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ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB)

ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB) . CH21 PS CLASS. Preparation of Organohalides. From ALKENES C=C [just review old lessons] FOR TERTIARY ALCOHOLS, we can simply use H-X (gas) X= Cl,Br in ether, 0°C . Preparation of Organohalides.

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ORGANOHALIDES + Nucleophilic Reactions (S N 1/2, E1/E2/E1cB)

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  1. ORGANOHALIDES + Nucleophilic Reactions (SN1/2, E1/E2/E1cB) CH21 PS CLASS

  2. Preparation of Organohalides • From ALKENES C=C [just review old lessons] • FOR TERTIARY ALCOHOLS, we can simply use H-X (gas) X=Cl,Br in ether, 0°C

  3. Preparation of Organohalides • FOR TERTIARY ALCOHOLS, we can simply use H-X (gas) X=Cl,Br in ether, 0°C • Follows SN1 so a carbocation is formed, • be careful with rearrangements!

  4. Preparation of Organohalides • FOR PRIMARY/SECONDARY ALCOHOLS: SOCl2 / PBr3

  5. Practice

  6. Alkyl Fluorides • Also from ALCOHOLS + • HF / Pryidine • (CH3CH2)2NSF3

  7. Grignard Reagents • Reaction of R-X with Mg over ether/THF to form R-Mg-X organometallic compound.

  8. Grignard Reagents: reduction of R-X

  9. More samples:

  10. Nucleophilic Reactions • R-X, alkyl halides are ELECTROPHILES (positive or electron-poor) • They react with NUCLEOPHILES/BASES (negative or electron-rich) • Either substitution • C-C-X becomes C-C-blah + X- • or elimination reactions • C-C-X becomes C=C + X-

  11. SUBSTITUTION REACTIONS • S – substitution: R-X + Nu  R-Nu + X- • N – Nucleophilic • 1 or 2  unimolecular or bimolecular rates • INVERSION (change of stereochemistry) CAN HAPPEN!

  12. Try this first…

  13. SN2  BIMOLECULAR • Bimolecular simply refers to the rate depending on BOTH reactants because of the nature of the mechanism • Rate = k[RX][Nu] • Rate depends on both because there is ONE SINGLE COLLISION BETWEEN RX and Nu to form a Nu-R-X transition state

  14. SN2  BIMOLECULAR SUBSTRATE 100% INVERSION OF STEREOCHEMISTRY OCCURS! LEAVING GROUP

  15. Factors that affect SN2 RXNS: • STERIC EFFECTS TO INCOMING Nu: • C=C-X (vinylic) and Ar-X (aryl) TOTALL UNREACTIVE

  16. Factors that affect SN2 RXNS: • THE NUCLEOPHILE

  17. Factors that affect SN2 RXNS: • THE LEAVING GROUP should be stable on its own as a free anion • Comparing halides, we go down the column

  18. Factors that affect SN2 RXNS: • Alcohols and fluorides usually do not undergo SN2 because OH- and F- aren’t good leaving groups • This is why we use SOCl2 and PBr3 … THEY CONVERT THE –OH INTO A BETTER LEAVING GROUP

  19. Factors that affect SN2 RXNS: • Reaction SOLVENT can also affect the reaction. • We prefer POLAR APROTIC SOLVENTS • POLAR but no –OH or –NH in the molecule (no H2O, NH3, etc…) • Polar protic solvents form a CAGE around Nu

  20. Practice

  21. Practice

  22. Practice

  23. Practice

  24. SN1  UNIMOLECULAR • Unimolecular: rate depends only on the substrate (mechanism), almost opposite of SN2 • Rate = k[RX] • Rate is only dependent on the slowest step which is the spontaneous dissociation of your leaving group. (molecules just don’t easily dissociate!)

  25. SN1  UNIMOLECULAR

  26. SN1  UNIMOLECULAR

  27. SN1  UNIMOLECULAR STEREOCHEM IS LOST, A RACEMATE FORM IS MADE, but usually not 50:50

  28. SN1  UNIMOLECULAR STEREOCHEM IS LOST, A RACEMATE FORM IS MADE, but usually not 50:50 An ION PAIR BLOCKS THE OTHER SIDE!

  29. Factors that affect SN1 RXNS: • SUBSTRATE:

  30. Factors that affect SN1 RXNS: • LEAVING GROUP: An –OH in acidic medium can become –OH2+ and leave as H2O which is very favorable

  31. Factors that affect SN1 RXNS: • NUCLEOPHILE: no effect, almost at all.

  32. Factors that affect SN1 RXNS: • SOLVENT: rates increase if you stabilize carbocation transition state. • POLAR PROTIC!

  33. Factors that affect SN1 RXNS: • SOLVENT: rates increase if you stabilize carbocation transition state. • POLAR PROTIC!

  34. PRACTICE

  35. PRACTICE

  36. PRACTICE

  37. PRACTICE

  38. PRACTICE

  39. PRACTICE

  40. PRACTICE

  41. Elimination Reactions • More compliated (different mechanisms) • The loss of H-X can lead to a MIXTURE of alkene products (C-C-X  C=C + HX) • But we can predict the most stable/major poduct • ZAITZEV’S RULE: base-induced eliminations will form more stable alkene

  42. E2 elimination • Again, bimolecular so a single collision between your Base B: and the alkyl halide.

  43. E2 elimination • Anti-periplanar is favored for transition state

  44. E2 elimination • Anti-periplanar is favored for transition state

  45. Practice

  46. Practice

  47. Practice

  48. Practice

  49. E1 reaction • Unimolecular, ALSO spontaneously forms carbocation, but then followed by loss of H+ (taken by a base B: and not an attack by Nu:) • COMPETES WITH SN1 reactions!

  50. E1 reaction

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