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54c) Fill in the blanks.

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... than are needed to synthesize alkenes. ... E2 Reactions and Alkyne Synthesis Alkyl Halides and ... Alkynes Reactions of Acetylide Anions Steric ... – PowerPoint PPT presentation

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Title: 54c) Fill in the blanks.


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54c) Fill in the blanks.
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Oxidation and Reduction
Oxidation of Alcohols
  • Alcohols are oxidized to a variety of carbonyl
    compounds.

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Oxidation and Reduction
Oxidation of Alcohols
  • Recall that the oxidation of alcohols to carbonyl
    compounds is typically carried out with Cr6
    oxidants, which are reduced to Cr3 products.
  • CrO3, Na2Cr2O7, and K2Cr2O7 are strong,
    nonselective oxidants used in aqueous acid (H2SO4
    H2O).
  • PCC is soluble in CH2Cl2 (dichloromethane) and
    can be used without strong acid present, making
    it a more selective, milder oxidant.

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Oxidation and Reduction
Oxidation of 2 Alcohols
  • Any of the Cr6 oxidants effectively oxidize 2
    alcohols to ketones.

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Oxidation and Reduction
Oxidation of 1 Alcohols
  • 1 Alcohols are oxidized to either aldehydes or
    carboxylic acids, depending on the reagent.

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Oxidation and Reduction
Oxidation of 1 Alcohols
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Alkyl Halides and Elimination Reactions
E2 Reactions and Alkyne Synthesis
  • A single elimination reaction produces a ? bond
    of an alkene. Two consecutive elimination
    reactions produce two ? bonds of an alkyne.

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Alkyl Halides and Elimination Reactions
E2 Reactions and Alkyne Synthesis
  • Two elimination reactions are needed to remove
    two moles of HX from a dihalide substrate.
  • Two different starting materials can be useda
    vicinal dihalide or a geminal dihalide.

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Alkyl Halides and Elimination Reactions
E2 Reactions and Alkyne Synthesis
  • Stronger bases are needed to synthesize alkynes
    by dehydrohalogenation than are needed to
    synthesize alkenes.
  • The typical base used is NH2 (amide), used as
    the sodium salt of NaNH2. KOC(CH3)3 can also be
    used with DMSO as solvent.

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Alkyl Halides and Elimination Reactions
E2 Reactions and Alkyne Synthesis
  • The reason that stronger bases are needed for
    this dehydrohalogenation is that the transition
    state for the second elimination reaction
    includes partial cleavage of the CH bond. In
    this case however, the carbon atom is sp2
    hybridized and sp2 hybridized CH bonds are
    stronger than sp3 hybridized CH bonds. As a
    result, a stronger base is needed to cleave this
    bond.

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Alkyl Halides and Elimination Reactions
E2 Reactions and Alkyne Synthesis
Figure 8.9 Example of dehydrohalogenation of
dihalides to afford alkynes
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Alkynes
Introduction to Alkyne ReactionsAcetylide anions
  • Because sp hybridized CH bonds are more acidic
    than sp2 and sp3 hybridized CH bonds, terminal
    alkynes are readily deprotonated with strong base
    in a BrØnsted-Lowry acid-base reaction. The
    resulting ion is called the acetylide ion.

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Alkynes
Reactions of Acetylide Anions
  • Acetylide anions react with unhindered alkyl
    halides to yield products of nucleophilic
    substitution.
  • Because acetylides are strong nucleophiles, the
    mechanism of substitution is SN2, and thus the
    reaction is fastest with CH3X and 10 alkyl
    halides.

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Alkynes
Reactions of Acetylide Anions
  • Steric hindrance around the leaving group causes
    2 and 3 alkyl halides to undergo elimination
    by an E2 mechanism, as shown with
    2-bromo-2-methylpropane.
  • Thus, nucleophilic substitution with acetylide
    anions forms new carbon-carbon bonds in high
    yield only with unhindered CH3X and 1 alkyl
    halides.

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Alkynes
Reactions of Acetylide Anions
  • Acetylide anions are strong nucleophiles that
    open epoxide rings by an SN2 mechanism.
  • Backside attack occurs at the less substituted
    end of the epoxide.

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