Chapter 15 Aldehydes and Ketones and Chiral Molecules

1
Chapter 15 Aldehydes and Ketones and Chiral
Molecules

• 15.6 Oxidation and Reduction
• 15.7 Addition Reactions

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Oxidation

• Aldehydes are easily oxidized to carboxylic
  acids.
• O O
• O
• CH3CH CH3COH
• Acetaldehyde Acetic acid

3
Hydrate formation

• Hydrate formation is the reason that aldehydes
  are oxidized (O-2Hoxidation) to carboxylic
  acids in aqueous media.
• O OH O
• H2O O
• RCH ? RC OH ? RCOH
• H

4
Tollens Test

• Tollens reagent, which contains Ag, oxidizes
  aldehydes, but not ketones.
• Ag is reduced to metallic Ag, which appears as
  a mirror in the test tube.

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Tollens reagent detects an aldehyde

• Tollens reagent (a solution of Ag in aqueous
  NH3) can be used to detect the presence of the
  aldehyde group in the unknown sample.
• As the oxidation of the aldehyde proceeds, silver
  metal is deposited on the walls of the reaction
  flask as a shiny mirrow.
• If upon addition of the Tollens reagent to the
  unknown, the test tube becomes silvery, the
  unknown is an aldehyde.
• RCHO 2Ag(NH3)2 2OH- ? RCOO- NH4 2Ag
  3NH3 H2O
• an Tollens salt of a silver
• aldehyde reagent carboxylic acid
  mirrow

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Benedicts Test

• Benedicts reagent, which contains Cu2, reacts
  with aldehydes that have an adjacent OH group.
• When an aldehyde is oxidized to a carboxylic
  acid, Cu2 is reduced to give Cu2O(s).

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Test for glucose (Benedicts test)
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Addition Reactions

• Polar molecules can add to the carbonyl groups of
  aldehydes and ketones.
• The negative part of the added molecule bonds to
  the positive carbonyl carbon.
• The positive part of the added molecule bonds to
  the negative carbonyl oxygen.
• ? ?- ? ?-
  
• CO XY COX
• Y

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Acetal Formation

• Alcohols add to the carbonyl group of aldehydes
  and ketones.
• The addition of two alcohols forms acetals.

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Hemiacetal Formation

• The addition of one alcohol to an aldehyde or
  ketone forms an intermediate called a hemiacetal.
• Usually, hemiacetals are unstable and difficult
  to isolate.

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Cyclic Hemiacetals

• Stable hemiacetals form when the CO group and
  the OH are both part of five- or six-atom carbon
  compounds.