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Chapter 7                                       Lecture                                       Alkyl Halides:              ...
Classes of Alkyl Halides                                          H H        • ___________ halides: H          C C Br     ...
IUPAC Nomenclature                                   4-(2-fluoroethyl)heptane© 2013 Pearson Education, Inc.     Chapter 6 ...
Common Names    CH2X2 is called methylene halide.    CHX3 is a haloform.    CX4 is carbon tetrahalide.    Common halogenat...
Compare                                             classification to                                             alcohols...
Uses of Alkyl Halides          • Industrial and household cleaners.          • Anesthetics:             • CHCl3 used origi...
Dipole Moments depends on           electronegativity and bond length    • Electronegativities of the halides:    • >>>>  ...
Boiling Points and Densities        • Greater intermolecular forces, higher b.p.           • Dipole-dipole attractions not...
Preparation (synthesis) of                         Alkyl Halides          • Free radical halogenation                 • __...
© 2013 Pearson Education, Inc.
Write the mechanism for the free radical                    bromination of propane                                 Chapter...
Synthesis of alkyl halides by                    allylicbromination           • Allylic radicals are resonance stabilized....
Mechanism of allylic bromination• N-Bromosuccinimide (NBS) is an allylic brominating agent.• Reacts close to room temperat...
Major Write mechanism© 2013 Pearson Education, Inc.   Chapter 6                  14
Reactions of Alkyl Halides© 2013 Pearson Education, Inc.   Chapter 6      15
The SN2 Reaction: Substitution                             Nucleophilic Bimolecular    • The___________atomon the alkyl ha...
Back-Side Attack in the SN2 Reaction    SN2 Energy Diagram    a_) Exergonic    b) ____________    c) Transition state rese...
New functional groups synthesized                   with the SN2 Reaction or RX© 2013 Pearson Education, Inc.   Chapter 6 ...
SN2 requires a _______ nucleophile                 Stronger nucleophiles give fast SN2  A negatively charged nucleophile i...
The more polarizable, the                           better Nu            Electrons in a larger atom are not as tightly hel...
The solvent       Dielectric constant: A measure of a solvent’s ability to insulate       opposite charges from one anothe...
SN2 reactions proceed faster in          aprotic solvents that “free” the Nu                                            Po...
Crown ethers solvate the cation, so the                nucleophilic strength of the anion                   increases (“Fr...
A good leaving group is required in                  substitution and elimination                            reactions    ...
Relative rates for SN2 depends on structure of              substrate (RX, electrophile)   Nucleophile approaches from   t...
Stereochemistry of SN2           SN2 reactions will result in an inversion of           configuration also called a Walden...
Write the mechanism for the reaction           or (S)-2-Chlorobutane with iodide ion                                 Chapt...
The SN1 Reaction      • The SN1 reaction is a ______________        nucleophilic substitution.      • _____________It has ...
SN1 Mechanism       Step 1: Formation of the       carbocation (rate-determining step)        Step 2: Attack of the nucleo...
Step 3: If the nucleophile was neutral, a third step                             (deprotonation) will be needed.          ...
Stereochemistry of SN1               enantiotopic phases of the carbocation   • Carbocations are_______hybridized and have...
Stereochemistry of SN1    The SN1 reaction produces mixtures of enantiomers. There    is usually slightly _______inversion...
examples of complete racemization have been observed, but                partial racemization with a slight excess of inve...
Factors that favor SN1 reactions          a) Nature of the electrophile (RX)  b) A better leaving group will increase the ...
Rearrangements can occur       • Carbocations can rearrange to form a more stable         carbocation: 1,2 Hydride shift: ...
Write mechanism for the following reaction© 2013 Pearson Education, Inc.   Chapter 6         36
SN1 or SN2 Mechanism?                                 SN2                    SN1          CH3X > 1º> 2º                   ...
Exercises© 2013 Pearson Education, Inc.
© 2013 Pearson Education, Inc.
© 2013 Pearson Education, Inc.
