Hydrogen Halide Addition

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