Chapter 4 : Stereochemistry B.sc 1st year Page : 8 Organic Chemistry
Geometrical isomers are also configurational isomers like optical isomers but differs from optical isomers by the fact that it results from restricted rotation about double bond or single bonds in cyclic compounds.
Geometrical Isomerism in Alkenes :
The carbon atoms of the carbon-carbon double bonds are spa hybridized. The re-bond formed by the sidewise overlapping of unhybridized p-orbitals of two carbons as above and below perpendicular to the plane of the molecule gives extra strength. Consequently, rotation about carbon-carbon double bond is restricted and relative position of atoms or groups attached to carbon atoms of the double bond are fixed. This makes two spatial arrangements are possible.
Cis-trans isomers :
The isomer in which two similar atoms or groups lie on the same side of the double bond is called ‘cis – isomer’ and the isomer in which two similar atoms or groups lie on the opposite side of the double bond is called ‘trans isomer.
Example : But-2-ene exists in two geometrical cis-trans isomers.
Cis-trans nomenclature is applicable only to those olefins in which similar atoms or groups are attached to both double bonded carbons. When all the four atoms or groups attached to double bonded carbons are different a new E. Z System of nomenclature is used.
Z-System of Nomenclature :
This system is based on the priority sequence rules. Originally developed by Cahn – Ingold and Prelog to describe the configurations about chiral center’s in a molecule.
- Assign priority order to both atoms or groups attached to each carbon atom of the double bond.
- If the atoms or groups of higher priority on each carbon atom of the double bond are on the opposite sides of the double bond, the configuration is designated as E(German; entgegen = opposite)
- If the atoms are groups of higher priority on each carbon atom of the double bond are on the same side of the double bond the configuration is designated as Z(German; zusammen = together). For examples :
If the organic compound contains two or more double bonds, capable of showing geometrical isomerism E, Z notations are given to each double bond.
Determination of Configuration in Geometrical isomers :
a) Physical Method:
The trans isomer generally has higher melting point and lower boiling point than as-isomer. Cis-isomers have higher solubility’s and dipole moments while trans-isomers do not.
(b) Chemical Methods :
Geometrical isomers can often be distinguished by reactions that lead to the formation of rings. Cis- isomer is expected to undergo ring closure much more readily than the transisomer. e.g. Maleic acid readily undergoes dehydration when heated to about 150°C to offered and anhydride whereas fumaric acid does not at this temperature. Fumaric acid must be heated to about 300°C to get the same anhydride, Thus, maleic acid is the cis-isomer.
(c) Optical activities of products :
Generally, different geometrical isomers for products have different optical properties. Since halogenation of alkenes is a trans-addition therefore trans-alkenes give meso products while cis-alkenes give racemic mixture.
Geometrical Isomerism in Oximes :
Oximes exhibit geometric isomerism due to restricted carbon-nitrogen double bond. However, here the prefixes syn (for cis) and anti (for trans) are conventionally used. In case of aldoxime a syn-isomer is the one in which H-atom and OH group lie on the same side of the double bond, whereas anti-isomer is the one in which H and OH group lie on the opposite sides of the double bond.
Preparation of Oximes :
Oximes are compounds formed by the condensation of aldehydes or ketones with hydroxylamine; NH,OH under weakly acidic conditions.
Geometrical Isomerism in Alicyclic compounds :
The presence of small rings like a double bond prevent rotation and as a result geometrical isomerism is possible. The small rings are generally anar whereas large ones have puckered(non linear) shape and can not show geometrical isomerism.