Transcript No Slide Title

Reduction and Oxidation Reactions
Oxidation states:
Cl3C-CCl3 + 2e-  Cl2C=CCl2 + 2ClPh-NO2 + 6H+ + 6e-  Ph-NH2 + 2H2O
SO42- + 9H+ + 8e-  HS- + 4H2O
6CO2 + 24H+ + 24e-  C6H12O6 + 6H2O
O2 + 4H+ + 4e-  2H2O
FeIIIP + e-  FeIIP
O
Fe
OH
+ 2H+ + 2e- 
O
OH
Redox Reactions
Chapter 14
Redox reactions may be abiotic or biological
“abiotic” means it does not involve a living organism
(but may involve biological molecules released from cells)
Common environmental redox reactions
Nernst equation:
RT  oxidized form
EE 
ln


nF
reduced form 
o
remember: DG = -nFE
Half-Reactions:
coupling of oxidation and reduction:
reductant:
FeIIP  FeIIIP + e-
E = -0.07 (pH 7)
oxidant:
CCl4 + e-  CCl3 + Cl-
E = 0.09 (favorable)
CCl3H + e-  CCl2H + Cl-
E = -0.19 (not favorable)
Note: [Cl-] = 10-3 M
Factors affecting
redox condition in
the environment:
Note: pE is a rotten way to
predict reaction rates
Reduction reactions are a
function of the identity and
conc. of reductants present.
Reduction
potentials
Reductive Dehalogenation
More halogens = more
favorable redox potential
Br easier to reduce than Cl
Aromatic halides more
difficult to dehalogenate than
aliphatic or vinyl
Oxidation reactions:
Most contaminants are stable with respect to abiotic
oxidation (O2 is a kinetically weak oxidant).
Groups that can be abiotically oxidized include:
mercaptans
R-SH
anilines
Ph-NH2
phenols
Ph-OH
Environmentally important oxidizers include:
Fe(III) and Mn(III/IV) minerals
Oxidation of phenols by Mn(III/IV) oxides:
form inner-sphere complex:
surface  Mn(III)-OH +ArOH  surface  Mn(III)-O-Ar + H2O
electron transfer:
surface  Mn(III)-O-Ar  surface  Mn(II),•O-Ar
release of phenoxy radical:
surface  Mn(II),•O-Ar + H2O  surface  Mn(II)-OH2 + •O-Ar
release of Mn(II):
surface  Mn(II)-OH2  Mn2+ (aq) + new site
Note the incredibly complicated kinetics, which depend on:
pH (speciation of oxide surface sites and phenol)
number of available sites
rate at which they are regenerated
affinity of phenol for sites
mass transfer?
Stone 1987
Reduction reactions:
Groups that can be reduced include:
halogens (I, Br, Cl, usually not F)
nitro (-NO2)
Environmentally important reductants include:
reduced sulfur species such as HS-
many minerals, esp. Fe(II) minerals (FeS, FeCO3)
Fe(II) adsorbed to mineral surfaces
Electron transfer mediators:
dissolved, complexed Fe(II) species
O
OH
quinones
+ 2H+ + 2e- 
O
OH
Reductive dehalogenation:
Important for nearly all chlorinated and brominated species.
