Ethers in the Liquid Phase

The fluorescent behavior, following optical excitation, of some liquid cyclic ethers, in particular p-dioxane and its solutions, has been studied by Hirayaraa et al. (183) and Halpern and Ware (184). Product studies on cyclic ethers are still in a preliminary state. There is a brief study by Pitts and co-workers (91) on oxetane in isoSctane and aqueous solutions. Some aspects of possible importance for photochemists working in p-dioxane as 

Fluorescence (, 3, 247 nm in neat p-dioxane) occurs with a quantum yield of 0.029 (184) in the liquid state only (183). Addition of water effects a diminution and a red shift while isooctane causes a blue shift, as well as a reduction in intensity. O2 (183) and N2O (187) are quenchers. It is thought that the fluorescence is from some form of excited aggregate whose composition varies with dilution, and that monomeric p-dioxane does not fluoresce (183). It has also been suggested that photodetachment of an electron might occur, and fluorescence might be a consequence of charge recombination (187). 

Ethylene and formaldehyde are the main products from oxetane (91). Preliminary quantum yield values of some products from p-dioxane (186) and the tetrahydrofurans (188) are presented in Tables 14 and 15. In the case of p-dioxane one is led to propose 

TABLE 14. Quantum yields of some products (A 185 nm) from neat liquid p-dioxane (186). 

N2O induces, however, the formation of hydroxydioxane as an important product (186). These facts seem to indicate that ionic processes, which are modified by the presence of N2O, contribute to product formation. 

---

V. PHOTOLYSIS OF ETHERS A. Open-Chain Ethers in the Liquid Phase... 

The Hg-sensitized photolysis of several ethers in the liquid phase was briefly studied by Tsao (264). The gas phase photosensitized decomposition of ethers has found much more interest. In Steacie s group before 1950 there were investigated diethyl ether (265), ethylene oxide (266), and dimethyl ether (267). Cvetanovic and co-workers studied ethylene oxide (268) and trans-2,3-epoxybutane (269). Subsequent work on dimethyl ether was done by Pottle et al. (259), Takezaki et al. (270), Loucks and Laidler (271), and Payette et al. (252). The Hg-sensitized... 

Comments denotes the volume fraction of 15-crown-5 ether in the liquid phase. 

The reaction occurs in the liquid phase at relatively low temperatures (about 50°C) in the presence of a solid acid catalyst. Few side reactions occur such as the hydration of isohutene to tertiary hutyl alcohol, and methanol dehydration and formation of dimethyl ether and water. However, only small amounts of these compounds are produced. Figure 5-8 is a simplified flow diagram of the BP Etherol process. 

In the liquid-phase process, high pressures in the range of 80-100 atmospheres are used. A sulfonated polystyrene cation exchange resin is the catalyst commonly used at about 150°C. An isopropanol yield of 93.5% can be realized at 75% propylene conversion. The only important byproduct is diisopropyl ether (about 5%). Figure 8-4 is a flow diagram of the propylene hydration process. ... 

Ewald20 23 made a few measurements on these systems at 80°C in order to show that the formation of a hydrogen bond between iodoform and an ether leads to an enhanced solubility in the gas phase as well as in the liquid phase. In the absence of such a bond, the solubility in methyl ether would be expected to be about the same as that in propane. Ewald found solubilities that were up to four times greater. 

The rate of the Diels-Alder reaction betweenp-benzoquinone and cyclopenta-diene was measured in SC-CO2 and subcritical CO2 [85]. Relative reaction rates at different pressures are reported in Table 6.14. On going from CO2 in the liquid phase (below 31 °Q to SC-CO2, the reactivity increased significantly only when the reaction was carried out under high pressure. At 30 °C and 60 bar the reaction was 1.36 times faster than when it was performed in diethyl ether at 30 °C and 1 bar. 

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

An extremely pure product results, when difluorochloro methane or difluoro-dichloro methane are used as solvents (79). Dichlorophosphoric acid is a fluid, colourless, very hygroscopic liquid, which is easily soluble in CHCI3, CCI4, Ethanol and Ether (6). In the liquid phase it is stable for some time at room temperature, whereas at 12 Torr there is no sign of decomposition up to 250 °C (6). According to the Raman spectra in the liquid it is dimeric in analogy to the carboxylic acids (20) ... 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

The data in Table 17.3 are for vapor pressure and vapor and liquid composition of solutions of methyl tert-butyl ether (1) and acetonitrile (2), (9). The symbol Xi represents the mole fraction of (1) in the liquid phase, and yi represents the mole fraction of (1) in the vapor phase. P is the equilibrium vapor pressure of the solution. The temperature is 313.15 K. 

For the comparison of hydroperoxides with methyl ethers (equation 2), we find there is enthalpy of formation data only for dimethyl ether, isopropyl methyl ether and t-butyl methyl ether (again ignoring the ethyl and propyl hydroperoxides). The enthalpies of formal reaction 2 for R = Me, i-Pr and f-Bu (two gas phase enthalpies of formation for f-BuOOH) are —53.1, —54.9 and —37.6 or —48.6 kJmoU, respectively, in the gas phase. In the liquid phase, the enthalpies of reaction are —7.4, —35 (from the estimated enthalpy of formation of isopropyl hydroperoxide) and —20.0 kJmoU, respectively. Because the enthalpy of formation deviations from linearity for dimethyl ether and methyl hydroperoxide might not be identical, the reaction enthalpy might not be consistent with those... 

