ch4-2021-class.ppt
- 1. John McMurry Chapter 4 Reactions of Alkenes and Alkynes Copyright © Cengage Learning. All rights reserved.
- 2. Reaction of alkene P112 Electrophilic addition reaction C =C C C 2 Various reagent
- 3. Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) 3
- 4. 4.1 Addition of HX to Alkenes: Hydrohalogenation 4
- 5. In the addition of HX to an alkene, the H attaches to the carbon with more hydrogens and the X attaches to the carbon with fewer hydrogens the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. In an unsymmetrical alkene, Markovnikov’s rule Regiospecific Only one product forms where two orientations are possible
- 6. When both double-bond carbon atomhave the same degree of substitution, a mixture of addition products results: 6 mixture
- 7. The more highly substituted carbocation is forms as intermediate rather than the less highly substituted one Markovnikov’s rule (restated) 7 3o X 1o
- 8. Worked Example 4.1 P115 What product would you expect from the reaction of HCl with 1-ethylcyclopentene? 8 Solution
- 9. Problem 4.1 P115 Predict the products of the following reactions: 9
- 10. Problem 4.2 What alkenes would you start with to prepare the following alkyl halide? (a) (b) (d) (c)
- 11. 4.3 Carbocation Structure and Stability Electronic structure of carbocation Stability: P116 planar 6 electrons 1 1
- 13. Problem 4.3 P117 Show the structures of the carbocation intermediates you would expect in the following reactions: 1 3
- 14. 4.4 Addition of H2O to Alkenes: Hydration Hydration of an alkene is the addition of H-OH to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol P117
- 15. Mechanism of the acid-catalyzed hydration P118 1 5
- 16. Newer methods for hydration
- 17. Worked Example 4.2 P119 Predicting the Product of an Alkene Hydration Reaction What product would you expect from acid-catalyzed addition of water to methylenecyclopentane? Solution 1 7
- 18. Problem 4.4 What product would you expect to obtain from the acid-catalyzed addition of water to the following alkenes? 1 8
- 19. Problem 4.5 What alkenes might the following alcohols be made from?
- 21. Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation Mechanism Oxymercuration supplementary
- 22. What products would you expect from oxymercuration of the following alkenes? supplementary Exercise 2 2
- 23. Borane (BH3) is a Lewis acid because the boron atom has only six electrons in its valence shell. Hydroboration & Oxidation supplementary 2 3
- 25. What alkenes might the following alcohols have been prepared from? (a) Oxymercuration of (b) Oxymercuration of Hydroboration of solution supplementary Exercise 2 5
- 26. What products would you obtain from reaction of 2,4- dimethylpent-2-ene with (a) BH3, followed by H2O2, OH- (b) Hg(OAc)2, followed by NaBH4 supplementary Exercise 2 6
- 27. How might you prepare the following alcohol? Solution Exercise 2 7
- 28. Show the structures of the products you would obtain by hydroboration/oxidation of the following alkenes: What alkenes might be used to prepare the following alcohols by hydroboration/oxidation? Exercise 2 8
- 29. 4.5 Addition of X2 to Alkenes: Halogenation Halogenation P120 Reactivity F2 > Cl2 > Br2 > I2 x
- 30. × anti stereochemistry Bottom side shielded from attack sole product
- 31. Problem 4.6 & 4.7 What product would you expect to obtain from addition of Br2 to 1,2-dimethylcyclohexene? Show the stereochemistry of the product. Show the structure of the intermediate. trans 3 1
- 32. 4.6 Reduction of H2 to Alkenes: Hydrogenation Heterogeneous process Reduction Increases electron density on carbon by: - forming C-H - or breaking one of these C-O C-N C-X Syn stereochemistry Catalyst: Pd/C or PtO2 Alkene + H2
- 33. Mechanism of Catalytic Hydrogenation atom H-H
- 34. Selective for C=C. No reaction with C=O, C=N Supplementary
- 35. Application 10% -15 % trans isomer R When eaten and digested, the free trans fatty acids are released, raising blood cholesterol levels and contributing to potential coronary problems Supplementary polyunsaturated liquid oils become solids
- 36. What product would you expect to obtain from catalytic hydrogenation of the following alkenes? Problem 4.8
- 37. 4.7 Oxidation of Alkenes: Hydroxylation, and Cleavage Oxidation decreases electron density on carbon by: - forming C-O C-N C-X - or breaking one of these C-H
- 38. in basic solution addition of one or more–OH group to a molecule. P125 Stereochemistry: syn Hydroxylation
- 39. Cleavage No H on C: C=O ; One H on C: COOH ;Two Hs on C: CO2 P126 Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C KMnO4 in acidic solution 3 9
- 40. Reduce molozonide to obtain ketones and/or aldehydes Cleavage reduction Ozonolysis supplementary Ozone 4 0
- 41. Worked Example 4.3 Predict the product of reaction of pent-2-ene with aqueous acidic KMnO4. 4 1
- 42. Worked Example 4.4 P126 Predicting the Structure of a Reactant Given the Products What alkene gives a mixture of acetone and propanoic acid on reaction with acidic KMnO4 ? Solution 4 2
- 43. Problem 4.9 Predict the product. 4 3
- 44. ? Problem 4.10 Propose structures for alkenes that yield the following products on treatment with acidic KMnO4 2 equiv CH3CH2COOH Hex-3-ene ? 4 4
- 45. A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization Alkenes react with radical catalysts to undergo radical polymerization 4.8 Addition of Radicals to Alkenes: Polymers P128 PE High pressure: 1000-3000 atm High temperature: 100-250 0C Radical catalyst: benzoyl peroxide
- 46. Free Radical Polymerization: STEP 1: initiation Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times STEP 2: Propagation 4 6
- 47. P128 Chain propagation ends when two radical chains combine STEP 3: Termination 2 R-CH2CH2 R-CH2CH2CH2CH2-R example Vinyl monomer radical polymerization 4 7
- 49. Worked Example 4.5 P129 Show the structure of poly(vinyl chloride), a polymer made from H2C=CHCl, by drawing several repeating units. Solution Problem 4.11 Show the structure of Teflon. The monomer unit is tetrafluyoroethylene, F2C=CF2 4 9
- 50. 4.9 Conjugated Dienes Conjugated dienes: they are more than one double & separated by only one single bond P130 Double bonds react independently
- 51. Conjugated dienes are somewhat more stable than nonconjugated dienes An orbital view of buta-1,3-diene. Each of the four carbon atoms has a p orbital,allowing for an electronic interaction across the C2-C3 single bond. P130
- 52. Electrophilic Additions to Conjugated Dienes 1,2-addition 1,4-addition
- 53. allylic Carbocations Addition of H+ leads to delocalized secondary allylic carbocation Conjugated dienes undergo electrophilic reactions to produce a mixture of products 2 4
- 54. 4.10 Stability of Allylic Carbocations: Resonance Resonance + +
- 55. Problem 4.12 Buta-1,3-diene H2C=CH-CH=CH2, reacts with Br2 to yield a mixture of 1,2- and 1,4-addition products. Show the structure of each.
