1. 7-1 Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson William H. Brown Christopher S. Foote Brent L. Iverson

2. 7-2 Alkynes Chapter 7

3. 7-3 Nomenclature yn  IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond  Common names: prefix the substituents on the triple bond to the word “acetylene” Common name: IUPAC name: DimethylacetyleneVinylacetylene 2-Butyne1-Buten-3-yne

4. 7-4 Cycloalkynes  Cyclononyne is the smallest cycloalkyne isolated it is quite unstable and polymerizes at room temp the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain

5. 7-5 Physical Properties  Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

6. 7-6 Acidity  The pK a of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1) terminal alkynes react with sodium amide to form alkyne anions

7. 7-7 Acidity terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA) because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion

8. 7-8 Alkylation of Alkyne Anions  Alkyne anions are both strong bases and good nucleophiles nucleophilic substitution alkylation  They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation because alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halides with 2° and 3° halides, elimination is the major reaction

9. 7-9 Alkylation of Alkyne Anions alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne

10. 7-10 Preparation from Alkenes dehydrohalogenation  Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne

11. 7-11 Preparation from Alkenes for a terminal alkene to a terminal alkyne, 3 moles of base are required

12. 7-12 Preparation from Alkenes allenea side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C A haloalkene (a vinylic halide) An allene An alkyne RCCCR R H R H X H NaNH 2 RC–C=CR -HBr CCC R RR H

13. 7-13 Allene  Allene:  Allene: a compound containing a C=C=C group the simplest allene is 1,2-propadiene, commonly named allene

14. 7-14 Allenes most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions

15. 7-15 Addition of X 2  Alkynes add one mole of bromine to give a dibromoalkene addition shows anti stereoselectivity

16. 7-16 Addition of X 2 the intermediate in bromination of an alkyne is a bridged bromonium ion

17. 7-17 Addition of HX  Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed 2,2-Dibromopropane2-BromopropenePropyne CH 3 CCH Br Br Br HBr CH 3 C=CH 2 CH 3 CCH 3 HBr

18. 7-18 Addition of HX the intermediate in addition of HX is a 2° vinylic carbocation reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product

19. 7-19 Addition of HX in the addition of the second mole of HX, Step 1 is reaction of the electron pair of the remaining pi bond with HBr to form a carbocation of the two possible carbocations, the favored one is the resonance-stabilized 2° carbocation

20. 7-20 Hydroboration  Addition of borane to an internal alkyne gives a trialkenylborane addition is syn stereoselective

21. 7-21 Hydroboration to prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia) 2 BH treatment of a terminal alkyne with (sia) 2 BH results in stereoselective and regioselective hydroboration

22. 7-22 Hydroboration  Treating an alkenylborane with H 2 O 2 in aqueous NaOH gives an enol enol:enol: a compound containing an OH group on one carbon of a carbon-carbon double bond an enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=O keto forms generally predominate at equilibrium tautomers tautomerismketo and enol forms are tautomers and their interconversion is called tautomerism

23. 7-23 Hydroboration hydroboration/oxidation of an internal alkyne gives a ketone hydroboration/oxidation of a terminal alkyne gives an aldehyde 3-Hexanone3-Hexyne 1. BH 3 2. H 2 O 2, NaOH O

24. 7-24 Addition of H 2 O: hydration  In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water + Propanone (Acetone) 1-Propen-2-ol (an enol) Propyne CH 3 CCH OH O HgSO 4 H 2 SO 4 H 2 OCH 3 C=CH 2 CH 3 CCH 3

25. 7-25 Addition of H 2 O: hydration Step 1: attack of Hg 2+ (an electrophile) on the triple bond (a nucleophile) gives a bridged mercurinium ion Step 2: attack of water (a nucleophile) on the bridged mercurinium ion intermediate (an electrophile) opens the three-membered ring

26. 7-26 Addition of H 2 O: hydration Step 3: proton transfer to solvent gives an organomercury enol Step 4: tautomerism of the enol gives the keto form

27. 7-27 Addition of H 2 O: hydration Step 5: proton transfer to the carbonyl oxygen gives an oxonium ion Steps 6 and 7: loss of Hg 2+ gives an enol; tautomerism of the enol gives the ketone

28. 7-28 Reduction  Treatment of an alkyne with hydrogen in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane

29. 7-29 Reduction  With the Lindlar catalyst, reduction stops at addition of one mole of H 2 this reduction shows syn stereoselectivity

30. 7-30 Hydroboration - Protonolysis  Addition of borane to an internal alkyne gives a trialkenylborane addition is syn stereoselective treatment of a trialkenylborane with acetic acid results in stereoselective replacement of B by H

31. 7-31 Dissolving Metal Reduction  Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity

32. 7-32 Dissolving Metal Reduction Step 1: a one-electron reduction of the alkyne gives a radical anion Step 2: the alkenyl radical anion (a very strong base) abstracts a proton from ammonia (a very weak acid)

33. 7-33 Dissolving Metal Reduction Step 3: a second one-electron reduction gives an alkenyl anion this step establishes the configuration of the alkene a trans alkenyl anion is more stable than its cis isomer Step 4: a second acid-base reaction gives the trans alkene + Na R CC R H R CC R H + Na + An alkenyl anion. :

34. 7-34 Organic Synthesis  A successful synthesis must provide the desired product in maximum yield have the maximum control of stereochemistry and regiochemistry do minimum damage to the environment (it must be a “green” synthesis)  Our strategy will be to work backwards from the target molecule

35. 7-35 Organic Synthesis  We analyze a target molecule in the following ways the carbon skeleton: how can we put it together. Our only method to date for forming new a C-C bond is the alkylation of alkyne anions (Section 7.5) the functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and how can they be changed to give the functional groups of the target molecule

36. 7-36 Organic Synthesis  We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis  Retrosynthesis:  Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials target molecule starting materials

37. 7-37 Organic Synthesis  Target molecule: cis-3-hexene

38. 7-38 Organic Synthesis starting materials are acetylene and bromoethane

39. 7-39 Organic Synthesis  Target molecule: 2-heptanone

40. 7-40 Organic Synthesis starting materials are acetylene and 1-bromopentane

41. 7-41 Alkynes End Chapter 7