CVEN 4424 Environmental Organic Chemistry

1. CVEN 4424Environmental Organic Chemistry Lecture 21 – Hydrolysis Reactions

2. Announcements • Reading • Chapter 13, Sections 13.3 for carboxylic acid, carbamate hydrolysis • Problem sets • PS 8 due Thursday April 17 • Office hours • Tuesday, 12:30-2, my office • Wednesday, 11-12 am, ECES 131 • Wednesday, 3-5 pm, ECCE 1B41 (TA Brett)

3. Hydrolysis of Acid Derivatives 2,4-dinitrophenylacetate N,N-dimethylchloroacetamide 4-nitrophenylN-methyl-N-phenyl carbamate parathion • Examples • carboxylic acid esters • carboxylic acid amides • carbamates • phospho- and thioesters

4. Hydrolysis of Acid Derivatives • Carboxylic acid amides • derivatives of formamide HC(=O)NH2 • less reactive than carboxylic acid esters • because –NR2R3 is poorer leaving group compared to –OR2R2R3NH = R2R3N- + H+ pKa ~ 10 • acid-catalyzed, base-catalyzed hydrolysis dominate

5. Hydrolysis of Acid Derivatives • Mechanisms • base-catalyzed hydrolysis • (neutral hydrolysis insignificant) • products • carboxylic acid • amine

6. Hydrolysis of Acid Derivatives log kh pH • Mechanisms • acid-catalyzed hydrolysis • amide more basic, accepts proton better • IAB of amide > IAB of ester (acid hydrolysis important at higher pHs for amide)

7. Hydrolysis of Acid Derivatives • Carbamates • derivatives of carbamic acidH2NC(=O)OH • ester and amide combined • herbicides, insecticides • acid-catalyzed hydrolysis unimportant • too acidic, won’t protonate • products • alcohol, HO-R3 • amine, HNR1R2 • CO2

8. Hydrolysis of Acid Derivatives ethyl-N,N-dimethyl carbamate 4-nitrophenyl-N-methyl-N-phenyl carbamate naphthyl-N-methyl carbamate Carbamate nomenclature

9. Hydrolysis of Acid Derivatives What is this carbamate called?A. methyl-N-phenyl-N-methyl carbamate B. N-cyclohexyl-N-dimethyl-carbamate C. methyl-N-cyclohexyl-N-methyl carbamate

10. Hydrolysis of Acid Derivatives What is this carbamate called?A. methyl-N-phenyl-N-methyl carbamateB. N-cyclohexyl-N-dimethyl-carbamateC. methyl-N-cyclohexyl-N-methyl carbamate

11. Hydrolysis of Acid Derivatives A. B. C. What does2,4,6-chlorophenyl-N-methyl carbamatelook like?

12. Hydrolysis of Acid Derivatives • Mechanisms • base-catalyzed hydrolysis • leaving group controlled bypKa • usually, –OR3has lower pKa than –NR1R2

13. Hydrolysis of Acid Derivatives • Predicting hydrolysis rates • Hammett relationship • benzoic acid esters • kH is the rate coefficient for the unsubstituted benzoic acid ester •  is a “susceptibility” parameter that must be evaluated for a set of benzoic acid esters • m,p is the Hammett substituent constant for meta- and para- substituents

14. Hydrolysis of Acid Derivatives • Predicting hydrolysis rates • Brønsted relationship • carbamates • pKa is the acidity constant of the -OR3 leaving group •  and c are parameters to be determined with a set of carbamate rate data

15. Hydrolysis of Acid Derivatives   • Carbamates • hydrolysis depends on amide substitution • N-substituted • alcohol is leaving group • very sensitive to alcohol pKa • N,N-substituted • much less sensitive to alcohol pKa

16. Hydrolysis of Acid Derivatives 4-nitrophenyl-N-methyl-N-phenyl carbamate • What is the half-life of this carbamate at pH 8? • leaving group is 4-nitrophenol, pKa = 7.15 • N,N-methylphenyl carbamate:

17. Hydrolysis of Acid Derivatives 4-nitrophenyl-N-methyl-N-phenyl carbamate • What is the half-life of this carbamate at pH 8? • kB = 10-3.09 M-1 s-1 = 8.2x10-4 M-1 s-1 • kh = kB[OH-] = 8.2x10-10 s-1 • t1/2 = 27 y

18. Hydrolysis of Acid Derivatives 4-nitrophenyl-N-phenyl carbamate • What is the half-life of this carbamate at pH 8? • 4-nitrophenol, pKa = 7.15 • N-phenyl carbamate:

