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CHAPTER 7 Haloalkanes

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  1. CHAPTER 7 Haloalkanes

  2. Haloalkane (alkyl halide):a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX

  3. Haloalkanes An alkane in which one or more H atoms is replaced with a halogen (F, Cl, Br, or I) CH3Br 1-bromomethane Br (methyl bromide) CH3CH2CHCH3 2-bromobutane Cl chlorocyclobutane Timberlake LecturePLUS 1999

  4. Nomenclature - IUPAC • locate the parent alkane • number the parent chain to give the substituent encountered first the lower number • show halogen substituents by the prefixes fluoro-, chloro-, bromo-, and iodo- and list them in alphabetical order with other substituents • locate each halogen on the parent chain

  5. Nomenclature • examples • Common names: name the alkyl group followed by the name of the halide

  6. Nomenclature • several polyhaloalkanes are common solvents and are generally referred to by their common or trivial names • hydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes

  7. Name the following: Timberlake LecturePLUS 1999

  8. Name the following: bromocyclopentane 1,3-dichlorocyclohexane Timberlake LecturePLUS 1999

  9. SN2 – Substitution Nucleophilic, Bimolecular • This is called a concerted reaction • Meaning that the bond breaking and the bond forming occur simultaneously • Is classified as bimolecular • Because both the haloalkane and the nucleophile are involved in the rate determining step. • S = substitution • N = nucleophilic • 2 = bimolecular (two species are involved in the rate-determining step)

  10. Recall • Nucleophile(nucleus loving): An electron rich species that seeks a region of low electron density (Nu). • Electrophile (electron loving): A low electron-density species that seeks a region of high electron density.

  11. SN2 – Substitution Nucleophilic, Bimolecular The nucleophile attacks the reactive center from the side opposite the leaving group; in other words it involves a backside attack by the nucleophile.

  12. SN2 • both reactants are involved in the transition state of the rate-determining step • the nucleophile attacks the reactive center from the side opposite the leaving group

  13. Press the slide show button to see the animation. Press ESC to finish. ENERGY PROFILE SN2 ANIMATION R .. C Cl: .. H CH3

  14. .. :Br: .. ENERGY PROFILE SN2 ANIMATION R .. C Cl: .. H CH3

  15. .. :Br: .. ENERGY PROFILE SN2 ANIMATION R .. C Cl: .. H CH3

  16. .. :Br: .. ENERGY PROFILE SN2 ANIMATION R .. C Cl: .. H CH3

  17. .. :Br: .. ENERGY PROFILE SN2 ANIMATION R .. C Cl: .. H CH3

  18. .. .. :Br Cl: .. .. ENERGY PROFILE SN2 ANIMATION R Transition State d- d- C H CH3 Activated Complex

  19. .. .. :Br :Cl: .. .. ENERGY PROFILE SN2 ANIMATION R C H CH3

  20. .. .. :Br :Cl: .. .. ENERGY PROFILE SN2 ANIMATION R C H CH3

  21. .. .. :Br :Cl: .. .. ENERGY PROFILE SN2 ANIMATION R C H CH3

  22. .. :Br .. ENERGY PROFILE SN2 ANIMATION R C H CH3

  23. SN1 – Substitution Nucleophilic, Unimolecular • S = substitution • N = nucleophilic • 1 = unimolecular (only one species is involved in the rate-determining step)

  24. SN1 – Substitution Nucleophilic, Unimolecular • In this reaction the bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile begins • This is classified as unimolecular • Only the haloalkane is involved in the rate-determining step • In other words, only the haloalkane contributes to the rate law governing the rate determining step

  25. SN1 • Step 1: ionization of the C-X bond gives a carbocation intermediate

  26. SN1 • Step 2: reaction of the carbocation (an electrophile, low electron density) with methanol (a nucleophile, high electron density) gives an oxonium ion • Step 3: proton transfer completes the reaction

  27. SN1 • For an SN1 reaction at a stereocenter, the product is a racemic mixture

  28. SN1 • the nucleophile attacks with equal probability from either face of the planar carbocation intermediate

  29. SN2 and SN1 • Are competing constantly, what determines what mechanism is a reaction going to prefer? 1.The structure of the nucleophile 2.The structure of the haloalkane 3.The leaving group 4.The solvent

  30. 1. The structure of the nuceophile • Refer to table 7.2 page 228 from your book to see the types of nucleophiles we deal with most commonly in this semester. • Nucleophilicity: a kinetic property measured by the rate at which a Nu attacks

  31. Nucleophilicity • Table 7.2

  32. 2. Structure of the Haloalkane • SN1 reactions • governed by electronic factors, namely the relative stabilities of carbocation intermediates • relative rates: 3° > 2° > 1° > methyl • SN2 reactions • governed by steric factors, namely the relative ease of approach of the nucleophile to the site of reaction • relative rates: methyl > 1° > 2° > 3°

  33. SN1 • SN1 will be favored if a tertirarycarbocation is involved, sometimes if a secondary carbocation is involved • SN1 will never be favored if a primary cabocation or methyl are involved

  34. SN2 • The less crowded site will always favor the SN2 mechanism • Will be favored if it involves a primary carbocation and methyl • Sometimes will be favored if a secondary carbocation is involved

  35. 3.Leaving group • Chlorine ion, bromine ion and Iodine ion make good leaving groups because of their size and Electronegativity help to stabilize the resulting negative charge • The ability of a group to function as a leaving group is related to how stable is as an anion • The most stable anion and the best leaving groups are the conjugate bases of strong acids!!!

  36. 4. The Solvent • Protic solvent: a solvent that contains an -OH group • these solvents favor SN1 reactions; the greater the polarity of the solvent, the easier it is to form carbocations in it

  37. 4. The Solvent • Aproticsolvent:does not contain an -OH group • it is more difficult to form carbocations in aprotic solvents • aprotic solvents favor SN2 reactions

  38. Summary of SN1 and SN2

  39. Elimination reactions • Dehydrohalogenation • These reaction require forcing conditions like a strong base and heat. • (Hydroxide ion or ethoxide ion) • Halogen is removed from one carbon of a haloalkane • And the hydrogen from the adjacent carbon • To form a double bond • (an alkene)

  40. b-Elimination • Zaitsev rule: the major product of a -elimination is the more stable (the more highly substituted) alkene

  41. E1 and E2 mechanisms • There are both examples of beta-elimination reactions • The difference is the timing of the bond-breaking and the bond-forming steps. • E1 stands for elimination and 1 for unimolecular • E2 stands for elimination and 2 for bimolecular

  42. E1 • The breaking for the halogen carbon bond has to be completely broken before any reaction occurs with the base • This is the slow determining step (the breaking of the halogen carbon bond)

  43. E1 Mechanism • Step 1: The breaking for the halogen carbon bond gives a carbocation intermediate • Step 2: proton transfer from the carbocation intermediate to a base (in this case, the solvent) gives the alkene

  44. E2 • The base removes a beta hydrogen at the same time that carbon halogen bond is broken • The rate of the reaction will depend both on the haloalkane and the base • The stronger the base the more likely it is that the E2 mechanism will be in operation

  45. E2 Mechanism • A one-step mechanism; all bond-breaking and bond-forming steps are concerted

  46. Table 7.6 • E2 is favored if you are dealing with a primary haloalkane • E2 is favored for secondary haloalkane if you have a really strong base • E1 is favored for secondary haloalkane if you have weak bases • E1 is favored for tertiary haloalkanes.