- 56. 4.11 Drawing and interpreting Resonance Forms It is a hybrid of the two resonance forms, so the structure is called a resonance hybrid
- 57. 驢 馬 騾 mule donkey horse mule = donkey horse mule donkey mule horse 5 7
- 58. = 5 8
- 59. Rules for Resonance Forms (2) Resonance forms differ only in the placement of their or nonbonding electrons (1) Individual resonance forms are imaginary, not real. The real structure is a hybrid of the different forms. 5 9
- 60. (3) Different resonance forms of a substance don’t have to be equivalent
- 61. (5) Resonance leads to stability. (4) Resonance forms must be valid Lewis structures and obey normal rules of valency.
- 62. Drawing Resonance Forms Strategy: Find the three-atom groupings that contain a multiple bond next to a p orbital. Solution: Exchange the position of the double bond and an elctron lone pair. P orbital supplement
- 63. Worked Example 4.6 Use resonance structures to explain why the two N-O bonds of nitromethane are equivalent. Solution
- 64. Problem 4.15 Draw resonance structures for the following species:
- 65. 4.12 Alkynes and Their Reactions P136 Alkyne: -yne 6-methyl-3-octyne 6-methyloct-3-yne 1-hepten-6-yne Hept-1-en-6-yne 4-methyl-7-nonen-1-yne 4-methylnon-7-en-1-yne Numbering the chain starts from the end nearer the first multiple bond (C=C, CC), whether double or triple. When there is a choice in numbering, C=C receive lower numbers than CC.
- 66. Problem 4.16 Give IUPAC names for the following compounds: 3,3-dimethylbut-1-yne 6-Methylhept-3-yne 5-Methylhex-2-yne Hept-2-en-5-yne
- 67. Conversion of Alkynes to cis-Alkenes Lindlar catalyst: Pd/CaCO3 Catalyst deactivated by treatment with lead acetate and quinoline, an aromatic amine Alkyne Reactions: Addition of H2
- 68. Bp of NH3: -33 ºC Conversion of Alkynes to trans-Alkenes liquid ammonia Reducing agent: sodium or lithium in liquid ammonia Trans alkene supplementary
- 69. Using any alkyne needed, how would you prepare the following alkenes? (a) trans-oct-2-ene (b)cis-hept-3-ene ( c) 3-methylpent-1-ene supplementary Exercise
- 70. Addition reactions of alkynes are similar to those of alkenes Intermediate alkene reacts further with excess reagent Alkyne Reactions: Addition of HX P137 Markovnikov Stereochemistry: trans
- 72. Alkyne Reactions: Addition of X2 Initial addition gives trans intermediate Product with excess reagent is tetrahalide P138
- 73. Predict the products for the following reaction. supplementary Exercise
- 74. Alkyne Reactions: Addition of H2O The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone P138 Addition of H-OH as in alkenes Mercury (II) catalyzes: Hydroboration-oxidation: Markovnikov non-Markovnikov rearrangement Mercury (II) catalyzes
- 75. tautomers tautomerism the movement of a proton more stable example Keto-enol Tautomerism supplementary
- 76. Hydration of Unsymmetrical Alkynes OH OH
- 78. OH O H Comparison of Hydration of Terminal Alkynes supplementary
- 79. What product would you obtain by hydration of the following alkynes? What alkynes would you start with to prepare the following ketones? & supplementary Exercise
- 80. What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction? supplementary Exercise
- 81. Alkyne Reactions: Formation of Acetylide Anions Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25) Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion P139
- 82. Alkylation of Acetylide Anions Acetylide ions can react as nucleophiles as well as bases Acetylide ion + a 1o alkyl halide larger alkynes P139 8 3
- 83. Worked Example 4.7 P140 What product would you obtain by hydration of 4-methylhex -1-yne? Solution 8 4
- 84. Worked Example 4.8 What alkyne and what alkyl halide would you use to prepare pent-1-yne? Solution
- 85. Problem 4.17 P140 What products would you expect from the following reactions?
- 86. Problem 4.18 Problem 4.19 What products would you obtain by hydration of oct-4- yne? What alkynes would you start with to prepare the following ketones by a hydration reaction?
- 87. Problem 4.20 Show the alkyne and alkyl halide from which the following products can be obtained. Where two routs look feasible, list both. solution
- 88. 8 9