Dissociative for aliphatic compounds (halogenated methanes and
ethanes)
CCl4 + e-  • CCl3 + Clnon-dissociative for aromatics:
Cl
Cl
+
Cl
e-
Cl
Cl
•-
•
Cl
+ ClCl
Probably dissociative for vinyl halides:
Cl2C=CCl2 + e-  Cl2C=CCl • + Cl-
Cl
One-electron reduction potentials
more halogens = more favorable reduction potential
CH2=CHCl
CHCl=CHCl (Z)
CHCl=CHCl (E)
CH2=CCl2
CHCl=CCl2
CCl2=CCl2
-1.010
-0.997
-0.954
-0.820
-0.674
-0.613
CH3CH2Cl
CH2ClCH2Cl
CH3CHCl2
CH2ClCHCl2
CHCl2CHCl2
CH3CCl3
CH2ClCCl3
CHCl2CCl3
CCl3CCl3
-0.678
-0.558
-0.354
-0.220
-0.175
-0.031
0.077
0.139
0.152
CH3Cl
CClF3
CH2ClF
CHClF2
CHCl2F
CH2Cl2
CCl2F2
CHCl3
CCl3F
CCl4
-0.729
-0.654
-0.618
-0.586
-0.488
-0.416
-0.298
-0.193
-0.074
0.090
Totten and Roberts, 2001
Reduction pathways
Hydrogenolysis:
RCl + 2e- + H+
RH + Cl -
Reductive b-elimination (usually favored):
H2ClC-CClH2 + 2e-
H2C=CH2 + 2Cl -
HClC=CClH + 2e-
HC CH + 2Cl -
Reductive a-elimination:
CCl4 + 2e-
CCl3 - + Cl -
CCl2 + 2Cl -
Cl
Cl
C C
Cl
Cl
PCE
2
1
Cl
Cl C C Cl
dichloroacetylene
Cl
C C
6
Cl
H
TCE
3
17
16
4
Cl
Cl C C H
chloroacetylene
8
5
Cl
H
Cl
Cl
H
C C
C C
C C
H
Cl
trans-1,2-DCE
H
H
cis-1,2-DCE
Cl
H
1,1-DCE
14
15
10
from Arnold and Roberts, 1998
11
Cl
H C C H
acetylene
Hypothesized reaction sequence for
reduction of chlorinated ethylenes and
related compounds by Zn(0). (also true for
reactions with vitamin B12, Fe(0), etc.)
9
7
13
H
C C
H
H
vinyl chloride
12
H
H
C C
H
H
ethylene
Reactant
Ethanes
CH3CH2Br
CH2BrCH2Br
CH3CH2Cl
CH3CHCl2
CH3CHClF
CH2ClCH2Cl
CH3CCl3
CH2ClCHCl2
CH2ClCHCl2
CH2ClCCl3
CH2ClCCl3
CHCl2CHCl2
CHCl2CHCl2
CHCl2CCl3
CHCl2CCl3
CCl3CCl3
Ethylenes
CH2=CHCl
CH2=CCl2
CHCl=CHCl (E)
CHCl=CHCl (Z)
CHCl=CCl2
CHCl=CCl2
CHCl=CCl2
CCl2=CCl2
Acetylenes
HCCCl
ClCCCl
Hydrogenolysis
Product
E2
h
CH3CH3
CH3CH2Br
CH3CH3
CH3CH2Cl
CH3CH2F
CH3CH2Cl
CH3CHCl2
CH2ClCH2Cl
CH3CHCl2
CH2ClCHCl2
CH3CCl3
CH2ClCHCl2
0.400
0.473
0.369
0.493
0.309
0.467
0.552
0.520
0.494
0.580
0.522
0.546
CHCl2CHCl2
CH2ClCCl3
CHCl2CCl3
0.648
0.614
0.680
CH2=CH2
CH2=CHCl
CH2=CHCl
CH2=CHCl
CHCl=CHCl (Z)
CH2=CCl2
CHCl=CHCl (E)
CHCl=CCl2
0.451
0.423
0.427
0.406
0.530
0.513
0.508
0.591
HCCH
HCCCl
0.499
0.561
Elimination
Product
E2 e
CH2CH2
0.772
CH2CH2
0.735
CH2CHCl
0.805
CH2CCl2
0.963
CHClCHCl (Z)
CHClCHCl (E)
CHClCCl2
0.945
0.924
1.063
CCl2CCl2
1.152
HCCH
HCCH
HCCCl
0.589
0.568
0.599
ClCCCl
0.629
Reductive elimination
is thermodynamically
favored over
hydrogenolysis
from Totten and
Roberts, 2001
Table 2-2. Percentage of reaction occurring via reductive elimination
Species
% Reductive Elimination
D E2=(Ered. elim-Ehydrogenolysis)
PCE
0.049
15( 2)a
TCE
0.075b
30( 4)
cis-DCE
0.086
85( 8)
trans-DCE
0.085
95( 2)
a
Uncertainties represent 95% confidence limits.
b
Using the value for the reduction of TCE to trans-DCE (the principal observed DCE
isomer).
For reactions of chlorinated ethylenes with Zn(0),
thermodynamic considerations can predict amount of
elimination product formed.
(from Arnold and Roberts 1998)
LFERs for reduction reactions:
E1 (when available)
LUMO
BDE (dissociative reactions)
E2’s?
