Sciencemadness Discussion Board

Finkelstein reaction

TIETSE - 5-5-2009 at 19:19


The classic Finkelstein reaction involves the conversion of an alkyl chloride or an alkyl bromide to an alkyl iodide by the addition of sodium iodide in acetone. (http://en.wikipedia.org/wiki/Finkelstein_reaction)


i wonder if this could be done with potassium iodide?

Paddywhacker - 5-5-2009 at 22:14

Yes. Potassium Iodide has considerable solubility in acetone, wheras KCl formed in the reaction does not, and that drives the reaction the way you want it.

A classic use is the preparation of diiodomethane from dichloromethane and KI in acetone.

TIETSE - 6-5-2009 at 06:52


One could use another ketone, eg cyclohexanone, or MEK.

DJF90 - 6-5-2009 at 10:46

Is that a question or a statement? It depends on the relative solubilities of the KI and the KBr/Cl in the ketone. Acetone is usually used because it works (generally)well, and has the bonus of being cheap and available.

TIETSE - 6-5-2009 at 11:39


sorry DJF90 , is a question, where i can find the solubilities of this salts in cyclohexanone, because is the most cheap ketone i could find.

DJF90 - 6-5-2009 at 12:39

Check the CRC handbook if you got one. Can't remember if theres one in the SM libray but you should look there. Ideally you want the KCl/KBr to be insoluble, and the KI (very) soluble. Consider using the sodium salts if they are more appropriate.

TIETSE - 6-5-2009 at 14:39


Thanks a lot DJF90, i am gonna check the CRC handbook.
if you got some articles of this reaction please send me the link.
thanks:)

Panache - 13-5-2009 at 00:12

crc quotes KI's solubility in RT acetone as 1.31g/100ml, whereas NaI is 40. I was intially surprised to hear someone above mention KI being suitable for the finkestein. it is not. It is a named reaction and specific in advocating only NaI. If you use KI succesfully you are performing a finklestein-like reaction, but your reaction times are going to be horrendous as the concentration of iodine available for the swap is very low.

[Edited on 13-5-2009 by Panache]

Paddywhacker - 13-5-2009 at 01:21

The reaction is driven not so much by the solubility of your starting iodide, but by the insolubility of the resulting chloride.

I have used KI in acetone and observed the KCl forming a rime on the insides of the glass container, so it works just fine.

What you want to call it is irrelevant.

Panache - 13-5-2009 at 19:16

Quote: Originally posted by Paddywhacker  


What you want to call it is irrelevant.


In that case I think we should then call it the abduyhfgfbshjhvdkalvbdklvn reaction from now on. Settled.

But your point is of course entirely correct, the mechanism being the same in both circumstances and i was just being a peadantic MF.
What were your reactions yields and times like cf NaI? What was your substrate?

Paddywhacker - 13-5-2009 at 21:58

Substrates were methylene chloride in one case and paradichlorobenzene in a second. Rhoduim archives has a writeup of the methylene chloride reaction.

Since I don't have NaI, I cannot compare, but there was no reason to complain about the reaction with KI. Excess KI dissolved as the KCl came out of solution. The only potential problem was the KI being enclosed within a KCl deposit, but that didn't happen.

Nicodem - 14-5-2009 at 13:26

Acetone or acetonitrile are usualy used as solvents in the Finkelstein reaction, only exceptionaly is methyl ethyl ketone used when a higher reflux temperature is desired. Never heard of cyclohexanone being used, neither do I see any reason for it to be used except maybe for its higher boiling point (but in most cases this only means it will be more difficult to remove from the product). For just about all primary alkyl halides the reflux temperature of acetone suffices (only for the stericaly hindered ones like neopentyl chloride it won't do). For the less reactive substrates like the secondary alkyl halides one can always use acetonitrile.
Quote: Originally posted by Paddywhacker  
Substrates were methylene chloride in one case and paradichlorobenzene in a second.

What exactly do you mean with paradichlorobenzene? No paradichlorobenzene reacts with KI in acetone as far as I know. What was the reference for this?
The Finkelstein reaction only works for alkyl halides, not aryl halides. With aryl halides the copper catalysis is required for the substitution to occur (likely via a SRN1 mechanism, but I'm not sure if this has been indisputably demonstrated). This reaction is sometimes called as "aromatic Finkelstein reaction" even though it has nothing to do with the Finkelstein reaction, because this is not an SN2 substitution at all. The mechanism of SRN1 reactions involves a nucleophilic substitution at an radical anion (thus the required catalysis by Cu(I) salts). Or else the mechanism can be based on transition metal catalysis like when some Pd-complexes are used for the halogen swap on aryl halides. This is yet a different mechanism (oxidative insertion of Pd in the Ar-X bond, displacement of the halide by another nucleophile and finally the reductive elimination of Pd). But in no way can the iodide displace chloride from any of the three possible dichlorobenzes just by refluxing in acetone - reaction conditions for a mechanism other than that operating in the SN2 substitution must be met. Only some very electron poor aryl chlorides can react with such nucleophiles like the iodide under such mild conditions (like the ortho- and/or para-nitrochlorobenzenes and similar) by the addition/elimination mechanism (again not via the Finkelstein reaction).

PS: There is another very recent thread on the Finkelstein reaction.

