The parameters that affect the stability of a series of quaternary ammonium and phosphonium salts R4M+Y" under phase-transfer catalysis (PTC) conditions in a chlorobenzene-aqueous NaOH two-phase system have been defined. Quaternary ammonium salts are much more stable than phosphonium derivatives. When the quaternary cation R4M+ is the same, the stability is in the order I > Br' » Cl". It dramatically increases either by diminishing the concentration of the base in the aqueous phase (from 50% to 15% aqueous NaOH) or by adding to the heterogeneous system a molar excess of the corresponding inorganic salt NaY. In all cases the degradation reactions are found to proceed in the organic phase via extraction of OH" as R4M+OH", interfacial phenomena being unimportant. As a consequence quaternary onium salts are stable in the presence of aqueous alkaline solutions provided that the extractability and/or the reactivity of OH" in the organic phase are minimized.Lipophilic quaternary ammonium and phosphonium salts R4M+Y" are widely used in organic synthesis as phase-transfer catalysts.1"4 5They are soluble even in low polarity solvents like benzene, toluene, chlorobenzene, and cyclohexane. In these media they give rise to loose ion 147-200.

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