1. Introduction Alkynes contain a triple bond. General formula is CnH2n-2
Two elements of unsaturation for each triple bond.
Some reactions are like alkenes: addition and oxidation.
Some reactions are specific to alkynes =>

2. Nomenclature: IUPAC Find the longest chain containing the triple bond.
Change -ane ending to -yne.
Number the chain, starting at the end closest to the triple bond.
Give branches or other substituents a number to locate their position =>

3. Name these:
propyne
5-bromo-2-pentyne
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4. Additional Functional Groups
All other functional groups, except ethers and halides have a higher priority than alkynes.
For a complete list of naming priorities, look inside the back cover of your text =>

5. Examples
4-methyl-1-hexen-5-yne
4-hexyn-2-ol
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6. Common Names Named as substituted acetylene. methylacetylene
isobutylisopropylacetylene
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7. Physical Properties Nonpolar, insoluble in water.
Soluble in most organic solvents.
Boiling points similar to alkane of same size.
Less dense than water.
Up to 4 carbons, gas at room temperature =>

8. Acetylene Acetylene is used in welding torches.
In pure oxygen, temperature of flame reaches 2800C.
It would violently decompose to its elements, but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it =>

9. Synthesis of Acetylene
Heat coke with lime in an electric furnace to form calcium carbide.
Then drip water on the calcium carbide.
coke
lime
*This reaction was used to produce light for miners’ lamps and for the stage => *

10. Electronic Structure The sigma bond is sp-sp overlap.
The two pi bonds are unhybridized p
overlaps at 90, which blend into a
cylindrical shape.
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11. Bond Lengths More s character, so shorter length.
Three bonding overlaps, so shorter.
Bond angle is 180, so linear geometry =>

12. Acidity of Alkynes
Terminal alkynes, R-CC-H, are more acidic than other hydrocarbons.
Acetylene  acetylide by NH2-, but not by OH- or RO-.
More s character, so pair of electrons in anion is held more closely to the nucleus. Less charge separation, so more stable =>

13. Acidity Table
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14. Migration of Triple Bond
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15. Forming Acetylide Ions
H+ can be removed from a terminal alkyne by sodium amide, NaNH2.
NaNH2 is produced by the reaction of ammonia with sodium metal.

16. Qualitative Test
Reagent is AgNO3 or CuNO3 in alcohol, or ammonia is added to form the complex ion.
The solid is explosive when dry.
Copper tubing is not used with acetylene =>

17. Bimolecular Nucleophilic Substitution
Figure: UN
Caption:
One of the best methods for synthesizing substituted alkynes is a nucleophilic attack by the acetylide ion on an unhindered alkyl halide.

18. Internal Alkynes from Acetylides
Acetylide ions are good nucleophiles.
SN2 reaction with 1 alkyl halides lengthens the alkyne chain.
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19. Alkyl Halide Must be 1 Acetylide ions can also remove H+
If back-side approach is hindered, elimination reaction happens via E2.
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20. Figure: UN
Caption:
Note:
A nucleophile will add to the carbon of a carbonyl forming the corresponding alkoxide which upon protonation gives an alcohol. The carbon atom of the carbonyl is partial positive and the oxygen has a partial negative charge.

21. Figure: UN
Caption:
Note:
Acetylide ion will attack ketones or aldehydes to form, upon protonation, tertiary and secondary alcohols respectively. The intermediate alkoxide is not isolated, the reaction is worked up under acidic conditions to protonate the alkoxide ions and convert them to alcohols.

22. Addition to Carbonyl
Acetylide ion + carbonyl group yields an alkynol (alcohol on carbon adjacent to triple bond).
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23. Formation of Primary Alcohol
Product is a primary alcohol with one more carbon than the acetylide.
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24. Reaction of Aldehydes
Product is a secondary alcohol, one R group from the acetylide ion, the other R group from the aldehyde.
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25. Reaction of Ketones
Product is a tertiary alcohol.
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26. Synthesis by Elimination
Removal of two molecules of HX from a vicinal or geminal dihalide produces an alkyne.
First step (-HX) is easy, forms vinyl halide.
Second step, removal of HX from the vinyl halide requires very strong base and high temperatures =>

27. Figure: UN
Caption:
Note:
Vicinal or geminal dihalides can be dehydrohalogenated by strong bases to produce vinyl halides. The vinyl halides can be isolated or can be reacted with a second equivalent of base to produce the corresponding alkyne. The second dehydrohalogenation requires extremely basic conditions and heat.

28. Reagents for Elimination
Molten KOH or alcoholic KOH at 200C favors an internal alkyne.
Sodium amide, NaNH2, at 150C, followed by water, favors a terminal alkyne.
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29. Addition Reactions Similar to addition to alkenes
Pi bond becomes two sigma bonds.
Usually exothermic
One or two molecules may add.
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30. Addition of hydrogen: three different reactions:
Reduction to alkane
Reduction to cis-alkene.
Reduction to trans-alkene =>

31. Reduction to Alkane
Add lots of H2 with metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated.

32. Lindlar’s Catalyst - Poisoned
Powdered BaSO4 coated with Pd, poisoned with quinoline.
H2 adds syn, so cis-alkene is formed.
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33. Figure: UN
Caption:
To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes.

