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The Haloform Reaction

The Haloform Reaction. Under basic conditions, halogenation of a methyl ketone often leads to carbon-carbon bond cleavage. Such cleavage is called the haloform reaction because chloroform, bromoform, or iodoform is one of the products. Example. The Haloform Reaction.

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The Haloform Reaction

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  1. The Haloform Reaction • Under basic conditions, halogenation of a methyl ketone often leads to carbon-carbon bond cleavage. • Such cleavage is called the haloform reaction because chloroform, bromoform, or iodoform is one of the products.

  2. Example

  3. The Haloform Reaction • The haloform reaction is sometimes used as a method for preparing carboxylic acids, but works well only when a single enolate can form.

  4. The Haloform Reaction • The haloform reaction is sometimes used as a method for preparing carboxylic acids, but works well only when a single enolate can form.

  5. Mechanism • First stage of a haloform reaction is substitution of all available hydrogens by halogen atoms.

  6. Formation of the tri-a-halomethyl ketone is followed by its hydroxide-induced cleavage Mechanism

  7. 18.8Some Chemical and StereochemicalConsequences of Enolization

  8. O H H H H O D D D D Hydrogen-Deuterium Exchange + 4D2O KOD, heat + 4DOH

  9. •• O •• – H H •• OD •• H H •• – •• O •• •• H H •• + HOD H •• Mechanism +

  10. •• O •• – H D •• + OD •• H H •• – •• O •• •• H •• H OD D H •• Mechanism Bridgehead Carbonyl Compounds?

  11. H O H3C CC6H5 C CH3CH2 Stereochemical Consequences of Enolization H3O+ 50% R50% S 50% R50% S 100% R H2O, HO–

  12. H H3C OH O H3C C CC6H5 CC6H5 C CH3CH2 CH3CH2 Enol is achiral R

  13. O C H3C OH C CC6H5 H CH3CH2 O H3C CC6H5 C CH3CH2 Enol is achiral H3C H CC6H5 S 50% CH3CH2 50% R

  14. H O H3C CC6H5 C CH3CH2 Results of Rate Studies • Equal rates for:racemizationH-D exchangebrominationiodination • Enol is intermediate and its formation is rate-determining

  15. 18.9The Aldol Condensation

  16. O O – •• •• RCH2CH RCHCH OH HOH •• •• •• Some thoughts... + + • A basic solution contains comparable amounts of the aldehyde and its enolate. • Aldehydes undergo nucleophilic addition. • Enolate ions are nucleophiles. • What about nucleophilic addition of enolate to aldehyde? •• – pKa = 16-20 pKa = 16

  17. Aldol Addition • This product is called an "aldol" because it is both an aldehyde and an alcohol

  18. Aldol Addition of Acetaldehyde

  19. Aldol Addition of n-Butanal

  20. Aldol Condensation

  21. Aldol Condensation of Butanal

  22. C C O O H C C OH C C Dehydration of Aldol Addition Product • dehydration of -hydroxy aldehyde can becatalyzed by either acids or bases

  23. C C O O H – C C •• OH OH C C Dehydration of Aldol Addition Product • in base, the enolate is formed NaOH

  24. C C O O H – C C •• OH OH C C Dehydration of Aldol Addition Product • the enolate loses hydroxide to form the ,-unsaturated aldehyde NaOH

  25. Aldol reactions of ketones • the equilibrium constant for aldol addition reactions of ketones is usually unfavorable

  26. Intramolecular Aldol Condensation even ketones give good yields of aldol condensation products when the reaction is intramolecular

  27. Information Suggested Problems: 18.26-18.37 New Chp. 15 & 17 problem set now available ----------------------------------------------------- Office Hour: Today, 3.30 P.M., SES 170 ----------------------------------------------------- Revise Chapter 5.7-5.12 (elimination reactions) Review 17-7-17.7 (carbonyl addition)

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