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17.4 How Aldehydes and Ketones React (Part III)

Main Menu. 17.4 How Aldehydes and Ketones React (Part III). 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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17.4 How Aldehydes and Ketones React (Part III)

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  1. Main Menu 17.4 How Aldehydes and Ketones React (Part III) 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 alcohol. Oxygen as the Nucleophilic Atom pKa = 15-16 pKa = -2 1° and 2° Alcohols function as weak acids, weak bases or nucleophiles. 3° Alcohols function as weak acids or weak bases.

  6. 2° and 3° Alcohols are too hindered to react. General reaction with primary alcohols: Reactions of Aldehydes and Ketones with Alcohols H+ 2 1° Alcohols Ketal or acetal General reaction with water: H+ Geminaldiol (gem-diol)

  7. Example: H+ Reactions of Aldehydes and Ketoneswith Water Mechanism:

  8. Examples: H+ Reactions of Aldehydes and Ketones with Alcohols 2 H+ 2 H+ 1

  9. Mechanism: Reactions of Aldehydes and Ketones with Alcohols hemiacetal acetal

  10. Synthesis using protecting group ? LiAlH4 or DIBAL will reduce the ketone as well. Application of Ketal or Acetal Ketal as the protecting group for ketone: • Ketals like ethers are relatively stable in basic condition.

  11. Mechanism for the Formation of Cyclic Ketal

  12. ? SOCl2 (PBr3 or PCl3) will convert carboxylic acids into acyl halides as well. More Examples on the Use of Protecting Groups Ester as the protecting group for carboxylic acid: • Ester will not react with SOCl2 (from compound 3 to compound 4).

  13. Amines under acidic condition may turn into an EWG in the form of ammoniim ions. More Examples on the Use of Protecting Groups Amide as the protecting group for amine: • Amide in compound 3 will act as EGD..

  14. Intramolecular reaction: formation of 5 or 6-membered rings Formation of Cyclic Hemiacetals or Ketals

  15. Intramolecular formation of 5 or 6-membered rings Cyclic Hemiacetals in Carbohydrates D-Glucose

  16. 1. What could be the reagent and reaction condition for the following transformation? (a) ethanol, NaOH (b) ethanol, H+ (c) methanol, NaOH (d) methanol, H+ (e) None of the above Learning Check 2. What should be the product from the following reaction?

  17. 3. What could be the product for the following reaction? Learning Check

  18. 4. Which hydroxy group when added to the aldehyde functional group (C-1) of open chain glucose will produce the cyclic hemiacetal form shown below? (a) The one that locates on C-2. (b) The one that locates on C-3. (c) The one that locates on C-4. (d) The one that locates on C-5. (e) None of the above Learning Check

  19. Main Menu 5. Which hydroxy group when added to the aldehyde functional group (C-1) of open chain glucose will produce the cyclic hemiacetal form shown below? Learning Check (a) The one that locates on C-2. (b) The one that locates on C-3. (c) The one that locates on C-4. (d) The one that locates on C-5. (e) None of the above

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