Properties
The disulfide bond is strong, with a typical bond dissociation energy of 60 kcal/mole (251 kJ mol-1). However, being about 40% weaker than C-C and C-H bonds, the disulfide bond is often the "weak link" in many molecules. Furthermore, reflecting the polarizability of divalent sulfur, the S-S bond is susceptible to scission by polar reagents, both electrophiles and especially nucleophiles:
- RS-SR + Nu- → RS-Nu + RS-
The disulfide bond is about 2.05 Å in length, about 0.5 Å longer than a C-C bond. Rotation about the S-S axis is subject to a low barrier. Disulfides show a distinct preference for dihedral angles approaching 90°. When the angle approaches 0° or 180°, then the disulfide is a significantly better oxidant.
Disulfides where the two R groups are the same are called symmetric, examples being diphenyl disulfide and dimethyl disulfide. When the two R groups are not identical, the compound is said to be an unsymmetric or mixed disulfide.
Although the hydrogenation of disulfides is usually not practical, the equilibrium constant for the reaction provides a measure of the standard redox potential for disulfides:
- RSSR + H2 → 2 RSH
This value is about -250 mV vs NHE (pH = 7). By comparison, the standard reduction potential for ferrodoxins is about -430 mV.
Read more about this topic: Disulfide Bond
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