Page 107 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

Electrochemistry 185

HOOH + HOO − [HOOOH] + HO − Some of the considerations for electron-transfer pro-
cesses that have been discussed are fundamental to
py
− (138)
9
2
n
s
k,3 × 10 M −1 −1 1/n [·pyOH] + H 2 O + O · the electrochemistry of these examples. Thus, reduc-
tive processes always involve the most electrophilic
The resultant HO· is trapped by the pyridine solvent to
(acidic, positive-charge density) center (substrate or sub-
−
yield a stable solution of O ·.
2 strate/matrix combination) that produces the least basic
In acetonitrile there is no evidence of a superoxide ion,
(nucleophilic) product. Under acidic conditions the pri-
either by ESR or by cyclic voltammetry, from the elec-
mary reactant often is the hydronium ion (H O) to give a
+
trochemical reduction of HOOH. This can be explained 3
hydrogen atom that couples with the substrate via covalent
by the slow rate of reaction of HO· with CH 3 CN (k,4 × bond formation, e.g.,
6
10 M −1 −1
s ), which favors direct disproportionation.
+
−
[HOOOH] + HO − H 2 O + [HOOO ] PhN NPh + 2H O + 2e −
3
− PhNHNHPh + 2H 2 O E p,c , −0.5VvsSCE.
HO +·O 2 ·
Me 2 SO
(139) (144)
The reduction of hydrogen peroxide in aqueous solution In contrast, oxidations always involve the most nucle-
appears to be analogous to that in CH 3 CN, with the mech- ophilic (basic, negative-charge density) center (substrate
anism represented by the reaction of Eq. (137) followed or substrate/base combination) that produces the least
by the reactions of Eqs. (138) and (139). Thus, the re- acidic (electrophilic) product. Under basic conditions (or
−
duction of HOOH yields H 2 and HOO , initially, in a neutral aqueous solutions) the primary reactant often is
one-electron step. The final products are the result of the the base [B ;HO , HOC(O)O , AcO , PhO ]togive
−
−
−
−
−
−
reaction of HOO and HOOH and are analogous to those an oxyl radical (e.g., HO·) that adds to the substrate or,
for the base-induced decomposition of HOOH. more often, abstracts a hydrogen atom, e.g.,
InacetonitrileHOOHisoxidizedto·O 2 ·viaanelectron-
−2e −
transfer/chemical/electron-transfer (ECE) mechanism, PhNHNHPh + 2HO − PhN NPh + 2H 2 O
MeCN
−e − H 2 O
HOOH [HOOH] · HOO·+H O E p,a , −1.1VvsSCE. (145)
+
+
3
E , +2.0 V vs NHE (140) 1. Elemental Sulfur (S 8 )
◦
−e −
+
◦
HOO · ·O 2 ·+ H O E , +0.6VvsNHE. (141) Although the cyclic voltammogram of S 8 (in dimethyl sul-
3
H 2 O
foxide at a gold electrode) appears to be a simple sequence
Although HOO − reacts rapidly with most organic sol- of two two-electron-per-S 8 , pseudo-reversible reductions,
vents, it persists long enough in pyridine to permit its controlled-potential coulometry at −0.7 V vs SCE indi-
2−
electrochemical oxidation via a similar ECE mechanism. cates that 2.7 electrons per S 8 are consumed to give S .
6
−e − HO − Controlled-potential coulometry at −1.5 V vs SCE con-
−
HOO − [HOO·] H 2 O + O · 2−
2 sumes 4.0 electrons per S 8 to produce two S ions. On the
4
◦
E , −0.37 V vs NHE (142) basis of these results and associated spectroscopic studies,
the reduction sequence in Me 2 SO involves two ECE steps
−e −
− ◦ (143)
O · ·O 2 · E , −0.66VvsNHE with ring opening and chain breaking the two chemical
2
steps,
C. Nonmetals e
c-S 8 e E p,c , 0.60 V vs SCE S 8
+
Although hydronium ion (H O) and dioxygen (·O 2 ·) are
3
the most studied of the molecules and ions without metal e
2

atoms, several of the molecules that contain sulfur, ni- S 8 E p,c, 1.29 V S 4 S 4
trogen, or carbon also are electroactive. The results for
representative examples illustrate the utility of electro- e
chemical measurements for the evaluation of the redox
thermodynamics and bond energies for nonmetals. In 2
S 4
particular, the electrochemistry for several sulfur com-
(146)
pounds [S 8 ,SO 2 ], nitrogen compounds [·NO, hydrazines
−

(RNHNHR )], and carbon compounds (CO 2 , CO) is sum- The appearance of the bright blue solution (S ·) during
3
marized and interpreted. the course of electrolysis is dramatic;