Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

JEE: Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

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Geometrical Isomerism 

  • Isomers which 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 Isomerism Notes | Study Chemistry for JEE - JEE


Conditions of geometrical Isomerism

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

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

(i.e. Geometrical Isomerism Notes | Study Chemistry for JEE - JEE, Geometrical Isomerism Notes | Study Chemistry for JEE - JEE, - N = N - or ring structure).

Due to the rigidity of double bond or the ring structure to rotate at the 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.

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

e.g.

(A) Geometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEE (Restricted Rotation)

 

(B) Geometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEE (Restricted Rotation)

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

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE and Geometrical Isomerism Notes | Study Chemistry for JEE - JEE are identical but not geometrical isomers.

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

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE, Geometrical Isomerism Notes | Study Chemistry for JEE - JEE or Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Examples of Geometrical isomers :

(I) Along Geometrical Isomerism Notes | Study Chemistry for JEE - JEE bond

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE   and  
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE   and
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

(II) Along Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE and  
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE    

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE  and
 Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE  and  
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE  and
 Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

(III) Along - N = N - bond

Geometrical Isomers of N2H2Geometrical Isomers of N2H2

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE    and  
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE and
Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

(IV) Along s bond of cycloalkane

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE and 

  Geometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEE and 

  Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE   and

Geometrical Isomerism Notes | Study Chemistry for JEE - JEEGeometrical Isomerism Notes | Study Chemistry for JEE - JEE  and 

  Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

(V) Along Geometrical Isomerism Notes | Study Chemistry for JEE - JEE 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.

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE


Configurational nomenclature in geometrical Isomerism:

 

Configuration

Criteria

Remarks

cis / trans

Similarity of groups

If the two similar groups are on same side of restricted bond the configuration is cis otherwise trans.

E/Z

Seniority of groups

If the two senior groups are on same side of the restricted bond the configuration is Z (Z = zusammen = together) otherwise E (E = entgegen = opposite).

 

Sequence rules : (Cahn - Ingold - Prelog sequence rules)

For deciding the seniority of groups, following rules are applied :

Rue I : The group with the first atom having higher atomic number is senior. According to this rule the seniority of 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 group is identical then second atom is observed for seniority.

e.g.

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

 

(B) Geometrical Isomerism Notes | Study Chemistry for JEE - JEE > Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

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 Notes | Study Chemistry for JEE - JEE &  - C º Y as if it were Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

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

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE > Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Rule V : Bond pair gets priority over lone pair.

Rule VI : Z > E & R > S.

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Difference between Optical and Geometrical Isomerism

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE


Number of Geometrical Isomers:

Number of geometrical isomers can be found by calculating the number of stereocentres in the compound. (stereocentre is defined as an atom or bond bearing groups of such nature that an interchange of any two groups will produce a 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)

 

Physical Properties of Geometrical Isomers :

Physical properties

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Remarks

Dipole moment

I > II

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

Boiling point

I > II

Molecules having higher dipole moment have higher boiling point 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.

 

Table

Physical properties

Geometrical Isomerism Notes | Study Chemistry for JEE - JEE

Dipole moment

I > II

Boiling point

I > II

Solubility (in H2O)

I > II

Melting point

I > II

Stability

I > II

 

The document Geometrical Isomerism Notes | Study Chemistry for JEE - JEE is a part of the JEE Course Chemistry for JEE.
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