Alkynes and alkenes are "soft bases" according to the principles of the HSAB theory and since protons* are "hard acids" they interact very poorly with the triple or double carbon-carbon bonds. On the other hand, the
Hg<sup>2+</sup> ion is a "soft acid" and therefore interact strongly with alkynes and alkenes. The interaction of Hg(II) with the triple
bond of the alkynes, in the presence of a nucleophile such as water, results in the oxymercuration reaction which gives an organomercuric intermediate that can rearrange to the corresponding mercury(II) enolate which is then
demercuriated by protonation thus liberating the corresponding aldehyde/ketone. So overall, the Hg(II) salts have the role of catalysts via
electrophilic catalysis. H2SO4 is generally used as the acid because HgSO4 is well soluble. Also, the
HSO<sub>4</sub><sup>-</sup> is a "hard base" and as such it does not poisons the catalyst (unlike the halides do).
The reaction of alkenes is somewhat different in that the protodemercuriation of the organomercury intermediate, as far as I know, is a slow reaction
even under strongly acidic media (perhaps via electrophilic substitution). Reductive demercuriation is therefore commonly used instead (with NaBH4,
for example) to give the alcohol from the alkene. This reaction is therefore not catalyzed by Hg(II), but Hg(II) salts need to be used in
stoichiometric amounts. Similar reactions involving nucleophiles other than water are also well known, but rarely used due to the need for
stoichiometric amounts of Hg salts and the hazards involved.
* Actually, there is no such thing as free protons in solution, they are always coordinated, as in H<sub>3</sub>O<sup>+</sup>
in water, but for the HSAB principles evaluation, the pKa's and related theoretical approaches, you need to consider the protons themselves, as what
is important is the strength of the interaction of the proton with the triple bond (in relation to other possible interactions).
PS: Please don't cross-post. If you feel you misplaced a thread, ask a moderator to move it rather than open another one. Also, referenceless threads,
beginner's or pedagogical topics belong to the Beginners section where I'm moving this. The Beginners section is specifically for pedagogical
discourse (for replies aimed at teaching you novel concepts, starting from basics).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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