1. Organic Chemistry 4 th Edition Paula Yurkanis Bruice Chapter 6 Reactions of Alkynes Introduction to Multistep Synthesis Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

2. Alkynes are hydrocarbons that contain a carbon–carbon triple bond General formula: C n H 2n–2 (acyclic); C n H 2n–4 (cyclic)

3. Nomenclature In common nomenclature, alkynes are named as substituted acetylenes

4. 3-bromo-2-chloro-4-octyne not 6-bromo-7-chloro-4-octyne because 2 < 6 A substituent receives the lowest number if there is no functional group suffix, or if the same number for the functional group suffix is obtained in both directions 1-bromo-5-methyl-3-hexyne not 6-bromo-2-methyl-3-hexyne because 1 < 2

5. The Structure of Alkynes A triple bond is composed of a  bond and two  bonds

8. Commercial Use of Ethyne

9. Acidity of a Hydrogen Bonded to an sp Hybridized Carbon pK a = 25 pK a = 44 pK a = 50

10. Electronegativities versus Acid Strengths relative base strength weakest base strongest base

11. The stronger the acid, the weaker its conjugate base top 252

12. Synthesis Using Acetylide Ions: Formation of C–C Bond

13. Addition Reactions of Alkynes

14. Sequential Addition of HCl

15. An alkyne is less reactive than an alkene

16. Relative Stabilities of Carbocations

17. Addition of Hydrogen Halides In the presence of 1 mole equivalent of HBr The secondary vinylic cation is more stable

18. In the presence of excess HBr, a carbocation is formed by adding the electrophile to the sp 2 carbon bonded to the greater number of hydrogen

19. A secondary vinylic cation is about as stable as a primary cation, therefore a pi-complex may be the actual reaction intermediate Many (but not all) alkyne addition reactions are stereoselective

21. Addition of Halogens to Alkynes

22. Addition of Water keto tautomer enol tautomer tautomerization

23. Examples of Water Addition

24. Hg 2+ is added to increase the rate of water addition to terminal alkynes

25. Hydroboration–Oxidation of Internal Alkynes

27. Hydroboration–Oxidation of Terminal Alkynes

28. Formation of Ketone versus Aldehyde

29. Addition of Hydrogen Formation of Cis Alkene

30. Conversion of Internal Alkynes to Trans Alkenes

31. Reason for trans addition:

32. Designing a Synthesis ? Example 1

35. Example 2

38. Example 3