In general, strong bases are strong                           nucleophiles,            but not all strong nucleophiles are...
Elimination Reactions          • Elimination reactions produce double bonds.          • Also called dehydrohalogenation (-...
The E1 Reaction “Nucleophile” acts as base.      •    __________ elimination.      •    Stepwise. ___________ intermediate...
E1 Mechanism       Step 1: The leaving group leaves to form a ____________          Step 2: Solvent abstracts a proton to ...
In elimination reactions, the major product of                  the reaction is the more stable, more                 subs...
E1 and SN1occur in solvolysis• Substitution results from _________attack on thecarbocation• Ethanol serves as a __________...
The E2 Reaction • ___________________. • Requires a __________ base. • This is a _________________ reaction: The proton   ...
The E2 Reaction  • Reactivity for alkyl halides:     3° > 2 ° > 1°  • Mixture may form, but     Zaitsevproduct     predomi...
E1 or E2 Mechanism?   • Tertiary > secondary             • Tertiary > secondary   • Base strength usually            • Str...
Substitution or Elimination?          • Strength of the nucleophile determines            order: Strong nucleophiles or ba...
20 halides: strong nucleophiles will promote SN2/ Strong bases E2.                       Weak nucleophiles/ bases promote ...
Predict the mechanisms and products of the                    following. reaction© 2013 Pearson Education, Inc.   Chapter ...
Predict the mechanisms and products of the                    following reaction.© 2013 Pearson Education, Inc.   Chapter ...
54© 2013 Pearson Education, Inc.
55© 2013 Pearson Education, Inc.
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    1. 1. Chapter 7 Lecture Alkyl Halides: Nucleophilic Substitution and Elimination © 2013 Pearson Education, Inc.© 2013 Pearson Education, Inc.
    2. 2. Classes of Alkyl Halides H H • ___________ halides: H C C Br Halogen is directly H H bonded to sp3 carbon. • Vinyl halides: Halogen H H is bonded C C H Cl to____carbon of alkene. I • ______halides: Halogen is bonded to sp2 carbon on _______ ring.© 2013 Pearson Education, Inc.Chapter 6 2
    3. 3. IUPAC Nomenclature 4-(2-fluoroethyl)heptane© 2013 Pearson Education, Inc. Chapter 6 3
    4. 4. Common Names CH2X2 is called methylene halide. CHX3 is a haloform. CX4 is carbon tetrahalide. Common halogenated solvents: ________ is methylene chloride. _______ is chloroform. ________is carbon tetrachloride.© 2013 Pearson Education, Inc. Chapter 6 4
    5. 5. Compare classification to alcohols and amines© 2013 Pearson Education, Inc. Chapter 6 5
    6. 6. Uses of Alkyl Halides • Industrial and household cleaners. • Anesthetics: • CHCl3 used originally as general anesthetic but it is toxic and carcinogenic. • CF3CHClBr is a mixed halide sold as Halothane. • Freons are used as refrigerants and foaming agents. • Freons can harm the ozone layer, so they have been replaced by low-boiling hydrocarbons or carbon dioxide. • Pesticides such as DDT are extremely toxic to insects but not as toxic to mammals. • Haloalkanes can not be destroyed by bacteria, so they accumulate in the soil to a level that can be toxic to mammals, especially humans.© 2013 Pearson Education, Inc. Chapter 6 6
    7. 7. Dipole Moments depends on electronegativity and bond length • Electronegativities of the halides: • >>>> • Bond lengths increase as the size of the halogen increases: • >>>> • Bond dipoles: C—Cl> C—F > C—Br > C—I 1.56 D 1.51 D 1.48 D 1.29 D Carbon—halogen bond is polar, carbon has partial _______ charge and can be attacked by ____________© 2013 Pearson Education, Inc. Chapter 6 7
    8. 8. Boiling Points and Densities • Greater intermolecular forces, higher b.p. • Dipole-dipole attractions not significantly different for different halides. • London forces greater for larger atoms. • Greater mass, higher b.p. • Spherical shape decreases b.p. (CH3)3CBr CH3(CH2)3Br 73 C 102 C • Alkyl fluorides and alkyl chlorides (thosewith just one chlorine atom) are less dense than water (1.00 g/mL). • Alkyl chlorides with two or more chlorine atoms are denser than water. • All alkyl bromides and alkyl iodides are denser than water.© 2013 Pearson Education, Inc. Chapter 6 8
    9. 9. Preparation (synthesis) of Alkyl Halides • Free radical halogenation • _________________more selective than __________________ • Free radical allylichalogenation • Halogen is placed on a carbon ________________to the double bond (allylic).© 2013 Pearson Education, Inc. Chapter 6 9
    10. 10. © 2013 Pearson Education, Inc.
