POC-3 UNIT-2

GEOMETRICAL ISOMERISM

• The isomerism which arises due to restricted (frozen) rotation about double bond in molecules OR about single bond in cyclic compounds is known as geometrical or cis-trans isomerism.

• Geometrical isomerism show by variety of compounds such as -

• Compound containing double bond C=C , C=N , N=N
• Compound containing cyclic structure
• Restricted Rotation.

• Condition for geometrical isomerism -

i) There should be restricted (frozen) rotation about a bond in the molecules.
ii) Both substituents on each carbon about which rotation is frozen (restricted) should be different.

• Nomenclature of Geometrical Isomers [Cis-Trans, EZ, Syn Anti System]

Geometrical Isomers can be nomenclatured by three methods

1. Cis Trans Nomenclature
2. E Z nomenclature
3. Syn Anti Nomenclature.

1) Cis Trans Nomenclature

Compounds show this nomenclature due to restricted rotation about carbon-carbon double bond.

• Compounds should have atleast one double bond b/w carbon carbon.
• Atleast one similar atom/group between both double bonded carbon.
• Restricted Rotation.

• Cis : The isomer in which the identical groups are on the same side of the double bond.

• Trans : The isomer in which the identical group are on the opposite sides of the double bond.

examples $\rightarrow$

2) E Z Nomenclature

This nomenclature of geometrical isomers is more general and can be applied to all compounds.

• Z (Zusammen $\rightarrow$ together)
• E (Entegegen $\rightarrow$ Opposite)

• It is based on Cahn-Ingold-Prelog system [Sequence Rule].
• The group of highest priority on each carbon atom is identified by using the sequence rules

(Z)-isomer (E)-isomer

• (Z) - If the highest priority group are on the same side of the double bond.
• (E) - If the highest priority group are on the opposite sides of the double bond.

3) Syn Anti Nomenclature

• This type of nomenclature (isomerism) show by those compounds which have atleast one C=N or N=N.

  Syn $\rightarrow$ Same
  Anti $\rightarrow$ Opposite

  • Syn - If H on Carbon and substitute on Nitrogen are on same side.
  • Anti - When H on Carbon and substitute on Nitrogen are on opposite sides

Methods of determination of configuration of geometrical isomers

• There are a number of methods to determine the configuration of geometrical isomers.

• These are depending on the nature of the compounds.
  i) Physical method
  ii) Cyclisation method
  iii) Conversion method

i) Physical Method

• The melting point and intensity of absorption of the cis-isomer are lower than those of the trans. Trans>Cis.
• The boiling point, solubility, heat of combustion, heat of hydrogenation, density, refractive index, dipole moments and dissociation constant of the cis-isomers are greater than those of the trans. Cis>Trans.
• So, By comparing these properties compound identified as cis or trans.

ii) Cyclisation Methods: (Methods of Cyclisation)

• A cis isomers undergo cyclisation much more readily than the trans isomers.

  Example : Maleic acid forms a cyclic anhydride (maleic anhydride) when heated, whereas fumaric acid does not form fumaric acid anhydride.

  

iii) Conversion Method: (method of conversion into compound of known configuration)

• In a number of cases it is possible to determine the configuration of geometrical isomers by converting them into compounds of known configuration.

  Example:

Conformational Isomerism in Ethane, n-butane and Cyclohexane

Conformational Isomers

These are different spatial arrangements of a molecules that are generated by rotation about single bonds.

• Use Newman Projection to these isomers.

• The structure of conformers is of two types
  i) Staggered - Torsional angle is $60^\circ$.
  ii) Eclipsed - Torsional angle is $0^\circ$.

• Newman projection - Used to represent 3D structure.

• Different conformations of the same molecules are sometimes called conformers/rotomer & conformational isomers.

• Torsional angle - The angle between the atom attached to the front and rear carbon atoms. (Dihedral angle).

