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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 Organic Chemistry William H. Brown & Christopher S. Foote

2 Alkynes Chapter 10

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 name “acetylene”

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

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

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

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

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

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

13 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 Preparation  Alkynes are also prepared by double dehydrohalogenation of geminal dihalides

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

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

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

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

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

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

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

27 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 Hydroboration  Hydroboration/oxidation of an internal alkyne gives a ketone

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

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

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

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

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

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

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

42 Organic Synthesis starting materials are acetylene and bromoethane

43 Organic Synthesis  Target molecule: 2-heptanone

44 Organic Synthesis starting materials are acetylene and 1-bromopentane

45 Prob 10.9 Predict bond angles about each highlighted atom.

46 Prob State the orbital hybridization of each highlighted atom.

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

48 Prob How many stereoisomers are possible for enanthotoxin?

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

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

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

52 Prob Propose a mechanism for this reaction.

53 Prob Show how to convert propene to each compound.

54 Prob Show how to bring about each conversion.

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

56 Prob 10.25

57 Prob Show how to bring about each conversion.

58 Prob Show how to bring about this conversion.

59 Alkynes End Chapter 10

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