17.5 Reactions of Aldehydes and Ketones: A Review and a Preview

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17.5Reactions of Aldehydes and KetonesA
Review and a Preview
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Reactions of Aldehydes and Ketones

• Already covered in earlier chapters
• reduction of CO to CH2
• Clemmensen reduction
• Wolff-Kishner reduction
• reduction of CO to CHOH
• addition of Grignard and organolithium reagents

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17.6Principles of Nucleophilic Addition to
Carbonyl GroupsHydration of Aldehydes and
Ketones
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Hydration of Aldehydes and Ketones
H2O
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Substituent Effects on Hydration Equilibria

• compared to H
• electronic alkyl groups stabilize
  reactants
• steric alkyl groups crowd product

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Equilibrium Constants for Hydration

• CO hydrate K
• CH2O CH2(OH)2 41 99.96
• CH3CHO CH3CH(OH)2 0.018 50
• (CH3)3CCHO (CH3)3CCH(OH)2 0.0041 19
• (CH3)2CO (CH3)2C(OH)2 0.000025 0.14

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When does equilibrium favor hydrate?

• when carbonyl group is destabilized
• alkyl groups stabilize CO
• electron-withdrawing groups destabilize CO

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Substituent Effects on Hydration Equilibria
OH
R

H2O
C
R
C
R
R
OH

• R CH3 K 0.000025
• R CF3 K 22,000

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Mechanism of Hydration (base)
Step 1

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Mechanism of Hydration (base)
Step 2

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Mechanism of Hydration (acid)
Step 1


H
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Mechanism of Hydration (acid)
Step 2



OH
C
OH
C



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Mechanism of Hydration (acid)
Step 3

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17.7Cyanohydrin Formation
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Cyanohydrin Formation

HCN

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Cyanohydrin Formation

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Cyanohydrin Formation

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Cyanohydrin Formation
H
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Cyanohydrin Formation
H
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Example
NaCN, water
then H2SO4
2,4-Dichlorobenzaldehyde cyanohydrin (100)
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Example
NaCN, water
then H2SO4
(77-78)

• Acetone cyanohydrin is used in the synthesis of
  methacrylonitrile (see problem 17.6).