Phase Transfer Catalysis (PTC) - Host-Guest Chemistry

Demonstration Experiment on Video

Phase Transfer Catalysis (PTC) - Host-Guest Chemistry

Objectives: Extraction of Methylene Blue and Potassium Permanganate from the Aqueous Phases,
Complexation with Sodium dodecylsulfate and Crown Ether, Host-Guest Complexes

Peter Keusch

German version

Chemicals:
Methylene Blue

methylene chloride (dichloromethane)
crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane (18-Crown-6)
cyclohexene
sodium dodecylsulfate
potassium permanganate

Apparatus and glass wares:
separation funnel with stopper 250 mL
2 beakers 250 mL
2 conical measures 350 mL
snap cap vial 20 mL
3 snap cap vials 50 mL
glass stirring rod
Pasteur pipette with rubber bulb

Hazards and safety precautions:

Methylene chloride is harmful if swallowed or inhaled. May be harmful by skin contact. Eye and skin irritant. Readily absorbed through the skin. Possibly carcinogenic in humans. Possible mutagen. Experimental reproductive effects.
18-Crown-6 is harmful if swallowed. Skin, eye and respiratory irritant
18-Crown-6 is harmful if swallowed. Skin, eye and respiratory irritant.
Potassium permanganate is harmful if swallowed. Irritant. Readily absorbed through skin.

Cyclohexene is highly flammable.

Safety glasses and protective gloves must be worn. Good ventilation required. The use of effective ventilation, such as a good fume hood, is essential when handling methylene chloride.

Experimental procedure:

Experiment 1: Extraction of Methylene Blue from the aqueous phase:

A spatula full of Methylene Blue is dissolved in a 200 mL beaker containing 150 mL of dest. water. The solution is transferred into a separation funnel and is mixed with methylene chloride. The separating funnel is shaken carefully. When the two phases completely separated, a spatula full of sodium dodecylsulfate is added. The two-phase system is shaken again.

Experiment 2: Extraction of permanganate from the aqueous phase:

350 mL of methylene chloride are mixed in a conical measure with 0.25 g of sodium permanganate. While stirring the solid-liquid system with a lass rod, 0.2 g of 18-Crown-6 is added. Half of the solution is poured into a second conical measure and mixed with cyclohexene.

Results:

Experiment 1: The aqueous upper layer shows a blue color. After the addition of sodium dodecylsulfate the lower organic phase is colored blue.

Video clip (Download RealPlayer .rm file)

Experiment 2: After the addition of 18-Crown-6 potassium permanganate goes into solution in the organic solvent. The purple solution reacts with cyclohexene to form a brown precipitate.

Video clip (Download RealPlayer .rm file)

Discussion and background:

Experiment 1: Water and methelene chloride are immiscible. Due to the "hard" negative charge of the chloride ion, Methylene Blue is unsoluble in the organic non-polar solvent. After the addition of sodium dodecylsulfate a lipophilic counter ion is available. A good mixing of the two phases is obtained by shaking the separation funnel. The lipophilic ion pair is transferred rapidly into the organic solvent.

Experiment 2: 18-Crown-6 is an example of a crown ether, which is a large polyether. 18-Crown-6 contains a cavity into which a metal ion such as a potassium ion fits snugly. The cation interacts with the crown ether via electrostatic interactions with the unshared electron pairs of the oxygen atoms. A "host-guest" complex is formed, with the ether as "host" and the ionic species as "guest". The complexed cation is dissolved in dichloromethane. Also the permanganate anion is forced into the organic solvent in order to ion-pair with the potassium ion. Thus, the complexed salt available in the organic solvent comprises of a strong solvated cation and a "naked" anion, which is especially reactive.

The substrate cxlohexene and the permanganate ion are brought together in the organic solvent by the crown ether. The permanganate ion oxidizes cyclohexene to adipic acid, and is reduced itself to o brown manganese dioxide.

Summary:

· A phase transfer catalyst allows mixing of water-soluble reactants with organic-soluble reactants in a two-phase mixture of solutions.

· The role of the catalyst is primarily that of an ion shuttle. The cation forms a tight ion pair with the reagent anion, permitting the anion to be transferred into the organic phase.

· The catalyst is a "true" catalyst in the classical sense and is not consumed in the reaction.

Index of Lecture Experiments