[MissPhosgene]

Hello,


   I am trying to do a nucleophilic substitution between p-bromophenol and 1-bromo-3-methyl-2-butene. Solvent is DMF and base is potassium carbonate.

After several hours I still have yet to see the emergence of a spot corresponding to product on TLC plates. I see spots corresponding to both starting materials in 10% EtOAc/hex.

I am considering adding potassium iodide because I think displacing the bromine with iodine may help get the reaction going. Finkelstein reactions are usually done in acetone with sodium iodide. The dielectric of acetone is 21 and the dielectric of DMF is 38, so solubility is not a problem.

Acetone and DMF are both polar aprotic solvents, but DMF is protophilic while acetone is protophobic. I'm not really sure how KI will act in DMF. 

Is it reasonable to try a Finkelstein in DMF despite the differences in solvent-types?

Thanks!

[Smrt guy]

Acetone is typically used because of the difference in solubility between NaI and NaBr or NaCl.  DMF probably will not have this advantage, but it should not cause problems.

[MissPhosgene]

Yes, you are right. I doubt it will drive the equilibrium via precipitation of KBr.

[orgopete]

I am surprised the DMF, potassium carbonate conditions do not effect the ether formation. I am skeptical that the iodide is needed because the bromide fails in this case. I would check my reagents or analyze this reaction by GC.

[MissPhosgene]

Orgopete,

 I definitely agree. I pulled the procedure from the literature, and checked the phenol and allyl bromide by NMR. Both are clean. The reaction just won't go, as far as I can tell by TLC. 

There is the possibility that the allyl bromide and the product move up the plates at the same rate, but that seems really doubtful. I expect the coupled product to have a higher rf value than the reactants.

I found another procedure in which Finkelstein conditions were utilized. If the way I tried doesn't work, then I suppose I will try it that way.

Thanks.

[stewie griffin]

There is the possibility that the allyl bromide and the product move up the plates at the same rate, but that seems really doubtful. I expect the coupled product to have a higher rf value than the reactants.

If you think the product may be cospotting with your starting allyl bromide you should be able to find out very easily by UV. I'm guessing your allyl bromide isn't UV active, however your coupled product would be. So instead of running the TLC and going straight for whatever stain you happen to use, check it under the UV lamp first. If the spot corresponding to allyl bromide is now UV active, then it's likely that the coupled product does cospot with your starting material.

[MissPhosgene]

The products do cospot! I was looking under UV yesterday and was seeing spots that corresponded with reactants. However, today I eluted a plate in hexanes and got a spot corresponding to product! . Very slow reaction.

[Doc Oc]

Phenols are poor nucleophiles in my experience.  Even when using reactive electrophiles like acid or sulfonyl chlorides, the reactions can take quite some time.

[MissPhosgene]

it was really slow.... but now 6.56 g (93%) of a yellow semi-solid. hooray!