    11. 11. Write the mechanism for the free radical bromination of propane Chapter 2 11© 2013 Pearson Education, Inc.
    12. 12. Synthesis of alkyl halides by allylicbromination • Allylic radicals are resonance stabilized. • Bromination occurs with good yield at the allylic position (sp3 C next to C C).© 2013 Pearson Education, Inc. Chapter 6 12
    13. 13. Mechanism of allylic bromination• N-Bromosuccinimide (NBS) is an allylic brominating agent.• Reacts close to room temperature.• The mechanism involves an ___________stabilized by ____________. Both allylic radicals can react with bromine.© 2013 Pearson Education, Inc. Chapter 6 13
    14. 14. Major Write mechanism© 2013 Pearson Education, Inc. Chapter 6 14
    15. 15. Reactions of Alkyl Halides© 2013 Pearson Education, Inc. Chapter 6 15
    16. 16. The SN2 Reaction: Substitution Nucleophilic Bimolecular • The___________atomon the alkyl halide is replaced with the ____________(HO-). • Since the halogen is more electronegative than carbon, the C—I bond breaks ________________ and the iodide ion leaves.© 2013 Pearson Education, Inc. Chapter 6 16
    17. 17. Back-Side Attack in the SN2 Reaction SN2 Energy Diagram a_) Exergonic b) ____________ c) Transition state resembles ____hybridization d) _____species participate in rds e) Rate law f) Stereochemistry ______________ 6© 2013 Pearson Education, Inc. Chapter 17
    18. 18. New functional groups synthesized with the SN2 Reaction or RX© 2013 Pearson Education, Inc. Chapter 6 18
    19. 19. SN2 requires a _______ nucleophile Stronger nucleophiles give fast SN2 A negatively charged nucleophile is ___________ than its neutral counterpart: In the same period, nucleophilicity ____________ from left to right: __________down periodic table, as size and polarizability increase:© 2013 Pearson Education, Inc. Chapter 6 19
    20. 20. The more polarizable, the better Nu Electrons in a larger atom are not as tightly held as the electrons in a smaller atom and can be used to form a bond much easier© 2013 Pearson Education, Inc. Chapter 6 20
    21. 21. The solvent Dielectric constant: A measure of a solvent’s ability to insulate opposite charges from one another: The greater the value of the dielectric constant of a solvent, the smaller the interaction between ions of opposite charge dissolved in that solvent.© 2013 Pearson Education, Inc.
