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Reactions of Alkynes' Introduction to Multistep Synthesis Chapter 6

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5-ethyl-2-methyl-2,4-heptadiene. Chapter 6. 11. IUPAC Nomenclature of Dienes ... 5-ethyl-2-methyl-2,4-heptadiene. or 5-ethyl-2-methyl-hepta-2,4-diene. Chapter 6. 12 ... – PowerPoint PPT presentation

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Title: Reactions of Alkynes' Introduction to Multistep Synthesis Chapter 6


1
Reactions of Alkynes. Introduction to Multistep
SynthesisChapter 6

2
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

3
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

4
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)

5
Alkynes
  • Common names of alkynes are based on
    substitution of the simplest alkyne, acetylene

6
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

7
IUPAC Nomenclature of Dienes
  • Find the longest chain containing both double
    bonds

1
2
3
4
5
3-butyl-1,4-pentadiene
8
IUPAC Nomenclature of Dienes
  • Use corresponding alkane name but replace the
    ne ending with diene

3-butyl-1,4-pentadiene
pentane changed to pentadiene
9
IUPAC Nomenclature of Dienes
  • Number in the direction that gives the lowest
    number to a double bond

1,5-heptadiene not 2,6-heptadiene
10
IUPAC Nomenclature of Dienes
  • List substituents in alphabetical order

5-ethyl-2-methyl-2,4-heptadiene
11
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
12
IUPAC Nomenclature of Dienes
13
Configurational Isomers of Dienes
14
IUPAC Multifunctional Compound Nomenclature
15
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

16
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

17
Relative Stabilities of Carbocations
  • Vinyl cations are one level less stable than
    alkyl cations

18
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).

19
Halogen Addition to Alkynes
  • Halogens add to alkynes as well as to alkenes
  • Excess halogen leads to the addition of a second
    equivalent

20
Addition of Water to Alkynes
  • Water adds to alkynes in the presence of acid to
    yield an enol

21
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
22
Mercuric-Ion-Catalyzed Addition of Water to
Alkynes
  • Terminal alkynes need catalytic help to undergo
    hydration.
  • Internal alkynes dont need the catalyst.

23
HydroborationOxidation
  • Hydroboration of an internal alkyne leads to a
    ketone

24
HydroborationOxidation
  • For terminal alkynes, a bulkier borane is needed
    to prevent addition of a second equivalent of
    borane to the resultant double bond

25
Addition of Water to an Alkyne
26
Addition of Hydrogen to an Alkyne
27
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

28
Syntheses Using Acetylide Ions
  • Alkylation reactions work best with primary
    alkyl halides and methyl halides

29
Introduction to Multistep Synthesis
  • The thought process is known as retrosynthetic
    analysis and is indicated by using open arrows

30
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

31
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

32
Introduction to Multistep Synthesis Practice
  • Use retrosynthetic analysis to make these
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