Kinetics and Mechanisms of Nonmetal Redox Reactions of

Kinetics and Mechanisms of Non-metal Redox Reactions of Oxyhalogens Kara E. Huff Hartz Department of Chemical Engineering Carnegie Mellon University

Oxyhalogen Redox Reactions Halogen(I) reactions proceed by the following mechanism: HOX + Y- + H+ W XY + H 2 O W X- + HOY + H+ where X = Cl, Br, I Y = Cl-, Br-, I-, NO 2 -, CN-, SO 32 -, Cl. O 2 -, Br. O 2 - How about halogen(III) reactions ? XO 2 -, HOX 2? 2

Halite Ions (XO 2 -, X = Cl, Br) n Halites often occur in oscillating reactions. 1 n Bromite is an intermediate for Br. O 3 - formation (a carcinogen) in drinking water. 2 Both Br. O 3 and Cl. O 2 - are regulated by the USEPA. 3 n Few mechanistic studies regarding Br. O 2 - exist, largely due to the lack of commercial sources and its instability. 1 (a) Orbán, M. ; Epstein, I. R. J. Phys. Chem. 1995, 99, 2358 -2362. (b) Faria, R. D. ; Lengyel, I. ; Epstein, I. R. J. Phys. Chem. 1993, 97, 1164 -1171. (c) De Kepper, P. ; Boissonade, J. ; Epstein, I. J. Phys. Chem. 1990, 94, 6525 -6536. 2 von Gunten, U. ; Oliveras, Y. Environ. Sci Technol. 1998, 32, 63 -70. 3 Fed. Regist. 1998, 63, 69390. 3

Halite/S(IV) Reactions XO 2 - + 2 S(IV) → X- + 2 S(VI) X = Cl, Br S(IV) = SO 32 -, SO 3 H-, SO 2 S(VI) = SO 42 n Lee and Lister 4 measured the kinetics of Br. O 2 -/SO 32 - reaction in basic solution ([OH-] > 0. 01 M) n Frerichs et al. 5 reported rate constants for Cl. O 2 -/S(IV) reaction from the unpublished data of Rushing and Thompson. n The Br. O 2 -/SO 32 - reaction rate is several orders of magnitude greater than the Cl. O 2 -/SO 32 - reaction rate. Br. O 2 - + SO 32 - → OBr- + SO 42 Cl. O 2 - + SO 32 - → OCl- + SO 42 - EBo = 1. 52 V EBo = 1. 60 V 4 Lee, C. L. ; Lister, M. W. Can J. Chem. 1979, 57, 1524 -1530. 5 Frerichs, G. A. ; Mlnarik, T. M. ; Grun, R. J. ; Thompson, R. C. J. Phys. Chem. A 2001, 105, 829 -837. 4

Kinetics Data Analysis t = time, kobs = observed pseudo-firstorder rate constant, f = final, i = initial, DA = initial – final absorbance Example of Stopped-Flow Spectroscopy Data 5

[SO 3]T Dependence of kobs [SO 3]T = [SO 32 -] + [SO 3 H-]+ [SO 2] This reaction is first order in [XO 2 -] and [SO 3]T. 6

Halite/S(IV) Reaction Stoichiometry Overall Reaction (confirmed by ion chromatography): XO 2 - + 2 SO 32 - → X- + 2 SO 42 XO 2 - + SO 32 - → OX- + SO 42 - (rds) H+ + OX- + SO 32 - → XSO 3 - + OH- (rapid)8 -9 XSO 3 - + H 2 O → X- + SO 42 - + 2 H+ (rapid)9 -10 8 Fogelman, K. D. ; Walker, D. M. ; Margerum, D. W. Inorg. Chem. , 1989, 28, 986 -993. 9 Troy, R. C. ; Margerum, D. W. Inorg. Chem. , 1991, 30, 3538 -3543. 10 Yiin, B. S. ; Margerum, D. W. Inorg. Chem. , 1988, 27, 1670 -1672. 7

General-Acid Dependence of Br. O 2 -/S(IV) Reaction The Br. O 2 -/S(IV) reaction rate is catalyzed by general acids (HA). 8

[H+] Dependence of Br. O 2 -/S(IV) Reaction A non-zero intercept shows that even as [H+] → 0, the Br. O 2 -/SO 32 reaction still occurs. 9

Bromite/Sulfite Reaction Mechanism HA = general acids such as H 2 O, H 3 O+, HCO 3 -, B(OH)3, HPO 42 -, H 2 PO 4 - 10

Rate Expression for Br. O 2 -/SO 32 - Reaction Hbf = acidic form of the buffer Kw = [H+][OH-] = 10 -13. 60 M 2 In the absence of buffer, the rate constant increases linearly with increasing [H+]. k 3 H 2 OK 1 = 4. 50(3) M-1 s-1 and k 4 H 2 OK 1 K 2/Kw = 3. 03(4) x 1012 M-2 s-1 In the presence of buffer at constant [H+], the rate constant increases linearly with increasing [Hbf]. To resolve the two buffer-catalyzed paths, kobs vs. [Hbf] data at varied [H+] is required. 11

Br. O 2 -/S(IV) Reaction has Two General-Acid Catalyzed Paths 12

Resolution of General-Acid Catalysis Rate Constants Slope (of krxn vs. [HCO 3 -]) = k 3 HCO 3 -K 1 + (k 4 HCO 3 -K 1 K 2/Kw)[H+] k 4 HCO 3 -K 1 K 2/Kw = 1. 8(3) 1013 M-3 s-1 k 3 HCO 3 -K 1 = 9(2) 103 M-2 s-1 13

