Reactions early stages

The phenomenon of intemiolecular exchange is very common. The loss of couplings to hydroxyl protons in all but the very purest etiianol samples was observed at a very early stage. Proton transfer reactions are still probably the most carellilly studied [14] class of intemiolecular exchange. 

The mechanism of the Hantzsch s synthesis was studied at a very early stage by several authors. The intermediates were generally assumed to be open-chain a-thioketones, but in a series of papers by Murav eva and Schukina (470, 490) the isolation of hydroxythiazolines from the reaction between a-haloketones and a variety of thioureas was reported. 

Cure kinetics of thermosets are usually deterrnined by dsc (63,64). However, for phenohc resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized dsc ceUs, the upper limit on temperature is ca 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of hquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret dsc scans in terms of the cure kinetics are supphed by instmment manufacturers. 

Oxidation. Aromatic amines can undergo a variety of oxidation reactions, depending on the oxidizing agent and the reaction conditions. For example, oxidation of aniline can lead to formation of phenyUiydroxylamine, nitrosobenzene, nitrobenzene, azobenzene, azoxybenzene or -benzoquinone. Oxidation was of great importance in the early stages of the development of aniline and the manufacture of synthetic dyes, such as aniline black and Perkin s mauve. 

Cellulose acetate with improved solubiUty properties can be prepared from low quaUty wood pulps by multistage addition of the components (97) or by intermpting the reaction in the early stages, filtering, and continuing the acetylation with fresh reactants (98,99). 

Theoretical plots of ee (substrate) and eep (product) as a function of c are shown in Figure 2a and b. It can be seen that the ee increases with the extent of conversion. Consequently the enantiomeric purity of the substrate can be increased by sacrificing the yield and carrying out the reaction to higher degrees of conversion. Conversely, if high purity product is required the conversion should be terminated at early stages. 

With these kinetic data and a knowledge of the reactor configuration, the development of a computer simulation model of the esterification reaction is iavaluable for optimising esterification reaction operation (25—28). However, all esterification reactions do not necessarily permit straightforward mathematical treatment. In a study of the esterification of 2,3-butanediol and acetic acid usiag sulfuric acid catalyst, it was found that the reaction occurs through two pairs of consecutive reversible reactions of approximately equal speeds. These reactions do not conform to any simple first-, second-, or third-order equation, even ia the early stages (29). 

Initial or early stage in the reaction of some thermosetting resins in which the material is still soluble in certain liquids and fusible referred to as resol. 

Figure 3-9 shows plots of Eqs. (3-135) and (3-136) for some hypothetical systems. Obviously the equilibrium approximation is poor in the early stages of the reaction, but in the later stages the assumption can be quite good. The preequilibrium assumption, applied to Scheme XIV, amounts to the statement that 2 is negligible relative to ki and i. 

The early stages of the reaction of the quaternary salt can be regarded as proceeding in a manner exactly analogous to that by which the isoxazoles themselves are degraded, the j8-oxoketene imine structure (148) being one mesomeric form of a compound which could alternatively be formulated as a nitrilium betaine. However, by contrast with the products from the isoxazoles (i.e., enolates of /3-keto-nitriles), this is electrically neutral and susceptible to further nucleophilic attack. 

The time-dependence of enantioselectivity in the reaction thiophenol with 3-cro-tonoyl-2-oxazolidinone catalyzed by l ,J -DBFOX/Ph-Ni(C104)2-3H2O at room temperature in THF is shown in Scheme 7.44. After 3 h, the yield of the thiol adduct is 70% with the enantioselectivity of 91% ee, but the enantioselectivity was 80% ee at the completion of reaction after 24 h (yield 100%). Although the catalyst maintains a high catalytic activity, and hence a satisfactory enantioselectivity, at the early stage of reaction, the deterioration of catalyst cannot be neglected thereafter even under neutral conditions. 

As discussed in CbapL 6, copper-mediated diasteteoselective addition and substitution reactions ate well studied metliods for the construction of chiral centers in organic molecules. Tlie development of coppet-mediated enantioselective substitution reactions, however, is still at an early stage. 

During the manufacturing process, if the grafting increases during early stages of the reaction, the phase volume will also increase, but the size of the particles will remain constant [146-148]. Furthermore, reactor choice plays a decisive role. If the continuous stirred tank reactor (CSTR) is used, little grafting takes place and the occlusion is poor and, consequently, the rubber efficiency is poor. However, in processes akin to the discontinuous system(e.g., tower/cascade reactors), the dispersed phase contains a large number of big inclusions. 

Logani and Smeltzer " have observed that, for Fe-1.5%Si at 1 000°C in CO/CO2, the initial slow reaction rate was followed by regions of linear behaviour due to the amorphous Si02 film being consumed by the growth of wustite-fayelite nodules during the early stages. These wustite-fayelite... 

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