CHEM 266 Midterm Review Session Nov 1 st , 2011. 7 pm to 9 pm

1. CHEM 266 Midterm Review SessionNov 1st, 2011. 7 pm to 9 pm Tutor: James Lee

2. Outline • Nomenclature • Bonding & Orbitals • Stereochemistry • Acid & Base • SN2 & SN1

3. Alkanes n = 9: nonanen = 10: Decane

4. Alkanes/ Hydrocarbons

5. Alkenes

6. Alkenes

7. Alkynes

8. Alkynes

9. Functional Groups

10. Functional Groups

11. Nonmenclature Priorities

12. Octet Rule

13. Lewis Structures

14. Lewis Structures

15. Formal Charges

16. Common Structures

17. Resonance Rules

18. Writing Resonance Structures

19. VSEPR Theory

20. VSEPR Theory • 4 e- sets: tetrahedral • Formula: AB4 • Bond Angle: 109.5o

21. VSEPR Theory

22. VSEPR Theory • 3 e- sets: trigonal planar • Formula: AB3 • Bond Angle: 120o

23. VSEPR Theory

24. VSEPR Theory • 2 e- sets: linear • Formula: AB2 • Bond Angle: 180o

25. Questions

26. What is the hybridization (sp3, sp2, sp) of a, b, c and d? sp3 sp3 sp2 sp2

27. Chirality Chiral: can NOT superimpose on its mirror image Achiral: can superimpose on its mirror image, has a plane of symmetry or or

28. Assigning Priorities: R or S **Assign HIGHER priority to atom w/ HIGHER atomic #** • Look down lowest priority group • Identify priorities on remaining 3 groups R S • R = clockwise • S = counter clockwise

29. Stereoisomerism Stereoisomer's:same atomic connectivity but with different geometries Enantiomers:Compounds which are not identical but mirror images of each other Examples: Diastereomers:Stereochemistry different at one C only Examples:

30. Sample Questions Identify all the chiral centers in the following molecules and determine how many possible stereoisomers each compound has (Possible number of stereoisomers equals 2n, where n = number of stereocenters, only applies to non-meso compounds) Note that this epoxide has defined stereochemistry None!!! Two undefined stereocenters 24 = 4 stereoisomers (two are meso)! Two defined stereocenters Seven undefined stereocenters 27 = 128 stereoisomers (discounting epoxide)!

31. How many chiral centers in Molecule X? Three

32. Is center “b” R or S? R

33. Assigning Priorities: Cis/Trans, E/Z **Assign HIGHER priority to atom w/ HIGHER atomic #** • Cis/Trans works for: • Simple alkenes (2 or fewer different substituents on a double bond; ONE double bond) • Cyclohexanes and other non-alkene compounds • E/Z works for: • More than 1 double bond • More than 2 different substituents on a double bond

34. Assigning Priorities: Cis/Trans, E/Z • Sample Question: • Assign E/Z or Cis/Trans to the following: E Z Z Cis

35. Newman Projections • Useful for visualizing conformations, torsional strain, and steric strain along one bond. • Torsional strain: associated with eclipsed/gauche conformations. • Steric strain: force that destabilizes a conformation due to hindering interactions among side chains • No torsional • No steric • High torsional • Almost no steric • No torsional • High steric • High torsional • High steric

36. Newman Projections • Related topic: conformational energy diagrams

37. Newman Projections • Which of the following compounds, when twisted a full 360 degrees about the bond shown, would produce an asymmetrical conformational energy diagram?

38. Fischer Projections • Used mostly in visualizing sugars • A simplification of VSEPR • All horizontal lines point out of the page D-Glucose

39. Fischer Projections OR • General Rules: • Most oxidized atom/group at the top • Longest carbon chain written vertically • Can NOT Rotate atoms (because changes stereochemistry) • All vertical lines show bonds going IN to page • All horizontal lines show bonds coming OUT of page

40. Fischer Projections What is the chirality (R or S) at each stereocenter? S R S

41. Fischer Projections • Which is the same as D-glucose? D-Glucose

42. Cyclohexane Conformations • Cyclohexane structures can adopt a maximum of four stable conformations (energy minima); two “chair” conformations, and two “boat” conformations. • Notice that in every one of these conformations, all carbon centers in the ring is tetrahedral. • In general, “chair” conformations are more stable than “boat” conformations.

43. Cyclohexane Conformations • In general, equatorial positions are favored over axial positions by R groups due to less hinderance. • In most cases, the chair conformation with the majority of its R groups on equatorial positions is favored energetically. • Axial positions are useful in antiperiplanar elimination reactions.

44. Cyclohexane Conformations • Which is the most stable conformation of cis-1,3-dibromocyclohexane?

45. Molecular Stability and Energy Stability of a Organic Compound is based on: • Steric Effects • Inductive Effects • Hyperconjugation • Resonance

46. Molecular Stability and Energy • Steric effects: • Physical disturbances, size, position of atoms • Ex: 1,3-diaxial interactions, staggered/eclipsed formation Which one is the more stable isomer?

47. Molecular Stability and Energy • Inductive Effects: • Transmission of charge, dipole moments, donating/withdrawing of electrons • Allows stabilization of charge through sigma bonds (don’t need to know details).

48. Molecular Stability and Energy • Hyperconjugation: • Specifically affects π bonds • Allows stabilization of charge through sp3-p interactions (don’t need to know details). • The more alkyl groups, the more stable the alkene. Trans is more stable than cis due to steric effects

49. Alkene Stability • 2 factors in order of importance: • Substitution • Cis/trans (steric strain) • Increasing alkene stability due to: • Hyperconjugation • Bond strength Trans is more stable than cis

50. Molecular Stability and Energy • Resonance • Delocalization of charge through π bonds • The more resonance structures possible the more stable Draw resonance structures for: