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20.4 Nucleophilic Substitution in Acyl Chlorides

20.4 Nucleophilic Substitution in Acyl Chlorides. O. O. (CH 3 ) 2 CHC OH. (CH 3 ) 2 CHC Cl. Preparation of Acyl Chlorides. from carboxylic acids and thionyl chloride (Section 12.7). SO Cl 2. +. +. SO 2. H Cl. heat. (90%). O. RC Cl. O. O. RCOCR'. O. RCOR'. O. RC NR' 2. O.

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20.4 Nucleophilic Substitution in Acyl Chlorides

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  1. 20.4Nucleophilic Substitutionin Acyl Chlorides

  2. O O (CH3)2CHCOH (CH3)2CHCCl Preparation of Acyl Chlorides from carboxylic acids and thionyl chloride(Section 12.7) SOCl2 + + SO2 HCl heat (90%)

  3. O RCCl O O RCOCR' O RCOR' O RCNR'2 O RCO– Reactions of Acyl Chlorides

  4. O O O O RCOCR' RCCl R'COH H O O via: R OCR' C Cl Reactions of Acyl Chlorides Acyl chlorides react with carboxylic acids to giveacid anhydrides: + + HCl

  5. O O CH3(CH2)5CCl CH3(CH2)5COH O O CH3(CH2)5COC(CH2)5CH3 Example + pyridine (78-83%)

  6. O O RCOR' RCCl H O via: R OR' C Cl Reactions of Acyl Chlorides Acyl chlorides react with alcohols to give esters: + + R'OH HCl

  7. O O C6H5COC(CH3)3 C6H5CCl Example pyridine + (CH3)3COH (80%)

  8. O O RCNR'2 RCCl H O via: R NR'2 C Cl Reactions of Acyl Chlorides Acyl chlorides react with ammonia and aminesto give amides: + + R'2NH + HO– H2O + Cl–

  9. O O C6H5CN C6H5CCl HN Example NaOH + H2O (87-91%)

  10. O O RCOH RCCl O O RCO– RCCl Reactions of Acyl Chlorides Acyl chlorides react with water to givecarboxylic acids (carboxylate ion in base): + + H2O HCl + + 2HO– Cl– + H2O

  11. O O RCOH RCCl H O R OH C Cl Reactions of Acyl Chlorides Acyl chlorides react with water to givecarboxylic acids (carboxylate ion in base): + + H2O HCl via:

  12. O O C6H5CH2COH C6H5CH2CCl Example + + H2O HCl

  13. O C6H5CCl Reactivity • Acyl chlorides undergo nucleophilic substitution much faster than alkyl chlorides. C6H5CH2Cl Relative rates ofhydrolysis (25°C) 1,000 1

  14. 20.5Nucleophilic Acyl Substitution in Carboxylic Acid Anhydrides • Anhydrides can be prepared from acyl chlorides as described in Table 20.1

  15. O O O O CH3COCCH3 O O O O Some anhydrides are industrial chemicals Aceticanhydride Phthalicanhydride Maleicanhydride

  16. O O H COH H C tetrachloroethane O 130°C C H H COH O O From dicarboxylic acids • Cyclic anhydrides with 5- and 6-membered rings can be prepared by dehydration of dicarboxylic acids + H2O (89%)

  17. O O RCOCR' O RCOR' O RCNR'2 O RCO– Reactions of Anhydrides

  18. O O O O RCOR' RCOCR RCOH Reactions of Acid Anhydrides Carboxylic acid anhydrides react with alcoholsto give esters: • normally, symmetrical anhydrides are used(both R groups the same) • reaction can be carried out in presence of pyridine (a base) or it can be catalyzed by acids + + R'OH

  19. O O O O RCOR' RCOCR RCOH H O R OR' C OCR O Reactions of Acid Anhydrides Carboxylic acid anhydrides react with alcoholsto give esters: + + R'OH via:

  20. O O + CH3CHCH2CH3 CH3COCCH3 OH O CH3COCHCH2CH3 CH3 Example H2SO4 (60%)

  21. O O O O RCNR'2 RCO– RCOCR H + O R'2NH2 R NR'2 C via: OCR O Reactions of Acid Anhydrides Acid anhydrides react with ammonia and aminesto give amides: + + 2R'2NH

  22. O O H2N CH(CH3)2 CH3COCCH3 O CH3CNH CH(CH3)2 Example + (98%)

  23. O O O RCOCR O O O RCOCR Reactions of Acid Anhydrides Acid anhydrides react with water to givecarboxylic acids (carboxylate ion in base): + H2O 2RCOH + + 2HO– 2RCO– H2O

