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Substitution and Elimination

Substitution and Elimination. Reaction of Alkyl Halides By: Ismiyarto, MSi. ALKIL HALIDA. Manfaat (Pestisida, Bahan Dasar Sintesis Alkohol, Alkena) Struktur (Metil, Primer, Sekunder, Tersier, Benzil dan Vinil) Reaksi (SN-2, SN-1, E-2 dan E-1). 7. Vinil Halida 8. Aril Halida.

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Substitution and Elimination

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  1. Substitution and Elimination Reaction of Alkyl Halides By: Ismiyarto, MSi

  2. ALKIL HALIDA • Manfaat (Pestisida, Bahan Dasar Sintesis Alkohol, Alkena) • Struktur (Metil, Primer, Sekunder, Tersier, Benzil dan Vinil) • Reaksi (SN-2, SN-1, E-2 dan E-1)

  3. 7. Vinil Halida 8. Aril Halida Dalam Pembahasan Tersendiri SN-2 PETA REAKSI ALKIL HALIDA SN-2 SN-2, SN-1 dan E-2 SN-2, SN-1 dan E-2 SN-2, SN-1 SN-2, SN-1 Metil Halida Alkil halida Primer Alkil Halida Sekunder Alkil Halida Tersier Alil Halida Benzil Halida

  4. Organic compounds with an electronegative atom or an electron-withdrawing group bonded to a sp3 carbon undergo substitution or elimination reactions - Substitution Elimination Halide ions are good leaving groups. Substitution reaction on these compounds are easy and are used to get a wide variety of compounds alkyl fluoride alkyl chloride alkyl bromide alkyl iodide

  5. Alkyl Halides in Nature Synthesized by red algae red algae Synthesized by sea hare a sea hare

  6. Substitution Reaction with Halides (1) (2) bromomethane methanol If concentration of (1) is doubled, the rate of the reaction is doubled. If concentration of (1) and (2) is doubled, the rate of the reaction quadruples. If concentration of (2) is doubled, the rate of the reaction is doubled.

  7. Substitution Reaction with Halides (1) (2) bromomethane methanol Rate law: rate = k [bromoethane][OH-] this reaction is an example of a SN2 reaction. S stands for substitution N stands for nucleophilic 2 stands for bimolecular

  8. Mechanism of SN2 Reactions The rate of reaction depends on the concentrations of both reactants. When the hydrogens of bromomethane are replaced with methyl groups the reaction rate slow down. The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of only one stereoisomer

  9. Mechanism of SN2 Reactions Hughes and Ingold proposed the following mechanism: Transition state Increasing the concentration of either of the reactant makes their collision more probable.

  10. Mechanism of SN2 Reactions Steric effect activation energy: DG2 activation energy: DG1 Energy reaction coordinate reaction coordinate Inversion of configuration (S)-2-bromobutane (R)-2-butanol

  11. Factor Affecting SN2 Reactions The leaving group relative rates of reactionpKa HX HO- + RCH2I RCH2OH + I- 30 000 -10 HO- + RCH2Br RCH2OH + Br- 10 000 -9 HO- + RCH2Cl RCH2OH + Cl- 200 -7 HO- + RCH2F RCH2OH + F-1 3.2 The nucleophile In general, for halogen substitution the strongest the base the better the nucleophile. pKa Nuclephilicity

  12. SN2 Reactions With Alkyl Halides an alcohol a thiol an ether a thioether an amine an alkyne a nitrile

  13. Substitution Reactions With Halides 1-bromo-1,1-dimethylethane 1,1-dimethylethanol Rate law: rate = k [1-bromo-1,1-dimethylethane] this reaction is an example of a SN1 reaction. S stands for substitution N stands for nucleophilic 1 stands for unimolecular If concentration of (1) is doubled, the rate of the reaction is doubled. If concentration of (2) is doubled, the rate of the reaction is not doubled.

  14. Mechanism of SN1 Reactions The rate of reaction depends on the concentrations of the alkyl halide only. When the methyl groups of 1-bromo-1,1-dimethylethane are replaced with hydrogens the reaction rate slow down. The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of two stereoisomers * a small rate is actually observed as a result of a SN2

  15. Mechanism of SN1 Reactions nucleophile attacks the carbocation slow C-Br bond breaks fast Proton dissociation

  16. Mechanism of SN1 Reactions Rate determining step Carbocation intermediate DG R++ X- + R-OH2 R-OH

  17. Mechanism of SN1 Reactions Inverted configuration relative the alkyl halide Same configuration as the alkyl halide

  18. Factor Affecting SN1 reaction • Two factors affect the rate of a SN1 reaction: • The ease with which the leaving group dissociate from the carbon • The stability of the carbocation The more the substituted the carbocation is, the more stable it is and therefore the easier it is to form. As in the case of SN2, the weaker base is the leaving group, the less tightly it is bonded to the carbon and the easier it is to break the bond The reactivity of the nucleophile has no effect on the rate of a SN1 reaction

