Organic Chemistry 6 th Edition Paula Yurkanis Bruice

1. Organic Chemistry 6th Edition Paula Yurkanis Bruice Chapter 6 The Reactions of Alkynes An Introduction to Multistep Synthesis

2. Acetylenes and Birth Control Note the “yn,” as in alkyne, in the generic name

3. An alkyne is a hydrocarbon that contains a carbon–carbon triple bond General formula: CnH2n–2 (acyclic); CnH2n–4 (cyclic)

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

5. In IUPAC nomenclature: • The yne suffix is assigned the lowest number. • Then substituents are assigned the lowest possible set of numbers.

6. Naming a compound that has more than one functional group… A compound containing two double bonds or two triple bonds:

8. Longest chain with both functional groups Alkene and Alkyne Mixed Functional Groups • Find the longest chain bearing both functional groups. • Assign ene and yne suffixes the lowest set of numbers.

9. If there is a tie between a double bond and a triple bond, the ene suffix is assigned the lower number: A chain is numbered to give the lowest possible number to the functional group with the higher priority:

10. Longest chain most functional groups Longest chain most functional groups Vinyl or ethenyl Ethynyl Complex Mixed Functional Group Examples Do not forget to designate alkene stereochemistry if necessary.

11. The Structure of Alkynes A triple bond is composed of a s bond and two p bonds

12. Z-Stereoisomer Gem regioisomer Electrophilic Addition of Hydrogen Halides to Alkynes One Equivalent of HCl: Excess HCl:

13. The initial addition of the proton can occur with equal ease to either of the sp carbons and the geminal regioisomer always results:

14. Stereoselectivity of Alkyne Addition Reactions Trans addition of hydrohalides HCl and HBr occur because of a -complex intermediate: Adding excess halide anion increases the yield of trans addition product.

15. Regioselectivity of Alkyne Addition Reactions Use Markovnikov’s rule for HCl and HBr additions to terminal alkynes:

16. The second hydrohalide addition affords a geminal dihalide:

17. The transition state for the alkyne addition is less stable than for an alkene addition:

18. Relative Stabilities of Carbocations High s character, cation interacts with positive nucleus. Less s character, more stable cation A vinylic cation (sp) is formed in the first step of the alkyne addition reaction whereas an alkyl cation (sp2) is formed in the first step of the alkene addition reaction

19. The halogens Cl2 and Br2 also add to alkynes:

20. Addition of Water to an Alkyne One method of synthesizing ketones.

21. Hg2+ is added to increase the rate of water addition to terminal alkynes:

22. Mechanism involves a prototropic shift: Acid Catalyzed Base Catalyzed

23. Two sets of  protons Two sets of tautomers Proton from solvent How to Draw Tautomers: Recognize protons  to a carbonyl and then protonate oxygen and deprotonate carbon to afford enol species: No protons  to a carbonyl, no enol tautomers:

24. Hydroboration–Oxidation of Internal Alkynes

25. Hydroboration–Oxidation of Terminal Alkynes Bulky borane required to distinguish terminal end from substituted end:

26. Formation of Ketone versus Aldehyde

27. Addition of Hydrogen Formation of Cis Alkene Catalytic reduction of an alkyne affords an alkane without buildup of the alkene intermediate: Use a “poisoned” catalyst developed by Lindlar to obtain the alkene:

28. The components of Lindlar catalyst:Palladium MetalCalcium CarbonateLead Oxide Origin of the cis stereochemistry: The catalyst The substrate The poison Delivery of hydrogen atoms to one side by the solid-phase catalyst Lindlar Catalyst

29. Lindlar’s Synthesis of Vitamin A

30. Conversion of Internal Alkynes to Trans Alkenes

31. Reason for trans stereochemistry: electron repulsion and steric hindrance. Repulsion Steric Hindrance Unstable Stable

32. Acidity of a Hydrogen Bonded to an sp Hybridized Carbon High s character, sp orbital penetrates nucleus. Lower s character, sp3 orbital penetrates nucleus to a lesser degree

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

34. The stronger the acid, the weaker its conjugate base: An amide anion cannot remove a hydrogen bonded to an sp2 or sp3 carbon.

35. Synthesis Using Acetylide Ions: Formation of C–C Bond Known as an SN2 reaction We can convert terminal alkynes to longer internal alkynes by addition of alkyl halides

36. Designing a Synthesis Example 1

37. Example 2

38. Example 3

39. Example 4

40. Example 5

41. Example 6

42. Example 7