Electra
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Can the Finkelstein reaction be exploited as a sort of in situ catalysis via halogen exchange?
http://en.wikipedia.org/wiki/Finkelstein_reaction
It is discussed how the halogen exchange can turn alkyl chlorides or bromides into alkyl iodides by a sort of equilibrium reaction that is driven
forward by the starting salt being soluble in the liquid while the final salt isn't. For instance, Sodium Iodide converting alkyl chloride into alkyl
iodide, forming Sodium Chloride which leaves the solution (in acetone).
Is it possible to treat this exchange as a sort of catalysis?
Alkyl chlorides are far more unreactive than alkyl iodides in various substitution reactions. Alkyl chlorides generally require heat while alkyl
iodides can undergo substitution reactions at room temperature. Would it be possible for an alkyl chloride to temporarily behave as an alkyl iodide
insitu if Sodium Iodide is added as a catalyst? If the Alkyl Iodide is indeed formed in equilibrium, which it should be, then it should be far more
prone to nucleophilic substitution than the alkyl chloride. The only real downside would be is that the catalysis may slow down if sodium chloride is
a byproduct of the substitution, but it should go forward regardless.
When I google this, I can't find much information. Has anyone ever utilized a technique like this?
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Aconite
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The iodide would speed up the reaction, however it would not be a true catalyst. A catalyst is regenerated (not consumed) and typically used in 1-10
mol% concentrations. Semantics.
Anyhow, use 1-1.5 meq of iodide. The reason it pushes the equilibrium reaction forward is by precipitation of the sodium chloride, effectively
removing it from the right side of your equilibrium equation. Per Le Chatalier's, the reaction is pulled forward to the alkyl iodide. The Alkyl iodide
undergoes further reaction with your nucleophile.
In essence, you are creating a more reactive intermediate in-situ. To keep it clean though, I'd recommend performing this step, filtering, and *then
adding slowly to* your nucleophile in solution. If your nucleophile is an amine, keeping it at a much higher concentration relative to your
electrophile will ensure that mono-alkylation dominates.
Cheers!
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Electra
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The iodide would not be consumed.... in the wiki article yes, but that is not the application I am talking about. It would defeat the ease and
efficiency of the idea I am putting forth if you had to bring acetone into the picture along with equimolar amounts of the NaI.
If NaOH is used as a base to deprotonate one of the substrates, alkyl chloride would become an alkyl iodide, which would then react with the
deprotonated substrate, forming H2O, NaI, and the substituted product. In such a reaction NaI is recycled, and thus, act as a catalyst.
What I am unsure about is whether or not there are any specific factors that would prevent the NaI from forming the Alkyl Iodide long enough for it to
undergo substitution. NaI could react rapidly with the NaOH deprotonated substrate. What I'm curious about is whether or not the equilibrium dynamics
between NaI and the Alkyl chloride would allow for this.
For instance a SN2 reaction with a phenol:
1) NaI + CH3-Cl <---> NaCl + CH3-I
2) CH3-I + NaOH + Ph-OH ---> Ph-O-CH3 + NaI + H2O
This reaction could also occur, although the reaction with the iodide would be much more rapid:
2) CH3-Cl + NaOH + Ph-OH -> Ph-O-CH3 + NaCl + H2O
If the NaI is used at a 5-10% ratio, it would be continually recycled, although, NaCl would build up in the solution as well.
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Nicodem
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Yes. It is called nucleophilic catalysis.
For a study on methylations via the nucleophilic catalysis, see DOI: 10.1021/ja00165a046.
Edit: Corrected the "nucleophic" typo.
[Edited on 18/4/2014 by Nicodem]
…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)
Read the The ScienceMadness Guidelines!
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Electra
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Nicodem, some of your posts do irritate me but I love how you are so forward and straight to the point  <3. Thank you! I just found a bunch of great papers on the subject. This will definitely come in handy.
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