1.3- dioxane, protonation

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

The poly(vinyl alcohol) made for commercial acetalization processes is atactic and a mixture of cis- and /n j -l,3-dioxane stereoisomers is formed during acetalization. The precise cis/trans ratio depends strongly on process kinetics (16,17) and small quantities of other system components (23). During formylation of poly(vinyl alcohol), for example, i j -acetalization is more rapid than /ra/ j -acetalization (24). In addition, the rate of hydrolysis of the trans-2iQ. -A is faster than for the <7 -acetal (25). Because hydrolysis competes with acetalization during acetal synthesis, a high cis/trans ratio is favored. The stereochemistry of PVF and PVB resins has been studied by proton and carbon nmr spectroscopy (26—29). 

THERMODYNAMIC STUDIES ON THE PROTONATION EQUILIBRIA OF SOME HYDROXAMIC ACIDS IN NaNOj SOLUTIONS IN WATER AND IN MIXTURES OF WATER AND DIOXANE... 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

In solvents containing low concentrations of water in acetic acid, dioxane, or sulfolane, most of the alcohol is formed by capture of water with retention of configuradon. This result has been explained as involving a solvent-separated ion pair which would arise as a result of concerted protonation and nitrogen elimination. ... 

The rates of removal of axial and equatorial protons from 4-t-butylcyclohexane in NaOD/dioxan have been measured by an NMR technique. The rate of removal of an axial proton is 5.5 times faster than for an equatorial proton. What explanation can you offer for this difference ... 

Complete exchange of protons in a sterically unhindered position a to a carbonyl group can be achieved by heating a solution of the ketone in O-deuterated solvents in the presence of an acid or base catalyst, the latter being the more effective. The most commonly used solvents are methanol-OD, ethanol-OD, and the aprotic solvent anhydrous tetrahydrofuran or dioxane mixed with deuterium oxide. Under alkaline conditions the exchange rate in 153 2 14,164 stcroids, for example, is usually... 

Thus, the reduction of tosylhydrazones with sodium borodeuteride in dioxane provides only monodeuterated analogs. For the insertion of two deuteriums it is necessary to first exchange the hydrazone proton and to carry out the reduction in aprotic or deuterated solvents. Under these conditions the reduction of the tosylhydrazone derivatives of 7- and 20-keto... 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

At 0.9 °C the rate of bromination of biphenyl relative to benzene was approximately 1,270, compared to 26.9 in the presence of mineral acid, and this latter value is fairly close to that obtained with 50 % aqueous dioxan. The possibility that the positive brominating species might be protonated bromine acetate, AcOHBr+, was considered a likely one since the reaction rate is faster in aqueous acetic acid than in water, but this latter effect might be an environmental one since bromination by acidified hypobromous acid is slower in 50 % aqueous dioxan than in... 

A number of investigations concerning the effect of added electrolytes and change in the solvent polarity have been reported (see ref. 25 pp. 17-19 for a more detailed account). The addition of neutral salts causes a large increase in the rate of reaction in both the one and two-proton mechanisms. In addition, the effect of increasing the water content in a dioxan-water solvent, provided that the concentration of water is above a certain threshold value, also produces a large increase in... 

Proton magnetic resonance (dioxane-d8) <5, multiplicity 7.98 (singlet). 

Dioxane will exist in the protonated form or as an S03 complex in the acid mix from which it will be liberated by alkali. The following process conditions have to be established for good-quality ethoxylated alcohols of the type C12-C14/ 15 2-3 EO. 

The addition of an alkene to formaldehyde in the presence of an acid catalyst is called the Prins reaction.Three main products are possible which one predominates depends on the alkene and the conditions. When the product is the 1,3-diol or the dioxane, the reaction involves addition to the C=C as well as to the C=0. The mechanism is one of electrophilic attack on both double bonds. The acid first protonates the C=0, and the resulting carbocation attacks the C=C ... 

The spectral properties of pentafluorophenylcopper te-tramer are as follows infrared (Nujol) cm. 1630 medium 1391 medium 1353 medium 1275 medium 1090,1081, and 1071 strong triplet 978 strong 785 medium fluorine magnetic resonance (tetrahydrofuran with trichlorofluoromethane as internal reference) 8 (multiplicity, number of fluorines, assignment, coupling constant J in Hz.) 107.2 (20-line multiple , 2, ortho F), 153.4 (triplet of triplets, 1, para F, J= 1.3 and 20), 162.3 (17-line multiplet, 2, meta F). Absorptions at 820-900, 1100-1125, and 1290 cm.- in the infrared spectrum and at 8 3.05 in the proton magnetic resonance spectrum indicate that dioxane is still present. 

Finally, silylation-amination of 5,6-diliydro-6-oxauracil 231 with excess diphenyl-methylamine 232/HMDS 2 and (NH4)2S04 for 17 h in boiling dioxane affords, via protonation of or of the persilylated intermediate 233 and subsequent addition of the amine to the 4-position, the cytosine analogue 234 in 74% yield [60] (Scheme 4.21). 

L Dioxane 101 0.45 Proton acceptor with weak orientation properties. Proton donor properties are absent. 

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