The Hydroxyl Radical - Free-Radical-Induced DNA Damage and Its Repair - page 47

With sulfoxides, • OH reacts mainly by addition to the S − O double bond [DMSO:

10 9 dm 3 mol −1 s −1 ]. The resulting adduct has not been
detected, because it decomposes very rapidly by

reaction (14); 92%; k = 7 ×

-fragmentation [reaction (15);
Dixon et al. 1964; Norman and Gilbert 1967; Veltwisch et al. 1980].

β

3.3
H-Abstraction

H bond dissociation energy (BDE) is 499 kJ mol −1 , while the C The HO H bonds

in saturated hydrocarbons are much weaker (BDE = 376-410 kJ mol −1 ; Berkowitz
et al. 1994; for a compilation, see Chap. 6). Thus, there is a considerable driving
force for H-abstraction reactions by • OH. On the other hand, vinylic hydrogens
are relatively tightly bound, and an addition to the C

− −

C double bond is always
favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the
case of ethene, no vinylic radicals are formed (Söylemez and von Sonntag 1980),
and with benzene and its derivatives the formation of phenyl-type radicals has
never been conclusively established.
Despite the considerable driving force for the H-abstraction reaction, there is
some remarkable selectivity. Primary hydrogens (

−

− CH 3 ) are less likely abstract-

ed than secondary ( ) ones (Asmus et al. 1973). In ad-

dition, neighboring substituents that can stabilize the resulting radical by elec-

− CH 2− ) and tertiary ( − CH −