What is the mechanism for the hydroxymercuration-demercuration of n-butyne?
1 Answer
Wow, what a throwback question; haven't seen this reaction in forever! For reference, my textbook calls this "Mercuric-ion-catalyzed hydration".
You should form 2-butanone.
MERCURIC-ION-CATALYZED HYDRATION: REACTION
Mercuric-ion-catalyzed hydration of an alkyne is a type of hydration reaction, adding water across the triple bond in a Markovnikov fashion.
Unlike for the oxymercuration of alkenes, it actually uses significantly different reactants due to the lesser comparative reactivity. Here's what's up...
The main differences are:
- For alkynes, we are in acidic conditions. For alkenes, we are not.
- For alkynes, we do not require two separate steps. For alkenes, we do.
- For alkynes, we have keto-enol tautomerization (not at the end, but in the MIDDLE of the mechanism. Weird!). For alkenes, we do not.
MERCURIC-ION-CATALYZED HYDRATION: MECHANISM
The mechanism goes as follows. For your reaction,
-
We form a mercury cyclopropene analog, known as a "
#pi# complex"; similar to a bromonium cation in alkene bromination.Mercury donates its
#5d# electron pair in this scenario (as#"Hg"^(2+)# , its configuration is#[Xe]4f^14 5d^10# ). -
Water attacks the more-substituted carbon, since the transition state stabilizes the more-substituted vinylic carbocation via hyperconjugation.
This is the step that makes this reaction a Markovnikov addition.
-
Another water molecule abstracts a proton from the protonated enol, which is a strong acid. This forms a mercuric enol.
-
The oxygen conjugates its
#sigma# electrons down and the#"C"="C"# #pi# electrons obtain a proton from the previously-generated hydronium.(If this didn't happen in one step, the electronegative oxygen would have been adjacent to a carbocation, a thermodynamically unstable scenario. It would have poorly attempted to draw electron density away from an electropositive atom.)
-
Mercury donates away its bonding electron pair (which it got in step 1), forming a
#pi# bond, while oxygen withdraws its#pi# electrons and breaking its#pi# bond. - A hydronium approaches, and we basically repeat step 4, except without the mercury ion.
- A water molecule grabs a proton to regenerate the acid catalyst.