Chemistry Reference
In-Depth Information
Table 7. 3.
Rate constants (in units of dm
3
mol
−
1
s
−
1
) of the reaction of some radicals with
thiols
Radical
Thiol
pH
Rate
constant
Reference
•
CH
3
7. 4
×
10
7
Dithiothreitol
7
Reid et al. (2002)
•
CH
2
OH
1. 3
×
10
8
6.8
×
10
7
4.7
×
10
7
2-Mercaptoethanol
Dithiothreitol
Cysteamine
10
7
7. 6
Karmann et al. (1969)
Redpath (1973)
Adams et al. (1968)
•
C(CH
3
)HOH
2.3
×
10
8
9.6
×
10
7
1.4
2-Mercaptoethanol
Dithiothreitol
Cysteamine
10
7
7. 6
Karmann et al. (1969)
10
8
×
Adams et al. (1968)
•
C(CH
3
)
2
OH
5.1
×
10
8
2.0
×
10
8
2-Mercaptoethanol
Dithiothreitol
10
7
Karmann et al. (1969)
•
CH
2
C(CH
3
)
2
OH
10
7
6.8
×
10
7
1.8
×
10
7
2-Mercaptoethanol
Dithiothreitol
Cysteamine
10
7
7. 6
8.2
×
Karmann et al. (1969)
Redpath (1973)
Adams et al. (1968)
•
CHRC(O)
−
Glutathione
8.4
Very slow
Tamba and Quintil-
iani (1984)
•
CH
2
C(O)H
Dithiothreitol
8.4
Very slow
Akhlaq et al. (1987b)
•
C(NH
2
)[CH
3
S(CH
3
)
2
]H
Lipoic acid
8.8
(1-2)
×
10
9
Hiller and Asmus
(1983)
7. 4 .1
Hydrogen and Electron-Transfer Reactions by Thiols/Thiolate Ions
366 kJ mol
−
1
, thiophenol: BDE
The S
330 kJ mol
−
1
; Armstrong 1999), and for this reason thiols can serve as H-donors
[reaction (26)]. Thus thiols can play an important role in the repair of free-radi-
cal-induced damage (for some early studies see Adams et al. 1967, 1968, 1969).
Some rate constants are compiled in Table 7.3. Compared to aqueous solutions,
the rate of H-transfer by thiols is slower in organic solvents (Tronche et al.
1996).
It is seen from this table that the rate of reaction increases
•
CH
2
C(CH
3
)
2
OH <
•
CH
2
OH <
•
C(CH
3
)HOH <
•
C(CH
3
)
2
OH despite the fact that the exothermicity of
the reaction
decreases
in the same direction. This unexpected behavior has been
discussed in terms of the charge and spin polarization in the transition state, as
determined by the AIM analysis, and in terms of orbital interaction theory (Reid
et al. 2002). Rate constants, calculated by transition state theory, were in good
agreement with the experimental data.
−
H bond is weak (alkylmercaptans: BDE
≈
≈
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