[Guy_Gardner]

I need informations about the reaction mechanism between mercaptans (thiols) and alkali (potassium hydroxide, sodium hydroxide, etc). Or it could be more practical : the conversion of mercaptan in a KOH or NaOH treater which is commonly used in a refinery.

Every information will be appreciated. Thank you very much.

[Albert]

SH-group is stronger than OH, for what concerns acidity. RSH is to ROH as H2S is to H2O. The S-H bond is weaker than the OH ones, and less polar (H-bonds are less important).
Thiols have pKa which vary from 9 to 12. Reacions between thiols and strong bases have high values of Keq:

R-SH + OH- <=-----> RS- + H2O

RS- is an extremely weak base and an excellent nucleophile for SN2 reactions, such as:

R-SH + R'-Br + OH- -> R-S- + R'-Br + H2O -> RSR' + Br-

which is one of the three ways of producing thioethers (aka sulfides) I know.

[Guy_Gardner]

Thank you very much, Albert. It's very helpful.

And what will happen with the K⁺ and RS^-? Will they form a bond?

[Albert]

what will happen with the K⁺ and RS^-? Will they form a bond?

I daresay no. RS-K+ has almost the same non-covalent weak interaction you recognize in RO-K+
This means ionic interaction weaker than, for example, NaCl.

[Guy_Gardner]

Albert, do you have any idea will the K⁺ and RS^- soluble in H₂O or not? If there are K⁺ and RS^- in two phase mixture, light hydrocarbon and water phase, in which phase will they be dissolved? Or will both of them can not be dissolved in either water or light hydrocarbon?

[Dude]

Keep in mind that mercaptides (RS-K+) are also susceptible to oxidation to yield disulfides (R-S-S-R) and sulfates.  Refineries generally trap H2S and thiols with base (sodium hydroxide), oxidize the salt in a reactor (air with temp in the 50-60 C range if I remember correctly) to the sulfates/sulfites and disulfides and then sent for wastewater treatment.  The K+RS- will be water soluble and have very limited solubility in an organic phase.  Disulfides, however, will be much more soluble in the organic phase than the water phase.  I don't remember the details to control the disulfide/sulfate ratio.  There should be some articles regarding sulfur removal from refinery streams in the Kirk Othmer Encyclopedia of Chemical Technology.

[Guy_Gardner]

Dude, thank you very much for your information.

Usually refineries use Merox Unit to oxidize mercaptan to form disulfide, but in this case we just want to know the capability of  KOH or NaOH treater if they have to be used to treat light liquid hydrocarbon which contains mercaptan and trace H2S. So, in my question mercaptan is more to be the point.

Thank you once again.