Conformation and configuration sujith

CONFORMATION AND CONFIGURATION . ALONG WITH CONFORMATION OF ETHANE AND PROPANE

1. CONFORMATION AND CONFIGURATION Presented by , SUJITH K P MSC CHEMISTRY KARUNYA UNIVERSITY COIMBATORE , TAMILNADU INDIA

2. CONTENTS CONFORMATION CONFIGURATION COMPARISONS B/W CONFORMATION AND CONFIGURATION CONFORMATION OF ETHANS CONFORMATIONS OF SUBSTITUTED ETHANS ( PROPANE )

3. CONFORMATION Conformation  is the spatial arrangement of atoms in a molecules that can come about through free rotation of atoms about single bond. E.g . Rotation about single bonds produce the cis-trans conformational isomerism  is a form of  stereoisomers  in which the  isomers can be interconverted just by rotations about formally single bonds

4. Conformational isomers These can be broadly classified into two different cases: Eclipse conformation Conformation in which hydrogen atoms attached to two carbons areas nearest to each other as possible is known as eclipsed Staggered conformation Conformation in which hydrogen atoms attached to two carbons are as far as possible with respect to each other is known as staggered The staggered conformation is thus relatively more stable in comparison to eclipse conformation as there are minimum repulsive force.

5. Representation of Eclipsed and Staggered Conformation : Sawhorse projections: In this kind of projection, the bond between carbon atoms is shown as a long straight line. The lower end of the line designates the front carbon atom whereas the upper end designates the rear carbon atom . Since each carbon atom in ethane is attached to three hydrogen atoms; each carbon atom has three lines attached designating C-H bonds inclined at an angle of120 ° to each other

6. Representation of Eclipsed and Staggered Conformation : Newman projections : In this projection, out of the two carbon atoms present in ethane one which is nearer is shown as a dot whereas the rear carbon atom is represented as a circle. The three hydrogen atoms attached to each carbon atom are represented with the help of three lines either bulging out of the circle or diverging of the dotted lines. These lines are inclined to each other at an angle of 120° to each other.

7. configuration Configuration  is permanent geometry. It refers to spatial arrangement of bonds and can be change only by breaking bonds. It also tells about how the atoms or molecules were connected E.g . L&D and R&S configuration

8. Difference between conformation and configuration CONFORMATION These are 3-D arrangements in space of atom in a molecule which are interconvertible by rotation around a single bond Interconversion does not require breaking and making of bond Since conformations are readily interconvertible , they cannot be isolated from each other . Their existence leads to the phenomenon of conformational isomerism These are exist only as mixture of different conformations CONFIGURATION These are 3-D arrangements in space of atom in a molecule which are not interconvertible by rotation around a single bond Interconversion is possible only through breaking and making of the bond Since configuration are not easily interconvertible , they can be isolated from each other Their existence is involved in the phenomena of geometrical and optical isomerism They are stored as pure substance.

9. CONFORMATION OF ETHANE Although there are seven sigma bonds in the ethane molecule, rotation about the six carbon-hydrogen bonds does not result in any change in the shape of the molecule because the hydrogen atoms are essentially spherical.  Rotation about the carbon-carbon bond, however, results in many different possible molecular conformations.

10. In order to better visualize these different conformations, it is convenient to use a drawing convention called the  Newman projection . STAGGERED CONFORMATION ECLIPSED CONFORMATION

11. The staggered conformation of ethane has two relationship based on their rotation of 180 and 60 ANTI RELATIONSHIP GAUCHE RELATIONSHIP The term ANTI and GAUCHE apply only to the bonds on adjacent carbon , and only to staggered conformation

12. Free Rotations Do Not Exist in Ethane The carbon-carbon bond is not  completely  free to rotate – there is indeed a small, 12 J/mol barrier to rotation that must be overcome for the bond to rotate from one staggered conformation to another . the 12 kJ/mol (2.9 kcal/mol) of extra energy present in the eclipsed conformation of ethane is called TORSIONAL STRAIN Torsional strain is due to repulsion between electron clouds in the C-H bonds as they pass close by each other in the eclipsed conformer

13.  H ere the staggered conformation has less potential energy while eclipsed has high potential energy . This means eclipsed is not stable since any slight rotation will leads to lower in energy. However , at any given moment the molecule is more likely to be in a staggered conformation The potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds, as shown below. staggered eclipsed

14. Animation of potential energy vs. dihedral angle in ethane

15. Thus we can say that…. the eclipsed conformation of ethane is 12 KJ/ mol less stable than the staggered The eclipsed conformation is destabilized by torsional strain Torsional strain is the destabilization that’s results from eclipsed bonds. The most stable form is staggered

16. CONFORMATION OF PROPANE Propane   is a three- carbon   alkane  with the molecular formula  C 3 H 8 . These are free rotation around the C-C single bond  

17. These rotation result in two extreme conformations , namely staggered and eclipsed This conformational analysis is similar to ethane However, the torsional strain in propane is little higher than that of ethane due to steric hindrance caused by methyl group in eclipsed conformation

18. In the eclipsed conformation of propane , vanderwaals repulsion occurs between the methyl group and hydrogen. So staggered conformation is most stable , because all the bonds are lying as far as possible. The barrier to rotation that results from this strain can be represented in a graph of potential energy versus degree of rotation in which the angle between the bonds on adjacent carbon atoms (the  dihedral angle ) completes one revolution. 14 kJ/ mol