Dioxane formation

Bu2SnO, toluene, reflux, 4 h Pd(Ph3P)4, THE, CH2=CHCH(OAc)2, rt, 1 h, 80-89% yield. In pyranoside protection, selectivity for 1,3-dioxane formation is generally observed, but dioxolanes are often formed. 

Moderate effectively the temperature rise due to the exothermic reaction to control dioxane formation and color ... 

No kinetic data are available for dioxane formation in a PET environment. Nevertheless, dioxane may become an important side-product when reactions are proceeding with long residence times of EG and DEG. Calculations of the... 

The most important use of oxiranes for the preparation of six-membered heterocycles is in 1,4-dioxane formation. 1,4-Dioxane, for example, is obtained in excellent yield by treatment of oxirane with dilute sulfuric acid (equation 157), and substituted dioxanes can be prepared in a similar manner. 1,4-Dioxanes can also be conveniently obtained by acid-catalyzed condensation of oxiranes with glycols (equation 158), while use of... 

Bohmeioser and Jenkner1841 noted that 1,4-dioxan formation r.m involve participation by one of two isolable intermediate oxonium complexes. The first is a fairly stable bisoxonium complex of , 4-clioxiui itself, whereas the seoond appears to be a very labile bisoxonium... 

Polymerization of ethylene oxide with stannic chloride also leads to a mixture of polymer and dioxane (2). However, in contrast with the borontrifluoride initiated reaction the polymers can reach a molecular weight of up to 20.000 and chain growth and dioxane formation seem to be parallel with each consuming about one half of the monomer. 

The major 1,3-dioxane could be separated by column chromatography but the diastereoisomeric 1,3-dioxolane derivatives could not be separated [Scheme 3.89],39 The strong preference for 1,3-dioxane formation is underscored in the benzylidenation of L-gulonolactone depicted in Scheme 3.90. Two different products formed depending on the reaction conditions, but in neither case was a 1,3-dioxolane ge tie rated.165... 

Evidence for the back-biting reaction is the formation of cyclic oligomers during the ring-opening polymerization of some heterocyclic monomers (81. 84-87). The classic example is dioxane formation during polymerization of ethylene oxide. 

These results indicate that the rate of polymerization is higher at lower conversions than the rate of 1,4-dioxane formation from the polymer. At certain conversions, i.e. at a certain polymerization degree, these rates become equal to each other and only the formation of 1,4-dioxane is observed. This is illustrated in Scheme (4-9) as a competition between bimolecular growth and unimolecular cyclization. Depletion of monomer decreases the rate of propagation, however, the rate of cyclization remains invariant. 

The less nucleophilic acetals copolymerize with vinyl compounds more readily. Perhaps, in these systems the alkoxycarbenium ions (... —OCH ) that coexist in equilibrium with oxonium ions facilitate copolymerization with vinyl compounds. Styrene copolymerizes with trioxane 51,52) and tetraoxane53). The latter system yields polytrioxane and trioxane-styrene copolymer together with 1,4-phenyl-1,3-dioxane. It was formed in 25% yield in ethylene dichloride at 30 °C after 1 hr using [BF3 OEtj] = 10-2 mol l-1, [styrene], = [tetraoxane = 0.5 mol l-1. The proposed mechanism of 4-phenyl- 1,3-dioxane formation is shown below (cf. also Chap. 7) ... 

Santoleri, J. J. (1998). The impact of thermal oxidation quench systems on dioxan formation, Presented at the 17th Annual International Conference on Incineration and Thermal Technologies, Salt Lake City, UT. 

Substantially higher yields and shorter batch times as compared to a conventional stirred reactor under otherwise similar conditions Safe and almost isothermal operation. Residual epoxide in the product is less than 50ppm. Much lower dioxane formation decreases the hazard. Relatively very high ratios of epoxide to base reactant are possible Safe operation outside the explosive limits through the use of nitrogen purge in the headspace. Dead-end operation results in practically no vent gas and hence relatively very low operating cost... 

The formation of the by-product 1.4 dioxane during the sulphation of alcohol ethoxylates has prompted a re-appraisal of the required quality standards for this feedstock. Consequently some of the material components which are considered to affect 1.4 dioxane formation should be included when establishing a specification with a supplier. The following information may be used as guideline ... 

Water content 0.1% max. 0.05% recommended Moisture forms sulphuric acid during sulphation which is responsible for hydrolysis and considered to catalyse 1,4-dioxane formation... 

Avoid pH < 7. Acid conditions enhance dioxane formation and, additionally, hydrolysis will occur (autocatalytic reaction) with potentially disastrous consequences. 

The ESP residues from all other organic feedstocks are not suitable for blending with normal production paste because of their high by-product levels and should be disposed of in a suitable way. ESP residues of feedstock other than alkylbenzene, notably residues from alcohol ether sulphation, should be neutralised immediately after collection to avoid the sharp increase in dioxane formation with time under extreme acidic conditions. The neutralisation should not take place in open vessels because of the risk of 1,4-dioxane liberation with subsequent potential operator exposure. There are various ways to solve the problem of disposal of the neutralised material ... 

Control of the level in the acid/gas separator ensures an effective seal between the liquid handling parts of the sulphonation plant and the exhaust gas treatment section. Several methods can be used, but the one which minimizes the hold-up and residence time distribution of acids, and consequently dioxane formation during ether sulphate manufacture is based on control of a variable speed gear pump. 

The cation template effect includes both the promotion of the assembly of the growing chain around the template before cyclisation and protection against degradation to dioxan. The positive charge on the cation prevents the formation of the oxonium salt which is the precursor for dioxan formation. Experiments on deu-terated dioxan showed involvement of the latter in the formation of the assembled chain consequently the equilibrium was effectively shifted to formation of com-plexed rings [15]. 

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