    22. 22. SN2 reactions proceed faster in aprotic solvents that “free” the Nu Polar aprotic solvents ________have acidic protons and therefore ________ hydrogen bond and Nu is “free” to attack • Polar protic solvents ________ acidic hydrogens (O—H or N— H) that can solvate the nucleophile, _________ their nucleophilicity. • Nucleophilicity in protic solvents increases as the size of the atom increases© 2013 Pearson Education, Inc. Chapter 6 22
    23. 23. Crown ethers solvate the cation, so the nucleophilic strength of the anion increases (“Free” to attack) • Fluoride becomes a good nucleophile.© 2013 Pearson Education, Inc. Chapter 6 23
    24. 24. A good leaving group is required in substitution and elimination reactions The best leaving groups are: • Electron-withdrawing, to polarize the carbon atom. • Stable (not a strong base) once they have left. • Polarizable, to stabilize the transition state.© 2013 Pearson Education, Inc. Chapter 6 24
    25. 25. Relative rates for SN2 depends on structure of substrate (RX, electrophile) Nucleophile approaches from the back side; It must overlap the back lobe of the C—X sp3 orbital. CH3X1°2°3° • Tertiary halides do not react via the SN2 mechanism, due to _________________hindrance.© 2013 Pearson Education, Inc. Chapter 6 25
    26. 26. Stereochemistry of SN2 SN2 reactions will result in an inversion of configuration also called a Walden inversion. The SN2 reaction is a ___________ nucleophilic substitution. It is a ______________ reaction Rate =____________________ _____________ of configuration occurs.© 2013 Pearson Education, Inc. Chapter 6 26
    27. 27. Write the mechanism for the reaction or (S)-2-Chlorobutane with iodide ion Chapter 6 27© 2013 Pearson Education, Inc.
    28. 28. The SN1 Reaction • The SN1 reaction is a ______________ nucleophilic substitution. • _____________It has a ____________ intermediate. • Racemization ___________ • Rate =____________ 6© 2013 Pearson Education, Inc. Chapter 28
    29. 29. SN1 Mechanism Step 1: Formation of the carbocation (rate-determining step) Step 2: Attack of the nucleophile. If the nucleophile is an uncharged molecule like water or an alcohol, the positively charged product must lose a proton to give the final uncharged product.© 2013 Pearson Education, Inc. Chapter 6 29
    30. 30. Step 3: If the nucleophile was neutral, a third step (deprotonation) will be needed. SN1 Energy Diagram ______________ Forming the carbocation is ______________ Step 2 is _______ with a ______activation energy. If the product is chiral a _____________is obtained Rate =© 2013 Pearson Education, Inc. Chapter 6 30
    31. 31. Stereochemistry of SN1 enantiotopic phases of the carbocation • Carbocations are_______hybridized and have ________ geometry. The lobes of the empty p orbital are perpendicular to the trigonal plane above and below. • Nucleophilic attack can occur from either side, producing mixtures of retention and inversion of configuration if the carbon is chiral.© 2013 Pearson Education, Inc. Chapter 6 31
    32. 32. Stereochemistry of SN1 The SN1 reaction produces mixtures of enantiomers. There is usually slightly _______inversion than retention of configuration.© 2013 Pearson Education, Inc. Chapter 6 32
    33. 33. examples of complete racemization have been observed, but partial racemization with a slight excess of inversion is more common.© 2013 Pearson Education, Inc.
    34. 34. Factors that favor SN1 reactions a) Nature of the electrophile (RX) b) A better leaving group will increase the rate of the reaction. c) Polar protic solvent is best because it can solvate both ions strongly through hydrogen bonding stabilizing the carbocation. This reactions usually are solvolysis.© 2013 Pearson Education, Inc. Chapter 6 34
    35. 35. Rearrangements can occur • Carbocations can rearrange to form a more stable carbocation: 1,2 Hydride shift: H-, 1,2- Methyl shifts: CH3- or phenyl Ph- shifts.© 2013 Pearson Education, Inc. Chapter 6 35
    36. 36. Write mechanism for the following reaction© 2013 Pearson Education, Inc. Chapter 6 36
    37. 37. SN1 or SN2 Mechanism? SN2 SN1 CH3X > 1º> 2º 3º> 2º Strong nucleophile Weak nucleophile (may also be used as solvent) Polar aprotic solvent Polar protic solvent Rate = k[alkyl halide][Nuc] Rate = k[alkyl halide] Inversion at chiral carbon Racemization No rearrangements Rearranged products© 2013 Pearson Education, Inc. Chapter 6 37
    38. 38. Exercises© 2013 Pearson Education, Inc.
    39. 39. © 2013 Pearson Education, Inc.
    40. 40. © 2013 Pearson Education, Inc.