  Examples - Ethane [$CH_3-CH_3$]

1) Ethane

When an ethane molecule rotates about its carbon-carbon single bond, two conformations :- staggered and eclipsed.

• Now, there are also use of energy during rotation.
• Eclipsed conformation higher in energy than the staggered conformation.
• There is some steric repulsion b/w the H atoms in the eclipsed conformation that is reduced in staggered conformation.

Due to this:
i) Eclipsed - least stable, extraa energy (torsional strain) - more internal energy
ii) staggered - Most stable, less internal energy.

• Rotation

(I) (III) (V) $\rightarrow$ Staggered (II) (IV) (VI) $\rightarrow$ Eclipsed

• ENERGY

2) Butane

$CH_3-CH_2-CH_2-CH_3$

It has three carbon-carbon single bond and the molecules can rotate about each of them.

• Rotation

It has six structure

(I, III, V) - staggered conformers
(II, IV, VI) - Eclipsed conformers

• Conformer I - Most stable staggered, because in which two methyl group are as far (distance). $\rightarrow$ Anti conformer (fully staggered).
• Conformer III & V - Gauche conformer (partial staggered)
• Conformer II & VI - Eclipsed (partial eclipsed)
• Conformer IV - Fully eclipsed (least stable)

• ENERGY DIAGRAM

$\leftarrow$ stability $\leftarrow$ fully staggered > Gauche > eclipsed > fully eclipsed.

3) Cyclohexane

• Acc. to Sachce & Mohr, cyclohexane shows 2 forms of structure
• Chair form $\rightarrow$ staggered conformers (stable more).
• Boat form $\rightarrow$ eclipsed conformers (less stable).
• Chair conformation is more stable than the boat conformation.

Energy diagram & Conformers

• Conformations of cycloed cyclohexane are as follow

  i) Chair form
  ii) Half chair form
  iii) Twist boat form
  iv) Boat form

i) Chair Form

• There is no steric hindrance, so it has minimum energy and maximum stability.
• It is staggered conformers (most stable).

ii) Half chair form

• It has both angle strain and torsional strain, so less stable than chair form.
• Energy $\rightarrow$ 46 kJ/mole.

iii) Twist boat form

• More stable than boat conformation by about 5.4 kJ/mol, but less stable than chair conformation by 23.4 kJ/mol.

  

iv) Boat form

• Eclipsed conformers
• There are steric interaction b/w the non-bonding atom due to this, the boat conformation is less stable than chair conformation and has higher energy content.

Stereo isomerism in biphenyl compounds (Atropisomerism) and conditions for optical activity

• Atropisomerism : "Isolable stereoisomers resulting from restricted rotation about single bonds are called atropisomers and phenomena called as atropisomerism.

Atropisomerism $\rightarrow$ Restricted Rotation around single bond $\rightarrow$ Stereoisomerism.

• There is restricted rotation about the bond between the two phenyl rings due to steric hindrance between the bulky ortho substituents.
• These phenyl rings lie in different planes which are perpendicular, thus the molecules becomes chiral and exhibits enantiomers.

• Conditions for optical activity & for atropisomerism.

  i) There should be any functional group/atom at ortho position of rings [substitution at ortho position with large size such as - Cl, Br, I, COOH, NO2, SO3H, CH3 etc-].

  ii) Each ring must be resolvable, for that $a \neq b$ and $c \neq d$.

Examples

Biphenyl atropisomers

Sterospecific and Stereoselective reaction

1) Stereospecific Reaction : [stereospecific synthesis]

• A reaction in which a particular stereoisomer react to give one specific stereoisomer of the product.
• In this reaction, stereoisomers gives different stereoisomere product. (stereospecificity). ea

Cis (Reactant) (Product) Trans

Trans (Reactant) (Product)

2) Stereoselective Reaction: [stereoselective synthesis]

• A reaction in which two or more stereoisomers formed possibly, but one stereoisomers is obtained more than the other.

Example :