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17.3 How Aldehydes and Ketones React (Part II)

Main Menu. 17.3 How Aldehydes and Ketones React (Part II). Electron rich (Lewis base, Nu). d -. d +. Electron deficient (Lewis acid, E + ). R = alkyl or aryl (C). Y = alkyl, aryl or H (class II) ( No leaving group ). 1. General mechanism in basic condition:.

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17.3 How Aldehydes and Ketones React (Part II)

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  1. Main Menu 17.3 How Aldehydes and Ketones React (Part II) Electron rich (Lewis base, Nu) d- d+ Electron deficient (Lewis acid, E+) R = alkyl or aryl (C) Y = alkyl, aryl or H (class II) (No leaving group)

  2. 1. General mechanism in basic condition: Nucleophilic Addition (Class II) 2. General mechanism in acidic condition:

  3. Important pKa to Remember

  4. 1. Carbon as the nucleophilic atom Basic condition Types of Nucleophile for Class II Carbonyl Groups pKa = 25 Acetylide ion pKa = 50 carboanion 2. Hydrogen as the nucleophilic atom Mostly basic condition hydride 3. Nitrogen as the nucleophilic atom Mostly acidic condition 1° and 2° amines 4. Oxygen as the nucleophilic atom Acidic condition 1° alcohols

  5. pKaof amines. Nitrogen as the Nucleophilic Atom pKa = 38 pKa = 9-10 1° and 2°Amines function as weak bases or nucleophiles. 3° Amines function as weak bases.

  6. General Reactions: Reactions of Aldehydes and Ketones with Amines H+ 1° Amines imine (Schiff base) H+ 2° Amines enamine

  7. Examples: Reactions of Aldehydes and Ketones with Primary Amines H+ H+

  8. Examples: Reactions of Aldehydes and Ketones with Secondary Amines H+ H+

  9. Mechanism for the formation of Imines

  10. Mechanism for the formation of Enamines

  11. Optimal pH for the Formation of Imines pKa = 9-10 pKa = -2 The highlighted protons are quite acidic. Rate of rxn Strong acid will protonate the amines. Optimal pH: 4 – 5 (ex: acetic acid) 0 4 5 10 pH

  12. H+ Formation of Imine Derivatives Hydroxylamine Oxime H+ semicarbazide semicarbazone H+ hydrazine hydrazone

  13. H2, Pd/C General reaction Application of Imine: Reductive Amination H+ NaBH3CN Generated in situ The reaction can be conducted in one-pot fashion. The use of NaBH3CN is important since it can tolerate the acidic condition.

  14. Examples of Reductive Amination

  15. Developed by the German chemist Ernst Otto Beckmann (1853–1923) Application of Oxime: Beckmann Rearrangement 2-azacycloheptanone (a lactam) oxime nylon

  16. Developed by a team of researchers at the Croatian pharmaceutical company Pliva in 1980. Beckmann Rearrangement: Synthesis of Azithromycin oxime Erythromycin Azithromycin lactam

  17. Mechanism of Beckmann Rearrangement + H+ - H+

  18. Application of Hydrazone: Deoxygenation of Carbonyl Groups the Wolff–Kishner reduction

  19. Mechanism of the Wolff–Kishner Reduction

  20. General reaction: Application of Hydrazone: Use of 2,4-DNP 2,4-Dinitrophenylhydrazine (2,4-DNP) The hydrazones have specific m.p.

  21. 1. What could be the best reagents for the following reaction? Learning Check 2. What could be the product for the following reaction? (a) I II III IV None of the above

  22. 3. What could be the products for the following reaction? Learning Check (a) I, II (b) I, II, IV (c) III, IV (d) II, III (e) None of the above

  23. Main Menu 4. What could be the product for the following reaction? Learning Check

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