Page 109 - 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 187

O acids HA] react with the ultimate nucleophile (the elec-
CO 2 Au(s) e Au I C (s) E p,c , 2.1 V vs SCE tron) directly or via its reaction product with hydronium
+
O ion (H O) or Brønsted acids (HA).
CO 2 3
For those electrophiles (El) that undergo direct electron-

Au(s) HC(O)O HOC(O)O e Au I C(O)OC(O)O (s) transfer reduction at an inert electrode (glassy carbon)
H 2 O
e [with the electron occupying the Lowest-Unoccupied-

Au(s) CO OC(O)O Molecular-Orbital (LUMO)], the reduction potential
(E red ) is a measure of their electron affinity and elec-
(156)
trophilicity [relative to that for H O(−2.10 V vs NHE
+
3

CO 2 Hg 2 (l) e Hg I Hg II C(O)O (s) E p,c , 2.3 V in aqueous media)] (the more positive the potential, the
more electrophilic the molecule).

e H 2 O e , 2.5 V
CO 2
El + e − El · E red (159)
−

Hg 2 (l) HC(O)O HO

[ O(O)C Hg II Hg II C(O)O (s)]
CO 2 Often, the solution matrix contains Lewis acids (e.g.,
+
H O) that are more electrophilic than the substrate
HOC(O)O 3
2 moleculeand,incombination,areevenmoreelectrophilic,
Hg 2 (l) CO CO 3
(157) e , H 3 O

El H 3 O e [HEl] HElH (E red ) EC.
6. Carbon Monoxide (CO) H 2 O H 2 O
(160)
At an activated gold electrode in alkaline (0.01 M NaOH)
−
aqueous solution CO is oxidized to CO 2− viaaHO -cen- The first-formed intermediate (with an unpaired electron)
3
tered ECE process, in combination with a second Lewis acid molecule has
even greater electron affinity and is reduced at a more

e , Au e
CO HO [Au I C(O)OH] (HO) 2 C(O) positive potential to give a voltammogram that appears
E 1/2 , 0.55 V vs SCE HO
to be the result of an irreversible two-electron reduction
2HO
process. In most cases it is an ECEC process in which each
2 electron transfer (E part of the ECEC mechanism) to the
CO 3 2 H 2 O.
+
Lewis acid (H O) is reversible to give a product (H·) that
(158) 3
forms a covalent bond with the substrate (H–El) (the C
Again, the activated gold surface stabilizes the carbon part of the mechanism).
radical intermediate [·C(O)OH] and facilitates the second Conversely, nucleophilic molecules (Nu) [Lewis bases;
−
electron-transfer oxidation of HO to HO· via coupling e.g., catechols, hydroquinones, phenols, alcohols, and thi-
to the carbon radical. ols (and their anions); aromatic hydrocarbons and amines
(benzene, toluene, pyridine, bipyridine, etc.)] can be oxi-
dized (1) by direct electron-transfer oxidation [Eq. (161)]
D. Organic Molecules [with the electron coming from the Highest-Occupied-
Molecular-Orbital (HOMO)] or (2) by coupling with the
Most organic reactions are Lewis acid/base processes that
−
oxidation product of H 2 O (or HO ), hydroxyl radical
involve the interaction of a nucleophilic center with an
(HO·) [Eq. (162)].
electrophilic center. Because electrochemistry provides
the ultimate nucleophile via the electrons at the cathode
−e −
surface and the ultimate electrophile via the electron holes Nu: Nu · E ox (161)
+
at the anode surface, it is the ideal methodology for the
characterization of the electrophilicity and nucleophilic- The potential (E ox ) for those nucleophiles that undergo
ity of molecules. Thus, the carbon centers of saturated direct electron-transfer oxidation at an inert electrode is
hydrocarbons (e.g., CH 4 ) are resistant to electrochem- a quantitative measure of their nucleophilicity (the more
ical reduction and oxidation because of their inert na- negative the potential the more nucleophilic the molecule;
ture (all valence electrons are stabilized in sigma bonds; see Table VI for representative values). In many cases wa-
an absence of any Lewis acid/base character). However, ter in the solvent matrix (or as the solvent) is more nucle-
organic molecules with electrophilic components [e.g., ophilic than the substrate molecule and, in combination,
alkyl-, aryl-, and acyl- halides; carbonyl groups; unsatu- is even easier to oxidize, which often results in an ECEC
rated and aromatic hydrocarbons; nitro groups; Brønsted oxidation process.