Third-liquid phase transfer catalysis

In 1984, Neumann and Sasson investigated the isomerization of allylanisole using polyethylene glycol as catalyst in toluene and aqueous KOH solution and observed a third-liquid phase formed between the aqueous and organic phases. Third liquid phase is reported to be obtained at specified conditions :

i.  For phenethyl bromide to styrene using tetra-butyl-ammonium-bromide (TBAB) under phase-transfer conditions third-liquid phase only formed under conditions of using TBAB and 40% of aqueous NaOH solution.

ii.  Solvents of different polarities and the amount of NaOH are two important factors in formation of third-liquid phase, the distribution of catalyst and the reaction rate

Advantages of third-liquid phase-transfer catalysis includes:

• higher reaction rates and selectivity
• easy separation of catalyst and product
• easy reuse and recovery of catalyst

Etherification reaction of aqueous sodium onitrophenoxide with 1-bromoctane can be carried out under third-liquid phase-transfer catalytic conditions. The reaction scheme shown in Fig 4 is proposed by Lin et al. [2].

 

Fig 4. Schematic showing mechanism for etherification reaction of aqueous sodium o- nitrophenoxide with 1-bromo-octane in third-liquid phase transfer catalysis

Industrial processes:

Cyanation of alkyl chlorides

Cyanation of alkyl chlorides is a major way to produce nitriles. In the traditional process, since R-Cl and NaCN are mutually immiscible, solvents (lower alcohols-water mixtures) are used for reaction to proceed. In this process product has to be separated from the solvent and the solvent is reused. Another disadvantage is that the wastes, produced in substantial quantities, have to be destroyed and disposed.

In PTC methodology, neat alkyl chloride containing ~1% molar catalyst is stirred with saturated solution of NaCN in water. Upon completion of reaction in organic phase, which is often the pure product, is separated and product can be subsequently purified or used as such. Aqueous phase, after separation of solid NaCl, can be reused by introduction of fresh NaCN. Hence only waste in this method is solid NaCl.

 

Text Reference:

•  M. Makosza, Phase-transfer catalysis. A general green methodology in organic synthesis, Pure Appl. Chem., 72 (2000) 1399–1403

•  K. Maruoka (ed), Asymmetric Phase Transfer Catalysis, WILEY-VCH GmbH & Co., 2008

•  C.M.Starks, C.L.Liotta, M.Halpern, Phase transfer catalysis : Fundamentals, Applications and Industrial Perspectives, Chapman & Hall, Inc. 1994

 

Journal reference

1. S. Baj, A. Chrobok, I. Gottwald, Applied Catalysis A: General 224 (2002) 89–95

2. P. Lin, H. Yang, Journal of Molecular Catalysis A: Chemical 235 (2005) 293–301