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Characteristics of s and p Bonds

The following three slides leads students to realize that free rotation around C=C bonds does not occur at room temperature. Characteristics of s and p Bonds. s bonds p bonds s bonds formed from end to end p bonds formed from side interaction to side interaction

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Characteristics of s and p Bonds

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  1. The following three slides leads students to realize that free rotation around C=C bonds does not occur at room temperature.

  2. Characteristics of s and p Bonds s bonds p bonds s bonds formed from end to end p bonds formed from side interaction to side interaction ______ orbital overlap ______ orbital overlap • more, less b. less, more c. same ______ bond ______ bond ______ reactive ______ reactive

  3. s bonds p bonds • s bonds formed from end to end p bonds formed from side interaction to side interaction • ______ orbital overlap ______ orbital overlap • more, less b. less, more c. same • stronger__ bond 80 – 90 kcal/mole weaker_ bond 66 kcal/mole • less__ reactivemore__ reactive

  4. Characteristics of s and p Bonds cylindrically symmetrical not cylindrically symmetrical ~ > 3-5 kcal/mole p orbitals must be in same required for rotation plane for  bond. What happens to the bond if the orbitals don't overlap?  a. Nothing b. bond becomes stronger c. bond breaks “Free" rotation How much energy is required occurs at room temp. for rotation?   Will free rotation occur at room temperature? a. no b. yes c. can’t predict

  5. Alkynes • Which has the greatest overall bond strength in the 3 structures? a. C-C bond of ethane b. C=C of ethylene c. CC of acetylene d. no difference

  6. Which compound will form preferentially? A or B

  7. Acid-Base Reactions When an alkene reacts wit H-X, this is also a Lewis Acid-Base reaction. Which is the Lewis acid and which is the Lewis base? • alkene = Lewis acid, H-X Lewis base • alkene = Lewis base, H-X = Lewis acid

  8. The next two slides explains the exothermicity of additions across p bonds.

  9. Overall Reaction: Favorable or Not • In this reaction, 1 s bond and 1 p bond is being replaced with 2 s bonds.

  10. Overall Reaction: Favorable or Not What would you expect to be stronger 1 s bond and 1 p bond or 2 s bonds? Would you expect the reaction to be favorable or unfavorable then? • 1 s bond and 1 p bond and the reaction would be favorable • 1 s bond and 1 p bond and the reaction would be unfavorable • 2 s bonds and the reaction would be favorable • 2 s bonds and the reaction would be unfavorable

  11. Forward Reaction : elimination of H-Br from bromoalkane to give alkene • Reverse Reaction : What is happening in the reverse reaction? a. SN2 reaction b. SN1 reaction c. E1 reaction d. E2 reaction e. Addition reaction

  12. Electron density is easily donated. Why? a. p bond weaker than s bond b. p electrons more accessible c. Both a and b d. Electrons on atom with high electronegativity

  13. Which of the following could initiate the cationic polymerization of styrene?

  14. Enter the #’s of the reagents in the proper sequence that will synthesize 1-methylcylcohexanol from 1-methylcyclohexene

  15. Enter the #’s of the reagents in the proper sequence that will synthesize 1-methylcylcohexanol from (bromomethyl)cyclohexane

  16. The next 3 slides compares the mechanism in a biological hydration to the recently covered acid catalyzed hydration.

  17. Citric Acid Cycle • aconitase : enzyme (m.w.  89000)

  18. Hydration of cis-Aconitateto Isocitrate

  19. Hydration of cis-Aconitate to Isocitrate After 180 Flip Is this the same mechanism as that shown for the addition of water to 2-methypropene using sulfuric acid as a catalyst? a. Yes b. No c. Can’t be distinguished

  20. The next 2 slides are part of one question.

  21. An Example of a Biological Electrophilic Addition • Biosynthesis of a-Terpineol (found in pine oil)

  22. Which of the following in not a step in the mechanism of the reaction?

  23. Hydroboration • Which way would the B-H bond prefer to add if steric hindrance wasn’t a factor?

  24. Which structures appears to have less electron repulsion between the atoms?

  25. What would be the structure of the polymer formed from cationic polymerization of 2-methylpropene?

  26. Which of the following is a step in the mechanism for the following reaction if it is acid catalyzed?

  27. Which of the following is a major product of the following reaction used in carbohydrate synthesis?

  28. Predict the major stable product of the following reaction.

  29. The next 5 slides asks students to explain experimental results after they have already learned something about electrophilic addition.

  30. Bottle contains bromine in water • Left test tube contains cyclohexane • Right test tube contains cyclohexene

  31. Bromine/H2O is added to each test tube

  32. After Shaking

  33. A Chemical Reaction Occurred • Which compound reacted? • Cyclohexane b. Cyclohexene • What type of bond most likely was involved in the reaction? a. C-H s bond b. C-C s bond c. p bond

  34. What might be a product of the reaction?(H2O is a complicating factor covered later)

  35. Mechanism of X2 Addition to Alkenes • In the first step, what is the reactive intermediate formed called? a. cyclic halide ion b. cyclic halidium ion c. cyclic carbocation d. cyclic halonium ion e. cyclic halocarbocation

  36. Mechanism of X2 Addition to Alkenes • What reaction does the second step look like? a. E2 b. Addition of HX c. SN2 d. SN1 • E1 (reaction of bromide anion with cyclic halonium ion)

  37. Mechanism of X2 Addition to Alkenes • Why does only the trans product form? a. The reactive intermediate is resonance stabilized. b. The reactive intermediate is stabilized by the alkyl groups. c. SN2 reactions require backside displacement. d. The trans product is more stable. e. The bromide ion is too unreactive.

  38. Predict the major product of the following reaction.

  39. Catalytic Hydrogenation of Alkenes Does this appear to be a syn or anti addition? a. syn b. anti c. neither

  40. Carbenes • Based on this structure, would you expect a carbene to be an electrophile or nucleophile? a. nucleophile b. electrophile c. neither d. both

  41. Oxidation of Alkenes to Diols • How could the trans diol be synthesized from cyclohexene?

  42. Predict the major stable product(s) of the following reaction.

  43. Predict the major stable product(s) of the following reaction.

  44. How could the oxirane be synthesized from but-2-yne?

  45. In the biosynthesis of fatty acids, the crotonyl ACP is converted to butyryl ACP. What type of reaction does it formally appear to be? a. dehydration b. hydration c. alkyl shift d. hydrogenation e. hydride shift

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