Molecular symmetry and chirality

Molecular symmetry and chirality: Symmetry in the molecules, Role of Symmetry in predicting chirality and chirality due to symmetry elements

1. DOS&R IN ORGANIC CHEMISTRY TUMKUR UNIVERSITY By PRUTHVIRAJ K Faculty DOS&R in Organic Chemistry KPR. DOS&R in ORGANIC CHEMISTRY TUT

2. KPR. DOS&R in ORGANIC CHEMISTRY TUT

3. KPR. DOS&R in ORGANIC CHEMISTRY TUT

4. KPR. DOS&R in ORGANIC CHEMISTRY TUT

5. KPR. DOS&R in ORGANIC CHEMISTRY TUT

6. KPR. DOS&R in ORGANIC CHEMISTRY TUT

7. KPR. DOS&R in ORGANIC CHEMISTRY TUT

8. SYMMETRY OPERATION? Symmetry Operation is a operation with restriction or specified rotation. It is a geometrical manipulation on the molecule to get equivalent of indistinguishable configuration Ex- Rotation, Reflection and Inversion, SYMMETRY ELEMENTS Symmetry Operation generates symmetry elements. It is a geometrical entity such as point, plane or line KPR. DOS&R in ORGANIC CHEMISTRY TUT

9. Symmetry Element Symbol Symmetry Operation Rotational axis or n-fold axis of Symmetry C n Rotate about an axis by 360 / θ Plane of symmetry σ Reflection in the Plane Improper rotational axis or n-fold alternating axis of symmetry S n Rotate about an axis by 360 / θ and reflect it in a place perpendicular to rotationa axis Inversion Centre or Centre of Symmetry i Invert it through the centre of symmetry or gravity Identical Element E Leave the molecule unchanged KPR. DOS&R in ORGANIC CHEMISTRY TUT

10. Rotate the molecule over an axis by some angle θ to get identical of original configuration n=( 360 / θ ) n= Number of times of rotation to get identical/original configuration θ = Angle of rotation KPR. DOS&R in ORGANIC CHEMISTRY TUT

11. n =( 360 / θ ) = 360/180= 2 ∴ H 2 O has C 2 rotational axis of symmetry n=( 360 / θ ) = 360/120 = 3 ∴ NH 3 has C 3 rotational axis of symmetry KPR. DOS&R in ORGANIC CHEMISTRY TUT

12. Principle rotational axis (High fold rotational axis n=Maximum) Simple or secondary rotational axis ( it is often are C 2 axes and in the order of principle axis ie number of C 2 =n) KPR. DOS&R in ORGANIC CHEMISTRY TUT

13. KPR. DOS&R in ORGANIC CHEMISTRY TUT

14. KPR. DOS&R in ORGANIC CHEMISTRY TUT

15. This element of symmetry is obtained by doing reflection operation KPR. DOS&R in ORGANIC CHEMISTRY TUT

16. KPR. DOS&R in ORGANIC CHEMISTRY TUT

17. Simple/Vertical Plane of Symmetry σ v Dihedral Plane of Symmetry σ d Horizontal Plane of Symmetry σ h Dihedral Plane of Symmetry σ d ? It is a special type of plane present in some molecules like allenes , staggered ethane, methane etc. A dihedral plane is one which bisects the angle subtended between two similar consecutive C 2 axes. KPR. DOS&R in ORGANIC CHEMISTRY TUT

18. KPR. DOS&R in ORGANIC CHEMISTRY TUT

19. The element of Symmetry is obtained by rotating a molecule through an angle of θ then taking a reflection in the plane perpendicular to the Rotational axis C n . S n = C n X perpendicular σ KPR. DOS&R in ORGANIC CHEMISTRY TUT

20. KPR. DOS&R in ORGANIC CHEMISTRY TUT

21. This element is obtained by inversion operation when all the atoms are inverted through the centre of molecule (centre of gravity) This operation requires that all the molecules or atoms lying outside centre of gravity should always occur in identical pairs and must be diagonally placed to each other KPR. DOS&R in ORGANIC CHEMISTRY TUT

