Stereochemistry of organic compounds-II

Geometrical isomerism  It is a type of stereoisomerism which is exhibited by certain compounds containing double bonds and arises due to hindered rotation about the double bond.  Due to hindered rotation around the c-c double bond, the relative positions of groups attached to doubly bond carbon atoms gets fixed. As a result, several higher alkenes can exist in two distinct isomeric forms which differ from each other in the relative distribution of groups in space around the double bond.

Conditions for geometrical isomerism There are two necessary conditions for a compound to exhibit geometrical isomerism: 1.The molecule must contain a carbon-carbon double bond, the rotation around which is strongly hindered. 2.Each of the two doubly bonded atoms should have two unlike groups attached to it as, for instance, abC=Cab.

E and Z nomenclature If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z

Conformations In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds (refer to figure on single bond rotation). [ Such isomers are generally referred to as conformational isomers or conformers and, specifically, as rotamers. Rotations about single bonds are restricted by a rotational energy barrier which must be overcome to interconvert one conformer to another. Conformational isomerism arises when the rotation about a single bond is relatively unhindered. That is, the energy barrier must be small enough for the interconversion to occur.

Conformational analysis of ethane The potential energy of an ethane molecule is minimum for the staggered conformation, increases with rotation and becomes maximum for eclipsed conformation, difference between the two extreme conformations being kJ/mol.

Conformational analysis of butane Four important conformations of n-butane are: anti, eclipsed, gauche and fully eclipsed. Order of stabilities: anti > gauche > eclipsed > fully eclipsed.

Conformations of cyclohexane Two important conformations of cyclohexane are: Chair conformation : this is most stable and therefore, the most preferred conformation of cyclohexane. This conformation is essentially free of strain and has minimum energy. In other words,this is most stable conformation of cyclohexane. Boat conformation : in boat form,the hydrogens on four of the carbons are eclipsed. As a result, there is considerable torsional strain. At the same time two hydrogens pointing towards each other at C1 and C4 (flagpole hydrogens) are very close together. This gives rise to Vander Waals strain in the molecule. Order of stability: chair > twist boat > boat > half chair.

Difference between conformation and configuration conformation  These are 3D arrangements in space of the atoms in a molecule which are interconvertible by rotation around a single bond.  Interconversion does not require breaking and making of bonds.  Their existence leads to the phenomenon of conformational isomerism.  Since conformations are readily interconvertible, they cannot be isolated from each other. Conformational isomers exist only as a mixture of different conformations. configuration  These are 3D arrangements in space of the atoms in a molecule which are not interconvertible by rotation around a bond.  Interconversion is possible only through breaking and making of bonds.  Their existence is involved in the phenomena of geometrical and optical isomerism (configurational).  Since configurations are not easily interconvertible, they can be isolated from each other and stored as pure substances.

Assignment  Differentiate between conformations and configurations.  Give the salient features of chair and boat conformations of cyclohexane.  What are the factors which contribute towards the stability of conformational isomers ?  Discuss how geometrical isomerism affects melting and boiling points?  What do you understand by conformers? Explain.  Write a note on conformations of ethane.  Write conformations of n-butane and discuss their relative stability?  In terms of eclipsing interactions explain why chair and twist boat conformations of cyclohexane are more stable than the boat conformations.