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10-1 10 Organic Chemistry William H. Brown & Christopher S. Foote.

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Presentation on theme: "10-1 10 Organic Chemistry William H. Brown & Christopher S. Foote."— Presentation transcript:

1 10-1 10 Organic Chemistry William H. Brown & Christopher S. Foote

2 10-2 10 Alkynes Chapter 10

3 10-3 10 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 name “acetylene”

4 10-4 10 Cycloalkynes  The smallest cycloalkyne isolated is cyclononyne the C-C-C bond angle about the triple bond is approximately 156°

5 10-5 10 Physical Properties  Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

6 10-6 10 Acidity  A major difference between the chemistry of alkynes and that of alkenes and alkanes is the acidity of the hydrogen bonded to a triply bonded carbon (Section 4.4E) the pK a of acetylene is approximately 25, which makes it a stronger acid than ammonia but weaker than alcohols

7 10-7 10 Acidity  Acetylene reacts with sodium amide to form sodium acetylide it can also be converted to its metal salt by reaction with sodium hydride or lithium diisopropylamide (LDA)

8 10-8 10 Acidity  Water is a stronger acid than acetylene; hydroxide ion is not a strong enough base to convert acetylene to its anion

9 10-9 10 Alkylation of Acetylides  Acetylide anions are both strong bases and good nucleophiles  They undergo S N 2 reactions with alkyl halides, tosylates, and mesylates to form new C-C bonds to alkyl groups; that is, they undergo alkylation because acetylide anions are also strong bases, alkylation is practical only with methyl and 1° halides with 2° and 3° halides, E2 is the major reaction.

10 10-10 10 Alkylation of Acetylides  Alkylation of acetylide anions is the most convenient method for the synthesis of terminal alkynes

11 10-11 10 Alkylation of Acetylides  Alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne

12 10-12 10 Alkylation of Acetylides  With 2° and 3° alkyl halides, E2 is the major reaction

13 10-13 10 Preparation  Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive E2 reactions and formation of an alkyne

14 10-14 10 Preparation  Alkynes are also prepared by double dehydrohalogenation of geminal dihalides

15 10-15 10 Preparation  An alkene can be converted to an alkyne for a terminal alkene to a terminal alkyne, 3 moles of NaNH 2 are required for an internal alkene to an internal alkyne, only 2 moles of NaNH 2 are required

16 10-16 10 Preparation  A side product may be an allene, a compound containing adjacent carbon-carbon double bonds, C=C=C

17 10-17 10 Allenes  Most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions

18 10-18 10 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

19 10-19 10 Reduction  With the Lindlar catalyst, reduction stops at addition of one mole of H 2 this reduction shows syn stereoselectivity

20 10-20 10 Reduction  Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity

21 10-21 10 Na/NH 3 (l) Reduction Step 1: a one-electron reduction of the alkyne gives a radical anion Step 2: an acid-base reaction gives an alkenyl radical and amide ion

22 10-22 10 Na/NH 3 (l) 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

23 10-23 10 Hydroboration  Addition of borane to an internal alkynes gives a trialkenylborane  Treatment of a trialkenylborane with acetic acid results in stereoselective replacement of B by H

24 10-24 10 Hydroboration  To prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia) 2 BH

25 10-25 10 Hydroboration  Treatment of a terminal alkyne with (sia) 2 BH results stereospecific and regioselective hydroboration

26 10-26 10 Hydroboration  Treatment of an alkenylborane with H 2 O 2 in aqueous NaOH gives an enol

27 10-27 10 Hydroboration  Enol:  Enol: a compound containing an OH group on one carbon of a C=C  The enol is in equilibrium with a compound formed by migration of a hydrogen atom from oxygen to carbon and the double bond from C=C to C=O tautomers tautomerism  The enol and keto forms are tautomers and their interconversion is called tautomerism the keto form generally predominates at equilibrium we discuss keto-enol tautomerism in detail in Section 16.11

28 10-28 10 Hydroboration  Hydroboration/oxidation of an internal alkyne gives a ketone

29 10-29 10 Hydroboration  Hydroboration/oxidation of a terminal alkyne gives an aldehyde

30 10-30 10 Addition of X 2  Alkynes add one mole of bromine to give a dibromoalkene addition shows anti stereoselectivity

31 10-31 10 Addition of X 2 The intermediate in bromination of an alkyne is a bridged bromonium ion

32 10-32 10 Addition of HX  Alkynes undergo regioselective addition of first one mole of HX and then a second mole to give a dibromoalkane

33 10-33 10 Addition of HX the intermediate in addition of HX is a 2° vinylic carbocation reaction of the vinylic cation with halide ion gives the product

34 10-34 10 Addition of H 2 O:hydration  In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water

35 10-35 10 Addition of H 2 O:hydration Step 1: attack of Hg 2+ gives a bridged mercurinium ion intermediate, which contains a three-center, two- electron bond Step 2: water attacks the bridged mercurinium ion intermediate from the side opposite the bridge

36 10-36 10 Addition of H 2 O:hydration Step 3: proton transfer to solvent Step 4: tautomerism of the enol gives the keto form

37 10-37 10 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

38 10-38 10 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

39 10-39 10 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 acetylide anions (Section 10.5) 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

40 10-40 10 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

41 10-41 10 Organic Synthesis  Target molecule: cis-3-hexene

42 10-42 10 Organic Synthesis starting materials are acetylene and bromoethane

43 10-43 10 Organic Synthesis  Target molecule: 2-heptanone

44 10-44 10 Organic Synthesis starting materials are acetylene and 1-bromopentane

45 10-45 10 Prob 10.9 Predict bond angles about each highlighted atom.

46 10-46 10 Prob 10.10 State the orbital hybridization of each highlighted atom.

47 10-47 10 Prob 10.11 Describe each highlighted carbon-carbon bond in terms of the overlap of atomic orbitals.

48 10-48 10 Prob 10.12 How many stereoisomers are possible for enanthotoxin?

49 10-49 10 Prob 10.13 Show how to prepare each alkyne from the given starting material.

50 10-50 10 Prob 10.15 Complete each acid-base reaction and predict whether the equilibrium lies toward the left or toward the right.

51 10-51 10 Prob 10.17 Draw a structural formula for the enol intermediate and the carbonyl compound formed in each reaction.

52 10-52 10 Prob 10.18 Propose a mechanism for this reaction.

53 10-53 10 Prob 10.21 Show how to convert propene to each compound.

54 10-54 10 Prob 10.22 Show how to bring about each conversion.

55 10-55 10 Prob 10.24 Propose mechanisms for Steps (1) and (2) and reagents for (3) and (4).

56 10-56 10 Prob 10.25

57 10-57 10 Prob 10.26 Show how to bring about each conversion.

58 10-58 10 Prob 10.31 Show how to bring about this conversion.

59 10-59 10 Alkynes End Chapter 10

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