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Organic Chemistry. William H. Brown Christopher S. Foote Brent L. Iverson. Alkynes. Chapter 7. D. imethylacetylene. V. inylacetylene. Nomenclature. IUPAC: use the infix - yn - to show the presence of a carbon-carbon triple bond

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Organic chemistry l.jpg

Organic Chemistry

William H. Brown

Christopher S. Foote

Brent L. Iverson


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Alkynes

Chapter 7


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D

imethylacetylene

V

inylacetylene

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 word “acetylene”

IUPAC name:

2-Butyne

1-Buten-3-yne

Common name:


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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


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Physical Properties

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


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Acidity

  • The pKa 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


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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


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Alkylation of Alkyne Anions

  • Alkyne anions are both strong bases and good nucleophiles

  • 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


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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


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Preparation from Alkenes

  • 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


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Preparation from Alkenes

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


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N

a

N

H

2

R

C

C

=

C

R

-

H

B

r

R

C

C

C

R

A haloalkene

(a vinylic halide)

Preparation from Alkenes

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

R

H

C

C

C

X

H

H

R

R

An allene

H

R

R

An alkyne


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Allene

  • Allene: a compound containing a C=C=C group

    • the simplest allene is 1,2-propadiene, commonly named allene


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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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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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B

r

B

r

H

B

r

H

B

r

C

H

C

C

H

C

H

C

=

C

H

C

H

C

C

H

3

3

2

3

3

B

r

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

Propyne

2-Bromopropene

2,2-Dibromopropane


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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


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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


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Hydroboration

  • Addition of borane to an internal alkyne gives a trialkenylborane

    • addition is syn stereoselective


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Hydroboration

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

  • treatment of a terminal alkyne with (sia)2BH results in stereoselective and regioselective hydroboration


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Hydroboration

  • Treating an alkenylborane with H2O2 in aqueous NaOH gives an 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

    • keto and enol forms aretautomersand their interconversion is called tautomerism


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1

.

B

H

3

2

.

H

O

,

N

a

O

H

2

2

Hydroboration

  • hydroboration/oxidation of an internal alkyne gives a ketone

  • hydroboration/oxidation of a terminal alkyne gives an aldehyde

O

3-Hexyne

3-Hexanone


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O

H

H

S

O

2

4

C

H

C

C

H

C

H

C

C

H

H

O

C

H

C

=

C

H

3

3

3

2

3

2

H

g

S

O

4

1-Propen-2-ol

Propanone

(an enol)

(Acetone)

Addition of H2O: hydration

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

O

+

Propyne


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Addition of H2O: hydration

  • Step 1: attack of Hg2+ (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


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Addition of H2O: hydration

  • Step 3: proton transfer to solvent gives an organomercury enol

  • 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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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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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


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Dissolving Metal 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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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)


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+

N

a

N

a

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

R

R

.

:

+

+

C

C

C

C

R

R

H

H

An alkenyl anion


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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 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


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target

starting

molecule

materials

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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Alkynes

End Chapter 7