Ch 17 Aldehyde and Ketone Responses - PowerPoint PPT Presentation

ch 17 aldehyde and ketone reactions l.
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Ch 17 Aldehyde and Ketone Responses PowerPoint Presentation
Ch 17 Aldehyde and Ketone Responses

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Ch 17 Aldehyde and Ketone Responses

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  1. Ch 17 Aldehyde and Ketone Reactions • Addition Reactions to Carbonyls • Hydration can be Acid or Base Catalyzed • General Hydration Reaction • Base Catalyzed Mechanism • Acid Catalyzed Mechanism

  2. Hydrations are reversible • K < 1 for ketones • K > 1 for formaldehyde and aldehydes with inductive accepting groups • K = 1 for unsubstituted aldehydes • Donating R groups of Ketones stabilize the carbonyl form • Accepting groups destabilize the carbonyl, which favors alcohol formation • This same trend will hold for other XY addition reactions

  3. Addition of Alcohols • ROH adds to carbonyls to give Hemiacetals • This reaction can be acid or base catalyzed • A Hemiacetal has an –OH and and –OR group on the same carbon • Only reactive carbonyls (those with accepting substitutents or formaldehyde) give a reasonable amount of the hemiacetal • Strain-free 5- and 6-membered cyclic hemiacetals are common and stable

  4. Acids Catalyze Acetal Formation • An acetal has 2 –OR groups on the same carbon • General acetal formation reaction: • Mechanism • All steps are reversible • We can shift the equilibrium position towards the acetal by adding excess R’OH or by removing H2O during the reaction

  5. Use of Acetals as Protecting Groups for Carbonyls • Acetals are relatively unreactive ether functional groups • To protect a reactive carbonyl group from a reaction you want to do on another part of the molecule (ex: Grignard Rxn) we can make it an acetal • Cyclic Acetals are more stable than acyclic ones • Entropy disfavors normal acetal formation (3 particles2 particles) • Entropy doesn’t disfavor cyclic acetals (2 particles2 particles) 4. Removing the protecting acetal group is achieved easily by adding H+

  6. A carbonyl compound can also be used to protect a diol • Thioacetals are even stronger protecting groups than acetals • Thioacetals are stable to acid • A thioacetal is formed in the presence of ZnCl2 • The dithio protecting group can be removed with HgCl2. The product Hg(SCH2CH2S) precipitates.

  7. Thioacetals can be Desulfurized by Raney Ni, H2. This is a very effective way to remove a carbonyl oxygen. • Addition of Substituted Amines to Carbonyl Groups • Imine (Schiff Base) Formation • An amine adds to a carbonyl to form a hemiaminal • The hemiaminal quickly loses H2O to form an Imine or Schiff Base • Condensation = reaction where 2 molecules are joined and H2O is lost

  8. Primary Amine is needed, and will react with an aldehyde or ketone • Special Imines aid in the identification of carbonyls • These imines are well characterized solid derivatives of the carbonyl • The melting points are known and can be used in identification • Oximes • Hydrazones

  9. Semicarbazones • Secondary Amines form Enamines with carbonyls • The nitrogen can’t form a double bond to the carbonyl carbon because it would then be quaternary and (+1) charged • H is lost from C, not from N as in the primary amine case II. Deoxygenation of Carbonyl Groups

  10. Review • Clemmenson Reduction • Thioacetal Formation and Reduction • Wolff-Kishner Reduction • Strong base converts hydrazones to hydrocarbons • Usual conditions: 1. H2NNH2, H2O, diethylene glycol (high boiling point alcohol) NaOH, Heat. 2. Water (Don’t isolate the hydrazone)

  11. Mechanism • Wolff-Kishner doesn’t affect acid sensitive groups (like Clemmenson does) or double bonds (like thioacetal does). • Addition of Carbon Nucleophiles to Carbonyls • Review: Alkyl Metal reagents add to carbonyls

  12. Cyanohydrins • Hydrogen Cyanide (HCN) reversibly adds to carbonyls • HCN is very toxic • Formation in situ by H+ addition to NaCN or HCN is safer • Example reactions: • Mechanism • The reaction is reversed by adding base

  13. The Wittig Reaction • Phosphorous Ylide is a stabilized carbanion • A Carbonyl plus an Ylide forms an alkene

  14. Wittig-formed alkenes are unambiguous • Wittig • Elimination 4. Mechanism

  15. Sometimes you do get mixtures of E/Z isomers • This reaction has no effect on many other functional groups: ethers, esters, halogens, alkenes, alkynes • Oxidation of Carbonyls • Baeyer-Villiger Oxidation • Ketones + Peroxycarboxylic Acids give Esters • Mechanism • Cyclic Ketone will give a cyclic ester product • Double bonds are unaffected • Unsymmetric Ketones lead to only one product: some groups migrate more easily than others = Migratory Aptitude

  16. Migratory Aptitudes: Methyl < primary < phenyl ~ secondary < cyclohexyl < tertiary • Oxidation Tests for Aldehydes • Fehling’s Test • Tollens’s Test