CONFORMATIONS BY Y.SURENDER KV,GUNTUR

1. CONFORMATIONS BY Y.SURENDER KV,GUNTUR

2. Conformations • Different arrangements of atoms within a molecule that can be converted into one another merely by rotation about single bonds.

3. Conformations Different internal arrangements of the atoms in a molecule that differ by rotation(s) about one or more of the single bonds. CONFORMATIONS ARE NOT ISOMERS Conformations are different arrangements of theSAME MOLECULE. The connection pattern of the atoms does not change. .

4. The Shapes of Molecules • The three-dimensional shapes of molecules result from many forces • A molecule may assume different shapes, called conformations, that are in equilibrium at room temperature (the conformational isomers are called conformers, emphasis on the first syllable) • The systematic study of the shapes molecules and properties from these shapes is stereochemistry • The field of stereochemistry is one of the central parts of organic chemistry and includes many important topics 4

5. Ethane: staggered conformation End View Side View Sawhorse projection

6. Ethane: eclipsed conformation End View Side View eclipsed projection

7. Representing Conformations • Sawhorse representations show molecules at an angle, showing a molecular model • C-C bonds are at an angle to the edge of the page and all C-H bonds are shown • Newmanprojections show how the C-C bond would project end-on onto the paper • Bonds to front carbon are lines going to the center • Bonds to rear carbon are lines going to the edge of the circle 7

8. Newman Projections 8

9. rear bonds stop at edge bonds on front carbon meet only the front carbon shows Two conformations of ethane: Newman Projections ECLIPSED STAGGERED H H H H H H H H H H H H Looking down the carbon-carbon bond.

10. CONFORMATIONS DIFFER IN ENERGY numerous other conformations in between EXTREMES 12.1 kJ/mol eclipsed 3 Kcal/mol ENERGY staggered 0 kJ/mol 0 Kcal/mol The staggered conformation has a lower energy.

11. Torsional Strain • Extra potential energy that a molecule possess due to the presence of eclipsed bonds. • Eclipsed ethane has torsional strain because of the electron pair repulsions between adjacent bonds. • Staggered ethane has no torsional strain.

12. etc ………. 3.0 Kcal 3.0 Kcal 0o 60o 120o 180o 240o 300o 360o Torsional strain versus angle of rotation about the carbon-carbon bond in ethane eclipsed Potential Energy ETHANE staggered Dihedral Angle

13. . PROPANE CH3-CH2-CH3 CH3 C1 - C2 H eclipsed ENERGY 3.4 Kcal/mol (14.2 KJ/mol) H CH3 staggered 0 Kcal/mol As with ethane, the staggered conformation has a lower energy. Potential Energy Curve is similar to Ethane (sinusoidal)

14. CH3 CH3 H H H H BUTANE C2 - C3 view . 1 2 3 4 We will look down the C2 - C3 bond.

15. Butane(anti conformation) end view side view no torsional strain or any other strain

16. Butane(gauche conformation) end view side view no torsional strain, but has steric strain

17. Steric Strain • Extra potential energy that a molecule possesses because groups are crowded together or are forced to approach each other to within the sum of their effective radii

18. Butane(syn conformation) end view side view has torsional and steric strain

19. CH3 CH3 SYN (0o) ANTI (180o) CH3 H H H H H H H H CH3 CH3 GAUCHE (60o) ECLIPSED (240o) CH3 H CH3 H CH3 H H H H H H CH3 CH3 ECLIPSED (120o) GAUCHE (300o) H H CH3 H H H CH3 H H H Conformations of butane: Newman projections

20. CH3 CH3 H H H H CH3 CH3 H H CH3 H H CH3 H H H H CH3 CH3 CH3 H H CH3 H H H H H H CH3 H H H H CH3 Eclipsed always higher energy than staggered! syn ECLIPSED ~6 HIGH ENERGY eclipsed 3.4 STAGGERED gauche 1.0 ( ) Kcal mol anti 0 LOW

21. Torsional strain (Kj) versus angle of rotation BUTANE