As was the case for the alkyl hydroperoxides in reaction 4, the enthalpies of the oxy-gen/hydrocarbon double exchange reaction 8 for dialkyl peroxides are different depending on the classification of the carbon bonded to oxygen. For R = Me, Et and f-Bu, the liquid phase values are —4, 24.6 and 52.7 kJmoR, respectively, and the gas phase values are 0.1, 25.7 and 56.5 kJmoR, respectively. For the formal deoxygenation reaction 9, the enthalpies of reaction are virtually the same for dimethyl and diethyl peroxide in the gas phase, —58.5 0.6 kJ moR. This value is the same as the enthalpy of reaction of diethyl peroxide in the liquid phase, —56.0 kJ moR (there is no directly determined liquid phase enthalpy of formation of dimethyl ether). Because of steric strain in the di-ferf-butyl ether, the enthalpy of reaction is much less negative, but still exothermic, —17.7 kJmol (Iq) and —19.6 kJmol (g). 

The synthesis of acetic acid (AcOH) from methanol (MeOH) and carbon monoxide has been performed industrially in the liquid phase using a rhodium complex catalyst and an iodide promoter ( 4). The selectivity to acetic acid is more than 99% under mild conditions (175 C, 28 atm). The homogeneous rhodium catalyst is also effective for the synthesis of acetic anhydride (Ac O) by the carbonylation of dimethyl ether (DME) or methyl acetate (AcOMe) (5-13). However, rhodium is one of the most expensive metals, and its proved reserves are quite limited. It is highly desirable, therefore, to develop a new catalyst as a substitute for rhodium. 

Thus 4-chlorophenyl 2,4,5-trichlorophenyl ether (48, Scheme 7) produced 4% of a mixture of the dibenzofurans 49 and 50. Only in the case of 2,3,4-trichlorophenyl 2,3,4,5,6-pentai hlorophenyl ether was production of dibenzofurans by formal loss of o,o -chlorine detected. Neither product was identified, but one is presumably the expected product, 1,2,3,4,8,9-hexachloro-dibenzofuran, and the other must be due to a rearrangement. Chlorination of diphenyl ether in the gas phase is unusual. At 300°C the major product is 4-chlorophenyl phenyl ether, as in the liquid phase, but as the temperature is increased (400-500°C), the amount of 4-chlorophenyl phenyl ether decreases at the expense of 3-chlorophenyl phenyl ether, and dibenzofuran is also produced. ... 

The immiscibility in the liquid phase was observed for [CjoCilm]Cl with water and for [C8Qlm]Cl wifh water and 1-octanol [51]. For both salts the solubility in 1-octanol was higher than that in water. Only [C8Cilm]Cl was liquid at room temperature (melting point, = 285.4 K) [51]. The binary mixtures of [Ci2Cilm]Cl with n-alkanes and ethers have shown a very flat liquidus curve, but only in [C42Cjlm]Cl + n-dodecane, or methyl 1,1-dimeth-ylether] the immiscibility in the liquid phase was observed for the very low solvent mole fraction [95]. 

In the Liquid-Phase Photofluorination [39,44] process the reactant is injected at a very slow constant rate into an inert fluorocarbon solvent which is saturated by fluorine and under U.V. irradiation. Conditions are chosen to ensure that the concentration of fluorine and fluorine radicals is always much higher than the concentration of the substrate. This method is only suitable for the perfluorination of substrates, such as partially fluorinated ethers (see Section 2.5) and amines (see Section 2.7), that are both soluble in perfluorocarbon solvents and can withstand such vigorous reaction conditions. 

However, care must be exercised in using molecular sieves for drying organic liquids. Appreciable amounts of impurities were formed when samples of acetone, 1,1,1-trichloroethane and methyl-r-butyl ether were dried in the liquid phase by contact with molecular sieves 4A (Connett Lab.Practice 21 545 1972). Other, less reactive types of sieves may be more suitable but, in general, it seems desirable to make a preliminary test to establish that no unwanted reaction takes place. For the principles of synthesis and identification see R. Szostak Molecular Sieves, Chapman Hall, London 1988, and for structure, synthesis and properties see R.Szostak Handbook of Molecular Sieves, Chapman Hall 1992. 

The physical properties of most acids (esters) and alcohols allow the reaction to be carried out either in the liquid or in the vapour phase. In the liquid phase, the effects of solvents and of transport phenomena may play a more important role than in the vapour phase. On the other hand, the side reactions (mainly the ether and/or olefin formation from the alco- TABLE 20 Reactants and inorganic catalysts used in kinetic studies of esterification (transesterification) ... 

Naphthols are readily dehydrated to the corresponding ethers at considerably lower temperatures (300° C) in the liquid phase in the presence of metal oxides (16, 17). 

The catalysts used successfully in the liquid phase in the order of their efficiency are anti-niony(V) chloride, aluminum tribromide, aluminum trichloride, and iron(Ill) chloride. Zinc(ll) chloride, tin(ll) chloride, boron trifluoride diethyl ether complex, and aluminum trifluoridc do not catalyze the dismutation. 

The reaction for making methyl-r-butyl ether proceeds quickly and highly selectively by reacting a mixed butene-butane fraction with methyl alcohol in the liquid phase on a fixed bed of an acidic ion-exchange resin catalyst (Fig. 1). 

---