100
1+2
kSA (L·m-2·hr-1)
10-1
10-2
4+6
10-3
10-4
3
12
5
11
7+8
10-5
R2(adj.)=0.958
SE=0.34
9+10
10-6
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
E1 (Volts)
Correlation of surface area normalized rate constants (kSA) for
reactions of chlorinated ethylenes with Zn(0) with one-electron
reduction potentials. (from Arnold and Roberts 1998).
101
100
10-2
2
3
-2
-1
kSA (L·m ·hr )
1
Hydrogenolysis
R2(adj.)=0.932
SE=0.52
10-1
10-3
6
4
12
10-4
11
5
8
10-5
10
10-6
9
7
Red. Elimination
R2(adj.)=0.990
SE=0.19
10-7
0.35
0.40
0.45
0.50
0.55
0.60
0.65
E2 (Volts)
Correlation of surface area normalized rate constants (kSA) for reactions of
chlorinated ethylenes with Zn(0) with two-electron reduction potentials for
hydrogenolysis and reductive elimination. from Arnold and Roberts 1998
Zhijie Liu, Eric A. Betterton, and Robert G. Arnold* ; Electrolytic Reduction of
Low Molecular Weight Chlorinated Aliphatic Compounds: Structural and
Thermodynamic Effects on Process Kinetics, Environmental Science &
Technology; 2000; 34(5); 804-811.
porous nickel cathode. The chlorinated ethenes reacted much faster than predicted from bond
enthalpy calculations and the alkane-based correlation, suggesting that alkenes are not
transformed via the same mechanism as the chlorinated alkanes. Dihalo-elimination was the
predominant pathway for reduction of vicinal polychlorinated alkanes. For chlorinated alkenes
and geminal chlorinated alkanes, sequential hydrogenolysis was the major reaction pathway.
1
log k vs.BDE
1
y = -0.0312x + 1.7917
R2 = 0.0956
0
log k
log k
-1
-2
-3
-4
65.0
0
alkenes
y = -0.1779x + 12.759
R2 = 0.8177
75.0
85.0
BDE (kcal/mol)
alkanes
95.0
log k vs E1
y = 0.9827x - 0.3954
R2 = 0.4563
-1
-2
y = 3.5853x - 0.1143
R2 = 0.8753
-3
-4
-1.5
-1.0
-0.5
E1
0.0
0.5
10
7+8
11
9+10
4+6
0.1
s
log(k St) in (M/hr)
1
R2(adj.) = 0.898
SE = 0.298
1+2
0.01
5
3
0.001
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
E1 (Volts)
Correlation of the kinetic parameter for chlorinated ethylene reduction by Fe(0)
with one-electron reduction potential (E1). What’s wrong with this picture?!
From Arnold and Roberts, 2000
Reductive dechlorination of PCBs (aromatics):
occurs in anaerobic biodegradation, but slow
reduces toxicity but doesn’t destroy backbone
more chlorines = more favorable reduction potential, but very little
thermodynamic data available
Regiospecificity:
bacteria generally dehalogenate meta and para positions
Woods, Trobaugh, & Carter, ES&T 1999--a model reductant, (vitamin
B12s ) dehalogenated all positions equally (products were predicted
based on thermodynamic calculations)
Microbial dechlorination of PCBs
• So far, seen only in aquatic
sediments
• Mediated by chloroflexi
– Use organochlorine compounds as
electron acceptors
– Some spp are also sulfate reducers
• Usually removes chlorines at meta
and para but not ortho positions
– Several pathways identified
Dechlorination pathways
From Bedard, 2003
PCBs 47 and 51 build up as intermediates
These congeners are
abundant in Aroclors
PCB 132
Flanked and doubly
flanked meta Cls
removed
PCB 51
Only unflanked para Cls remain
PCB 153
Flanked meta
Cls removed
Cannot be
further
dechlorinated by
this pathway
PCB 47
PCBs 47 and 51 noted as recalcitrant products by Magar et
al. (ES&T 2005) in Lake Hartwell sediments
Further dechlorination of PCBs 47 & 51
by a different pathway (bacterial consortium?)
PCB 51
PCB 47
PCB 19
Unflanked para
Cls removed
PCB 4
End products of
dechlorination
(Seen in Upper Hudson
River and elsewhere)