17.8 Acetal Development - PowerPoint PPT Presentation

17 8 acetal formation l.
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17.8 Acetal Development

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  1. 17.8Acetal Formation

  2. Some reactions of aldehydes and ketones progressbeyond the nucleophilic addition stage Acetal formation Imine formation Enamine formation Compounds related to imines The Wittig reaction

  3. R O C •• •• R' HOH R •• •• HO O H C •• •• R' Recall Hydration of Aldehydes and Ketones

  4. R O C •• •• R' R"OH R •• •• R"O O H C •• •• R' Alcohols Under Analogous Reactionwith Aldehydes and Ketones Product is called a hemiacetal.

  5. R •• •• R"O O H C •• •• R' R •• •• R"O O H C •• •• R' Hemiacetal reacts further in acid to yield an acetal Product is called an acetal. ROH, H+ Product is called a hemiacetal.

  6. O CH CH(OCH2CH3)2 Example + 2CH3CH2OH HCl + H2O Benzaldehyde diethyl acetal (66%)

  7. O H2C CH2 O O C (CH2)5CH3 H Diols Form Cyclic Acetals + CH3(CH2)5CH HOCH2CH2OH benzene p-toluenesulfonic acid + H2O (81%)

  8. In general: Position of equilibrium is usually unfavorablefor acetal formation from ketones. Important exception: Cyclic acetals can be prepared from ketones.

  9. O Example C6H5CH2CCH3 + HOCH2CH2OH benzene p-toluenesulfonic acid H2C CH2 O + O H2O (78%) C CH3 C6H5CH2

  10. Mechanism of Acetal Formation First stage is analogous to hydration andleads to hemiacetal acid-catalyzed nucleophilic addition of alcohol to C=O

  11. H R •• O O H C O •• •• •• •• + H R Mechanism

  12. R O •• •• H Mechanism H •• + O O C •• •• H R

  13. R O •• •• H Mechanism •• + O C H

  14. Mechanism R •• •• O O C •• + H H

  15. R •• •• O O C •• + H H •• O •• R H Mechanism

  16. R •• •• O O C •• •• H Mechanism H + O •• R H

  17. Mechanism of Acetal Formation Second stage is hemiacetal-to-acetal conversion involves carbocation chemistry

  18. H R •• •• O H O O C •• •• + •• H R Hemiacetal-to-acetal Stage

  19. H O •• •• R Hemiacetal-to-acetal Stage R H •• •• + O O C •• H

  20. Hemiacetal-to-acetal Stage R H •• •• + O O C •• H

  21. H •• O •• H Hemiacetal-to-acetal Stage R •• O + C ••

  22. R •• O + C •• Hemiacetal-to-acetal Stage R + O C •• Carbocation is stabilized by delocalizationof unshared electron pair of oxygen

  23. R R •• •• O + C O •• •• H Hemiacetal-to-acetal Stage

  24. R R •• •• + O O C •• H Hemiacetal-to-acetal Stage

  25. R R •• •• + R O O C •• •• H O •• H Hemiacetal-to-acetal Stage

  26. R R •• •• O O C •• •• Hemiacetal-to-acetal Stage R •• H O + H

  27. OR" O + C 2R"OH R C R' + H2O R R' OR" Hydrolysis of Acetals mechanism: reverse of acetal formation;hemiacetal is intermediate application: aldehydes and ketones can be "protected" as acetals.

  28. 17.9Acetals as Protecting Groups

  29. O CH CH3CCH2CH2C O CCH3 CH3CCH2CH2C Example The conversion shown cannot be carried outdirectly... 1. NaNH22. CH3I

  30. O C: CH3CCH2CH2C because the carbonyl group and thecarbanion are incompatible functionalgroups. –

  31. Strategy 1) protect C=O 2) alkylate 3) restore C=O

  32. O CH H2C CH2 O O C CH CH3 Example: Protect + HOCH2CH2OH CH3CCH2CH2C benzene p-toluenesulfonic acid CH2CH2C

  33. H2C CH2 O O C CH2CH2C CCH3 CH3 H2C CH2 O O C CH CH3 Example: Alkylate 1. NaNH22. CH3I CH2CH2C

  34. H2C CH2 O O C CH2CH2C CCH3 CH3 O CCH3 CH3CCH2CH2C Example: Deprotect H2O HCl + HOCH2CH2OH (96%)