Reactions of Alkynes' Introduction to Multistep Synthesis Chapter 6

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Reactions of Alkynes. Introduction to Multistep
SynthesisChapter 6

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Contents of Chapter 6

• General Formulas of Alkynes and Nomenclature
• Multifunctional Group Nomenclature
• Structure and Properties of Alkynes
• Reactivity Considerations
• Addition of Hydrogen Halides and Halogens
• Other Additions
• Hydrogenation of Alkynes
• Acidity of Hydrogens Bonded to sp Carbons
• Introduction to Multistep Synthesis

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Alkynes

• If the triple bond is at the end of the chain,
  the alkyne is known as a terminal alkyne
• Terminal alkynes must be treated with special
  reagents (disiamylborane or HgSO4 with H2O/H2SO4

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Alkynes

• If the triple bond is not at the end of the
  chain, the alkyne is known as an internal alkyne
• Internal alkynes can be treated with normal
  alkene reagents (BH3 and H2SO4/H2O)

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Alkynes

• Common names of alkynes are based on
  substitution of the simplest alkyne, acetylene

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IUPAC Nomenclature of Alkynes

• Find the longest chain containing the triple bond
  and change the corresponding -ane ending to
  -yne
• The chain is numbered in direction that gives the
  triple bond, the lower number
• If the same number for the triple bond is
  obtained in numbering from both directions, the
  number for the substituent nearest the chain end
  is minimized

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IUPAC Nomenclature of Dienes

• Find the longest chain containing both double
  bonds

1
2
3
4
5
3-butyl-1,4-pentadiene
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IUPAC Nomenclature of Dienes

• Use corresponding alkane name but replace the
  ne ending with diene

3-butyl-1,4-pentadiene
pentane changed to pentadiene
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IUPAC Nomenclature of Dienes

• Number in the direction that gives the lowest
  number to a double bond

1,5-heptadiene not 2,6-heptadiene
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IUPAC Nomenclature of Dienes

• List substituents in alphabetical order

5-ethyl-2-methyl-2,4-heptadiene
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IUPAC Nomenclature of Dienes

• Place numbers indicating the double bond
  positions either in front of the parent compound
  or in the middle of the name immediately before
  the diene suffix

5-ethyl-2-methyl-2,4-heptadiene or
5-ethyl-2-methyl-hepta-2,4-diene
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IUPAC Nomenclature of Dienes
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Configurational Isomers of Dienes
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IUPAC Multifunctional Compound Nomenclature
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IUPAC Multifunctional Compound Nomenclature

• The longest chain has to go past the
  highest-priority functional group
• High prio group has lowest possible number
• If not highest priority NH2 is amino and OH is
  hydroxy substituent
• General form is n-substit-n-alken-n-yn-n-groupsuff
  ix

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Reactivity Considerations

• The hydrohalogenation product is an alkene which
  can undergo a second electrophilic addition
  reaction
• First halogen follows alkene hydrohalogenation
  regioselectivity rules
• Second halogen goes on same carbon as first
  halogen

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Relative Stabilities of Carbocations

• Vinyl cations are one level less stable than
  alkyl cations

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Hydrogen Halide Addition in the presence of
Peroxide

• Hydrogen peroxide has same effect on hydrogen
  bromide addition to an alkyne as to an alkene
  (reversed regioselectivity).

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Halogen Addition to Alkynes

• Halogens add to alkynes as well as to alkenes
• Excess halogen leads to the addition of a second
  equivalent

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Addition of Water to Alkynes

• Water adds to alkynes in the presence of acid to
  yield an enol

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Addition of Water to Alkynes

• However the initially formed enol reacts further
  to produce a ketone

Such isomers, differing only in the placement of
a hydrogen atom, are called tautomers
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Mercuric-Ion-Catalyzed Addition of Water to
Alkynes

• Terminal alkynes need catalytic help to undergo
  hydration.
• Internal alkynes dont need the catalyst.

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HydroborationOxidation

• Hydroboration of an internal alkyne leads to a
  ketone

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HydroborationOxidation

• For terminal alkynes, a bulkier borane is needed
  to prevent addition of a second equivalent of
  borane to the resultant double bond

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Addition of Water to an Alkyne
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Addition of Hydrogen to an Alkyne
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Acidity of a Hydrogen Bonded to an sp Carbon

• The conjugate bases have the following relative
  base strength because the stronger the acid, the
  weaker the conjugate base

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Syntheses Using Acetylide Ions

• Alkylation reactions work best with primary
  alkyl halides and methyl halides

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Introduction to Multistep Synthesis

• The thought process is known as retrosynthetic
  analysis and is indicated by using open arrows

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Introduction to Multistep Synthesis -
Considerations

• Alkene products made from alkynes
• Carbonyl products made from alkynes
• Alkane products made from alkenes
• Know reactions pgs 259-260 thoroughly
• Understand issues involved in proper choice of
  reagents thoroughly

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Introduction to Multistep Synthesis Reagent
Issues

• Regioselectivity of HX alkene/alkyne rxn
• Regioselectivity of 2 HX alkyne rxn
• Regioselectivity of HBr/peroxide rxn
• Regioselectivity of oxidative hydroboration
• Stereospecificity of halogenation rxn
• Regio/stereo/specificity of X2/ROH or X2/water
  rxn
• Stereospecificity of H2/Lindlar reduction
• Stereoselectivity of Na/NH3 reduction
• H2 with Pd/C, Pt/C, or Ni does complete reduction
• Terminal alkynes need disiamylborane or HgSO4
• Carbocation rearrangements prevented with
  Hg(OAc)2
• RX Na--R rxn works best with primary halide

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Introduction to Multistep Synthesis Practice

• Use retrosynthetic analysis to make these