[Edited on 14/5/2009 by Nicodem]

Paddywhacker - 15-5-2009 at 02:32

re p-dichlorobenzene, I must confess mea culpa because I have not verified the reaction products.

What happned was that just before Christmas I sealed 0.1 moles C6H4Cl2, 0.25 moles KI and 250 ml acetone in a flask. Then I started to remodel my lab and haven't gotten back to it, but in February, for a couple of weeks I dropped a stirring bar in and let it stir in a 50 degrees bath. During that stirring I observed a rime of crystals forming on the glass surface, similar to what I saw with dichloromethane. I think that is KCl, and that some substitution did take place, but I'll need to work up the mixture to prove it. The flask isn't going anywhere, and I'll get back to it when I finish renovations.

I've got an open mind on the issue, but the evidence is that there is something happening.

Mystery of the Finkelstein reaction

franklyn - 9-7-2010 at 08:10

" Ask an Impertinent question and you are on your way
to the pertinent answer " - Jacob Bronowski

http://www.ch.ic.ac.uk/rzepa/blog/?p=439

.

peach - 13-7-2010 at 05:15

There's an amazing discussion of how the Finkelstein interacts with solvents here ------> CLICK ME

I highly recommend a read

ScienceSquirrel - 13-7-2010 at 13:17

Quote: Originally posted by Paddywhacker  
re p-dichlorobenzene, I must confess mea culpa because I have not verified the reaction products.

What happned was that just before Christmas I sealed 0.1 moles C6H4Cl2, 0.25 moles KI and 250 ml acetone in a flask. Then I started to remodel my lab and haven't gotten back to it, but in February, for a couple of weeks I dropped a stirring bar in and let it stir in a 50 degrees bath. During that stirring I observed a rime of crystals forming on the glass surface, similar to what I saw with dichloromethane. I think that is KCl, and that some substitution did take place, but I'll need to work up the mixture to prove it. The flask isn't going anywhere, and I'll get back to it when I finish renovations.

I've got an open mind on the issue, but the evidence is that there is something happening.


I will try and keep an open mind as well but if you have substituted the chlorine atoms for iodine under the conditions that you are using it would require a radical addition to organic chemistry theory.

densest - 13-7-2010 at 16:26

@S-S - the unexpected substitution of I for Cl or Br using NaI or KI in acetone is documented in references mentioned earlier in this thread. KI is at least 10,000 times more soluble in acetone than KCl. That's what tips the reaction the "wrong way". Any KCl formed is immediately removed, so the improbable reaction goes forward.

Very, very interesting reading about the 3-D local order when NaI or KI dissolves in acetone. I didn't hear anything like that back in the '70s and '80s.... chemistry has had major successes predicting and explaining reactions since then.

peach - 14-7-2010 at 09:58

I was just running a Finkelbuster today. I made LiI by heating lithium with the iodine pellets. Then dissolved that in the minimum amount of solvent to carry the salt. When I woke up the next day. Some of the acetone had evaporated and it had formed a kind of watery gel. I couldn't help but picture those diagrams.

I was using the Finkelstein to swap TBAB to TBAI. Seems to have worked. I'll post some pictures when it's finished.

[Edited on 14-7-2010 by peach]

Paddywhacker - 14-7-2010 at 12:33

Tetrabutyl ammonium bromide (TBAB) is an ionic bromide salt. Finklestein will not do anything.

You can replace the bromide ion with iodide ion by using an iodide-loaded anion exchange resin column.

peach - 14-7-2010 at 18:32

You've spelled Finkelstein wrong, someone told me to try this and it produced an insoluble precipitate upon addition.

Hoveland - 28-7-2010 at 17:30

Since this is in beginnings, I assume I am entittled to ask a stupid question here.

Would adding dry Ammonium Nitrite dissolved in ethanol into the acetone, along with the akyl chloride and iodide, make a nitro-alkane?

Would using sodium propyl sulfide NaSC3H7 and
coppric nitrate dissolved in ethanol make propyl nitrate, by precipitating out CuS ?

[Edited on 29-7-2010 by Hoveland]

fusso - 11-9-2018 at 16:38

Quote: Originally posted by Hoveland  
Since this is in beginnings, I assume I am entittled to ask a stupid question here.

Would adding dry Ammonium Nitrite dissolved in ethanol into the acetone, along with the akyl chloride and iodide, make a nitro-alkane?

Would using sodium propyl sulfide NaSC3H7 and
coppric nitrate dissolved in ethanol make propyl nitrate, by precipitating out CuS ?

[Edited on 29-7-2010 by Hoveland]
Found this wonderful question that still don't have replies. Not sure if a similar question has replies so I'll just bump this. Also, why would one want to use amm nitrite?

[Edited on 12/09/18 by fusso]

[Edited on 12/09/18 by fusso]

JJay - 11-9-2018 at 16:54

I don't think that would work... I think it would form tetraalkylammonium salts and acid, decomposing the ammonium nitrite and exploding.

Heptylene - 12-9-2018 at 00:54

Ammonium nitrite is unstable, so unless you make it in situ from barium nitrite and ammonium sulfate I don't think this would be practical. Even then I don't know if it would work.

I know silver nitrite can be used for this purpose (Victor Meyer reaction) with an alkyl iodide to make the corresponding nitroalkane. The side-product is silver iodide. This method is expensive since you need silver, and the silver iodide takes a bit of work to recycle.