Chemistry Reference
In-Depth Information
Cr VI
(
)
+
S
Cr VI -S
(
)
(6.21)
Cr VI
(
)
− →
S
Cr V S
(
)
+
•
(6.22)
Cr VI
(
)
− →
S
Cr IV P
(
)
+
.
(6.23)
The first step is commonly the formation of the Cr(VI)-S complex, followed
by either a one-electron reduction to form Cr(V) and the substrate radical
(S
•
) or a two-electron reduction to form Cr(IV) and the product (P). The fol-
lowing set of subsequent reactions can occur (reactions 6.24-6.31):
2Cr IV
(
)
→
Cr V Cr III
(
)
+
(
)
(6.24)
Cr IV Cr VI
(
)
+
(
)
→ 2
Cr V
(
)
(6.25)
Cr IV S
(
)
+ →
Cr III R
(
)
+
•
(6.26)
•
Cr VI R
(
)
+ →
Cr V P
(
)
+
(6.27)
Cr IV S
(
)
+ →
Cr II
(
)
+
P
(6.28)
Cr VI Cr II
(
)
+
(
)
→
Cr V Cr III
(
)
+
(
)
(6.29)
•
Cr V S
(
)
+ →
Cr IV R
(
)
+
(6.30)
Cr V S
(
)
+ →
Cr III
(
)
+
P
.
(6.31)
The reductions of Cr(VI) by substrates differ depending on the involvement
of Cr(V) or Cr(IV) in the steps of the mechanisms. Cr(V) can be formed by (1)
disproportionation of the Cr(IV) (reaction 6.24), (2) the reaction of Cr(IV)
with Cr(VI) (reaction 6.25), and (3) the one-electron transfer from Cr(VI) by
either a radical (Eq. 6.27) or Cr(II) (reaction 6.29). Cr(IV) can oxidize a sub-
strate by a one-electron pathway to form a radical (reaction 6.26) and by a
two-electron pathway to yield the oxidized product of the substrate (reaction
6.28). The reduction of Cr(V) can proceed by either a one-electron or a two-
electron transfer pathway (reactions 6.30 and 6.31). Therefore, different mech-
anisms occur depending on the reaction conditions as well as the concentrations
of the chromium species and the substrate. The stabilization of the intermediate
Cr(V/IV) species by substrates results in complex species, which can also influ-
ence the reaction mechanisms [59]. These complex species may also react with
the substrate and the components of the reaction medium [59, 60].
6.1.4 Reactivity of Cr Species
In this section, the reduction of Cr(VI) with reductants of biological impor-
tance is discussed. The importance of these reactions will be discussed in the
next section.
6.1.4.1 Fe(II) Complexes.
The reduction of Cr(VI) by biological Fe(II)
complexes (e.g., heme proteins) may result in Cr(V) complexes. The reaction
Search WWH ::
Custom Search