Kara E. Huff Hartz

Department of Chemical EngineeringCarnegie Mellon University

Kinetics and Mechanisms of Non-metal Redox

Reactions of Oxyhalogens

2

Oxyhalogen Redox Reactions

Halogen(I) reactions proceed by the following mechanism:

HOX + Y- + H+ XY + H2O

XY + H2O X- + HOY + H+

where X = Cl, Br, IY = Cl-, Br-, I-, NO2

-, CN-, SO32-, ClO2

-, BrO2-

How about halogen(III) reactions ? XO2

-, HOX2?

3

Halite Ions (XO2-, X = Cl, Br)

Halites often occur in oscillating reactions.1

Bromite is an intermediate for BrO3- formation (a

carcinogen) in drinking water.2 Both BrO3- and

ClO2- are regulated by the USEPA.3

Few mechanistic studies regarding BrO2- 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.

4

Halite/S(IV) Reactions

Lee and Lister4 measured the kinetics of BrO2-/SO3

2- reaction

in basic solution ([OH-] > 0.01 M)

Frerichs et al.5 reported rate constants for ClO2-/S(IV) reaction

from the unpublished data of Rushing and Thompson.

The BrO2-/SO3

2- reaction rate is several orders of magnitude

greater than the ClO2-/SO3

2- reaction rate.

XO2- + 2S(IV) → X- + 2S(VI)

X = Cl, Br S(IV) = SO32-, SO3H-, SO2 S(VI) = SO4

2-

BrO2- + SO3

2- → OBr- + SO42- EB

o = 1.52 VClO2

- + SO32- → OCl- + SO4

2- EBo = 1.60 V

4Lee, C. L.; Lister, M. W. Can J. Chem. 1979, 57, 1524-1530. 5Frerichs, G. A.; Mlnarik, T. M.; Grun, R. J.; Thompson, R. C. J. Phys. Chem. A 2001, 105, 829-837.

5

Kinetics Data Analysis

t = time, kobs = observed pseudo-first-order rate constant, f = final, i = initial, A = initial – final absorbance

tkA)e(fAtA obs

tkei][XOt][XO obs22

][XOdt]d[XO

2obs2

k

0.961 mM BrO2-, 30.8(2) mM SO3

2-, 70.0 mM [CO3]T, p[H+] 9.53

= 1.0 M (NaClO4), 25.0(1) oC

Time, ms

0 20 40 60 80 100A

bsor

banc

e at

295

nm

(0.9

62 c

m c

ell)

0.00

0.02

0.04

0.06

0.08

0.10

(Average of 10 pushes)

Af = 2.56(1) x 10-3

A = 0.1083(1) kobs = 49.40(3) s-1

Example of Stopped-Flow Spectroscopy Data

6

[SO3]T Dependence of kobs

[SO3]T = [SO32-] + [SO3H-]+ [SO2]

This reaction is first order in [XO2-] and [SO3]T.

[SO3]T, M

0.00 0.05 0.10 0.15 0.20

k obs

(BrO

2- ), s-1

0

1

2

3

4

5

6

104 x

kob

s (C

lO2- ),

s-1

0

2

4

6

8

10

[SO3]T, mM0 1 2 3

k obs

(BrO

2- ), s

-1

0

200

400

600

800

p[H+] 7.10

p[H+] 11.55p[H+] 9.53 10

4 x k

obs(

ClO

2- ), s-1

7

Halite/S(IV) Reaction Stoichiometry

Overall Reaction (confirmed by ion chromatography):XO2

- + 2SO32- → X- + 2SO4

2-

XO2- + SO3

2- → OX- + SO42- (rds)

H+ + OX- + SO32- → XSO3

- + OH- (rapid)8-9

XSO3- + H2O → X- + SO4

2- + 2H+ (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.

8

General-Acid Dependence of BrO2-/S(IV) Reaction

The BrO2-/S(IV) reaction rate is catalyzed by general acids (HA).

9

[H+] Dependence of BrO2-/S(IV) Reaction

A non-zero intercept shows that even as [H+] → 0, the BrO2-/SO3

2- reaction still occurs.

