1. Structural isomers Structural isomers (constitutional isomers): Compounds with the same molecular formulas but different arrangements of the atoms. Example: Draw the structural isomers for C 4 H 10 2401

2. CH 3 CH 2 CH 2 CH 3 butane 2402

3. CH 3 CH 2 CH 2 CH 3 butane CH 3 CHCH 3 2-methylpropane CH 3 (the 2 is redundant in this name) 2403

4. Example: Draw the structural isomers for C 5 H 12 2404

5. Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane 2405

6. Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane CH 3 CH 2 CHCH 3 2-methylbutane CH 3 (2 is redundant) 2406

7. Example: Draw the structural isomers for C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 pentane CH 3 CH 2 CHCH 3 2-methylbutane CH 3 (2 is redundant) CH 3 CH 3 CCH 3 2,2-dimethylpropane CH 3 (each 2 is redundant) 2407

8. 2408

9. 2409

10. Example: Draw the structural isomers for C 2 H 6 O 2410

11. Example: Draw the structural isomers for C 2 H 6 O CH 3 CH 2 OH ethanol 2411

12. Example: Draw the structural isomers for C 2 H 6 O CH 3 CH 2 OH ethanol CH 3 OCH 3 methoxymethane (dimethyl ether) 2412

13. Exercise: Draw and name all the structural isomers for C 6 H 14 (Answer there are 5). 2413

14. Exercise: Draw and name all the structural isomers for C 6 H 14 (Answer there are 5). The number of structural isomers increases significantly as the number of carbon atoms increases. For example, C 20 H 42 has 366,319 isomers. 2414

15. Number of carbons Number of isomers for alkanes 1 1 2 1 3 1 4 2 5 3 6 5 7 9 8 18 9 35 10 75 20 366,319 30 4,111,846,763 40 62,491,178,805,831 2415

16. Stereoisomerism 2416

17. Stereoisomerism Stereoisomerism: Isomers having the same molecular formula and the same atom-to- atom bonding, but the atoms differ in their arrangement in space. 2417

18. Stereoisomerism Stereoisomerism: Isomers having the same molecular formula and the same atom-to- atom bonding, but the atoms differ in their arrangement in space. Geometric isomers: Isomers having the same atom-to-atom bonding, but the atoms differ in their arrangement in space. 2418

19. Examples: The trans and cis isomers of 1,2-dichloroethene. 2419

20. Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene 2420

21. Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene cis-1,2-dichloroethene 2421

22. Examples: The trans and cis isomers of 1,2-dichloroethene. trans-1,2-dichloroethene (b.p. 48 o C, m.p. -50 o C) cis-1,2-dichloroethene (b.p. 60 o C, m.p. -80 o C) 2422

23. An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer 2423

24. An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer common name: cisplatin 2424

25. An example from inorganic chemistry. NH 3 Cl NH 3 Cl Pt Pt NH 3 Cl Cl NH 3 cis isomer trans isomer common name: cisplatin Only the cis isomer is an effective chemotherapy agent. 2425

26. Optical Isomers - Chirality 2426

27. Optical Isomers - Chirality Polarized Light: Plane polarized light consists of electromagnetic waves with the electric component vibrating in one direction. 2427

28. 2428

29. Optical Isomer: An isomer that causes rotation of the plane of polarization of light when passed through the substance. 2429

30. 2430

31. Chiral (sounds like ki ral): An object that cannot be superimposed on its mirror image is called chiral. 2431

32. 2432

33. 2433

34. mirror plane 2434

35. mirror plane Can superimpose these two molecules; trichloromethane is achiral. 2435

36. mirror plane 2436

37. mirror plane Cannot superimpose these two molecules; bromochlorofluoromethane is chiral. 2437

38. Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. 2438

39. Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. The simplest case is a tetrahedral carbon bonded to four different groups. 2439

40. Enantiomers: A chiral molecule and its non- superimposable mirror image are called enantiomers. The simplest case is a tetrahedral carbon bonded to four different groups. Chiral molecules lack molecular symmetry. 2440

41. 2441

42. Lactic acid has optical isomers. 2442

43. Naming chiral centers The R, S convention 2443

44. Naming chiral centers The R, S convention R, S system. A system for specifying the absolute configuration of a chiral center was developed by Cahn, Ingold, and Prelog and is named after them. It is also referred to as the R, S system. 2444

45. Naming chiral centers The R, S convention R, S system. A system for specifying the absolute configuration of a chiral center was developed by Cahn, Ingold, and Prelog and is named after them. It is also referred to as the R, S system. The orientation of the atoms/groups attached to a chiral center are assigned using a set of priority rules. In simplified form the rules are as follows. 2445

46. Priority rules 1. Each atom bonded to the chiral center is assigned a priority number on the basis of increasing atomic number, e.g. 2446

47. Priority rules 1. Each atom bonded to the chiral center is assigned a priority number on the basis of increasing atomic number, e.g. increasing priority H CH 3 NH 2 OH F Cl 1 6 7 8 9 17 2447

48. 2. If the priority cannot be assigned on the basis of the first atom in the group attached to the chiral center, examine the next atom in the group, and so on, e. g. increasing priority CH 2 H CH 2 CH 3 CH 2 OH 1 6 8 2448

49. 2. If the priority cannot be assigned on the basis of the first atom in the group attached to the chiral center, examine the next atom in the group, and so on, e. g. increasing priority CH 2 H CH 2 CH 3 CH 2 OH 1 6 8 3. Note that priority is assigned on the basis of the first point of difference, not on the size of the group attached. For example, CH 2 Cl has higher priority than CH 2 CH 2 CH 2 CH 2 CH 3. 2449

50. Assigning R and S to a chiral center 1. For each of the four atoms/groups attached to the chiral center, assign a priority order from highest (1) to lowest (4) using the priority order from the previous slides. 2450

51. Assigning R and S to a chiral center 1. For each of the four atoms/groups attached to the chiral center, assign a priority order from highest (1) to lowest (4) using the priority order from the previous slides. 2. Orient the molecule so that the atom/group with lowest priority (4) is directed away from you, and the three other atoms/groups project towards you. 2451

52. Assigning R and S to a chiral center 3. If the three atoms/groups facing you have a priority assignment order that increases clockwise the configuration of the chiral center is assigned as R. If the assignment of priority order increases anticlockwise, the configuration of the chiral center is assigned as S. 2452

53. Assigning R and S to a chiral center 3. If the three atoms/groups facing you have a priority assignment order that increases clockwise the configuration of the chiral center is assigned as R. If the assignment of priority order increases anticlockwise, the configuration of the chiral center is assigned as S. R S 2453 4 2 1 3 4 2 1 3

54. Example: Give the name of the following 2454

55. Example: Give the name of the following increasing priority H F Cl Br 1 9 17 35 Priority order: 4 3 2 1 2455

56. The three top priority atoms attached to the chiral center go in clockwise order, so the name of the compound is: (R)-bromochlorofluoromethane 2456 H (4) Br (1) Cl (2) (3) F

57. Example: Give the name of the following CH 3 2457

58. Example: Give the name of the following CH 3 increasing priority H C O F 1 6 8 9 Priority order: 4 3 2 1 2458

59. The three top priority atoms attached to the chiral center go in anticlockwise order, so the name of the compound is: 2459 H (4) OH (2) (1) F CH 3 (3)

60. The three top priority atoms attached to the chiral center go in anticlockwise order, so the name of the compound is: (S)-1-fluoroethanol 2460 H (4) OH (2) (1) F CH 3 (3)