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Hydrogen Halide Addition

Hydrogen Halide Addition. The addition of a hydrogen halide to an alkyne follows Markovnikov’s rule because a secondary vinylic cation is more stable than a primary vinylic cation. Hydrogen Halide Addition. When excess hydrogen halide is present, a second equivalent is added

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Hydrogen Halide Addition

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  1. Hydrogen Halide Addition • The addition of a hydrogen halide to an alkyne follows Markovnikov’s rule because a secondary vinylic cation is more stable than a primary vinylic cation Chapter 5

  2. Hydrogen Halide Addition • When excess hydrogen halide is present, a second equivalent is added • Markovnikov’s Rule also is followed for the second addition, forming a geminal dihalide Chapter 5

  3. Hydrogen Halide Addition in the presence of Peroxide • Hydrogen peroxide has same effect on hydrogen bromide addition to an alkyne as to an alkene Chapter 5

  4. Halogen Addition to Alkynes • Halogens add to alkynes as well as to alkenes • Excess halogen leads to the addition of a second equivalent Chapter 5

  5. Addition of Water to Alkynes • Water adds to alkynes in the presence of acid to yield an enol Chapter 5

  6. Addition of Water to Alkynes • However the initially formed enol reacts further to produce a ketone Such isomers, differing only in the placement of a hydrogen atom, are called tautomers Chapter 5

  7. Mercuric-Ion-Catalyzed Addition of Water to Alkynes Chapter 5

  8. Hydroboration–Oxidation • Hydroboration of an internal alkyne leads to a ketone Chapter 5

  9. Addition of Water to an Alkyne Chapter 5

  10. Addition of Hydrogen to an Alkyne Chapter 5

  11. Addition of Hydrogen to an Alkyne Chapter 5

  12. Acidity of a Hydrogen Bonded to an sp Carbon • The acidities of hydrocarbons can be compared with the acidities of compounds having hydrogen attached to other second-row elements Chapter 5

  13. Acidity of a Hydrogen Bonded to an sp Carbon • The conjugate bases have the following relative base strength because the stronger the acid, the weaker the conjugate base Chapter 5

  14. Acidity of a Hydrogen Bonded to an sp Carbon Relative electronegativities of carbon sp > sp2 > sp3 The amide ion is strong enough to remove the proton from an sp carbon Chapter 5

  15. Acidity of a Hydrogen Bonded to an sp Carbon Hydroxide ion is too weak to remove a proton from an sp carbon Chapter 5

  16. Syntheses Using Acetylide Ions • Alkylation reactions work best with primary alkyl halides and methyl halides Chapter 5

  17. Introduction to Multistep Synthesis • Starting with 1-butyne, how would you prepare the following ketone? • At this point the only way we know to form a ketone is to add water to an alkyne in the presence of a catalyst Chapter 5

  18. Introduction to Multistep Synthesis • If the alkyne used has identical substituents on both sp carbons, only one ketone will result • 3-Hexyne can be obtained from the starting material by removing the hydrogen bonded to the sp carbon and reacting with an alkyl halide Chapter 5

  19. Introduction to Multistep Synthesis • Overall we have a two-step synthesis Chapter 5

  20. Introduction to Multistep Synthesis • In designing a synthesis it is best to start from the product and work backwards as we did in this example Chapter 5

  21. Introduction to Multistep Synthesis • The thought process is known as retrosynthetic analysis and is indicated by using open arrows Chapter 5

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