34. Na in Liquid Ammonia Use dry ice to keep ammonia liquid.
As sodium metal dissolves in the ammonia, it loses an electron.
The electron is solvated by the ammonia, creating a deep blue solution.
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35. Mechanism
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36. Figure: UN
Caption:
Note:
Alkynes can add one or two equivalents of halogen across the triple bond. If only one mole of halogen is used the product obtained will be the dihaloalkene. This addition will not be stereoselective and mixtures of cis and trans isomers will be obtained.

37. Addition of Halogens
Cl2 and Br2 add to alkynes to form vinyl dihalides.
May add syn or anti, so product is mixture of cis and trans isomers.
Difficult to stop the reaction at dihalide.
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38. Figure: UN
Caption:
Note:
One or two molecules of hydrogen halides can be added to an alkyne to form vinyl halides or geminal dihalides, respectively. When a terminal alkyne is used, the addition of HX follows Markovnikov's rule.

39. Addition of HX HCl, HBr, and HI add to alkynes to form vinyl halides.
For terminal alkynes, Markovnikov product is formed.
If two moles of HX is added, product is a geminal dihalide.
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40. Figure: UN
Caption:
Note:
By using peroxides, hydrogen bromide can be added to a terminal alkyne anti-Markovnikov. The bromide will attach to the least substituted carbon giving a mixture of cis and trans isomers.

41. HBr with Peroxides
Anti-Markovnikov product is formed with a terminal alkyne.
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42. Hydration of Alkynes
Mercuric sulfate in aqueous sulfuric acid adds H-OH to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone.
Hydroboration-oxidation adds H-OH with an anti-Markovnikov orientation, and rearranges to an aldehyde =>

43. Figure: UN
Caption:
Note:
Water can be added across the triple bond in a reaction analogous to the oxymercuration-demercuration of alkenes. The hydration is catalyzed by the mercuric ion. In a typical reaction a mixture of mercuric acetate in aqueous sulfuric acid is used. The addition produces an intermediate vinyl alcohol (enol) that quickly tautomerizes to the more stable ketone or aldehyde.

44. Mechanism for Mercuration
Mercuric ion (Hg2+) is electrophile.
Vinyl carbocation forms on most-sub. C.
Water is the nucleophile.
an enol
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45. Figure: UN
Caption:
Note:
Enols are not stable species so the proton from the alcohol shifts to the neighboring carbon and the double bond shifts from the C=C to the C=O position. This process is an equilibrium between the two forms with the keto form being favored, and it is called tautomerization.

46. Enol to Keto (in Acid) Add H+ to the C=C double bond.
Remove H+ from OH of the enol.
A methyl ketone
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47. Figure: UN
Caption:
Note:
In acidic solution a proton adds to the methylene group of the enol. The intermediate is stabilized by resonance. In the second step of the reaction a hydroxyl proton is abstracted by a water molecule forming the keto form.

48. Figure: UN
Caption:
Note:
Alkynes can be hydrated anti-Markovnikov by using the hydroboration-oxidation reaction. A hindered alkyl borane needs to be used to prevent two molecules of borane to add to the triple bond. A molecule of borane adds to the triple bond to form a vinyl borane. If a terminal alkyne is used, the borane will add to the least substituted carbon.

49. Hydroboration Reagent
Di(secondary isoamyl)borane, called disiamylborane.
Bulky, branched reagent adds to the least hindered carbon.
Only one mole can add =>

50. Hydroboration - Oxidation
B and H add across the triple bond.
Oxidation with basic H2O2 gives the enol.
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51. Enol to Keto (in Base) H+ is removed from OH of the enol.
Then water gives H+ to the adjacent carbon.
An aldehyde
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52. Oxidation of Alkynes Similar to oxidation of alkenes.
Dilute, neutral solution of KMnO4 oxidizes alkynes to a diketone.
Warm, basic KMnO4 cleaves the triple bond.
Ozonolysis, followed by hydrolysis, cleaves the triple bond =>

53. Figure: UN
Caption:
Note:
Under neutral conditions, potassium permanganate can oxidize a triple bond into an a-diketone. The reaction used aqueous KMnO4 to form a tetrahydroxy intermediate which loses two water molecules to produce the diketone.

54. Reaction with KMnO4 Mild conditions, dilute, neutral
Harsher conditions, warm, basic
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55. Ozonolysis
Ozonolysis of alkynes produces carboxylic acids (Alkenes gave aldehydes and ketones)
Used to find location of triple bond in an unknown compound =>

56. Figure: UN
Caption:
Note:
If potassium permanganate is used under basic conditions or if the solution is heated too much, an oxidative cleavage will take place and two molecules of carboxylic acids will be produced.

57. Product of KMnO4 oxidation