L26 TOPIC 8. ADDITIONS TO ALKENES AND ALKYNES (Chapter 8 and parts of chapters 7, 10 and 11)

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

HYDROGENATION OF ALKENES AND ALKYNES Prob: 7.40-42; 8.30e,g-i,37,45 Hydrogenation of Alkenes

A Practical Application of Hydrogenation – Hydrogenated Oils and Fats Soybean Oil Mixture of saturated and various cis-unsaturated hydrocarbon chains. Low melting (-16 °C) because cis C=C impede crystallization. http://www.fb.com/nefb/ag-ed/soybean.gif Margarine Mixture of saturated hydrocarbon chains (some C=C kept to adjust consistency). Saturated alkyl chains pack together (crystallize). http://www.nicssa.co.uk/Contentpages/Fitness&Exercise/contentweightetc.htm http://www.bassriverchairs.com/new_prodMargarine.html

Hydrogenation of Alkynes Complete Hydrogenation cis-Semihydrogenation

Dissolving Metal Reduction

PRS PREVIEW OF REACTIVITY Alkenes and Alkynes are Weakly _____ and Weakly _____________ S:8.1 PRS

PRS Which of the following mechanistic steps is most likely to occur? 1 2 3 4 PRS

ADDITION OF HYDROGEN HALIDES TO ALKENES AND ALKYNES Prob: 8.27(28) a,b,f,j, 29b-g,35 Overview

Addition of H-Hal to Alkenes Mechanism

PRS 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. PRS

PRS Which of the following cannot be made by Markovnikov addition of HBr to an appropriate alkene? 1 2 3 4 PRS

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.

PRS Which of the following is the most stable carbocation? 1 2 3 4 PRS

Stereochemistry of H–Hal Addition to Alkenes

PRS Anti-Markovnikov Addition of H-Hal to Alkenes Mechanistic Rationale for Anti-Markovnikov Addition PRS

PRS Which of the following radicals is more stable? 1 2 PRS

Addition of H-Hal to Alkynes HWeb

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SYNTHESIS OF ALCOHOLS BY HYDRATION OF ALKENES Prob: 8.27(28)c,32, 58,11.33 35(a-g),38a,d, 39,46,48 Addition of Water to Alkenes PRS

PRS Which of the following mechanistic steps is most likely to occur? 1 2 PRS

Proceeds via Markovnikov addition PRS

PRS Which of the following cannot be made by Markovnikov hydration of an appropriate alkene? 1 2 3 4 PRS

Rearrangements During Acid Catalyzed Hydration of Alkenes Intermediate carbocations can rearrange.

Oxymercuration-Demercuration of Alkenes e.g., Step 1: Oxymercuration Step 2: Demercuration

Mechanism PRS

PRS Which of the following cannot be made in high yield by oxymercuration-demercuration of an appropriate alkene? 1 2 3 4 PRS

Hydroboration-Oxidation: Anti-Markovnikov Hydration of Alkenes e.g., Step 1: Hydroboration Step 2: Oxidation

Mechanism: Regiochemistry

Stereochemistry PRS

PRS Which of the following can be made by hydroboration-oxidation of an appropriate alkene? 1 only A 2 only B 3 only B and C 4 only B and D PRS

Hydration of Alkynes (covered in 16.5B)

SYNTHETIC STRATEGIES Prob:7.24 Problem: Synthesize cis-1-cyano-2-methylcyclohexane (shown below) using any hydrocarbon.

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. HWeb

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ADDITION OF BROMINE AND CHLORINE TO ALKENES AND ALKYNES Prob: 8.30c,d,31,54 Overview

Addition of Hal2 to Alkenes Stereochemistry PRS PRS

PRS What is the relationship between the following pair of compounds? 1 identical 2 enantiomers 3 diasteromers 4 consitutional isomers PRS

PRS What is the relationship between the following pair of compounds? 1 identical 2 enantiomers 3 diasteromers 4 consitutional isomers PRS

Mechanism

Formation of Halohydrins

Mechanism PRS

PRS Which of the following bromohydrins can be prepared from an appropriate alkene? 1 2 3 4 PRS

Addition of Hal2 to Alkynes HWeb

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ADDITION OF CARBENES TO ALKENES What is a Carbene? What is a Carbenoid? PRS

PRS How many electrons does the carbon atom of methylene (carbene) have in its valance shell, what is the formal charge? :CH2 1 4, +2 2 4, 0 3 6, +2 4 6, 0 5 6, -1 6 8, -1 PRS

Additions to Alkenes Mechanism

EPOXIDES S:11.13-11.15 Synthesis Prob 11.43,44 MMPP mCPBA

Nucleophilic Ring-opening Stereochemistry

Regiochemistry

Addition of Other Nucleophiles PRS

PRS What is the major organic product of the following sequence of reactions? 1 2 3 4 PRS

OXIDATION OF ALKENES AND ALKYNES Prob: 8.27(28)k-n, 30j,k,46,47 Syn-Hydroxylation of Alkenes

Oxidative Cleavage of Alkenes Permanganate Oxidation

Ozonolysis PRS

PRS PRS What is the structure of compound A in the following scheme? 1 2 3 4 PRS

HWeb 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? HWeb

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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) L30 S:10.10

Chain-Growth (Chain Propagation) End of Growth (Chain Termination)

Common Chain-Growth Polymers

SYNTHETIC STRATEGIES Add to your chart of organic reactions Prob: 8.21-26,44 Add to your chart of organic reactions No more reactions! For now!!!

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?

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?

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: 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

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.

HWeb 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 HWeb

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