Alkynes

1. Alkynes

2. Hydrocarbons with a carbon–carbon triple bond are alkynes. Noncyclic alkynes have the molecular formula CnH2n-2. Acetylene (HC≡ CH) is the simplest alkyne. Compounds with the triple bond at the end of a carbon chain (RC≡ CH) are monosubstituted, or terminal, alkynes. Disubstituted alkynes (RC≡ CR′) have internal triple bonds. Alkynes

3. In the late 19th century calcium carbide was formed by heating coke and limestone. Sources of Alkynes The calcium carbide was hydrolysed to form acetylene.

4. One alternative synthesis is the dehydration of ethylene. The endothermic reaction favors acetylene at high temperature. Sources of Alkynes Enzymes known as acetylenases catalyze this reactionin nature.

5. Some bacteria produce dynemicin A which has been shown to cleave DNA. Naturally Occurring Alkynes Diacetylene has been identified asa component of the atmospheresof Uranus, Neptune and Pluto. Some fatty acids contain an alkyne, e.g. stearolic acid.

6. Replace the suffix –ane for the corresponding alkane with –yne. Naming Alkynes

7. Compounds that contain both an alkene and an alkyneare numbered to give the first multiple bond the lowest number and are named as an –enynes. Naming Alkynes hept-1-en-5-yne

8. Alkynes resemble alkanes and alkenes in physical properties.Low density and low water solubility.Boiling points similar to the corresponding alkane. Physical Properties of Alkynes

9. Acetylene is linear. Structure of Alkynes Cyclononane, the smallest stable cycloalkyne is strainedas the C-C≡ C-C unit is clearly bent.

10. The C≡ C has two p-bonds and one s-bond. The s-bond is formed by overlap of sp orbitals. The p-bonds areformed by overlap of p orbitals. The p-bonds are orthogonal to each other – shown in (b) and (c). Bonding in Alkynes

11. The electrostatic potential maps shows the high electron density (red) associated with the p-bonds. Bonding in Alkynes

12. Bonding in Alkanes, Alkenes and Alkynes

13. Bonding in Alkanes, Alkenes and Alkynes For the series ethane → ethylene → acetylene the general trends are: 1. The geometry at carbon changes from tetrahedral → trigonal planar → linear. 2. The C-C and C-H bonds become shorter and stronger. 3. The acidity of the C-H bonds increases.

14. C-H bond lengths are shortest for hydrogens bonded toa sp C. The sp orbital has higher s character than sp2orbitals so it is closer to the carbon atom. C-H Bond Lengths

15. C-H bonds of alkanes, alkenes and alkynes are very weak acids. The pKa of methane is estimated to be 60. Acidity of Alkanes, Alkenes and Alkynes Because it is derived from a very weak acid the conjugate base, a carbanion, is a very strong base.

16. The effective electronegativity of carbon increases with increasing s-character (sp3>sp2>sp) therefore the pKadecreases in the same order. Acidity Trend

17. After deprotonation the acetylide electron pair is in a sporbital. Acidity of Alkynes Terminal alkynes RC≡ CH are similar to acetylene in acidity.

18. In order to form the acetylide anion the base must bestrong enough. The conjugate acid of the base should have pKa greater than the pKa of the alkyne.Hydroxide is too weak and the equilibrium lies to the left. Acid-Base Reactions of Alkynes Amide is strong enough and the equilibrium lies to the right.

19. Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes

20. Acetylide anions react with methyl and primary alkyl halides to form substituted alkynes. Acetylide Anions as Nucleophiles This is an SN2 type of reaction.

21. Acetylide Anions as Nucleophiles Examples include: The reagents are listed above and below the reaction arrow in the order they are used. So, in the first reaction,acetylene is first deprotonated with sodium amide in ammonia and then reacted with ethyl bromide.

