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Organic Chemistry. William H. Brown & Christopher S. Foote. Alkynes. Chapter 10. Chapter 10. Nomenclature. 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”. Cycloalkynes.

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Organic chemistry l.jpg

Organic Chemistry

William H. Brown & Christopher S. Foote


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Alkynes

Chapter 10

Chapter 10


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Nomenclature

  • 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”


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Cycloalkynes

  • The smallest cycloalkyne isolated is cyclononyne

    • the C-C-C bond angle about the triple bond is approximately 156°


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Physical Properties

  • Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton


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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 pKa of acetylene is approximately 25, which makes it a stronger acid than ammonia but weaker than alcohols


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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)


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Acidity

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


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Alkylation of Acetylides

  • Acetylide anions are both strong bases and good nucleophiles

  • They undergo SN2 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.


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Alkylation of Acetylides

  • Alkylation of acetylide anions is the most convenient method for the synthesis of terminal alkynes


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Alkylation of Acetylides

  • Alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne


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Alkylation of Acetylides

  • With 2° and 3° alkyl halides, E2 is the major reaction


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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


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Preparation

  • Alkynes are also prepared by double dehydrohalogenation of geminal dihalides


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Preparation

  • An alkene can be converted to an alkyne

    • for a terminal alkene to a terminal alkyne, 3 moles of NaNH2 are required

    • for an internal alkene to an internal alkyne, only 2 moles of NaNH2 are required


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Preparation

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


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Allenes

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


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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


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Reduction

  • With the Lindlar catalyst, reduction stops at addition of one mole of H2

    • this reduction shows syn stereoselectivity


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Reduction

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


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Na/NH3(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


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Na/NH3(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


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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


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Hydroboration

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


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Hydroboration

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


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Hydroboration

  • Treatment of an alkenylborane with H2O2 in aqueous NaOH gives an enol


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Hydroboration

  • 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

  • The enol and keto forms aretautomersand their interconversion is called tautomerism

    • the keto form generally predominates at equilibrium

    • we discuss keto-enol tautomerism in detail in Section 16.11


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Hydroboration

  • Hydroboration/oxidation of an internal alkyne gives a ketone


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Hydroboration

  • Hydroboration/oxidation of a terminal alkyne gives an aldehyde


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Addition of X2

  • Alkynes add one mole of bromine to give a dibromoalkene

    • addition shows anti stereoselectivity


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Addition of X2

  • The intermediate in bromination of an alkyne is a bridged bromonium ion


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Addition of HX

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


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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


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Addition of H2O:hydration

  • In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water


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Addition of H2O:hydration

  • Step 1: attack of Hg2+ 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


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Addition of H2O:hydration

  • Step 3: proton transfer to solvent

  • Step 4: tautomerism of the enol gives the keto form


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Addition of H2O:hydration

  • Step 5: proton transfer to the carbonyl oxygen gives an oxonium ion

  • Steps 6 and 7: loss of Hg2+ gives an enol; tautomerism of the enol gives the ketone


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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


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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


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Organic Synthesis

  • We use a method called a retrosynthesis and use an open arrow to symbolize a step in a retrosynthesis

  • Retrosynthesis: a process of reasoning backwards from a target molecule to a set of suitable starting materials


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Organic Synthesis

  • Target molecule: cis-3-hexene


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Organic Synthesis

  • starting materials are acetylene and bromoethane


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Organic Synthesis

  • Target molecule: 2-heptanone


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Organic Synthesis

  • starting materials are acetylene and 1-bromopentane


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Prob 10.9

Predict bond angles about each highlighted atom.


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Prob 10.10

State the orbital hybridization of each highlighted atom.


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Prob 10.11

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


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Prob 10.12

How many stereoisomers are possible for enanthotoxin?


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Prob 10.13

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


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Prob 10.15

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


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Prob 10.17

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


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Prob 10.18

Propose a mechanism for this reaction.


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Prob 10.21

Show how to convert propene to each compound.


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Prob 10.22

Show how to bring about each conversion.


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Prob 10.24

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



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Prob 10.26

Show how to bring about each conversion.


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Prob 10.31

Show how to bring about this conversion.


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Alkynes

End Chapter 10


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