Formulas for imaging of stereo isomers

Conformational Isomerism

Conformational isomers are stereoisomers that have different arrangements of their atoms in space due to free rotation about a covalent σ-bond.

e.g.

Conformational isomers interconvert into each other without breaking of chemical bonds. Unlike structural isomers, different conformers can’t usually be isolated, because theyinterconverttoo rapidly. This kind of isomers can be fined by physical and chemical methods only.

 

Geometrical Isomerism

Geometrical isomers are stereoisomers that have different arrangements of their atoms in space due to restricted rotation about a covalent bond.

Rotation of a carbon atom of a C = C bond through an angle of 90° causes the breaking of the p bond.

 

Geometrical isomers have different physical and chemical properties.

 

Physical properties	cis -But-2-ene	trans -But-2-ene
Melting point (°C)	–139	–106
Boiling point (°C)

trans -But-2-ene has higher melting point

Þ more regular and symmetrical structure

Þ molecules pack more compactly in crystal lattice

Þ difficult to break the lattice

Þ higher melting point

 

cis -But-2-ene has higher boiling point because it has net dipole moment

Þ molecules are held together by dipole-dipole interactions

Þ trans -isomer has no net dipole moment, their molecules are held by instantaneous dipole-induced dipole interactions

Þ dipole-dipole interactions are stronger

Þ cis -but-2-ene has higher boiling point

Another example:

cis -butenedioic acid and trans -butenedioic acid

Physical properties	cis -Butenedioic acid	trans -Butenedioic acid
Melting point (°C)
Solubility in water (gram of solution per 100 g of water at 25 °C)	78.8	0.7

trans -butenedioic acid has higher melting point
and form more extensive intermolecular hydrogen bonding.

While the cis -isomer forms intramolecular hydrogen bonding
Þ less extensive intermolecular hydrogen bonding
Þ molecules are less strongly held
Þ lower melting point

Although trans -butenedioic acid can form more extensive hydrogen bonds with water molecules, cis- butenedioic acid is more soluble in water than the trans -isomer because of the greater dipole moment.

cis - and trans -butenedioic acids have different chemical properties

Enantiomerism

• Enantiomerism occurs in those compounds whose molecules are chiral.

• A chiral molecule is one that is not superimposable with its mirror image.

• The chiral molecule and its mirror image are enantiomers.

 

sp ³-hybridized carbon atom with two or more identical groups attached is achiral and contains a plane of symmetry

sp ³- hybridized carbon atom with four different groups attached is chiral and do not contain a plane of symmetry

E.g. butan-2-ol