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Alkenes II 1. Types of Rxn.: elimination, addition, substitution

Chapter 6. Alkenes II 1. Types of Rxn.: elimination, addition, substitution 2. Mechanisms: How rxn’s of molecules occur. 3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective, Inductive effects, most stable R + , R 3 C + > R 2 HC + > RH 2 C + > H 3 C +

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Alkenes II 1. Types of Rxn.: elimination, addition, substitution

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  1. Chapter 6 Alkenes II 1. Types of Rxn.: elimination, addition, substitution 2. Mechanisms: How rxn’s of molecules occur. 3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective, Inductive effects, most stable R+, R3C+ > R2HC+ > RH2C+ > H3C+ B. Hydration H+/H2O C. Rearrangements D. Bromonium ions, (anti addition) E. Halohydrin formation, (anti) F. H+ - hydration, Hg+2-hydration - (anti) 4. Hydroboration syn addition 5. Oxidation (syn) 6. Reduction (syn), olefin and diene stability 7. Reactions relative to stereocenters 21

  2. Chapter 6 synthesis reaction types olefin additions mechanisms energy progress of rx specific additions and reactions

  3. Chapter 6 synthesis reaction types olefin additions energy progress of rx specific additions and reactions

  4. Reaction (Rx): A --->B Mechanism: “how” B A Y X Z • Rx mechanism describes: • how rx occurs • which bonds - broken/formed

  5. Addition Reaction: What is a rx? HOW What is a mechanism?

  6. Electrophilic Additions “R” = any alkyl group; i.e. methyl, ethyl, etc. primary 1o secondary 2o tertiary 3o

  7. Three Classes of Reaction: 1. Addition - increase in ‘atoms bonded’ at 2 (adjacent) carbons. 2. Elimination - decrease in ‘atoms bonded’ at 2 (adjacent) atoms. 3 . Substitution - replace 1 group/atom by another. Rxn’s of Alkenes additions to the C=C break 1-π, form 2- bonds (Table 6.1)

  8. Electrophilic Additions  Hydrohalogenation  Hydration Halogenation Halohydrination Oxymercuration Hydroboration Diol formation (oxidation) Hydrogenation etc.

  9. graph showing the changes in energy during a reaction starting materials products E Diagams • potential • energy B energy A reaction coordinate (progress of reaction)

  10. activation energy Ea heat of reaction (energy given off) energy reaction coordinate A-X + Y products A + X-Y starting materials

  11. transition state (t) = an energy maximum “believed” to be formed - unstable species of maximum energy transition state activation energy energy heat of reaction energy absorbed SM C + Z-X products C-Z + X reaction coordinate

  12. PE diagrams intermediates - energy minima betweent#s. intermediate 1st transition state t1 t2 2nd transition state • potential • energy H products SM reaction coordinate

  13. small Ea large Ea PE diagrams large Ea, few collisions, slow reactions smallEa, fast rx t1 t2 • potential • energy heat of reaction products SM reaction coordinate

  14. H H H Cl H Cl H C H C H + C + HCl H H C C H C H CH CH CH 3 3 3 “trace” A. Addition of HCl,HBr, HI MAJOR Markovnikov’s rule - H+ (electrophile) to carbon with most H’s - less substituted carbon “regioselective” (explain next).

  15. E+ (others later) (+) Mechanism of Electrophilic Addition Base is C=Cfunctional group H+ (E+, electrophile) is acids base -system +H carbocation R+

  16. (+) Carbocation trigonal/120o/sp2 Lewis base + Lewis acid +H [ E(+) ]

  17. (+) Carbocation Lewis base + Lewis acid +H

  18. Which intermediate carbocation is easier to form? How Regioselective

  19. Which intermediate carbocation is easier to form? ts1 ts2 PE SM Product(s) Why Regioselective

  20. < H H C H C H < H CH3 1 alkyl gp H-C CH3 1o < H3C CH3 C 2o CH3 3o Alkyl - groups slightlyelectrondonating - stabilize + charge. H H C H no alkyl gps

  21. B. Addition of H2O: Acid-Catalyzed Hydration of Alkenes 1. Catalyst H2SO4 (or H3PO4) 2.H+adds to least substituted end of alkene 3.“ -OH” adds to more substituted end regioselective

  22. H H H acid-base Mechanism: H + O O H H H H H H C H H H C H C H+ transfer H 1. O + H C O + CH H 3 CH H 3 H H H H H H C O H + H -H+ 2. H C C product H O H C CH 3 H CH 3 Hydration - Addition of H2O (~HX addition) Rx:

  23. Slow or “rate determining,” highest Ea’s Selective for 2o R+ over 1o R+ + + + + R3C > R2CH > RCH2>CH3 Why regioselective?

  24. Chapter 6 synthesis hydration rearrangements mercury hydration hydroboration other additions halogenation halohydration

  25. 2o R+ E 3o R+ C. carbocation characteristic - rearrangements major normal and rearranged product major mechanism?

  26. 1,2- H: migration 2o R+ E 3o R+ rearranged normal rx mech.

  27. 2o R+ E 3o R+ Hydration, Rearrangements can occur - R+ major (mechanism)? Rearrangements - problems in the book and a web worksheet

  28. H+ = proton from H2SO4, H3PO4, HClO4, etc. (or H3O+) examples: number of regioisomers/stereoisomers 1 / 0 1 / 0 1 / 4 2 / 8 (4 each) plus RR

  29. Mech.: Hydration, Rearrangements can occur - R+ major

  30. D. Bromination & Chlorination (F2 too reactive, I2 poor to OK) anti addition

  31. Antiaddition mechanism, then stereochemistry Br-Brhas very weak bond

  32. Antiaddition Notice: (1) No stereochemistry (2) No rearrangement

  33. Anti Addition: Stereochemistry trans-diaxial addition - ‘stereospecific’, bromonium ion controls anti addition

  34. Chapter 6 synthesis halohydration mercury hydration hydroboration other additions

  35. HOBr Issues: Why selective for 2o alcohol (vs 1o)? Why no RR (note -H)?

  36. HOBr Why 2o - not 1o?

  37. 1o OH HOBr

  38. HOBr 2oR+ like > 1oR+-like end

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