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Chapter 8

Chapter 8. Alkenes and Alkynes II Addition Reactions. Introduction: Additions to Alkenes. An addition to a C-C double bond. Electrophilic Addition. Electrophilic Addition of Hydrogen Halides to Alkenes-Mechanism. Electrophilic Addition of Hydrogen Halides to Alkenes Markovnikov’s Rule.

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Chapter 8

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  1. Chapter 8 Alkenes and Alkynes II Addition Reactions

  2. Introduction: Additions to Alkenes • An addition to a C-C double bond

  3. Electrophilic Addition

  4. Electrophilic Addition of Hydrogen Halides to Alkenes-Mechanism

  5. Electrophilic Addition of Hydrogen Halides to AlkenesMarkovnikov’s Rule

  6. Example

  7. The Rate Determining step • Step 1 is the rate determined step • Formation of carbocation

  8. Theoretical Explanation of Markovnikov’s Rules

  9. Theoretical Explanation of Markovnikov’s Rules

  10. A mechanism for the reaction • Predict the products and show the full mechanism

  11. Modern Statement of markovnikov’s Rule • In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon of the double bond so as to yield the more stable carbocation as intermediate

  12. Regioselective Reactions • When a reaction that can potentially yield two or more constutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioselective • HX + unsymetrical alkene  more than product

  13. An exception of Markovnikov’s Rule • When alkenes are treated with HBr in presence of peroxide, an anti-Markovnikov addition

  14. 8.3 Stereochemistry of the Ionic Addition to an Alkene • The carbocation is formed in the first step of the addition is trigonal planar • achiral

  15. 8.4 Addition of Sulfuric Acid Alkenes • When alkenes are treated with cold concentrated sulfuric acid, they dissolve because they react with electrophilic addition to form alkyl hydrogen sulfate

  16. Alcohols from Alkyl Hydrogen Sulfate • Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating them with water • Markovnikov addition of H- and -OH

  17. example • Provide mechanistic explanation for the following observation • The addition of hydrogen chloride to 3-methyl-1-butene produces two products: 2-chloro-3-methylbutane and 2-chloro-2-methylbutane • Give the structure and name of the product that would be obatained from ionic addition of IBr to propene

  18. example • In one industrial synthesis of ethanol, ethene is first dissolved in 95% sulfuric acid. In a second step of water is added and the mixture is heated. Out the reaction involved

  19. 8.5 Addition of Water to Alkene:Acid-Catalyzed Hydration • Method for preparation of low molecular weight alcohols • Usually regioselective

  20. Mechanism

  21. Rate determining step However, formation of 1o carbocation does not take place

  22. Rearrangement • The carbocation formed initially invariable rearranges to a more stable one if possible • Provide the full mechanism

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