Aldehydes and Ketones II. Oxidation and Reduction: Synthesis

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Aldehydes and Ketones II. Oxidation and
Reduction Synthesis

• Chapter 17

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Assignment for Chapter 17

• Sections 17.0 through 17.2
• Section 17.3 -- SKIP
• Sections 17.4 through 17.12
• Section 17.13 through 17.15 -- SKIP
• Sections 17.16 through 17.20
• Problems

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Problem Assignment

• In-Text Problems
• 17-1 through 17-11
• 17-15 through 17-18
• End-of-Chapter Problems
• 1 through 4 (all parts)

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Reduction of a Carbonyl Group to an Alcohol
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What If We Had the Worlds Simplest Nucleophile?
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• If we could make this work, we would have means
  of reducing the carbonyl group.
• Go back and re-read the Special Topics box at the
  beginning of Chapter 14.

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Lithium Aluminum Hydride
Lithium aluminum hydride, LiAlH4, is a very
powerful hydride donor, and thus it is an
excellent reducing agent.
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Outline
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Lithium Aluminum HydrideReduction of
Aldehydesand Ketones
These reactions typically go in very high yield.
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Learn it this way
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Aluminum is required to coordinate with the
oxygen of the carbonyl group. For example, in the
absence of aluminum (using sodium hydride, NaH as
the reducing agent), there is no reduction.
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You Must Be Careful!
LiOH Al(OH)
4 H2
LiAlH
4 H
O
4
2
3
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A similar, but less reactive, reducing agent is
sodium borohydride, NaBH4.
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Sodium Borohydride Reduction of Aldehydes and
Ketones
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Example
In general, the reduction should proceed equally
easily from either side, yielding two
mirror-image products.
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Sodium Borohydride Reduction of Camphor
(endo)
(exo)
In this case, approach by borohydride from the
upper side is sterically hindered by the bridge
methyl group. The result is a predominance of
exo alcohol.
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Sodium Borohydride Reduction of Norcamphor

• Here, it is the underside of the molecule that is
  more sterically hindered, so the favored alcohol
  is the endo alcohol.
• Similar results are obtained with lithium
  aluminum hydride.

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Another example
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Contrast these two reactions
Note that sodium borohydride is more selective.
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Catalytic Hydrogenation ofAldehydes and Ketones
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Reduction of a Carbonyl Group to a Methylene Group
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We are trying to accomplish...
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Clemmensen Reduction
Zn(Hg) is a zinc amalgam
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• The mechanism of the Clemmensen reduction
  involves the formation of some sort of
  zinc-ketone complex.
• This reaction is a good method for the reduction
  of acid-stable carbonyl compounds (Note that the
  reaction takes place in the presence of
  concentrated HCl).

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Example of a Clemmensen Reduction
Notice that the reduction does not affect the
carboxyl group
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Wolff-Kishner Reduction
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• The mechanism involves the formation of a
  hydrazine adduct, which loses water to form a
  hydrazone.
• The hydrazone then rearranges to lose a molecule
  of nitrogen gas, leaving behind a carbanion.
• The carbanion picks up a proton from the solvent
  to form the product.

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This will never show up on any of my exams!
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The Wolff-Kishner reduction is useful for the
reduction of carbonyl compounds that are stable
in strong base. (Note that the reaction takes
place in the presence of concentrated KOH or
other base)
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Example
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Desulfurization
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• Advantages High yields
  Mild and nearly neutral
  reaction conditions
• Disadvantages Stinky!
  Raney Nickel is tricky to prepare

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Example of a Raney Nickel Desulfurization
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