Hg-free Exams? Not quite

Homework assignment #6 contained an important announcement, but many students who worked on the homework either ignored the announcement or didn’t fully understand it (and many students may not have even seen the announcement given the small number of completed assignments that were turned in).

Here’s the announcement quoted in its entirety and verbatim:

Reagent Alert: I will not cover, and I am not going to test you on, the chemistry of Hg(OAc)2 + H2O, then NaBH4, but Loudon uses this reagent extensively in practice problems. Every time you run into this reagent, simply substitute another reagent in its place: H2O + cat. H2SO4.

And here’s what I saw on HW #6:

  • occasionally replacement of the entire oxymercuration-reduction reagent with H2O + cat. H2SO4 (correct behavior)
  • more often, replacement of parts of the oxymercuration-reduction reagent with H2O + cat. H2SO4. This is incorrect. Do not draw hybrid reagents like “H2O + cat. H2SO4 then NaBH4”. In fact, do not resort to oxymercuration-reduction in this class at all.

But that’s not all. This reagent-alert does not mean that you should never draw Hg-containing reagents. The activity for class #33 – Alkyne addition reactions: hydrohalogenation and hydration describes the use of salts containing Hg+2 as a method for promoting hydration of alkynes.

Bottom-line: draw H2O + cat. H2SO4 to hydrate alkenes (Markovnikov) and draw H2O + Hg+2 + H2SO4 to hydrate alkynes (Markovnikov).

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