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Chapter 9

Chapter 9. Substitution and Elimination Reactions of Alkyl Halides. Alkyl halides are classified as primary ( 1 ° ), secondary ( 2 ° ), or tertiary ( 3 ° ), depending on the number of carbons bonded to the carbon with the halogen atom.

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Chapter 9

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  1. Chapter 9 Substitution and Elimination Reactions of Alkyl Halides

  2. Alkyl halides are classified asprimary(1°), secondary(2°), or tertiary (3°), depending on the number of carbons bonded to the carbon with the halogen atom. • The halogen atom in halides is often denoted by the symbol “X”.

  3. Nomenclature of Alkyl Halides

  4. Name these alkyl halides.

  5. Name the alkyl halides.

  6. Interesting Alkyl Halides

  7. Alkyl halides are weak polar molecules.

  8. Reactions of Alkyl Halides

  9. General Features of Nucleophilic Substitution Three components are necessary in any substitution reaction.

  10. Negatively charged nucleophiles like HO¯ and HS¯ are used. • When a neutral nucleophile is used, the substitution product bears a positive charge.

  11. To draw any nucleophilic substitution product: • Find the sp3 hybridized carbon with the leaving group. • Identify the nucleophile, the species with a lone pair or  bond. • Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom that is involved in bond breaking or bond formation.

  12. Try this similar problem - Page 260 Problem 30

  13. What alkyl halides don’t react?

  14. SN2 Reaction - 80% ethanol B r C H O H + C H B r + N a O H 3 3 20% water 55oC rate = k2[RBr][NaOH] - H O 80% ethanol - C H B r B r C H O H + 3 3 20% water Rate dependence of the reaction is interpreted in a way that we expect a bimolecular reaction with a concerted mechanism substitution nucleophilic SN2 bimolecular

  15. ENERGY PROFILE Mechanism E N E R G Y

  16. ENERGY PROFILE Mechanism E N E R G Y

  17. ENERGY PROFILE Mechanism E N E R G Y

  18. ENERGY PROFILE Mechanism E N E R G Y

  19. ENERGY PROFILE Mechanism E N E R G Y

  20. ENERGY PROFILE Mechanism E N E R G Y

  21. ENERGY PROFILE Mechanism E N E R G Y

  22. Energy Profile of SN2

  23. Stereochemistry of SN2 Reactions

  24. Effect of Alkyl Groups on SN2 Reactions

  25. Try a similar problem - Page 260 Problem 29, 34 Page 256 Problem 28

  26. SN1 Mechanism C H C H 3 3 - 80% ethanol N a O H B r H C C B r + H C C O H + 3 3 55oC 20% water C H C H 3 3 rate = k1[RBr] Rate depends only on substrate concentration. Two independent steps that differ significantly in speed. Unimolecular substitution SN1 nucleophilic unimolecular

  27. SN1 Mechanism of SN1 Reactions Key features of the SN1 mechanism are that it has twosteps, and carbocations are formed as reactive intermediates. C H C H 3 3 - 80% ethanol N a O H B r H C C B r + H C C O H + 3 3 20% water C H C H 3 3 slow O H fast C H 3 - + H C C B r + 3 C H 3

  28. SN1

  29. SN1 Bond gets longer

  30. SN1 Bond gets longer and longer Rehybridization sp3  sp2

  31. SN1 Bond gets longer and longer Rehybridization sp3  sp2

  32. SN1 Bond gets longer and longer Rehybridization sp3  sp2 sp2-Hybridized intermediate formed

  33. SN1 Bond gets longer and longer Rehybridization sp3  sp2 sp2-Hybridized intermediate formed Nucleophile approaches Rehybridization sp3  sp2 takes place

  34. SN1 Bond gets longer and longer Rehybridization sp3  sp2 sp2-Hybridized intermediate formed Nucleophile approaches Rehybridization sp3  sp2 takes place

  35. SN1 Bond gets longer and longer Rehybridization sp3  sp2 sp2-Hybridized intermediate formed Nucleophile approaches Rehybridization sp3  sp2 takes place Bond forms

  36. SN1 Bond gets longer and longer Rehybridization sp3  sp2 sp2-Hybridized intermediate formed Nucleophile approaches Rehybridization sp3  sp2 takes place Bond forms

  37. Energy Profile of SN1 carbocation intermediate transition state 1 transition state E 2 N E activation energy2 2 R activation energy1 G Y step 1 step 2 starting D H material product REACTION COORDINATE

  38. Stereochemistry of SN1 Reactions

  39. Effect of Alkyl Groups on SN1 Reactions • This trend is exactly opposite to that observed in SN2 reactions.

  40. Elimination reactions

  41. E2 Reaction THIS REACTION IS KNOWN AS b-ELIMINATION

  42. Regioselectivity of E2 Reactions WHAT HAPPENS IF THERE IS MORE THAN ONE b-HYDROGEN?

  43. Relative Stability of Alkenes

  44. The Zaitsev (Saytzeff) Rule • When alkyl halides have two or more different  carbons, more than one alkene product can be formed. • The Zaitsevrule predicts that the major product in  elimination has the more substituted double bond. This is the more stable alkene.

  45. Stereoselectivity of E2 Reactions • The E2 reaction is stereoselective because the more stable stereoisomer is formed preferentially.

  46. Try this similar problem - Page 260 Problem 33 Page 247 Problem 15 (Solve #17 the same time you complete #15) Page 248 Problem 17 Page 256 Problem 27 Page 262 Problem 49, Problem 55

  47. The E1 Elimination Reaction (two steps) weak base carbocation B : H H slow :X C C + C C + step one X 3o > 2o > 1o favored if a stabilized carbocation is formed step two unimolecular fast rate = k[RX] C C

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