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 Acetylide ion
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. 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)
H3O+

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 OR 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