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CVEN 4424 Environmental Organic Chemistry

CVEN 4424 Environmental Organic Chemistry. Lecture 21 – Redox Reactions and Kinetics . Announcements. Reading Chapter 14, Sections 14.1, 14.2 for redox introduction, kinetics Problem sets PS 8 due next Thursday Office hours Tuesday, 12:30-2, my office Wednesday, 11-12 am, ECES 131

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CVEN 4424 Environmental Organic Chemistry

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  1. CVEN 4424Environmental Organic Chemistry Lecture 21 – Redox Reactions and Kinetics

  2. Announcements • Reading • Chapter 14, Sections 14.1, 14.2 for redox introduction, kinetics • Problem sets • PS 8 due next Thursday • Office hours • Tuesday, 12:30-2, my office • Wednesday, 11-12 am, ECES 131 • Wednesday, 3-5 pm, ECCE 1B41 (TA Brett)

  3. Redox Reactions

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

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

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

  7. Redox Kinetics • Equilibrium • pe, EH, rG • will the reaction go? • Kinetics • how fast will the reaction go? • time scales • Redox kinetics • rate depends on EH • Marcus theory • electron transfer • outer-sphere complexes

  8. Redox Kinetics • Equilibrium • pe, EH, rG • will the reaction go? • Kinetics • how fast will the reaction go? • time scales • Redox kinetics • rate depends on EH(really??) • Marcustheory • electron transfer • outer-sphere complexes

  9. Redox Kinetics • Redox reaction steps • sorption/desorption to/from NOM • adsorption to reactive surfaces • electron transfer • two electrons to form a stable product • regeneration of oxidant or reductant

  10. Redox Kinetics • Example: Reduction of nitrobenzene to aniline • 3 two-electron steps • EH0(W) = 0.42 V for the six-electron reaction • reduction should “go” at EH below 0.42 V • (high enough electron activity to make rxn go) N(III) N(I) N(-I) N(-III)

  11. Redox Kinetics • Electrons donated by hydrogen sulfide • first electron transfer is slow (Figure 14.4) • HS- gives up e- readily (high pe) • ArNO2 takes on electron only at high electron activity (low pe)

  12. Redox Kinetics • Nitrobenzene reduction to aniline • thermodynamically favored, but very slow... Dunnivant et al. (1992) ES&T26, 2133

  13. Redox Kinetics • Nitrobenzene reduction to aniline • thermodynamically favored, but very slow... • ...unless natural organic matter is present Dunnivant et al. (1992) ES&T26, 2133

  14. Redox Kinetics • Nitroaromatic reduction • natural organic matter • functional groups that resemble hydroquinone, mercaptohydroquinone, etc. • less negative redox potentials • HS- = HS• + e- EH0(W) = -1.06 V • DOMred = DOMox + e- -0.5 < EH0(W) < 0.3 V • NOM acts as redox catalyst • [NOM] does not change EH0(W) = -0.28 V

  15. Redox Kinetics 1,4-benzoquinone hydroquinone 1,4-naphthoquinone 1,4-naphthohydroquinone

  16. Redox Kinetics

  17. bulkox mediatorox pollutantox - n e- + n e- - n e- + n e- bulkred mediatorred pollutantred Redox Kinetics • Electron transfer mediators • variety of compounds • transition metal complexes (Fe(II) porphyrin) • NAD/NADH; AQDS/AHDS • lawsone, juglone • natural organic matter

  18. Redox Kinetics AQDS AHDS lawsone juglone

  19. Reduction Reactions (Eqn. 14-38) (Eqn. 14-42) • Predicting kinetics • Rate LFERs • kR redox potential (Marcus) • EH1; usually first electron transfer • kR  (Hammett)

  20. bulkox mediatorox pollutantox - n e- + n e- - n e- + n e- bulkred mediatorred pollutantred Redox Kinetics • Nitroaromatic reduction • estimate the half-life of 2,4-dinitrotoluenein the presence of hydrogen sulfide and NOM 5 mg L-1“Hyde County NOM” S(s) 2,4-dinitrotoluene H2S(aq) 2-nitro-4-aminotoluene

  21. Redox Kinetics • Nitroaromatic reduction • rate expression • relating rate coefficient to redox potential • but what is redox potentialof 2,4-dinitrotoluene? 2,4-dinitrotoluene 3-nitro-4-methylnitrobenzene

  22. Redox Kinetics • Nitroaromatic reduction 0.57 (0.73-0.16)

  23. Redox Kinetics • Nitroaromatic reduction • redox potential for 2,4-dinitrotoluene • NOM rate coefficient

  24. Redox Kinetics • Nitroaromatic reduction • first-order k for 2,4-dinitrotoluene • half-life

  25. Reduction Reactions • reductive dehalogenation • R—X R—H • R—CHX—CHX—R  R—CH=CH—R • nitroaromatic reduction • Ar—NO2 Ar—NH2 • aromatic azo reduction • Ar-N=N-Ar 2 Ar—NH2 • sulfoxide reduction • R—S(=O)—R  R—S—R • quinone reduction • O=Ar=O HO—Ar—OH

  26. Reduction Reactions • Reduction of carbon: reductive dehalogenation • R—X R—HCCl4 + H+ + 2 e- = CHCl3 + Cl- R—CHX—CHX—R  R—CH=CH—R

