What's the mechanism for this isotopic substitution reaction?

I'm curious to know what's the mechanism of: $$\ce{HF + D^+ \rightarrow DF + H^+}$$

• Maybe you should include your own thoughts on this? – tschoppi Dec 8 '13 at 14:02
• @tschoppi. I have an idea (probably stupid) but I would prefer not to contaminate the discussion with it. – Claude Leibovici Dec 8 '13 at 14:13
• Well, you got my answer. I'd be curious to hear about your approach. – tschoppi Dec 8 '13 at 14:53
• The use of a double arrow is appropriate in this case. – bobthechemist Dec 9 '13 at 3:37

To answer this question you should understand the kinetic isotope effect.

Since you deliver no real data about reaction rate constants I assume that the dissociation of $\ce{HF}$ is faster (has a larger $k_\text{dis}$) than the dissociation of $\ce{DF}$. I postulate the following mechanism:

$\ce{HF + D+ <=>[k_\text{dis}^\ce{HF}] H+ + F- + D+ <=>[][k_{\text{dis}}^\ce{DF}] DF + H+}$

Essentially it is a pretty simple reaction mechanism, but it tends to forming $\ce{DF}$ over $\ce{HF}$ because the deuterium dissociates more slowly.

• I was thinking about an intermediate (HFD)+ based on some quantum mechanical calculations I performed. – Claude Leibovici Dec 8 '13 at 14:58
• Hm, given the high strength of the HF bond that is also entirely possible. Depending on the medium, of course. – tschoppi Dec 8 '13 at 15:04
• @ClaudeLeibovici That is probably a good description for a gas phase reaction. However, when inspecting the reaction in a condensed phase, calculations must be done much more carefully as bare hydrogen nuclei are extremely polarizing and will drag along one or more solvating entities with them. – Nicolau Saker Neto Dec 8 '13 at 19:28
• Actually, $\ce{F-}$ isn't that stable, and one would expect a concerted mechanism. – sencer Dec 10 '13 at 2:35

You know this happens in water all the time...what do you mean by mechanism? this is just proton jumping or hopping. The kinetics are are a side issue.