Zensate - 20-4-2014 at 16:57
I read that TBAI is great to use in substitution reactions that use Alkyl Chlorides and NaOH, since the Alkyl Chloride is converted to an Alkyl Iodide
intermediate which can readily react with a present aq. nucleophile that is carried over from the aq. phase by the quaternery cation. This is supposed
to be very effective.
I also read much about how the Iodide ion acts as a catalyst poison and can completely stop reactions from occurring by tying up the quaternary
ammonium cation. If an alkyl chloride, turned iodide, reacts with a NaOH nucleophile, then NaI would (supposedly) be formed, leaving TBAI as TBACl.
Would this not "poison'' the TBACl? Or is sodium chloride formed instead of sodium iodide, in spite of the hydroxide being attacked by the
intermediate alkyl iodide?
I'm just confused on how Iodide can be a phase transfer catalyst poison, yet iodide quaternary ammonium salts be some of the most effective PTC by
nature of halogen exchange with alkyl chlorides/bromides.
The only explanation I can think of is that the iodide in TBAI never really leaves the quat but enables an alkyl iodide to exist in equilibrium with
the initial alkyl chloride, causing the substitution to go forth as it normally would as if the NaOH-Nucleophile was reacting directly with the alkyl
chloride... though I have no evidence for this. Strangly, sources also claim that alkyl iodides themselves poison PTC's, so this explanation of the
alkyl iodide intermediate not being a trule alkyl iodide yet reacting as one is my only explanation... :/
[Edited on 21-4-2014 by Zensate]