10

Bromite/Sulfite Reaction Mechanism

24

23 SOBr rapidSOHOBr

332

232 )BrSO(OSO BrO

1

1-

k

k

OH)BrSO(HOOH)BrSO(O 2322

332

2

2-k

k

OHAOBrSOHA)BrSO(O 33

32HA3k

OHAOBrSOHA)BrSO(HO 232

32HA4 k

24

rapid23 SOH2OBrOHOBrSO

OH HOBr OHOBr -rapid

2 HA = general acidssuch as H2O,H3O+, HCO3

-,B(OH)3, HPO4

2-,H2PO4

-

11

Rate Expression for BrO2-/SO3

2- Reaction

Hbf = acidic form of the bufferKw = [H+][OH-] = 10-13.60 M2

In the absence of buffer, the rate constant increases linearly with increasing [H+].

k3H2OK1 = 4.50(3) M-1 s-1 and k4

H2OK1K2/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.

]H][Hbf[]Hbf[]H[]SO[ w

21Hbf

413

w21

OH4

1OH

3T3

obs 22

KKKkKk

KKKkKk

k

12

BrO2-/S(IV) Reaction has Two General-Acid

Catalyzed Paths

13

Resolution of General-Acid Catalysis Rate Constants

Slope (of krxn vs. [HCO3-]) =

k3HCO3-K1 + (k4

HCO3-K1K2/Kw)[H+]

k4HCO3-K1K2/Kw = 1.8(3) 1013 M-3 s-1

k3HCO3-K1 = 9(2) 103 M-2 s-1

OHAOBrSOHCO)BrSO(O 3-

33

32-HCO3

3k

OHAOBrSOHCO)BrSO(HO 23-

32

32-HCO3

4 k

14

[H+] Dependence of BrO2-/S(IV) Reaction (p[H+] 5.9- 7.2)

]H[]PO][H[H][H1

]PO][H[H][H

T]3[SOobs

a2

a242

42

K

K

dfe

dfed

k

d = k1(BrO2-/SO3

2-) =3.0(5) 107 M-1 s-1

e = k4H2OK2/k-1Kw =

1.5(3) 105 M-1

f = k4H2PO4-K1K2/Kw =

1.3(3) 1013 M-3 s-1

15

p[H+] Dependence of Chlorite/S(IV) Reaction

Data are curve fit to:

The ClO2-/SO3

2- direct reaction rate is negligible.

The ClO2-/S(IV) reaction

rate is NOT catalyzed by buffer salts.

2a1a2a1

2

T3obs

][H][H][H][H

][SO

KKKbak

4 5 6 7 8 9 10 11

10-4

x k

obs/[

SO

3]T,

M-1

s-1

0

2

4

6

8

10

p[H+]6 7 8 9 10 11

k rxn

, M-1

s-1

0

50

100

150

200

p[H+]

16

ClO2-/S(IV) Reaction Mechanism

11Horner, D. A.; Connick, R. E. Inorg. Chem. 1986, 25, 2414-2417

a2 -23SO H

rapid H3SO K

3HOSOOCl2HOSO-2ClO 6k

HSOClSOHOCl 24

rapid23

Rate Expression:

9.4]HSO[

]HOSO[

3

2s

K (ref. 11)

k6 = 5.5(3) M-1 s-1

k5 = 6.26(4) x 106 M-1 s-1

a1 H H3SOrapid O2H 2SO K

3SOOCl52SO-

2ClO k

2H24SOrapid

3SO O2H

)Sa1

Sa2][HS

a12]([H

][HS1

Sa1

S6

2][H5

T]3[SOobs

KKK

KKK

kk

k

17

Halite/S(IV) Mechanism ComparisonBrO2

-/S(IV) Prefers SO3

2- over SO3H- reaction

General-acid catalyzed BrO2

- acts as electrophile and Br atom expands its octet; SO3

2- acts as nucleophile

OBr+ transfer mechanism with Br-S bonded adducts

OBr

OS

OO

OH

A

3-+

-Cl O S

OO

OHO

2-- +

ClO2-/S(IV)

Prefers SO3H- over SO32-

reaction Specific-acid catalyzed ClO2

- acts as nucleophile; SO32-

acts as electrophile and S atom expands its octet

Oxygen-atom transfer mechanism with O-S bonded adducts

18

Summary of Rate Constants25.0±0.1 oC, = 1.0 M (NaClO4)

p[H+] 4.0-12.6

Huff Hartz, et al., Inorg. Chem. 2003, 42, 78-87.