  24. O O O RCOCR H O R OH C OCR O Reactions of Acid Anhydrides Acid anhydrides react with water to givecarboxylic acids (carboxylate ion in base): + H2O 2RCOH

  25. O O COH O COH O O Example + H2O

  26. 20.6Sources of Esters

  27. O CH3COCH2CH2CH(CH3)2 Esters are very common natural products • also called "isopentyl acetate" and "isoamyl acetate” contributes to characteristic odor of bananas 3-methylbutyl acetate

  28. O CH2OCR' O RCOCH CH2OCR" O Esters of Glycerol • R, R', and R" can be the same or different • called "triacylglycerols," "glyceryl triesters," or "triglycerides" • fats and oils are mixtures of glyceryl triesters

  29. O CH2OC(CH2)16CH3 O CH3(CH2)16COCH CH2OC(CH2)16CH3 O Esters of Glycerol Tristearin: found in many animal and vegetable fats

  30. O CH2(CH2)6CH3 O H H Cyclic Esters (Lactones) (Z)-5-Tetradecen-4-olide(sex pheromone of female Japanese beetle)

  31. Preparation of Esters • Fischer esterification (Sections 15.8 and 19.14) • from acyl chlorides (Sections 15.8 and 20.4) • from carboxylic acid anhydrides (Sections 15.8and 20.6) • Baeyer-Villiger oxidation of ketones (Section 17.16)

  32. 20.7Physical Properties of Esters

  33. CH3 CH3CHCH2CH3 OH CH3CHCH2CH3 Boiling Points boilingpoint • Esters have higher boiling points than alkanes because they are more polar. • Esters cannot form hydrogen bonds to other ester molecules, so have lower boiling points than alcohols. 28°C O 57°C CH3COCH3 99°C

  34. CH3 CH3CHCH2CH3 OH CH3CHCH2CH3 Solubility in Water Solubility(g/100 g) • Esters can form hydrogen bonds to water, so low molecular weight esters have significant solubility in water. • Solubility decreases with increasing number of carbons. ~0 O 33 CH3COCH3 12.5

  35. 20.8Reactions of Esters:A Review and a Preview

  36. Reactions of Esters • with Grignard reagents (Section 14.10) • reduction with LiAlH4 (Section 15.3) • with ammonia and amines (Sections 20.12) • hydrolysis (Sections 20.10 and 20.11)

  37. 20.9Acid-Catalyzed Ester Hydrolysis

  38. O O H+ + RCOR' H2O RCOH Acid-Catalyzed Ester Hydrolysis is the reverse of Fischer esterification • maximize conversion to ester by removing water • maximize ester hydrolysis by having large excess of water • equilibrium is closely balanced because carbonyl group ofester and of carboxylic acid are comparably stabilized + R'OH

  39. O + H2O CHCOCH2CH3 Cl HCl, heat O + CH3CH2OH CHCOH Cl Example (80-82%)

  40. Mechanism of Acid-CatalyzedEster Hydrolysis • Is the reverse of the mechanism for acid-catalyzed esterification. • Like the mechanism of esterification, it involves two stages: • 1) formation of tetrahedral intermediate (3 steps) • 2) dissociation of tetrahedral intermediate (3 steps)

  41. O + RCOR' H2O OH OR' RC OH First stage: formation of tetrahedral intermediate • water adds to the carbonyl group of the ester • this stage is analogous to the acid-catalyzed addition of water to a ketone H+

  42. O RCOH OH OR' RC OH Second stage: cleavage of tetrahedralintermediate + R'OH H+

  43. Mechanism of formationoftetrahedral intermediate

  44. H H O •• + H H •• + H O O •• •• H RC O R' •• •• Step 1 •• O •• RC O R' •• ••

  45. •• H O •• RC + O R' •• •• + H O RC O R' •• Step 1 • carbonyl oxygen is protonated because cation produced is stabilized by electron delocalization (resonance) ••

  46. •• OH H •• + RC O •• H OR' •• •• •• + H O H RC O •• •• H O R' •• •• Step 2

  47. •• OH H •• + H RC O •• O H •• •• OR' •• •• H •• OH H •• H RC O •• + •• O H •• OR' •• •• H Step 3

  48. Cleavage of tetrahedralintermediate

  49. •• OH •• •• H RC OH •• + O •• •• O H •• R' H •• OH •• •• RC OH H •• O O H •• •• •• R' + H Step 4

  50. •• OH •• •• RC OH •• + O H •• R' •• OH •• •• + O RC H •• R' + •• OH •• Step 5

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