Geometrical Isomerism Class 11 Notes | EduRev

Chemistry Class 11

Class 11 : Geometrical Isomerism Class 11 Notes | EduRev

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Geometrical Isomerism 

Definition :

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.

Conditions of geometrical Isomerism

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

(i.e. Geometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRev, - 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.

e.g.

(A) Geometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRev (Restricted Rotation)

 

(B) Geometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRev (Restricted Rotation)

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

Geometrical Isomerism Class 11 Notes | EduRev and Geometrical Isomerism Class 11 Notes | EduRev are identical but not geometrical isomers.

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

Geometrical Isomerism Class 11 Notes | EduRev, Geometrical Isomerism Class 11 Notes | EduRev or Geometrical Isomerism Class 11 Notes | EduRev

Examples of Geometrical isomers :

(I) Along Geometrical Isomerism Class 11 Notes | EduRev bond

Geometrical Isomerism Class 11 Notes | EduRev   and  
Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev   and
Geometrical Isomerism Class 11 Notes | EduRev

(II) Along Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev and  
Geometrical Isomerism Class 11 Notes | EduRev    

Geometrical Isomerism Class 11 Notes | EduRev  and
 Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev  and  
Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev  and
 Geometrical Isomerism Class 11 Notes | EduRev

(III) Along - N = N - bond

Geometrical Isomerism Class 11 Notes | EduRev    and  
Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev and
Geometrical Isomerism Class 11 Notes | EduRev

(IV) Along s bond of cycloalkane

Geometrical Isomerism Class 11 Notes | EduRev and 

  Geometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRev and 

  Geometrical Isomerism Class 11 Notes | EduRev

Geometrical Isomerism Class 11 Notes | EduRev   and

Geometrical Isomerism Class 11 Notes | EduRevGeometrical Isomerism Class 11 Notes | EduRev  and 

  Geometrical Isomerism Class 11 Notes | EduRev

(V) Along Geometrical Isomerism Class 11 Notes | EduRev 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 Class 11 Notes | EduRev

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 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 Class 11 Notes | EduRev > Geometrical Isomerism Class 11 Notes | EduRev

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 Class 11 Notes | EduRev &  - C º Y as if it were Geometrical Isomerism Class 11 Notes | EduRev

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

Geometrical Isomerism Class 11 Notes | EduRev > Geometrical Isomerism Class 11 Notes | EduRev

Rule V : Bond pair gets priority over lone pair.

Rule VI : Z > E & R > S.

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 Class 11 Notes | EduRev

Remarks

Dipole moment

I > II

cis-isomer has 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 larger intermoleculer 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 are less stable. In cis isomer the bulky groups are closer they have larger vander Waals' strain.

 

Table

Physical properties

Geometrical Isomerism Class 11 Notes | EduRev

Dipole moment

I > II

Boiling point

I > II

Solubility (in H2O)

I > II

Melting point

I > II

Stability

I > II

 

Unsolved problem : Find the number of geometrical isomers in

(A)

Geometrical Isomerism Class 11 Notes | EduRev

(B)

Geometrical Isomerism Class 11 Notes | EduRev


Unsolved problem : Compare the physical properties (m, b.p., m.p., solubility & stability) in the geometrical isomers of CH3 - CH = CH - CN.

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