Alkynes

1. Alkynes • Alkynes • Nomenclature • Synthesis • Reactions

2. Alkynes • Alkynes • Hydrocarbons that contain carbon-carbon triple bond • sp hybridized carbons • linear geometry • Molecular formula • CnH2n-2 • Less common than alkenes in nature or in pharmaceutical applications acetylene

3. Alkynes • Physical Properties: • Similar to alkanes and alkenes • Nonpolar • Insoluble in water • Densities ~0.6 – 0.8 g/mL • Acetylene • Most important commercial alkyne • Fuel for oxyacetylene welding torches • Thermodynamically unstable • Stored in cylinders filled with crushed firebrick wet with acetone

4. IR • Terminal alkynes exhibit the following peaks in an IR spectrum: • sp C – H at ~ 3300 cm-1 • C Ξ C at ~ 2150 cm-1 (weak) • Internal alkynes do not have an sp C-H so the peak at 3300 cm-1 is absent. • Internal alkynes may or may not exhibit a peak at ~2150 cm-1 for the triple bond.

7. Alkyne Nomenclature • To name alkynes: • Find the longest continuous chain that includes the triple bond • Change the ending of the parent alkane to “yne” • Number the chain from the end closest to the triple bond • Designate the position of the triple bond using its lower-numbered carbon atom • Number and name substituents

8. Alkyne Nomenclature Example: Give the IUPAC name for the following compounds.

9. Alkyne Nomenclature Example: Draw the following compounds. • (R)-2-chloro-3,3-dimethylnon-4-yne • 4,5-dimethyl-1-hexyne

10. Synthesis of Alkynes • Terminal alkynes are much more acidic than other hydrocarbons. • Acidity increases as the amount of s character increases • sp > sp2 > sp3

11. Synthesis of Alkynes • Terminal acetylenes can be deprotonated using a very strong base such as sodium amide (NaNH2) to form the corresponding acetylide ion. • Hydroxide ion and alkoxide ions are not strong enough to produce the acetylide ion. Acetylide ion

12. Synthesis of Alkynes • Acetylide ions are strong bases and strong nucleophiles. • Can serve as nucleophile in SN2 reactions • Addition of an acetylide ion to alkyl halides • Addition of an acetylide ion to a carbonyl compound

13. Synthesis of Alkynes • Alkylation of Acetylide Ions • SN2 addition of acetylide ion to a methyl or 1o alkyl halide forming a new alkyne with a longer carbon chain

14. H3O+ Synthesis of Alkynes • Addition of Acetylide Ion to Carbonyl Group • acetylide ion adds to the electrophilic carbon in a carbonyl producing an alkynol (an alcohol with a triple bond in the backbone)

15. Synthesis of Alkynes Examples:

16. Synthesis of Alkynes Example: Draw the product for the following reaction.

17. Synthesis of Alkynes • Synthesis of Alkynes by Double Dehydrohalogenation • loss of two molecules of HX from a vicinal or geminal dihalide

18. Synthesis of Alkynes • Loss of the second molecule of HX (from the vinyl halide) requires extremely basic conditions: • KOH (fused)/200oC • most stable internal alkyne formed by base catalyzed rearrangement • NaNH2/150oC • terminal alkyne formed • Limited use due to extreme conditions and rearrangements

19. Reactions of Alkynes • The pi bonds of alkynes are electron rich • undergo addition reactions readily • similar to alkenes • Alkynes can add up to two moles of a reagent to the triple bond • Forms the corresponding alkane

20. Reactions of Alkynes • Catalytic Hydrogenation • formation of an alkane by addition of hydrogen to the triple bond • Reagents: • H2 • Pt, Pd, or Ni

21. Reactions of Alkynes • Catalytic hydrogenation involves an alkene as an intermediate • cannot stop at the alkene under these conditions

22. Reactions of Alkynes • Catalytic Hydrogenation to cis-Alkene • formation of cis-alkene using a poisoned (partially deactivated) catalyst • Reagents • H2 • Lindlar’s catalyst (Pd/BaSO4/quinoline) • Ni2B (nickel boride) OR

23. Reactions of Alkynes • Examples:

24. Reactions of Alkynes • Metal-NH3 Reduction to trans-Alkene • formation of trans-alkene using Na/NH3 • involves radical anion and radical intermediates • trans isomer forms because the trans radical intermediate is more stable

25. Reactions of Alkynes • Example:

26. Reactions of Alkynes • Addition of Halogens (X2) • addition of Cl2, Br2, or I2 across the triple bond • dihaloalkene intermediate • difficult to stop at the alkene • most commonly used to form the tetrahaloalkane by addition of 2 moles of halogen

27. Reactions of Alkynes • Example:

28. Reactions of Alkynes • Addition of HX • addition of HX to the triple bond • monohalogenated alkene intermediate • difficult to stop at the alkene • Markovnikov product formed

29. Reactions of Alkynes • Internal alkynes give a mixture of products: • first mole of HX can form 2 structural isomers (with each structural isomer present as the E and Z isomer also)

30. Reactions of Alkynes • Hydration of Alkynes to Ketones • Reagents: • HgSO4/H2SO4/H2O • Markovnikov product • produces ketone

31. Reactions of Alkynes • Hydration of Alkynes to Aldehydes • Reagents: • (1) Sia2BH (di-sec-isoamylborane) (2) H2O2/NaOH • Anti-Markovnikov product • converts terminal alkyne to aldehyde

32. Reactions of Alkynes

33. Reactions of Alkynes • Examples: H

34. Reactions of Alkynes Example: Predict the products of the following reactions:

35. Reactions of Alkynes Example: Predict the product of the following reactions. H