[Monoamine]

I am trying to convert an bromoalkane into an idodoalkane, and I came across a procedure called the "Finkelstein reaction".

In short: You dissolve NaI in acetone together with your bromo (or chloro or fluoro)-alkane, and use the fact that, unlike NaI, the NaBr, NaCl and NaF salts aren't soluble in acetone, which drives the reaction to completion. I tried to find out what the exact reaction conditions are so I know temperature, reaction time, etc..., but the only information I could find is from this paper:

The facile preparation of alkenyl metathesis synthons
T. W. Baughman, J. C. Sworen, K. B. Wagener, Tetrahedron, 2004, 60, 10943-10948.

In it, they use 1.7 eqiv. NaI for their chloroalkene, and run the reaction at reflux for.... THREE DAYS!! (Yield: 89%)

Ok, granted, they use a haloalkane not a haloalkane, and a chlorinated one at that, not a brominated one, but three days?? That seems extraordinarily long for such a simple reaction...

Maybe periodically interrupting the reaction and filtering off the insoluble KBr precipitate would slightly speed up the reaction, by shifting the equilibrium? This might also provide a way to monitor how far the reaction has progressed, since once 1.0 equiv KBr is collected, the reaction should be done.

So I was wondering if anyone here has ever tried this reaction with a bromoalkane, instead of a chloroalkene, and what reaction conditions were used? Also, I only have KI, but I'm guessing that shouldn't make a difference?

Thank you.

[rolnor]

You should use NaI, KBr is more soluble in acetone than NaBr. Usually roomtemp over night is OK, you dont need reflux.

[wildfyr]

I've done it, very common reaction, and definitely use NaI, what rolnor says about potassium salts is important. You want to run it a long time to get very high conversion because theres a good chance you can't separate the different haloalkanes unless they are distillable.

[Babcock_Hall]

In my hands the reaction is sometimes sluggish with chloroalkanes but somewhat more facile with bromoalkanes.  @OP, How does the insolubility of NaBr play into the overall completeness of the reaction?  The concentration of reagents is usually high, in my experience.  Occasionally this reaction is run in 2-butanone, but I could only speculate as to why.

[wildfyr]

Probably because you can cook 2-butanone (usually called MEK or methyl ethyl ketone) a little hotter.

[Monoamine]

I went ahead and tried it with KI anyway, since I didn't have any NaI around, and it worked just fine. I did however, use a 3x molar excess of KI. The fact that the end product was EtI and not EtBr could be seen both by its boiling point (when I distilled off the solvent) and because I did a quick AgNO₃ test to check that I truly had an iodinated species, not a brominated one.
And you're right, it didn't take nearly three days. Overnight was enough for a total transformation (unless the EtBr boils off with the acetone as an azeotrope or something...)

The link below shows a picture of the silver nitrate test (left: Iodoethane. Right: Bromoethane)
https://ibb.co/MNmxNL3

Cheers