  19. Comparison SN1 – SN2

  20. Kestabilan Karbokation

  21. Elimination Reactions 1-bromo-1,1-dimethylethane 2-methylpropene Rate law: rate = k [1-bromo-1,1-dimethylethane][OH-] this reaction is an example of a E2 reaction. E stands for elimination 2 stands for bimolecular

  22. The E2 Reaction A proton is removed Br- is eliminated The mechanism shows that an E2 reaction is a one-step reaction

  23. Elimination Reactions 1-bromo-1,1-dimethylethane 2-methylpropene Rate law: rate = k [1-bromo-1,1-dimethylethane] this reaction is an example of a E1 reaction. E stands for elimination 1 stands for unimolecular If concentration of (1) is doubled, the rate of the reaction is doubled. If concentration of (2) is doubled, the rate of the reaction is not doubled.

  24. The E1 Reaction The base removes a proton The alkyl halide dissociate, forming a carbocation The mechanism shows that an E1 reaction is a two-step reaction

  25. Products of Elimination Reaction 50% 30% 80% 2-butene 20% 2-bromobutane 1-butene The most stable alkene is the major product of the reaction for both E1 and E2 reaction The greater the number of alkyl substituent the more stable is the alkene For both E1 and E2 reactions, tertiary alkyl halides are the most reactive and primary alkyl halides are the least reactive

  26. ELIMINATION REACTIONS:ALKENES, ALKYNES

  27. Elimination Reactions Dehydrohalogenation (-HX) and Dehydration (-H2O) are the main types of elimination reactions.

  28. Dehydrohalogenation (-HX)

  29. The E2 mechanism This reaction is done in strong base at highconcentration, such as 1 M NaOH in water. _

  30. Kinetics • The reaction in strong base at high concentration is second order (bimolecular): Rate law: rate = k[OH-]1[R-Br]1

  31. The E1 mechanism This reaction is done in strong base such as 0.01 M NaOH in water!! Actually, the base solution is weak!

  32. Kinetics • The reaction in weak base or under neutral conditions will be first order (unimolecular): • Rate law: rate = k [R-Br]1 • The first step (slow step) is rate determining!

  33. The E2 mechanism • Mechanism • Kinetics • Stereochemistry of reactants • Orientation of elimination (Zaitsev’s rule) • Stereochemistry of products • Competing reactions

  34. E2 mechanism This reaction is done in strong base at high concentration, such as 1 M NaOH in water.

  35. Kinetics of an E2 reaction • The reactions are second order (bimolecular reactions). • Rate = k [R-Br]1[Base]1 second order reaction (1 + 1 = 2) High powered math!!

  36. d- d- Transition State energy Reaction coordinate

  37. Stereochemistry of reactants • E2 reactions must go by an anti elimination • This means that the hydrogen atom and halogen atom must be 180o (coplanar) with respect to each other!! • Draw a Newman projection formula and place the H and X on opposite sides.

  38. Stereochemistry of E2 Reaction H and Br are anti structure in conformation!!!!!!!!!

  39. (S,S)-diastereomer

  40. This one is formed!

  41. (R,S)-diastereomer

  42. This one is formed!

  43. Orientation of elimination: regiochemistry/ Zaitsev’s Rule • In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogenwhich is lost will come from the more highly-branchedb-carbon. More branched Less branched A. N. Zaitsev -- 1875

  44. Product formed from previous slide More substituted alkene is more stable!!!!!!!!

  45. Typical bases used in E2 reactions High concentration of the following >1M If the concentration isn’t given, assume that it is high concentration! • Na+-OH • K+-OH • Na+-OR • Na+-NH2

  46. Orientation of elimination: regiochemistry/ Zaitsev’s Rule Explaination of Zaitsev’s rule: When you remove a hydrogen atom from the more branched position, you are forming a more highly substituted alkene.

  47. Stereochemistry of products • The H and X must be anti with respect to each other in an E2 reaction! • You take what you get, especially with diastereomers! See the previous slides of the reaction of diastereomers.

  48. Competing reactions • The substitution reaction (SN2) competes with the elimination reaction (E2). • Both reactions follow second order kinetics!

  49. The E1 mechanism • Mechanism • Kinetics • Stereochemistry of reactants • Orientation of elimination (Zaitsev’s rule) • Stereochemistry of products • Competing reactions

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