans)

IV: (trans, cis)

(II) Compound with similar ends with even stereocentres

2n-1 + 2n/2 - 1

CH3 - CH = CH - CH = CH - CH3

3

I: (cis, cis)

II: (trans, trans)

III: (cis, trans)

= (trans, cis)

(III) Compound with similar ends with odd stereocentre

 

2n-1 + 2n-1/2

CH3 – CH = CH – CH = CH – CH = CH – CH3

6

I: (cis, cis, cis)

II: (cis, cis, trans)

= (trans, cis, cis)

III: (cis, trans, trans)

= (trans, trans, cis)

IV: (trans, trans, trans)

V: (cis, trans, cis)

VI: (trans, cis, trans)

 

Question for Geometrical Isomerism
Try yourself:
Which of the following compounds can exhibit geometrical isomerism?
View Solution

Physical Properties of Geometrical Isomers :

Physical properties

 I(cis), II(trans)

Remarks

Dipole moment

I > II

cis-isomer has a resultant dipole moment while in trans-isomer dipole moments cancel out.

Boiling point

I > II

Molecules having higher dipole moments have higher boiling points due to the larger intermolecular force of attraction.

Solubility (in H2O)

I > II

More polar molecules are more soluble in H2O.

Melting point

II > I

More symmetric isomers have higher melting points due to better packing in crystalline lattice & trans isomers are more symmetric than cis.

Stability

II > I

The molecule having more Vander Waals' strain is less stable. In cis isomer, the bulky groups are closer they have larger Vander Waals' strain.

 

 

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FAQs on Geometrical Isomerism - Chemistry Class 11 - NEET

1. What is geometrical isomerism?
Ans. Geometrical isomerism is a type of stereoisomerism where molecules have the same molecular formula but differ in their spatial arrangement due to restricted rotation around a double bond or a ring. This leads to different physical and chemical properties of the isomers.
2. What are the conditions for geometrical isomerism?
Ans. Geometrical isomerism occurs when there is restricted rotation around a double bond or a ring. The conditions for geometrical isomerism include having a double bond or a ring in the molecule, presence of different groups attached to the double bond or ring, and the absence of a plane of symmetry.
3. Can you provide some examples of geometrical isomers?
Ans. Examples of geometrical isomers include cis-trans isomerism in alkenes, where the substituents are arranged differently around the double bond. For example, cis-2-butene and trans-2-butene are geometrical isomers. Another example is cis-trans isomerism in cycloalkanes, where the substituents are arranged differently around the ring.
4. How is configurational nomenclature used in geometrical isomerism?
Ans. Configurational nomenclature is used to distinguish between different geometrical isomers. The prefixes "cis" and "trans" are used to describe the relative positions of substituents on a double bond or a ring. "Cis" indicates that the substituents are on the same side, while "trans" indicates that the substituents are on opposite sides.
5. How many geometrical isomers can a molecule have?
Ans. The number of geometrical isomers a molecule can have depends on the number of different substituents around the double bond or ring. For a molecule with two different substituents, it can have two geometrical isomers (cis and trans). For a molecule with three different substituents, it can have four geometrical isomers (two cis and two trans). The number of possible geometrical isomers increases with the number of different substituents.
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