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20.12 Amides

20.12 Amides. Physical Properties of Amides. Amides are less reactive toward nucleophilic acyl substitution than other acid derivatives. Physical Properties of Amides. Amides are capable of hydrogen bonding. Physical Properties of Amides. Amides are less acidic than carboxylic acids.

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20.12 Amides

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  1. 20.12Amides

  2. Physical Properties of Amides Amides are less reactive toward nucleophilic acyl substitution than other acid derivatives.

  3. Physical Properties of Amides Amides are capable of hydrogen bonding.

  4. Physical Properties of Amides Amides are less acidic than carboxylic acids. Nitrogen is less electronegative than oxygen.

  5. Preparation of Amides Amides are prepared from amines by acylationwith: • Acyl chlorides (Table 20.1) • Anhydrides (Table 20.2) • Esters (Table 20.5)

  6. O O RCO RCOH Preparation of Amides Amines do not react with carboxylic acids to giveamides. The reaction that occurs is proton-transfer(acid-base). • If no heat-sensitive groups are present, the resulting ammonium carboxylate salts can be converted to amides by heating. + – + R'NH3 + R'NH2

  7. O O RCO RCOH O RCNHR' Preparation of Amides Amines do not react with carboxylic acids to giveamides. The reaction that occurs is proton-transfer(acid-base). + – + R'NH3 + R'NH2 heat + H2O

  8. O H2N COH O CNH Example + 225°C + H2O (80-84%)

  9. 20.13Hydrolysis of Amides

  10. O O RCNHR' RCOH Hydrolysis of Amides Hydrolysis of amides is irreversible. In acid solution the amine product is protonated to give an ammonium salt. + + + R'NH3 + + H2O H

  11. O O RCNHR' RCO Hydrolysis of Amides In basic solution the carboxylic acid product is deprotonated to give a carboxylate ion. – – + R'NH2 + HO

  12. O O CH3CH2CHCNH2 CH3CH2CHCOH + – NH4 HSO4 Example: Acid Hydrolysis H2O + H2SO4heat (88-90%)

  13. O NH2 CH3CNH O CH3COK Br Br Example: Basic Hydrolysis KOH + H2Oheat (95%)

  14. Mechanism of Acid-CatalyzedAmide Hydrolysis • Acid-catalyzed amide hydrolysis proceeds viathe customary two stages: • 1) Formation of tetrahedral intermediate2) Dissociation of tetrahedral intermediate

  15. O + RCNH2 H2O OH NH2 RC OH First Stage: Formation of Tetrahedral Intermediate • Water adds to the carbonyl group of the amide. • This stage is analogous to the acid-catalyzed addition of water to a ketone. H+

  16. O + RCOH NH4 OH NH2 RC OH Second Stage: Cleavage of TetrahedralIntermediate + H+

  17. Mechanism of FormationofTetrahedral Intermediate

  18. H •• O H O •• •• + H RC NH2 •• H •• + H O O •• •• H RC NH2 •• Step 1

  19. •• H O •• RC + NH2 •• + H O RC NH2 •• Step 1 • Carbonyl oxygen is protonated because cation produced is stabilized by electron delocalization (resonance).

  20. •• OH H •• + RC O •• H NH2 •• •• + H O H RC O •• •• H NH2 •• Step 2

  21. •• OH H •• + H RC O •• O H •• •• NH2 •• H •• OH H •• H RC O •• + •• O H •• NH2 •• H Step 3

  22. Cleavage of TetrahedralIntermediate

  23. •• OH •• •• H RC OH •• + O •• •• H2N H H •• OH H •• RC O •• •• H H2N O H •• •• + H Step 4

  24. •• OH •• •• RC OH •• + H2N H •• OH •• NH3 + RC •• + •• OH •• Step 5

  25. •• OH •• •• RC OH •• + H2N H •• + OH H3O •• NH3 RC •• + •• OH •• Step 6 + NH4 +

  26. •• •• + OH OH •• RC RC + •• •• OH OH •• •• Step 6

  27. •• H H O + H •• O •• •• H H O RC •• •• OH •• + O H RC •• OH •• Step 6 ••

  28. Mechanism of Amide Hydrolysisin Base • Involves two stages: • 1) Formation of tetrahedral intermediate2) Dissociation of tetrahedral intermediate

  29. O + RCNH2 H2O OH NH2 RC OH First Stage: Formation of Tetrahedral Intermediate • Water adds to the carbonyl group of the amide. • This stage is analogous to the base-catalyzed addition of water to a ketone. HO–

  30. O RCO OH NH2 RC OH Second Stage: Cleavage of TetrahedralIntermediate – + NH3 HO–

  31. Mechanism of FormationofTetrahedral Intermediate

  32. •• O •• H RC O •• •• – •• NH2 •• – •• O H •• •• RC O •• •• NH2 •• Step 1

  33. •• O H H •• – •• O RC O •• •• •• •• H NH2 •• – •• •• O H O •• H •• •• H RC O •• •• NH2 •• Step 2

  34. Dissociation ofTetrahedral Intermediate

  35. •• OH •• •• H RC OH •• + O – •• •• H2N •• H •• OH H •• RC O •• •• H H2N O H •• •• •• Step 3

  36. – •• O •• •• H •• •• O H O •• •• H RC NH3 O H •• •• •• Step 4 •• H O •• •• RC OH •• + H3N

  37. •• O •• RC – O •• •• •• •• O •• RC NH3 O H •• •• •• Step 5 HO–

  38. 20.14Lactams

  39.    -Caprolactam*: used toprepare a type of nylon  N O H *Caproic acid is the common name for hexanoic acid. Lactams Lactams are cyclic amides. Some are industrialchemicals, others occur naturally.

  40. O C6H5CH2CNH  S CH3  CH3 N O CO2H Penicillin G: a -lactam antibiotic Lactams Lactams are cyclic amides. Some are industrialchemicals, others occur naturally.

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