    41. 41. In general, strong bases are strong nucleophiles, but not all strong nucleophiles are basic. • Basicity is defined by the equilibrium constant for abstracting a proton. • Nucleophilicity is defined by the rate of attack on the electrophilic carbon atom© 2013 Pearson Education, Inc. Chapter 6 41
    42. 42. Elimination Reactions • Elimination reactions produce double bonds. • Also called dehydrohalogenation (-HX).© 2013 Pearson Education, Inc. Chapter 6 42
    43. 43. The E1 Reaction “Nucleophile” acts as base. • __________ elimination. • Stepwise. ___________ intermediate. • Solvolysis reaction • Rate = • The E1 and SN1 reactions have the same conditions so a mixture of products will be obtained.© 2013 Pearson Education, Inc. Chapter 6 43
    44. 44. E1 Mechanism Step 1: The leaving group leaves to form a ____________ Step 2: Solvent abstracts a proton to form an __________© 2013 Pearson Education, Inc. Chapter 6 44
    45. 45. In elimination reactions, the major product of the reaction is the more stable, more substituted double bond: Zaitsev’s rule. tetrasubstituted trisubstituted disubstituted monosubstituted© 2013 Pearson Education, Inc. Chapter 6 45
    46. 46. E1 and SN1occur in solvolysis• Substitution results from _________attack on thecarbocation• Ethanol serves as a ___________ in the elimination and as a______________ in the substitution.A carbocation can:React with its own leaving group to return to the reactant.React with a nucleophile to form a substitution product (SN1).Lose a proton to form an elimination product (E1).Rearrange to give a more stable carbocation, then react further.© 2013 Pearson Education, Inc. Chapter 6 46
    47. 47. The E2 Reaction • ___________________. • Requires a __________ base. • This is a _________________ reaction: The proton is abstracted, the double bond forms, and the leaving group leaves, all in one step.© 2013 Pearson Education, Inc. Chapter 6 47
    48. 48. The E2 Reaction • Reactivity for alkyl halides: 3° > 2 ° > 1° • Mixture may form, but Zaitsevproduct predominates • The halide and the proton to be abstracted must be anti-coplanar ( = 180º) (orbitals aligned so they can begin to form a pi bond in the transition state and steric hindrance between the base and the leaving group is minimized)© 2013 Pearson Education, Inc. Chapter 6 48
    49. 49. E1 or E2 Mechanism? • Tertiary > secondary • Tertiary > secondary • Base strength usually • Strong base required weak. Solvolysis • Good ionizing solvent • Polar aprotic solvent (polar protic) • Rate = k[alkyl halide] • Rate =k[alkylhalide][base] • Zaitsevproduct • Zaitsevproduct • No required geometry • Coplanar leaving groups (usually anti) • Rearranged products • No rearrangements© 2013 Pearson Education, Inc. Chapter 6 49
    50. 50. Substitution or Elimination? • Strength of the nucleophile determines order: Strong nucleophiles or bases promote bimolecular reactions. • 10 halides usually undergo SN2. • 30halides are a mixture of SN1, E1, or E2. They cannot undergo SN2. • High temperature and bulky bases favor elimination.© 2013 Pearson Education, Inc. Chapter 6 50
    51. 51. 20 halides: strong nucleophiles will promote SN2/ Strong bases E2. Weak nucleophiles/ bases promote SN1/E1. • Bromide and iodide are good examples of strong nucleophiles, weak bases • Ethoxide/ethanol is a better base than a nucleophile • Terbutoxide/terbutyl alcohol can not act as a nucleophile • Ethanol (polar protic solvent solvolysis© 2013 Pearson Education, Inc. Chapter 6 51
    52. 52. Predict the mechanisms and products of the following. reaction© 2013 Pearson Education, Inc. Chapter 6 52
    53. 53. Predict the mechanisms and products of the following reaction.© 2013 Pearson Education, Inc. Chapter 6 53
    54. 54. 54© 2013 Pearson Education, Inc.
    55. 55. 55© 2013 Pearson Education, Inc.
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