  27. Reduction Reactions • Reduction of carbon: quinone reduction • O=Ar=O  HO—Ar—OH

  28. Reduction Reactions • Reduction of nitrogen: nitroaromatic reduction • Ar—NO2 Ar—NH2 • aromatic azo reduction • Ar-N=N-Ar 2 Ar—NH2

  29. Reduction Reactions sulindac NSAID • Reduction of sulfur • sulfone reduction • R—S(=O)2—R  R—S(=O)—R • sulfoxide reduction • R—S(=O)—R  R—S—R

  30. Reduction Reactions • Need electron donors for reduction • aqueous species • H2S, HS- • Fe(II) • Mn(II) • NH3 • minerals, etc. • sulfide minerals (e.g., pyrite) • Fe(II) minerals (e.g., biotite) • Fe(0) in remediation

  31. reduction oforganiccontaminants oxidation of reductants

  32. Reduction Reactions • Reductive dehalogenation: • reducing conditions (anoxic) • promote breakdown of RX, removal of halogens • usually, products are less toxic • aliphatic (C1 and C2) – fast • olefinic (R=R-X) – relatively fast • aromatic (Ar-X) – slow(e.g., PCBs, chlorinated benzenes) • most reductants capable of reducing RX • high redox potentials mean easy to reduce

  33. Reduction Reactions chloroform yield range 10 to 90% • Reductive dehalogenation • products depend on reductant • example: carbon tetrachloride, CCl4 • two-electron transfer reductant favors chloroform • sulfide as reductant carbon disulfide

  34. Reduction Reactions cellulose viscose • Carbon disulfide and rayon • viscose through slit  cellophaneviscose through spinneret rayon • significant toxicity to nervous system • Parkinson’s disease

  35. Reduction Reactions • Reductive dehalogenation • rate affected strongly by • ease of breaking the C—X bond • C—X bond strength • stability of carbon radical formed • electron-withdrawing substituents stabilize carbon radical • rate affected weakly by • tendency of C—X to accept electron (electronegativity) • stabilization of carbon radical by geminal halogen

  36. Reduction Reactions Rank the following three halogenated ethanes by order of reductive dehalogenation rate from slowest to fastest:

  37. Reduction Reactions slowest fastest < <

  38. Reduction Reactions • Predicting kinetics – one example • Reduction of haloaliphatics • kR bond strength and electron withdrawal • BS: C—X bond strength (kcal mol-1) • *: Taft constant (Table 12.4, SGI, 1993) • BE: C—C or C=C bond energy (kcal mol-1) Piejenburg et al. (1991) Science of the Total Environment 109/110, 283-300.

  39. Reduction Reactions

  40. Reduction Reactions Piejenburget al. (1991) Science of the Total Environment 109/110, 283-300.

  41. Reduction Reactions • Example: 1,1,2,2-tetrachloroethanea.k.a., “sym-tetrachloroethane” • C-X bond strength, BS = 81 kcal mol-1 • stronger C-X bond (e.g., C-F)  slower rate • Taft constant, * = 4.20 • more electron-withdrawing  faster rate • C-C bond energy, BE = 63.2 kcal mol-1 • stronger C-C bond  faster rate

  42. Reduction Reactions • Example: 1,1,2,2-tetrachloroethanea.k.a., “sym-tetrachloroethane” • BS = 81 kcal mol-1 • * = 4.20 • BE = 63.2 kcal mol-1

  43. Reduction Reactions • Zero-valent iron • permeable reactive barrier remediation • other metals also(Sn, Zn, etc.) • iron metal is oxidized • “rusting” • corrosion • Fe0 = Fe2+ + 2 e- • EH0(W) ~ -0.6 V • Promotes reductivedechlorination

  44. Reduction Reactions • Permeable reactive barrier • maintain redox potential (Fe0 supply) • avoid clogging • Commercialization • ARS Technologies

  45. CCl4 + H+ + 2 e- = CHCl3 + Cl- Fe(0) = Fe2+ + 2 e- CCl4 + H+ + Fe(0) = CHCl3 + Cl- + Fe2+ CHCl3 + H+ + Fe(0) = CH2Cl2 + Cl- + Fe2+ CH2Cl2 + H+ + Fe(0) = CH3Cl + Cl- + Fe2+ CH3Cl + H+ + Fe(0) = CH4 + Cl- + Fe2+

  46. Reduction Reactions • Demonstrations • halogenated alkanes, alkenes • DDT, DDD, DDE • pentachlorophenol • triazines (e.g., atrazine, RDX) • metals • Cr(VI), As(V) • U(IV)

  47. Reduction Reactions • Fe(0) reductionof chlorinated compounds: • methanes • ethanes • propanes • ethenes (Johnson et al., 1996, ES&T30, 2634-2640)

  48. Reduction Reactions • Accounting for Fe(0) surface area • better correlation indicates surface process (Johnson et al., 1996, ES&T30, 2634-2640)

  49. Reduction Reactions • Rate of reduction by Fe(0) • depends on compound concentration • depends on iron surface area, as (m2 g-1) • kobs(h-1)= kSA(L m-2 h-1) aFe(0) (m2 L-1) (Johnson et al., 1996, ES&T30, 2634-2640)

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