Organic Chemistry

1
Alkynes
Chapter 7
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7.2 Nomenclature

• A. IUPAC use the infix -yn- to show the presence
  of a carbon-carbon triple bond
• B. Common names prefix the substituents on the
  triple bond to the word acetylene

IUPAC name
2-Butyne
1-Buten-3-yne
Common name
3
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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7.3 Physical Properties, Table 7-1

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

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7.4 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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pKa values, Table 4-1
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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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7.5 A. 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 whereas 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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B. 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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Preparation from Alkenes

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

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 CCC 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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7.6 A. 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. 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-Bromopropene
Propyne
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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7.7 A. Hydroboration

• Addition of borane to an internal alkyne gives a
  trialkenylborane
• addition is syn stereoselective

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Hydroboration

• Treating an alkenylborane with H2O2 in aqueous
  NaOH gives an enol

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Enols

• 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 CC to CO
• keto forms generally predominate at equilibrium
• keto and enol forms are tautomers and their
  interconversion is called tautomerism

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Hydroboration

• to prevent dihydroboration with terminal alkynes,
  it is necessary to use a sterically hindered
  dialkylborane, such as (sia)2BH
• a terminal alkyne treated with (sia)2BH results
  in stereoselective and regioselective
  hydroboration

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Hydroboration

• hydroboration/oxidation of an internal alkyne
  gives a ketone
• hydroboration/oxidation of a terminal alkyne
  gives an aldehyde

O
3-Hexanone
3-Hexyne
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B. 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

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Addition of H2O hydration

• 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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7.8 A. 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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B. 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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C. 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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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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7.9 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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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