19. Hydrolysis of Acid Derivatives 4-nitrophenyl-N-phenyl carbamate • What is the half-life of this carbamate at pH 8? • kB = 105.38 = 2.4x105 M-1 s-1 • kh = kB[OH-] = 0.24 s-1 • t1/2 = 3 s

20. Hydrolysis of Acid Derivatives • Phosphoric and thiophosphoric acid esters • pentavalent phosphorus – P(V) • insecticides, fire retardants • two spots for nucleophilic substitution • at the phosphorus • at the carbon in R1, R2, or R3

21. Hydrolysis of Acid Derivatives trimethylphosphate diethyl p-nitrophenyl phosphate (Paraoxon) dimethyl-S-(2-ethylmercaptoethyl) dithiophosphate (Thiometon) diethyl p-nitrophenylthiophosphate (Parathion) Nomenclature

22. Hydrolysis of Acid Derivatives • Mechanisms • nucleophile attacks P • base-catalyzed • OH- strong nucleophile (108 for P, not 104.2) • nucleophile attacks C • usually H2O

23. Hydrolysis of Acid Derivatives thiometon • Thiophosphoric acid thioester • internal nucleophilic substitution (SNi)

24. Hydrolysis of Acid Derivatives Problem 13-9

25. Hydrolysis of Acid Derivatives

26. Hydrolysis of Acid Derivatives para- values becauseleaving group is a phenol

27. Hydrolysis of Acid Derivatives para- values becauseleaving group is a phenol

28. Hydrolysis of Acid Derivatives • Hammett relationship • intercept of -3.29 close to log kB = -3.33 for compound with –H substituents • susceptibility value is = 1.18

29. Hydrolysis of Acid Derivatives • Brᴓnsted relationship • value of  is 0.517

30. Hydrolysis of Acid Derivatives • Hypothesis correct? • good correlation with  and pKa means goodness of leaving group, not electrophilicity (low electron density) of P, controls rate of hydrolysis

31. Hydrolysis of Acid Derivatives Scatter at low pKa values best leaving groups may indicate some SN1 character poor correlation with SN2 at higher pKa

33. Redox Reactions Oxidation state

34. Redox Reactions C(I) Oxidation state

35. Redox Reactions C(I) -elimination C(I) Oxidation state

36. Redox Reactions C(I) -elimination C(0) C(I) reduction Oxidation state

37. Redox Reactions C(I) -elimination C(0) C(I) reduction reductive dehalogenation (dihalo-elimination) half-cell reaction Oxidation state

38. Redox Reactions • Redox reaction keys • only a few functional groups involved • carbon (carboxy, quinone, halo-aliphatics) • nitrogen (nitro, amine, azo) • sulfur (disulfide, thiol, ..., sulfonate) • mechanisms not always clear • may be several steps • depicted just as electron transfer in half-cell rxn • electron donors, acceptors not always known • more difficult to determine reaction rates

39. Redox Reactions Redox refresherwhere n is the number of electrons

40. Redox Reactions another convention, EH (redox potential)with T = 298 K, R = 8.314 J mol-1 K-1,F = 96,490 J V-1 mol-1:substituting for pe and pe0:

41. Redox Reactions • EH0(W) • W designates “conditions typical of natural waters” • set pH to 7; {H+} = 10-7 M • set other concentrations to values common in natural waters; e.g., • [Cl-] = 10-3 M • [Br-] = 10-5 M • [HCO3-] = 10-3 M • [Ox] and [Red] = 1 (standard state)

42. Redox Reactions for standard state • Example of EH0(W) • What is EH at standard state for this reaction?CCl4 + H+ + 2 e- = CHCl3 + Cl-EH0 = 0.79 V

43. 10-7 M 10-3 M Redox Reactions • Example of EH0(W) • What is EH in typical natural water for this reaction?CCl4 + H+ + 2 e- = CHCl3 + Cl-EH0 = 0.79 V

44. Redox Reactions (as long as species like Cl-, Br-, and HCO3- not in reaction) Converting from pe0 to pe0(W)

45. Redox Reactions

46. Redox Reactions pe = -log{e-} 20 strongest oxidants lower{e-} 10 0 higher{e-} strongest reductants -10

47. Redox Reactions pe = -log{e-} 20 lower{e-} easier to reduce faster reduction 10 0 higher{e-} harder to reduce slower reduction -10

48. Redox Reactions pe = -log{e-} 20 lower{e-} hardest to oxidize slowest oxidation 10 0 higher{e-} easiest to oxidizefastest oxidation -10

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