Solvent effects ethylene oxide reactions

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

Crown ethers are heterocyclic chemical compounds that, in their simplest form, are cyclic oligomers of ethylene oxide. The essential repeating unit of any simple crown ether is ethyleneoxy, i.e., —CH2CH20—, which repeats twice in dioxane and six times in 18-crown-6. Crown ethers can activate enzymes for use in organic solvents through two methods (a) direct addition of 18-crown-6 to the reaction solvent [93], or (b) co-lyophilization of the enzyme with 18-crown-6, the latter being the most effective [94, 95]. 

An example of mechanism (154) is the reaction of 2-chloroethanol with hydroxide ion to form ethylene oxide. Various lines of evidence, including the actual size of the solvent isotope effect (Ballinger and Long, 1959 Swain et al., 1959), indicate the sequence (159) to (160) as the reaction mechanism... 

In this bicyclic case the palladium and methoxyl groups are trans to each other 1X>. A cis stereochemistry would have been expected on the basis of the ethylene oxidation mechanism. Trans-addition, however, is unusually favorable in the bicyclic examples. Although addition to the exo positions is generally strongly preferred, it cannot occur here if the favorable chelating effect of the second double bond is to be obtained. As a result, only the solvent methanol can attack from the exo side. The endo cis adduct has not been prepared and it conceivably could rearrange to the trans isomer even if it were formed initially. Clearly, more work needs to be done on the stereochemistry of the addition reactions. 

Allyl chloride, crotyl chloride, and 1,3-dichloropropane show bimolecu-lar reactions with water and are 8x2 reactions. The effect of solvent is shown by the hydrolysis of benzyl chloride which is bimolecular in 50 per cent aqueous acetone and almost unimolecular in water. The formation of ethylene oxide from ethylene chlorohydrin is a reversible reaction ... 

Ethylene Oxide The anionic polymerization of ethylene oxide is complicated by the association phenomenon and the participation of ion-pair and free ion intermediates in the propagation reactions [129, 130]. Simple lithium alkoxides are strongly associated into hexamers and tetramers even in polar media such as THE and pyridine [130]. As a consequence, lithium alkoxides are unreactive as initiators for the anionic polymerization of oxiranes. Association effects can be minimized by effecting polymerizations in alcohol media or in dipolar aprotic solvents. 

The solubility of terephthalic acid in the above-mentioned solvents is very low, which means that the acid must diffuse continuously from the solid particules to the solution where the reaction takes place. In such a case, the first question which arises is does the diffusion control the kinetics of the overall process In all cases, the authors claimed that the reaction rate is never affected by the amount of undissolved terephthalic acid and that the reaction proceeds through a chemical kinetic control. Under the experimental conditions used by Bhatia et al. the diffusion rate of terephthalic acid from the solid particles to the solution is 9.5x 10 mol cm" s at 100 °C and that of ethylene oxide from the gas phase to the liquid is 19.4 x 10" mol cm" s" . These values are far above the rate of formation of the diester(bishydroxy-ethylterephthalate), as this is only 5.84 x 10" mol cm" s" . Moreover, the independence of the reaction rate on the mass transfer effects was shown by varying the values of some parameters (e.g., ethylene oxide flow-rate, stirrer-speed, particule size, terephthalic acid charge) in a large range. 

Living polymerizations with reversible chain transfer (6) can be effected for alkoxide-initiated polymerizations of ethylene oxide in the presence of alcohol ([ROH]/[NaOR] 10) in solvents snch as dioxane (95,96). Narrow molecular weight distributions are obtained becanse, although there is formally a chain-transfer reaction between OH-ended polymers and alkoxide-ended polymers, the equilibrium between these two types of chain ends is rapid and reversible such that all chains participate imiformly in chain growth as described in Reference 97. 

The most studied example of 0 -3 participation is probably the base-catalyzed hydrolysis of 2-chloroethanol to produce ethylene oxide. As mentioned above, the reaction is catalyzed by hydroxide but not by water. Studies of the solvent isotope effect along with spectroscopic and conductivity measurements have confirmed the postulated two-step process. The reaction is often used as a stereospecific route to the more hindered epoxide derived from an olefin (via initial halohydrin addition). The carbohydrate field serves as a rich field for the application of epoxide opening and closing reactions. In this respect, Cerny and co-workers have observed that (193), (194), and (195) undergo hydroxide-catalyzed epoxide formation with relative rates of 1 23.3 180, respectively. 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifiuoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction may be carried out in an inert solvent such as hexane at about 60°C. The copolymer is obtained as a slurry and is collected, washed, and dried. 

Fig. 5.31 Polymerization of methyl methacrylate (MMA) by high-speed stirring of poly-(ethylene oxide) (PEO) solution (a) effect of monomer concentration on polymerization rate (PEO 0.4 gm/dl, stirring speed 30,000 rpm) (b) effect of monomer (MMA) concentration on intrinsic viscosity of reaction mixture (PEO 4 gm/dl, stirring speed 30,000 rpm, solvent benzene) (c) effect of PEO concentration on polymerization rate (d) effect of PEO concentration on intrinsic viscosity of reaction mixture (stirring speed 30,000 rpm) [147].

Ethylene oxide/styrene block copolymers have been further free-radical copolymerized with other ethylenically unsaturated compounds such as methyl methacrylate and methacrylic acid in benzene, tetrahydrofuran, and dimethylformamide (176). Correlations were made between reactivity ratio and solvent dielectric constant, as well as between solubility parameters of reaction solvent and growing polymer chains with marked effects apparent. Gel permeation chromatography of diblock and triblock copolymers based on polystyrene and poly(ethylene oxide) has revealed interesting molecular characteristics (177). Such block copolymers have an amphiphilic character. In aqueous solution, the polymers form spherical micells with a polystyrene core and a poly(ethylene oxide) outer sheath. The investigations used an aqueous-methanolic solution and were able to ascertain block copolymer structures and to estimate the impurities in the diblock copolymer. 

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