22. Obtained by identity operation Doing nothing or leave it alone operation Cartesian Coordinates & Symmetry Elelments For i= Centre of gravity must coincide with the origin of the Cartesian Coordinates The principle rotational axis is taken as Z axis. If molecule is planar if the Z axis lies in the plane then X axis is taken as perpendicula to the Plane and Y axis is along the plane KPR. DOS&R in ORGANIC CHEMISTRY TUT n=(360 / θ ) θ = 0 or 360

23. KPR. DOS&R in ORGANIC CHEMISTRY TUT

24. KPR. DOS&R in ORGANIC CHEMISTRY TUT Notation (Schoen Flies) Order of Point Group (h) Example (Only Organic Molecules) C 1 1 Hexaheline C s 2 Phenol, Benzopyrene C i 2 1,1 2,2 dichloro dibromo ethane C 2 2 Hexahelicine C 3 3 [3]- Rotane C 4 4 [4]- Rotane C 2v 4 1.2 Dichloro Benzene, Pyridine C 3v 6 1,1,1 trichloro ethane C 2h 4 Trans 1,2 dichloroethene , Glyoxime D 2 4 Skew Ethlyne , Skew Biphenyl D 3 6 Gauche Ethane

25. KPR. DOS&R in ORGANIC CHEMISTRY TUT Notation (Schoen Flies) Order of Point Group (h) Example (Only Organic Molecules) D 2h 8 Ethene , Napthalene , Anthracene D 3h 12 Borazole D 5h 20 Cyclopentadiene D 6h 24 Benzene, Chromocene D 2d 8 Allene D 3d 12 Staggered Ethane D 3d 20 Ferrocene S n 4 Tri Methoxy silane

26. KPR. DOS&R in ORGANIC CHEMISTRY TUT

27. KPR. DOS&R in ORGANIC CHEMISTRY TUT

28. A molecule can have only one mirror image. If the image is superposable on the original, the molecule is called achiral. If it is not superposable , the molecule and its mirror image form two distinct species called enantiomers giving rise to a type of stereoisomerism known as enantiomersim . Such molecules are called chiral and the two enantiomers are said to differ in their chirality. Chirality is necessary and sufficient condition for the occurrence of enantiomersim and is determined by absence of improper rotational axis of symmetry ( S n axis of any order) in the molecule. KPR. DOS&R in ORGANIC CHEMISTRY TUT

29. All the molecules belonging to the point groups C 1, C n , and D n lack reflection symmetry and are chiral while molecules belonging to the rest of the point group are achiral. KPR. DOS&R in ORGANIC CHEMISTRY TUT

30. KPR. DOS&R in ORGANIC CHEMISTRY TUT

31. Symmetry Element Symbol Symmetry Operation Rotational axis or n-fold axis of Symmetry C n Rotate about an axis by 360 / θ Plane of symmetry σ Reflection in the Plane Improper rotational axis or n-fold alternating axis of symmetry S n Rotate about an axis by 360 / θ and reflect it in a place perpendicular to rotationa axis Inversion Centre or Centre of Symmetry i Invert it through the centre of symmetry or gravity Identical Element E Leave the molecule unchanged Four symmetry operations namely, rotation, reflection, inversion and rotation reflection are described through which similar atoms and groups in a molecule can be interchanged to give indistinguishable from the original . By classifying the molecules bases on their symmetry elements we could tell whether they are chiral or achiral KPR. DOS&R in ORGANIC CHEMISTRY TUT

32. Biphenyls- Adamentanes - KPR. DOS&R in ORGANIC CHEMISTRY TUT

33. Ansa compounds- Cyclophanes - KPR. DOS&R in ORGANIC CHEMISTRY TUT

34. Trans cyclooctene - Catenanes and Rotanes - KPR. DOS&R in ORGANIC CHEMISTRY TUT