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Can anybody help me and explain the mechanism of reaction here? And also I was wondering when disulfide bond is made, can molecule dissociate, return to the previous state and make a disulfide bond when it gets to acidic environment again?

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    $\begingroup$ Exactly which part of the mechanism are you finding difficult? Most of it is relatively straightforward chemistry. In the first step you protonate the pyridine-type nitrogen in the benzimidazole ring, then do a nucleophilic attack with the pyridine. The third step is "just" a nucleophilic substitution on sulfur, etc. It's basically the same few reactions that you learn when starting out organic chemistry except that it's a bigger molecule with some heteroatoms instead of carbons. $\endgroup$ – orthocresol Feb 22 '17 at 11:46
  • $\begingroup$ @orthocresol I think OP might be confused in the first step; in the diagram, protonation of the N is skipped. That's what makes the aromatic lose resonance states, and that's what makes the C a good electrophile for any configuration that restorores that dual ring resonance. $\endgroup$ – sqykly Feb 22 '17 at 19:03
  • $\begingroup$ @orthocresol it retains aromaticity, but there are fewer resonance states if you can't move that double bond around the ring without knocking off that H+ $\endgroup$ – sqykly Feb 22 '17 at 19:56
  • $\begingroup$ @orthocresol (an aside not regarding OP) can we have resonance states with such different dipoles and asymmetries, or is that in fact a different species? And in that picture, the C is a bullseye for a nucleophile with or without an added proton. Together, this has made the first step confusing for me, and I agree that it's important to know which step OP is referring to. $\endgroup$ – sqykly Feb 22 '17 at 20:47
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    $\begingroup$ @orthocresol Sulfoxides can be chiral — but I don’t think that was what OP was asking ;) $\endgroup$ – Jan Feb 22 '17 at 23:17