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Created by Professor William Tam & Dr. Phillis Chang Ch. 7 - 1 Chapter 7 Alkenes and Alkynes I: Properties and Synthesis. Elimination Reactions of Alkyl.

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Presentation on theme: "Created by Professor William Tam & Dr. Phillis Chang Ch. 7 - 1 Chapter 7 Alkenes and Alkynes I: Properties and Synthesis. Elimination Reactions of Alkyl."— Presentation transcript:

1 Created by Professor William Tam & Dr. Phillis Chang Ch Chapter 7 Alkenes and Alkynes I: Properties and Synthesis. Elimination Reactions of Alkyl Halides

2 Ch About The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

3 Ch Introduction  Alkenes ●Hydrocarbons containing C=C ●Old name: olefins

4 Ch  Alkynes ●Hydrocarbons containing C ≡ C ●Common name: acetylenes

5 Ch The (E) - (Z) System for Designating Alkene Diastereomers  Cis-Trans System ●Useful for 1,2-disubstituted alkenes ●Examples:

6 Ch ●Examples

7 Ch Cl is cis to CH 3 and trans to Br  (E) - (Z) System ●Difficulties encountered for trisubstituted and tetrasubstituted alkenes

8 Ch  The Cahn-Ingold-Prelog (E) - (Z) Convention ●The system is based on the atomic number of the attached atom ●The higher the atomic number, the higher the priority

9 Ch  The Cahn-Ingold-Prelog (E) - (Z) Convention ●(E) configuration – the highest priority groups are on the opposite side of the double bond  “E ” stands for “entgegen”; it means “opposite” in German ●(Z) configuration – the highest priority groups are on the same side of the double bond  “Z ” stands for “zusammer”; it means “together” in German

10 Ch ●Examples

11 Ch ●Examples

12 Ch ●Other examples

13 Ch ●Other examples

14 Ch Relative Stabilities of Alkenes  Cis and trans alkenes do not have the same stability crowding Less stableMore stable

15 Ch A.Heat of Reaction  Heat of hydrogenation ●∆H° ≃ -120 kJ/mol

16 Ch Enthalpy ≈ ≈ ≈ 7 kJ/mol 5 kJ/mol  H° = -115 kJ/mol  H° = -127 kJ/mol  H° = -120 kJ/mol

17 Ch B.Overall Relative Stabilities of Alkenes  The greater the number of attached alkyl groups (i.e., the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability.

18 Ch  Relative Stabilities of Alkenes

19 Ch  Examples of stabilities of alkenes

20 Ch Cycloalkenes  Cycloalkenes containing 5 carbon atoms or fewer exist only in the cis form

21 Ch  Trans – cyclohexene and trans – cycloheptene have a very short lifetime and have not been isolated

22 Ch  Trans – cyclooctene has been isolated and is chiral and exists as a pair of enantiomers

23 Ch Synthesis of Alkenes via Elimination Reactions  Dehydrohalogenation of Alkyl Halides  Dehydration of Alcohols

24 Ch Dehydrohalogenation of Alkyl Halides  The best reaction conditions to use when synthesizing an alkene by dehydrohalogenation are those that promote an E2 mechanism

25 Ch A.How to Favor an E2 Mechanism  Use a secondary or tertiary alkyl halide if possible. (Because steric hinderance in the substrate will inhibit substitution)  When a synthesis must begin with a primary alkyl halide, use a bulky base. (Because the steric bulk of the base will inhibit substitution)

26 Ch  Use a high concentration of a strong and nonpolarizable base, such as an alkoxide. (Because a weak and polarizable base would not drive the reaction toward a bimolecular reaction, thereby allowing unimolecular processes (such as S N 1 or E1 reactions) to compete.

