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L TOPIC 8. ADDITIONS TO ALKENES AND ALKYNES (Chapter 8 and parts of chapters 7, 10 and 11)
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OBJECTIVES 1. Describe mechanisms for addition reactions of alkenes and alkynes Predict the structure of the product(s) of addition reactions Understand the mechanisms which explain the regiochemical and stereochemical outcome of addition reactions Use combination of substitution, elimination and addition chemistry to propose synthetic routes to useful value-added compounds
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~MAKE FLASHCARDS AFTER EACH LECTURE~
ADDITION REACTIONS Lecture 1 Lecture 2 Lecture 3 Lecture 4 Lecture 5 Addition of H2 - H2/Pd (alkene, alkyne) - H2/Lindlar catalyst (alkyne → cis alkene) - dissolving metal reduction (alkyne → trans alkene) Addition of H-Hal - Markovnikov - anti-Markovnikov Addition of H2O - H3O+ (regiochemistry) - Oxymercuration-demercuration (regiochemistry, stereochemistry) - Hydroboration-oxidation (regiochemistry, stereochemistry) - hydration of alkynes (regiochemistry) Addition of Hal2 - Hal2 (regiochemistry, stereochemistry) - Hal2/H2O (regiochemistry) Generation and reaction of carbenes Epoxidation Nucleopihilic ring opening reactions of epoxides (H2O, ROH, HCN, RMgHal) syn-Hydroxylation Oxidative cleavage of alkenes and alkynes - KMnO4 - Ozonolysis Radical Polymerization ~MAKE FLASHCARDS AFTER EACH LECTURE~
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HYDROGENATION OF ALKENES AND ALKYNES
Prob: ; 8.30e,g,i, 37,45 Hydrogenation of Alkenes
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Hydrogenation of Alkynes
Complete Hydrogenation cis-Semihydrogenation
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Dissolving Metal Reduction
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PRS PREVIEW OF REACTIVITY
Alkenes and Alkynes are Weakly _____ and Weakly _____________ S:8.1 PRS
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ADDITION OF HYDROGEN HALIDES TO ALKENES AND ALKYNES
Prob: 8.27,28a,b,f,j,29b-g,35 Overview
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Addition of H-Hal to Alkenes
Mechanism
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Regiochemistry Markovnikov’s Rule In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the _____________ substituted carbon and the X groups adds to the _____________ substituted carbon.
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PRS The Basis of Markovnikov’s Rule: Stability of Carbocations
The more substituted carbocation is formed. Remember hyperconjugation! Mechanistic Restatement of Markovnikov’s Rule In the ionic addition of HX to an unsymmetrical alkene, protonation gives the more stable carbocation as an intermediate.
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Stereochemistry of H–Hal Addition to Alkenes
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Anti-Markovnikov Addition of H-Hal to Alkenes
Mechanistic Rationale for Anti-Markovnikov Addition
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Addition of H-Hal to Alkynes
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~ MAKE FLASHCARDS ~ Hydrogenation Dissolving metal reduction
Hydrogenation over Lindlar catalyst Markovnikov addition of H-Hal Anti-Markovnikov addition of H-Hal …..more next lecture
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SYNTHESIS OF ALCOHOLS BY HYDRATION OF ALKENES
Prob: 8.27c,32, ,35(a-g),38(a,d), 39,46,48 Addition of Water to Alkenes PRS
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Proceeds via Markovnikov addition
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Rearrangements During Acid Catalyzed Hydration of Alkenes
Intermediate carbocations can rearrange.
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Oxymercuration-Demercuration of Alkenes
e.g., Step 1: Oxymercuration Step 2: Demercuration
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Mechanism
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Hydroboration-Oxidation: Anti-Markovnikov Hydration of Alkenes
e.g., Step 1: Hydroboration Step 2: Oxidation
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Mechanism: Regiochemistry
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Stereochemistry
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Hydration of Alkynes (covered in 16.5B)
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SYNTHETIC STRATEGIES Prob:7.24 Problem: Synthesize cis-1-cyano-2-methylcyclohexane (shown below) using any hydrocarbon.