[H+] Dependence of Br. O 2 -/S(IV) Reaction (p[H+] 5. 9 - 7. 2) d = k 1(Br. O 2 -/SO 32 -) = 3. 0(5) 107 M-1 s-1 e = k 4 H 2 OK 2/k-1 Kw = 1. 5(3) 105 M-1 f = k 4 H 2 PO 4 -K 1 K 2/Kw = 1. 3(3) 1013 M-3 s-1 14

p[H+] Dependence of Chlorite/S(IV) Reaction Data are curve fit to: The Cl. O 2 -/SO 32 - direct reaction rate is negligible. The Cl. O 2 -/S(IV) reaction rate is NOT catalyzed by buffer salts. 15

Cl. O 2 -/S(IV) Reaction Mechanism Rate Expression: (ref. 11) k 6 = 5. 5(3) M-1 s-1 k 5 = 6. 26(4) x 106 M-1 s-1 11 Horner, D. A. ; Connick, R. E. Inorg. Chem. 1986, 25, 2414 -2417 16

Halite/S(IV) Mechanism Comparison Br. O 2 -/S(IV) Cl. O 2 -/S(IV) § Prefers SO 32 - over SO 3 H§ Prefers SO 3 H- over SO 32 reaction § General-acid catalyzed § Specific-acid catalyzed § Br. O 2 - acts as electrophile and § Cl. O 2 - acts as nucleophile; SO 32 Br atom expands its octet; SO 32 acts as electrophile and - acts as nucleophile S atom expands its octet § Oxygen-atom transfer § OBr+ transfer mechanism with O-S bonded Br-S bonded adducts 17

Summary of Rate Constants 25. 0± 0. 1 o. C, m = 1. 0 M (Na. Cl. O 4) p[H+] 4. 0 -12. 6 Br. O 2 -/S(IV) Cl. O 2 -/S(IV) k 1(Br. O 2 -/SO 32 -) = 3. 0(5) 107 M-1 s-1 k 5(Cl. O 2 -/SO 2) = 6. 26(4) 106 M-1 s-1 k 3 H 2 OK 1 = 4. 50(3) M-1 s-1 k 6(Cl. O 2 -/SO 3 H-) = 5. 5(3) M-1 s-1 k 4 H 2 OK 1 K 2/Kw = 3. 03(4) 1012 M-2 s-1 k 3 HPO 42 -K 1 = 1. 5(2) 103 M-2 s-1 k 3 HCO 3 -K 1 = 9(2) x 103 M-2 s-1 k 4 HCO 3 -K 1 K 2/Kw = 1. 8(3) x 1013 M-3 s-1 k 4 B(OH)3 K 1 K 2/Kw = 8. 1(5) x 1013 M-3 s-1 k 4 H 2 PO 4 -K 1 K 2/Kw = 1. 3(3) x 1013 M-3 s-1 Huff Hartz, et al. , Inorg. Chem. 2003, 42, 78 -87. 18

Acknowledgements Dale W. Margerum (Purdue University) n Jeffrey S. Nicoson (Endocyte) n Lu Wang (Nova. Cal Pharmaceuticals) n Funding: NSF and Emerson Kampen Foundation n 19

OBr+ Transfer Mechanism HA HA 20

Stopped-Flow Spectroscopy DS = drive syringe PMT = photomultiplier tube RS = receiving syringe Typical deadtime = 2 -3 ms First-order rate constants (kobs) = 0. 005 – 800 s-1 21

Oxygen-Atom Transfer Mechanism 22

Halogenate/S(IV) Reaction Mechanisms Br. O 3 - + SO 32 - → Br. O 2 - + SO 42 - Ebo = 1. 81 V Cl. O 3 - + SO 32 - → Cl. O 2 - + SO 42 - Ebo = 1. 68 V Br. O 3 -/S(IV) and Cl. O 3 -/S(IV) reactions rates are slow in basic solution, but increase as S(IV) is protonated. 12 -16 Br. O 3 -/S(IV) rate constant does not depend on buffers. 13 Oxygen Atom Transfer: Cl. O 2 -, Cl. O 3 -, Br. O 3 X+ or OX+ Transfer: Br. O 2 -, HOCl, HOBr (12) Szirovicza, L. ; Boga, E. Int. J. Chem. Kin. 1998, 30, 869 -874. (13) Williamson, F. S. ; King, E. L. J. Am. Chem. Soc. 1957, 79, 5397 -5400. (14) Gleason, E. H. ; Mino, G. ; Thomas, W. M. J. Am. Chem. Soc. 1957, 61, 447 -450. (15) Dobrynin, N. A. ; Dymarchuk, N. P. ; Mishchenko, K. P. Zh. Obsch. Khim. 1969, 39, 2157 -2164. (16) Menédez, S. M. Rev. Fac. Cienc. , 1968, 9, 119 -213. 23

Brønsted-Pedersen Plot for Br. O 2 -/SO 32 - Reaction k. HA = rate constant Ka = acid dissociation constant of HA p = number of equivalent acidic sites in HA q = number of equivalent basic sites in Alog Ga = constant a = slope 24

Steady-state behavior of Br. O 2 -/SO 32 - Reaction Intermediates (p[H+] 7. 2 -5. 9) [SS]T = [(O 2 Br. SO 3)3 -] + [(HO 2 Br. SO 3)2 -] 25