27 Ch  Sodium ethoxide in ethanol (EtONa/EtOH) and potassium tert- butoxide in tertbutyl alcohol (t-BuOK/t- BuOH) are bases typically used to promote E2 reactions  Use elevated temperature because heat generally favors elimination over substitution. (Because elimination reactions are entropically favored over substitution reactions)

28 Ch  Examples of dehydrohalogenations where only a single elimination product is possible 6B.Zaitsev’s Rule

29 Ch  Rate = ̶̶̶ H a ̶̶̶ H b (2 nd order overall)  bimolecular

30 Ch  When a small base is used (e.g. EtO ⊖ or HO ⊖ ) the major product will be the more highly substituted alkene (the more stable alkene)  Examples:

31 Ch  Zaitsev’s Rule ●In elimination reactions, the more highly substituted alkene product predominates  Stability of alkenes

32 Ch Mechanism for an E2 Reaction β α EtO ⊖ removes a  proton; C − H breaks; new  bond forms and Br begins to depart Partial bonds in the transition state: C − H and C−Br bonds break, new  C−C bond forms C=C is fully formed and the other products are EtOH and Br ⊖

33 Ch G2‡G2‡ G1‡G1‡

34 Ch  Hofmann’s Rule ●Most elimination reactions follow Zaitsev’s rule in which the most stable alkenes are the major products. However, under some circumstances, the major elimination product is the less substituted, less stable alkene 6C.Formation of the Less Substituted Alkene Using a Bulky Base

35 Ch ●Case 1: using a bulky base EtO ⊖ (small base) t BuO ⊖ (bulky base)

36 Ch ●Case 2: with a bulky group next to the leaving halide more crowded β-H less crowded β-H

37 Ch  Zaitsev Rule vs. Hofmann Rule ●Examples

38 Ch ●Examples

39 Ch  The 5 atoms involved in the transition state of an E2 reaction (including the base) must lie in the same plane  The anti coplanar conformation is the preferred transition state geometry ●The anti coplanar transition state is staggered (and therefore of lower energy), while the syn coplanar transition state is eclipsed 6D.The Stereochemistry of E2 Reactions

40 Ch

41 Ch  Orientation Requirement ●H and Br have to be anti periplanar (trans-coplanar) ●Examples Only H is anti periplanar to Br

42 Ch  E2 Elimination where there are two axial β hydrogens (a) (b) Both H a and H b hydrogens are anti to the chlorine in this, the more stable conformation

43 Ch  E2 elimination where the only axial β hydrogen is from a less stable Conformer Menthyl chloride (more stable conformer) Elimination is not possible for this conformation because no hydrogen is anti to the leaving group Menthyl chloride (less stable conformer) Elimination is possible for this conformation because the green hydrogen is anti to the chlorine

44 Ch The transition state for the E2 elimination is anti coplanar 2-Menthene (100%)

45 Ch Acid-Catalyzed Dehydration of Alcohols  Most alcohols undergo dehydration (lose a molecule of water) to form an alkene when heated with a strong acid

46 Ch  The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate ●Primary alcohols are the most difficult to dehydrate. Dehydration of ethanol, for example, requires concentrated sulfuric acid and a temperature of 180°C Ethanol (a 1 o alcohol)

47 Ch ●Secondary alcohols usually dehydrate under milder conditions. Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165–170°C

48 Ch ●Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used. tert-Butyl alcohol, for example, dehydrates in 20% aqueous sulfuric acid at a temperature of 85°C

49 Ch ●The relative ease with which alcohols will undergo dehydration is in the following order:

50 Ch  Some primary and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration

51 Ch ●Notice that the carbon skeleton of the reactant is while that of the product is

52 Ch A.Mechanism for Dehydration of 2 o & 3 o Alcohols: An E1 Reaction  Consider the dehydration of tert-butyl alcohol ●Step 1

53 Ch ●Step 2 ●Step 3

54 Ch B.Carbocation Stability & the Transition State  Recall most stable least stable

55 Ch

56 Ch C.A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction alkene 1 o alcohol acid catalyst protonated alcohol conjugate base

57 Ch Carbocation Stability & Occurrence of Molecular Rearrangements 8A.Rearrangements during Dehydration of Secondary Alcohols

58 Ch  Step 1

59 Ch  Step 2

60 Ch  Step 3 The less stable 2 o carbocation rearranges to a more stable 3 o carbocation.

61 Ch  Step 4 (a) or (b) (a) (b) (a)(b) (major)(minor) less stable alkenemore stable alkene

62 Ch  Other common examples of carbocation rearrangements ●Migration of an allyl group

63 Ch ●Migration of a hydride

64 Ch B.Rearrangement after Dehydration of a Primary Alcohol

65 Ch spsp 2 sp 3 9.The Acidity of Terminal Alkynes pK a = 25pK a = 44pK a = 50 Acetylenic hydrogen  Relative basicity of the conjugate base