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Problem: Female houseflies attract males with a pheromone which has been shown to be Z-9-tricosene (C23H46). Synthesize this compound starting with organic compounds containing 13 or less carbon atoms.
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ADDITION OF BROMINE AND CHLORINE TO ALKENES AND ALKYNES
Prob: 8.30(c,d), 31,54 Overview
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Addition of Hal2 to Alkenes
Stereochemistry PRS PRS
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Mechanism
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Formation of Halohydrins
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Mechanism
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Addition of Hal2 to Alkynes
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ADDITION OF CARBENES TO ALKENES
What is a Carbene? What is a Carbenoid?
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Additions to Alkenes Mechanism
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EPOXIDES S: Prob: 11.43,44 Synthesis MMPP mCPBA
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Nucleophilic Ring-opening
Stereochemistry
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Regiochemistry
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Addition of Other Nucleophiles
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OXIDATION OF ALKENES AND ALKYNES
Prob: 8.27(k,n),30(j,k),46,47 Syn-Hydroxylation of Alkenes
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Oxidative Cleavage of Alkenes
Permanganate Oxidation
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Ozonolysis
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Oxidative Cleavage of Alkynes
Problem: Compound X (C10H18) gave pentanoic acid upon reaction with basic potassium permanganate followed by acidification. What is the structure of compound X?
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RADICAL POLYMERIZATION OF ALKENES (CHAIN-GROWTH POLYMERS)
About 60 x 109 pounds of ethylene are manufactured and used each year, primarily to make polyethylene Roll of Radical Initiator (Chain Initiation) L S:10.10
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Chain-Growth (Chain Propagation)
End of Growth (Chain Termination)
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Common Chain-Growth Polymers
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SYNTHETIC STRATEGIES Add to your chart of organic reactions
Prob: ,44 Add to your chart of organic reactions No more reactions! For now!!!
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DESIGNING SYNTHESES One Step Syntheses Two Step Syntheses
This is just the opposite of learning reactions - just think backwards. Although one step syntheses are relatively easy, they are harder than learning reactions. Two Step Syntheses This is involves considerable imagination and is quite different than “just the opposite of learning reactions”. You must be able to conceive of the intermediate. However, there is a direct link between the intermediate and both the product and the starting material. Think “retro” (retrosynthesis) - how do you synthesize the product from anything? Can you then synthesize your proposed synthetic intermediate from the starting material?
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Three Step Syntheses Three design of three-step organic syntheses requires a little more imagination and can be a little more difficult than two-step syntheses. You must be able to conceive of two intermediates. In this case there may not be a clear link between intermediate one and the product or intermediate two and the starting material. Again “think retro”. Can you think of ANY compounds that could be converted to the product by chemistry you know? Can you then think of one or more compounds that could be converted to these proposed intermediates by chemistry you know? Can any of these second proposed intermediates be synthesized from the starting material?
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Problem: Solomons 8.31(b). Show how 1-butyne could be synthesized from each of the following: (b) 1-chlorobutane
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Problem:
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Problem:
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Problem:
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Problem: Propose a synthesis of 1-methyl-1,2-cyclopentanediol (with the two hydroxy groups trans to one another) from methylcyclopentane. Prob: 11.30,35, 37,38,46
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Problem: An insect physiologist required 4Z-hexen-2-ol for the study of insect pheromones. Propose a synthesis of 4Z-hexen-2-ol from propene.
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TOPIC 8 ON EXAM 4 Types of Questions
- Describe addition reaction mechanisms - Predict the products of addition reactions The problems in the book are good examples of the types of problems on the exam. Preparing for Exam 4: - Work as many problems as possible. - Work in groups. - Do the “Learning Group Problem” at the end of the chapter. - Work through the practice exam
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