Phase Transfer Catalysis

Phase transfer catalysis (PTC) refers to reaction between two substances, located in different immiscible phases, in the presence of catalyst. Phase transfer catalysis is mainly used for synthesis of organics such as pharmaceuticals, dyes, chemicals etc. In this process one phase acts as a reservoir of reacting anions. The second phase, which is the organic phase, contains the organic reactants and catalysts generating lipophilic cations. The reacting anions enter the organic phase in ion pairs with lipophilic cations of the catalyst. Since the phases are mutually immiscible the reaction does not proceed unless the catalyst, usually a tetra alkyl ammonium salt, Q⁺X^– is present. The catalyst transfers reacting anions into the organic phase in form of lipophilic ion-pairs produced according to the ion-exchange equilibrium (1a). In the organic phase the anions react such as in nucleophilic substitution as shown in 1(b) where alkyl halides undergo nucleophilic substitution. A variety of other reactions with participation of inorganic anions such as addition, reduction, oxidation, etc. can take place efficiently using this methodology.

The advantages of phase transfer catalysis can be summarized as:

• Elimination of organic solvents
• Elimination of dangerous, inconvenient and expensive reactants
• high reactivity and selectivity of the active species
• high yields and purity of products
• simplicity of the procedure
• low investment cost
• low energy consumption
• minimization of industrial wastes
• mild reaction conditions

Suitable catalysts for PTC are those having highly lipophilic cation (that is cation having strong affinity for organics). The quaternary ammonium or phosphonium salts are most extensively used as phase transfer catalyst.

Examples of phase transfer catalyst are as follows:

1. Tetra-alkyl-ammonium or phosphonium salts

• Tetra-n-butyl ammonium bromide
• Methyl- triethyl ammonium chloride

2. Tri-ethyl-benzyl-ammonium chloride

Displacement reaction of 1-chloroctane with aqueous sodium cyanide is accelerated many thousand-fold by the addition of hexa-decyltributyl-phosphonium bromide as phase-transfer catalyst. Tremendous reactivity enhancement occurs due to generation of quaternary phosphonium cyanide, which makes cyanide anion organic soluble and sufficiently nucleophilic. In the absence of hexa-decyltributyl-phosphonium bromide phase transfer catalyst, no reaction occurs.

 

Mechanism

Mechanistic aspects of phase-transfer catalysis remain obscure, mainly due to difficulty of investigating biphasic systems and the many complex parameters that must be analyzed. Two pathways are proposed

1. Starks extraction mechanism
2.  Makosza interfacial mechanism

Starks extraction mechanism

According to this mechanism phase transfer catalyst moves back and forth across the organic and aqueous phases. The onium salt (Q⁺X^–) equilibrates with inorganic base (MOH) in aqueous phase, and extracts hydroxide into organic phase. Onium hydroxide (Q⁺OH^–) then abstracts hydrogen from the acidic organic compound to give the reactive intermediate Q⁺R^–.

Q⁺X^– = tetra alkyl ammonium or phosphonium salts

MOH = inorganic base

Fig 1. Schematic diagram showing Starks extraction mechanism