18-Crown added water

The rates of reaction of benzyl bromide and benzyl chloride with potassium cyanide were studied as a function of added water, in the presence and absence of 18-crown-6 (Equation 1). These heterogeneous reactions were carried out in toluene (50 mL) at 85 C and 25 C. 

The data in Tables I-III clearly indicate that the amount of added water had a marked effect on the rate of reaction. In the presence of 18-crown-6, addition of minute quantities of water caused a dramatic increase in rate. Beyond this maximum, as the quantity of... 

Table X The Rate of Reaction of Benzyl Broalde with Potasslua Cyanide at 85 C In the Presence and Absence of 18-Crown 6 as a Function of Added Water...

It has already been mentioned that in the absence of added water the reaction kinetics follows a pseudo zero-order rate profile the rate-controlling step under these conditions appears to involve the complexation of the crown in the organic phase with the salt in the solid phase. In contrast to this, in the presence of small quantities of water the reaction kinetics follows a pseudo first-order rate profile. Thus it appears that the water facilitates the interaction between the crown and the salt by forming an omega phase since the displacement process now becomes the rate-controlling step. The phase region where the displacement process actually takes place is not certain at this juncture. 

Figure 4. The Effect of Added Water on the Concentration of 18-Crown-6 in Toluene at Ambient Temperature.

The use of dicyclohexyl 18-crown-6 (in place of 18-crown-6) produced the same rate profile with respect to the quantity of added water. It was found that 30% of the decyclohexyl 18-crown-6 remained in the organic phase upon addition of 0.25 ml of water 70% of the crown was associated with the water-salt phase. 

The effects of added water on the rates of displacement of benzyl bromide and benzyl chloride with KCN salt in toluene catalyzed by 18-crown-6 were reported [145], It was observed that a small amount of water considerably increased the reaction rates compared to the anhydrous conditions and that the rate increased sharply to a maximum value in the presence of an optimum amount of added water. An important observation was that under anhydrous conditions, the reaction followed zero-order kinetics while in the presence of added water it followed first-order kinetics. It was suggested that the initial small amounts of added water coated the surface of the salt particle, which extracted the crown ether from the organic phase to form a new interfacial region called the omega (cd) phase. It was believed that the catalytic reaction took place mainly in the omega phase, since the quantity of added water corresponding to the maximum quantity of crown ether on the surface of the salt particles correlated well with the optimum quantity of added water. 

Table 13.3.19 Effect of added water on the concentration of 18-crown-6 ether in toluene at room temperatures...

In 1976, Johns, Ransom and Reese reported improvements in the previously reported syntheses of 18-crown-6 and 15-crown-5. By using tetraethylene glycol rather than triethylene glycol and the correspondingly shorter dichloride (2.5 equivalents of the latter) in concert with KOH (no water added), they were able to realize a 6% yield improvement in the synthesis of 18-crown-6 over the previously published method . The improvement in the yield of 15-crown-5 was of somewhat greater interest, being 38% compared to Liotta s previous report of 15% . ... 

Light-driven membrane transport. Cations may be transported through liquid membranes using crown ethers. For example, a typical system is of the type water-phase(I)/organic-phase/water-phase(II). The metal ion is added to water-phase(I) and the crown ether to the organic phase (to yield the liquid membrane). The crown acts as carrier for metal ions from water-phase(I) across the liquid membrane phase into water-phase(II). There have now been a very large number of studies of this type reported and a fuller discussion of this topic is given in Chapter 9. 

A. 1,1-Dibromo-2,2-bis(chloromethyl)cyclopropane (1). Into a 1-L, threenecked, round-bottomed flask, equipped with an efficient mechanical stirrer, a thermometer, and a condenser equipped with a potassium hydroxide drying tube, are placed 54.1 g (0.403 mol) of 3-chloro-2-(chloromethyl)propene (Note 1), 212 g (0.805 mol) of bromoform (Note 2), 1.70-2.00 g (14.4-16.9 mmol) of pinacol (Note 3), and 1.45 g (3.94 mmol) of dibenzo-18-crown-6 (Note 4). With very vigorous stirring (Note 5), 312 g of an aqueous 50% sodium hydroxide solution that has been cooled to 15°C is added in one portion. The reaction mixture turns orange, then brown, then black within 5 min, and the temperature of the reaction mixture begins to rise. Within 20 min, the internal reaction temperature is 49-50°C at which point the reaction flask is cooled with a room-temperature water bath, and the reaction temperature decreases to ca. 

Polymerization Method. To a solution of 5.18 mmole of HFB or PFB and 5.18 mmole of the appropriate bisphenol or bisthiophenol in 20 ml of solvent was added 22.4 mmole anhydrous of K2CO3 and 1.43 mmole of 18-crown-6 ether. The magnetically stirred, heterogenous mixture was heated in an oil bath and maintained under N2. Upon cooling to room temperature, the mixture was slowly poured into ca. 150 ml of methanol and was vigorously stirred. The filtered solids were washed three times in a blender with 300-ml portions of distilled water. The solids were air dried and subsequently placed in a vacuum oven (80 ) for 24 hr. Where soluble, the polymers obtained were characterized by IR and PMR analysis. Elemental analyses for all polymers were satisfactory. Polymer solubility was determined in THF, DMF, dioxane, toluene, m-cresol, chloroform, and sulfuric acid. The percent insoluble polymer was determined gravimetrically. Inherent viscosities of soluble polymers were determined in ca. 0.5% wt. solutions in either chloroform or THF. 

Surface-active crown ethers are distinctly differ from usual type of nonionics in salt effect on the aqueous properties, due to the selective complexing ability with cations depending on the ring size of the crown. As shown in Figure 3 (22), the cloud point of the crowns is selectively raised by the added salts. This indicates that the degree of cloud point increase is a measure of the crown-complex stability in water (23). 

Boc-18-crown-6-L-DOPA (4 66.4 g, 0.133 mol) was treated for 2h with 4M HQ in dioxane. After concentration to dryness under vacuo, the amino acid hydrochloride obtained (57.96 g, 0.133 mol) was dissolved in a H20/MeCN (1 9) mixture. DIPEA (69 mL) and Fmoc-OSu (49.80 g, 0.148 mol) were then added portionwise to the soln which was stirred mechanically for 3 h. The solvents were removed under reduced pressure and the residual oil was dissolved in CH2C12. The organic phase was washed with 1 M HC1 then water. After drying (MgS04), the solvent was removed and the residue was triturated with Et20 to give 5 as a white solid yield 95% mp 110-114°C TLC Rj 0.30 (silica gel, CHCl/MeOH/AcOH 70 15 15) [a]D25 -14.8 (c 1, DMF). 

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