66 Ch  Comparison of acidity and basicity of 1 st row elements of the Periodic Table ●Relative acidity ●Relative basicity

67 Ch Synthesis of Alkynes by Elimination Reactions  Synthesis of Alkynes by Dehydrohalogenation of Vicinal Dihalides

68 Ch  Mechanism

69 Ch  Examples

70 Ch  Synthesis of Alkynes by Dehydrohalogenation of Geminal Dihalides

71 Ch Replacement of the Acetylenic Hydrogen Atom of Terminal Alkynes  The acetylide anion can be prepared by

72 Ch  Acetylide anions are useful intermediates for the synthesis of other alkynes  ∵ 2 nd step is an S N 2 reaction, usually only good for 1 o R’  2 o and 3 o R’ usually undergo E2 elimination

73 Ch  Examples SN2SN2E2

74 Ch Hydrogenation of Alkenes  Hydrogenation is an example of addition reaction

75 Ch  Examples

76 Ch Hydrogenation: The Function of the Catalyst  Hydrogenation of an alkene is an exothermic reaction ●∆H° ≃ -120 kJ/mol

77 Ch

78 Ch A. Syn and Anti Additions  An addition that places the parts of the reagent on the same side (or face) of the reactant is called syn addition

79 Ch  An anti addition places parts of the adding reagent on opposite faces of the reactant

80 Ch Hydrogenation of Alkynes  Using the reaction conditions, alkynes are usually converted to alkanes and are difficult to stop at the alkene stage

81 Ch A. Syn Addition of Hydrogen: Synthesis of cis-Alkenes  Semi-hydrogenation of alkynes to alkenes can be achieved using either the Ni 2 B (P-2) catalyst or the Lindlar’s catalyst ●Nickel boride compound (P-2 catalyst)  ●Lindlar’s catalyst  Pd/CaCO 3, quinoline

82 Ch  Semi-hydrogenation of alkynes using Ni 2 B (P-2) or Lindlar’s catalyst causes syn addition of hydrogen ●Examples

83 Ch B. Anti Addition of Hydrogen: Synthesis of trans-Alkenes  Alkynes can be converted to trans- alkenes by dissolving metal reduction  Anti addition of dihydrogen to the alkyne

84 Ch  Example anti addition

85 Ch  Mechanism radical anionvinyl radical vinyl aniontrans alkene

86 Ch An Introduction to Organic Synthesis 16A. Why Do Organic Synthesis?  To make naturally occurring compounds which are biologically active but difficult (or impossible) to obtain TAXOL Anti-tumor, anti-cancer agent

87 Ch TAXOL  Isolated from Pacific Yew tree Leaves usually appear on separate male and female trees Cones and Fruit seedpollen cones

88 Ch  Approved by the U.S. Food & Drug Administration in 1992 for treatment of several types of cancer, including breast cancer, lung cancer, and melanoma  An estimation: a 100-year old yew tree must be sacrificed in order to obtain 300 mg of Taxol, just enough for one single dose for a cancer patient  Obviously, synthetic organic chemistry methods that would lead to the synthesis of Taxol would be extremely useful TAXOL

89 Ch B. Retrosynthetic Analysis

90 Ch  When doing retrosynthetic analysis, it is necessary to generate as many possible precursors, hence different synthetic routes, as possible

91 Ch C. Identifying Precursors  Synthesis of

92 Ch  Retrosynthetic Analysis disconnection 1 disconnection 2 S N 2 on 1 o alkyl halide: good S N 2 on 2 o alkyl halide: poor  will get E2 as major pathway

93 Ch  Synthesis

94 Ch D. Raison d’Etre Summary of Methods for the Preparation of Alkenes (Dehydration of alcohols) (Dissolving metal reduction of alkynes) (Semi- hydrogenation of alkynes) (Dehydrohalogenation of alkyl halides)

95 Ch Summary of Methods for the Preparation of Alkynes (Dehydrohalogenation of geminal dihalide) (Deprotonation of terminal alkynes and S N 2 reaction of the acetylide anion) (Dehydrohalogenation of vicinal dihalide)

96 Ch  END OF CHAPTER 7 


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