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How to identify Geometrical Isomers. Organic compounds which have same molecular formula but different non-interchangable geometry due to different orientation of atoms in space are called Geometrical Isomers. Example.

CH3

H

CH3

H

CH3

H

H

CH3

cis-but-2-ene trans)-but-2-ene Geometrical Isomers are

1. different compounds i.e non-superimposable i.e. cannot be interconverted without bond dissociation

2. A type of configurational isomers i.e. they differs in orientation at least at one carbon atom. Or ( stereo center)

3. Being isomers they have same molecular formula , empirical formule , % composition 4. They have a defined geometry. 5. As they geometrically differs therefore have different dipole moments , mp, bp, vapour

pressure , solubility. Geometrical Isomerism is found in compounds which have

a. Planner geometry (real or virtual) b. Different substituent on either side of imaginary or real planner geometry. c. No interconvertibility ( Presence of Pi bond , continuous pi bond etc.

Planner geometry (real or virtual) :-It is found in following cases.

1. Alkene.

CH3

H

H

CH3

sp2

carbon

2.Alkene (cumulene with odd no of pi bonds)

CH3

HC C CH3

H .

CH3

HC C CH3

H X

Y

ZZYY

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SINHA I. I.T. CHEMISTRY

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3 Cycloalkanes

CH3

H

CH3

H

XZ plane

4. Odd no Spiranes

CH3CH3

HH 5.Odd no combination of spiro & cumulene

CH3CH3

H H

6.Cycloalkene with carbon greater than 8

H

H

H

Hcis trans 7 Fused rings also have cis & trans orientation

H

H

cis

CH3

CH3

trans

Notation system:

1. cis & trans. 2. Syn and anti 3. E & Z. 1. cis & trans cis and trans notation is used only if there are two identical groups on either

side of the planer geometry.

Ex. cis trans none 2. Syn and Anti notation is used only if there are C=N or N=N bond with different

substituents ( Historical approach used by fisher for Oximes).

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Ex.

N

OHCH3

H

N

OHH

CH3

N

OHCH3

CH3anti syn none

N N

CH3CH3

syn

N N

CH3

CH3anti

3.E and Z notations are introduced to designate geometrical isomers of any type.

Ex.

Cl

Br

CH3

H

Br

Cl

CH3

HE Z1.

1. 1. 1.

1. represent high priority group No. of geometrical units and no of geometrical isomers. Total no of geometrical isomers depends on 1. no of geometrical units 2. symmetry/ unsymmetry 3. other structural limitations.

Ex.

CH3

CH3

C C CH3

CH3

CH3

1.

CH3

H

C C CH3

CH3

CH3

2.

CH3

H

C C H

H

CH3

3.

C H 3

H

C H 3

H

CH 3

H

CH 3

H4.

CH3

H

CH3

H

CH3

H

5.

6.CH3

H

CH3

CH3

H

7.

CH3

H

CH3

H

8.

CH3

H

CH3

H

CH3 H

CH3 H

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9.

CH3

H

CH3

H H

H

CH3

CH3

H

10

CH3

H

CH3

H

C C

H

C C

H

CH3

H

CH3

H

11. H

CH3

HH

CH3

H

12. H

CH3

H

CH3

HCH3

HH

13.14. 15.