22. Acetylide anions are very basic so they act as bases if the alkyl halide is secondary or tertiary. Elimination Reactions with Acetylide Anions

23. Alkynes can be prepared by double dehydrohalogenation of either geminal dihalides or vicinal dihalides. Preparation of Alkynes by Elimination

24. Three equivalents of base are required if the alkyne is terminal since a terminal alkyne is deprotonated by the base as soon as it is formed. Protonation of the acetylide anion is then required as a second step. Preparation of Alkynes by Elimination

25. Vicinal dibromides are formed on bromination of an alkene so this gives us a way to transform an alkeneinto an alkyne. From Alkenes to Alkynes

26. Reactions of alkynes are similar to reaction of alkenes:hydrogenation, hydration, halogenation, ozonolysis. Reactions of Alkynes

27. Alkynes are reduced to alkanes using hydrogen and a transition metal catalyst. Hydrogenation of Alkynes For example:

28. The heat of hydrogenation of 1- and 2-butyne can be used to determine the relative stability of an internal and a terminal alkyne (both yield butane as product). Relative Stability of Alkynes Less heat is released by 2-butyne so that is the more stable isomer.

29. Metal catalyzed hydrogenation is a syn addition process. Stereochemistry of Hydrogenation

30. Lindlars catalyst is a partially deactivated catalyst that was developed for partial hydrogenation of alkynes to alkenes. Hydrogenation with Lindlars Catalyst Disubstituted alkynes yield cis-alkenes.

31. Group I metals in liquid ammonia reduce alkynes to trans alkenes exclusively. Metal-Ammonia Reduction of Alkynes The key intermediate is the vinyl radical which prefers theless hindered trans conformation.

32. Step 1. Electron transfer. Mechanism of Metal-Ammonia Reduction Sodium atoms dissolved in liquid ammonia dissociate into sodium ions and electrons, both solvated by ammonia. The solvated electrons are represented in e–(am). Step 2. Proton transfer.

33. Step 3. Electron transfer. Mechanism of Metal-Ammonia Reduction Step 4. Proton transfer.

34. Hydrogen halides add to alkynes to form alkenyl halides. Addition of Hydrogen Halides The reaction follows Markovkovs Rule (the proton adds to the carbon that initially has the most hydrogens).

35. Markovnikov addition suggests formation of a vinyl cation: Mechanism of Addition to Alkynes

36. Kinetics studies however suggest the involvement of two equivalents of HX: rate = k[alkyne][HX]2 A possible mechanism is: Mechanism of Addition to Alkynes

37. Excess HX leads to geminal dihalides. Addition of HX to Alkynes Since each addition of HX follows Markovnikov’s ruleboth protons are added to the same carbon.

38. Anti-Markovnikov addition of HBr is performed with peroxide initiation. Free Radical Addition of HBr

39. Hydration of alkynes yields a special alcohol – an enol. The enol has an –OH attached to an alkene carbon. Hydration of Alkynes The enol rapidly isomerizes to the keto form. This equilibration is keto-enol tautomerism. Tautomers are constitutional isomers that equilibrate bymigration of an atom or group.

40. Step 1. Protonation. Mechanism of Conversion of an Enol to a Ketone Resonance forms: Step 2. Deprotonation.

41. In general the keto form is more stable so it will be drawnas the product of a hydration reaction. Hydration of Alkynes The reaction follows Markovnikov’s rule so terminal alkynes yield methyl ketones not aldehydes.

42. Two molecules of halogen react with one alkyne to yielda tetrahaloalkane. Addition of Halogens to Alkynes

43. An anti dihaloalkene can be isolated if exactly one equivalent of halogen is used. Addition of Halogens to Alkynes

44. We can write simple reactions to produce these usefulmonomers from acetylene. What Could be Made From Acetylene but Isn’t In reality they are actually made from ethylene because itis so much cheaper and easier to prepare than acetylene!

45. Ozonolysis also cleaves alkynes. The alkyne carbons are tranformed into carboxylic acid carbons. The end carbon of a terminal alkyne becomes carbonic acid which dissociatesto CO2 and H2O. Ozonolysis of Alkynes

46. Alkynes in Synthesis and Retrosynthesis

47. Applications of Alkynes in Synthesis Example: Outline a synthesis of 1,2-epoxybutane using ethyl bromide and acetylene as sources for all the carbon atoms. The epoxide may be formed from the corresponding alkene: The alkene could be made from the corresponding alkyne which could itself be prepared from an acetylide anion:

48. Applications of Alkynes in Synthesis Now list the reactions in the “forward” direction starting from acetylene.