BrO2-/S(IV) ClO2

-/S(IV)k1(BrO2

-/SO32-) = 3.0(5) 107 M-1 s-1 k5(ClO2

-/SO2) = 6.26(4) 106 M-1 s-1

k3H

2OK1 = 4.50(3) M-1 s-1 k6(ClO2

-/SO3H-) = 5.5(3) M-1 s-1

k4H

2OK1K2/Kw = 3.03(4) 1012 M-2 s-1

k3HPO

42-K1 = 1.5(2) 103 M-2 s-1

k3HCO

3-K1 = 9(2) x 103 M-2 s-1

k4HCO

3-K1K2/Kw = 1.8(3) x 1013 M-3 s-1

k4B(OH)

3K1K2/Kw = 8.1(5) x 1013 M-3 s-1

k4H

2PO

4-K1K2/Kw = 1.3(3) x 1013 M-3 s-1

19

Acknowledgements

Dale W. Margerum (Purdue University) Jeffrey S. Nicoson (Endocyte) Lu Wang (NovaCal Pharmaceuticals) Funding: NSF and Emerson Kampen

Foundation

20

OBr+ Transfer Mechanism

Br SOO

O

O-

+ A- + OH-

Br SOO

O

O-

+ A- + H2OO

BrHO

SOO

O

2-

BrO2- + SO3

2- OBr

OS

OO

O

3-

H2O

Br SOO

O

O-

OBr- + SO42- + 2H+H2O

k3HAK1

K2

k4HA

rapid

+ OH-

HA

HA

21

Stopped-Flow Spectroscopy

S o u rc e ,M o n o c h ro m e te r

Sto pS w itch

PMT R SD S 1

D S 2

D riv eP la te C ell

L ig h tM ix er

Typical deadtime = 2-3 ms

First-order rate constants (kobs) = 0.005 – 800 s-1

DS = drive syringe PMT = photomultiplier tubeRS = receiving syringe

22

Oxygen-Atom Transfer Mechanism

ClO2- + HOSO2

- Cl O SOO

OHO

2-

OCl- + HOSO3-

ClO2- + SO2

Cl O SO

O

-

OOCl- + SO3

SO3 + H2O SO42- + 2H+

SO42- + H+ pKa = 1.10HOSO3

-

rapid

23

Halogenate/S(IV) Reaction MechanismsBrO3

- + SO32- → BrO2

- + SO42- Eb

o = 1.81 VClO3

- + SO32- → ClO2

- + SO42- Eb

o = 1.68 V

Oxygen Atom Transfer: ClO2-, ClO3

-, BrO3-

X+ or OX+ Transfer: BrO2-, 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.

BrO3-/S(IV) and ClO3

-/S(IV) reactions rates are slowin basic solution, but increase as S(IV) is protonated.12-16

BrO3-/S(IV) rate constant does not depend on buffers.13

24

Brønsted-Pedersen Plot for BrO2-/SO3

2- Reaction

kHA = rate constantKa = acid dissociation constant of HA

p = number of equivalent acidic sites in HAq = number of equivalent basic sites in A-

log Ga = constant

= slope

aa logGpqαlogp

HAlog Kk

log(Ka/p)

-15 -12 -9 -6 -3 0

log(kH

Aq/

p)

0

3

6

9

12

15

18

= 0.88(7)

k3HAK1

k4HAK1K2/Kw

= 0.3(1)

Figure 5

OBr

OS

OO

OH

A

3-O

BrO

SOO

OH

A

2-

H

25

Steady-state behavior of BrO2-/SO3

2- Reaction Intermediates (p[H+] 7.2-5.9)

[SS]T = [(O2BrSO3)3-] + [(HO2BrSO3)2-] ])BrSO[(O

]][OH)BrSO[(HO3

32

232

2

K

]PO][H)3BrSO[(HOPOH])BrSO[(HOOH])BrSO[(OOHrate 422

24242

32243

3223 kkk

][H)H3(SOa

)H3(SOa

][Hw1

]-4PO2[H12-

4PO2H4][H

w1-2KO2H

41

])[Hw1

]-4PO2[H12-

4PO2H4][H

w1-2KO2H

4(1

T]3[SOobs

K

K

KkKKk

Kkk

KkKKk

Kkk

kk