Page 112 - 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

190 Electrochemistry

2. Quinones, Semiquinones, and Catechols Neutral catechols (H 2 Cat) and hydroquinones (H 2 Q) are
much more resistant to electron removal because of their
All molecules with unsaturated bonds (olefins, acetylenes,
release of a proton via the solvent matrix; the more basic
aromatics, carbonyls, quinones, etc.) have a degree of
the solvent, the less positive the oxidation potential.
electrophilicity and electron affinity. Within a class, the
extent of conjugation increases the electron affinity (re-
duction of benzene occurs at a less negative potential H 2 Cat e [HSQ ] e Q (E p,a ) DMF , 0.97 V
sol sol
than 1-butene), and the presence of unsaturated carbon- (E p,a ) AN , 1.19 V
Hsol Hsol
oxygen (carbonyl) functions within a conjugated system
(173)
(e.g., quinones; O O ) also enhances the electron affinity
of the molecule. Inacetonitrile,theresidualH 2 Oisthestrongerbase,which
The cyclic voltammogram for 3,5-di-tert-butyl-o- results in the formation of H O.
+
3
quinone (3,5-DBTQ) has a first reduction that is a re- The electrochemistry of quinones is surprisingly sim-
versible one-electron process, followed by a second one- ilar to that of dioxygen. It is as if a conjugated carbon
electron reduction, which can be reversible in rigorously link is inserted between two oxygen atoms (Q → SQ ·→
−
2−
anhydrous media to give catechol dianion (3,5-DTBC ). HCat vs ·O 2 ·→ O ·→ HOO ;Q → H 2 Cat vs ·O 2 ·→
−
−
−
2
HOOH).
e −
−
3,5-DTBQ + e − 3,5-DTBSQ . 3,5-DTBC 2−
(167)
3. Carbonyl Groups, Olefins,
Although DTBC 2− is a strong base that is hydrolyzed by and Aromatic Hydrocarbons
residual H 2 O,
Unsaturated carbon centers possess a limited degree of
−
−
DTBC 2− + H 2 O DTBCH + HO , (168) electrophilicity and will accept an electron at potentials
significantly more negative than their chloro derivatives
it is also a strong reductant that can reduce H 2 O in DMF. (usually at least −2.5 V vs SCE in rigorously anhydrous
solvents). When water is present, its reduction is syner-
DTBC 2− + H 2 O DTBSQ ·+ 1/2H 2 + HO − gistically facilitated via unsaturated carbon. For example,
−
(169) in acetonitrile at a glassy-carbon electrode,
In many cases, the second reduction step of quinones
[Eq. (167)] is irreversible and due to the facilitated reduc- H 2 O e [H ] HO E p,c , –3.9 V vs SCE
tion of residual H 2 O.
(174)
−
3,5-DTBSQ ·+ H 2 O + e − 3,5-DTBCH + HO −
−
(170)
e NP E p,c , 2.6 V vs SCE
+
In the presence of hydronium ions (H O), the reduction
3
of the quinones is an irreversible two-electron process (175)
(ECEC), with the first step being the more difficult (re-
quiring the more negative potential). NP H 2 O e [HNP ] e H 2 NP
H 2 O

3,5-DTBQ H O e [ 3,5-DTBSQH ] HO HO
3

H 2 O e , H 3 O
3,5-DTBCH 2 E p,c , ~ 2.0 V vs SCE (176)
H 2 O
(171) Similar synergism is observed for other Brønsted acids
(HA) in the presence of unsaturated carbon centers.
The oxidation potentials for the fully reduced forms of
quinones (catechols; H 2 Cat, HCat , and hydroquinones;
−
−
H 2 Q, HQ ) indicate that the first electron removal is the PhCH(O) HA e [PhCHOH] e PhCH 2 OH
HA
most difficult [e.g., 3,5-DTBCH − in DMF] in an irre-
A A
versible two-electron oxidation via an ECE mechanism.
(177)
−e − −e − For aromatic hydrocarbons, the more extensive the conju-
HCat − HSQ· Q + H 2 Cat
HCat − gation is, the lower the LUMO and the less negative the
E p,a , +0.14 V vs SCE (172) potential for direct electron transfer