Aldehydes and Ketones

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Aldehydes and Ketones

• Part II

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Carbonyl vs. Cyano Group
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Carbonyl vs. Cyano Group
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Hydrolysis of a Nitrile

• An alcohol contains a hydroxyl (-OH) group.
• A nitrile contains a cyano (-CN) group.
• Nitrile is the family name a nitrile contains a
  cyano group. You recognize that a nitrile
  contains nitrogen.

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Hydrolysis of a Nitrile

• Hydrolysis means clevage (olysis) by water
  (hydro).
• How does water cleave a nitrile?
• Water adds to p bonds. A cyano group has two p
  bonds, so two water molecules add to one cyano
  group, making a gem-diol.
• What do gem-diols do?

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How does Water Add to a Nitrile?

• Water can be considered H and OH-.
• The OH- adds to the C of the cyano.
• The H adds to the N of the cyano.
• A p bond remains, so the process is repeated OH-
  to C and H to N.
• The result a gem-diol (and amino group).

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Addition of Water to a Nitrile
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Compare

• The addition of water to a carbonyl group of an
  aldehyde or ketone
• With
• The addition of water to the cyano group of a
  nitrile

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Go Back to the Previous Slide and Ask Yourself
the Following Question

• What are the similarities and differences in the
  addition of water to a carbonyl and cyano group?

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Key to Both Reactions

• Identifying the positive and negative poles of
  the reactants
• Getting the first step correct by bonding the
  positive end (H) of the reagent water to the
  negative end (O) of the substrate and the
  negative end (OH-) of the reagent to the positive
  end (C) of the substrate, while keeping four
  bonds to carbon.

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Comparison

• Similarities
• Mode of addition is the same.
• H adds to heteroatom
• OH adds to carbon
• A p bond is cleaved

• Differences
• Carbonyl has one p bond, cyano group has two
• Water (H, OH) add twice

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For Cyano A Second H2O Adds
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A Geminal Diol Loses Water

• Two groups bonded to the same carbon atom are
  geminal (twins).
• A geminal or gem-diol is two ols (OH groups)
  bonded to the same C atom.
• Gem-diols are generally less stable than the
  corresponding carbonyl group, so gem-diols lose
  water to form or reform a carbonyl group.

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Oxidation-Reduction Reactions

• General Principles
• Oxidation is the gain of oxygen or loss of
  hydrogen.
• Reduction is the gain of hydrogen or loss of
  oxygen.

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Oxidation is an Increase in Oxidation Number

• The oxidation number of any covalently bonded
  atom is found by assigning valence electrons to
  that atom and subtracting that number of valence
  electrons from the atoms group number in the
  periodic table.
• The group number gives that atoms normal number
  of valence electrons.
• The oxidation number is the difference between
  the normal number of valence electrons and the
  number of assigned electrons.

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Procedure for Finding an Oxidation Number of a
Covalent Atom

• Show all valence electrons with a Lewis
  Structure.
• Assign bonding valence electrons to the more
  electronegative atom of the pair sharing those
  electrons.
• Assign nonbonding valence electrons to the atom
  that has (owns) them .
• Subtract the sum of assigned bonding and
  nonbonding electrons from the atoms group number
  in the periodic table.

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Oxidation Numbers for Water
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Oxidation Numbers for Methane
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Oxidation Numbers for Carbon Dioxide
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Alternative Method

• H will be 1 in hydrocarbons and derivatives, and
  O will be 2.
• Since the sum of oxidation numbers is the overall
  charge of the chemical species, the oxidation
  state of a single carbon can be found
  mathematically.
• However, for two or more carbon atoms this method
  gives an average oxidation number.

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Oxidation Numbers of Underlined Atoms
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Oxidation Numbers of Underlined Atoms
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Find the Oxidation Number of the Underlined Atom
in Each of the following Structures.
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Solutions
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Preparation of Aldehydes and Ketones

• Oxidation of Io alcohols to aldehydes
• Oxidation of IIo alcohols to ketones

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Preparation of Aldehydes from Io Alcohols
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Preparation of Aldehydes from Io Alcohols

• Requires a mild oxidizing agent.
• A strong oxidizing agent makes an acid.
• The oxidation number of C changes from 1 to 1.
• The alcohol loses two Hs.
• Loss of Hs is oxidation.

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Mild Oxidizing Agents

• PCC pyridinum chlorochromate (pyridine poisons
  chromium 6 somewhat, reducing its oxidizing
  power)
• Swern (a mans name) DMSO (dimethylsulfoxide),
  oxalyl chloride, and triethylamine.

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Complete the following Equations
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Preparation of Aldehydes from Io Alcohols
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Oxidation Methodology

• Carbon atoms that contain oxygen and hydrogen are
  candidates for further oxidation.
• Remove an H bonded to the C that contains O and
  replace the H with OH.
• When you get a gem-diol, dehydrate (lose water
  and make a carbonyl group.

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Oxidizable Carbon Atoms
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Mild Oxidation of Io Alcohol
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Strong Oxidizing Agents

• KMnO4 and K2Cr2O7 are strong oxidizing agents.
• They contain Mn7 and Cr6, respectively.
• The transition metal ions are reduced, generally
  to Mn4 and Cr3.
• KMnO4 is purple and K2Cr2O7 is orange.
• Mn4 is brown and Cr3 is green.

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Strong Oxidation of Io Alcohol
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Strong Oxidation of Aldehyde
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Mild or Strong Oxidation of IIo Alcohol
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Summary of Oxidation Reactions

• Io Alcohols can be oxidized to aldehydes with
  mild oxidizing agents and to acids with strong
  oxidizing agents.
• Aldehydes can be oxidized to acids with strong
  oxidizing agents.
• IIo Alcohols can be oxidized to ketones with mild
  or strong oxidizing agents.
• Methodology is the same for all.

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Ozone (O3) as an Oxidizing Agent

• Ozone, an allotrope of oxygen, is a powerful
  oxidizing agent.
• It cleaves both the sigma and pi bonds of a
  double bond.
• Each half of the clevage gains an oxygen with a
  double bond (i.e., a carbonyl group).
• Aldehydes formed by this reaction will be
  oxidized to acids unless a reducing agent is
  added before workup of the reaction.
• Reducing agents are zinc (dust) or
  dimethylsulfide (Me2S).

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Ozonolysis

• Ozonolysis literally means clevage with ozone.
  Step 1 of an ozonolysis is a reaction of a double
  bond with O3.
• If Zn/HCl or (CH3)2S is used in Step 2, then any
  aldehydes produced will not be oxidized to acids.
• If Zn/HCl or (CH3)2S is not used in Step 2, then
  any aldehydes produced will be oxidized to acids.

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Oxidative vs. Reductive Workup

• When Zn/HCl or Me2S is not added, the reaction is
  an oxidative workup (aldehydes will be oxidized,
  thus oxidative).
• When Zn/HCl or Me2S is added, the reaction is a
  reductive workup (aldehydes will not be oxidized,
  because these reducing agents neutralize any
  excess ozone, thus reductive).

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Ozonolysis of (Z)-2-Butene with Oxidative Workup
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Summary Ozonolysis

• Ozone cleaves double bonds CC to give two
  carbonyl compounds.
• If one or both products are aldehydes, they may
  be isolated with a reductive workup, or they may
  be further oxidized to acids with an oxidative
  workup.
• If one or both products are ketones, they are not
  further oxidized.
• Thus, ozonolysis is a way to convert alkenes into
  aldehydes and ketones.

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Ozonolysis of (Z)-2-Butene with Reductive Workup