Reaction completion time

The complex interaction between mass transfer and reaction kinetics requires determination of mixing sensitivity for virtually all heterogeneous reactions for which competitive and/or consecutive reactions are possible to be sure of successful scale-up. This requirement is prompted by issues of both 1) overall reaction completion time and 2) undesired reactions in the films around the discontinuous phase(s). The following set of guidelines may be useful in evaluating mixing sensitivity and for scale-up. 

Figure 5-25B. For a first order reaction, the ratio of time in a continuous tank to the time in a batch tank for various percentages of reaction completion. By permission, Oidshue, J. Y. [29].

Note. Substrate variation resulted in reaction times of 3-80 h, with reaction completion being monitored by g.l.c. 

Search for More Active Catalyst. An extensive screening effort was undertaken to find a catalyst more active than Et-DuPhos-Rh. As a result of this effort, Et-FerroTane-Rh and some other competitive catalysts were found. The reactivity of Et-FerroTane-Rh and Et-DuPhos-Rh, is presented in Figure 3.9. The reaction rate with Et-FerroTane-Rh catalyst is very high with a small induction period, and the total time for reaction completion is drastically less than with Et-DuPhos-Rh. 

Table Al.l Extent of reaction completion for a first order reaction at different time intervals corresponding to different multiples of x and tll2...

The two time constants x and tV2 define time intervals in which a specific extent of reaction has been completed. In some applications one may wish to define a time point associated with a certain other extent of reaction completion. That is, how much time is required for the reaction to go to, say, 75% or 90% completion. This can be calculated using rearranged forms of Equations (A.16) through (A.21). For convenience, in Table Al.l we tabulate the extent of reaction completion for different time intervals, as multiples of x and ty2. 

Reaction complete Reduce time until conversion is still complete to maximize... 

It should be pointed out that many of these reports compared reaction times of MW heated reactions with times previously reported in the literature for the same reactions under conventional heating. Unfortunately the conventionally heated reactions are often complete in times which are much shorter than those quoted and it is important to perform direct and careful comparisons between MW and thermal reactions, using the same quantities of reagents and solvents and the same reaction temperature. 

One of the most attractive features of borohydride reductions is that under micro-wave-enhanced conditions they can be performed in the solid state, and rapidly. We were attracted by the work of Loupy [57], and in particular Varma [58, 59] who has shown that irradiation of a number of aldehydes and ketones in a microwave oven in the presence of alumina doped NaBH4 for short periods of time led to rapid reduction (0.5-2 min) in good yields (62-93%). In our study [60] seven aldehydes and four ketones were reduced (Tab. 13.3). Again reduction was complete within 1 min, the products were of high purity (>95%), of high isotopic incorporation (95%, same as the NaBD4) and the reactions completely selective. 

Fitting data for the fraction of a reaction complete as a function of time to the kinetic models is often... 

AP, is 20.8 kPa after 665 s. If only ether is present initially, and the increase in pressure after a long time (reaction complete) is 82.5 kPa, what is the partial pressure of ether, pe, after 665 s State any assumptions made. 

Therefore, to obtain complete stereoselectivity, the entire glycosylation process has to be performed in a highly controlled manner. In this particular case, the control is achieved by the use of extremely mild catalyst (I NBr), although very reactive substrates and prolonged reaction at times are required. 

In autocatalytic reaction the graph plotted between the rate of reaction and time shows a sigmoid curve as shown in Fig. 6.1. As the product (catalyst) concentration increases the rate increases, and reaches to a maximum when the reaction is complete. 

This method is similar to continuous flow method except that the rate of flow is continuously varied and the analysis is made at a fixed point along the observation tube. Since the rate of flow changes with time, the reaction mixtures arriving at observation point have different time. In the accelerated flow method the output from a photo electric colorimeter is fed to a cathode-ray oscilloscope, which sweeps out a complete time-concentration record which may be photographed. The method is useful for very rapid enzyme reactions and requires only small quantities of reactants. 

With the availability of numerical ODE solvers, exercises of the kind just presented are superfluous. While the results are the same for large parts of the data, numerical integration delivers a complete analysis that covers the whole reaction from time 0 to the end. 

In a dehydration reaction (Scheme 12.4), the IR band of the formamide carbonyl group at 1684 cm in (7) decreased and eventually converted to the isonitrile band at 2150 cm in (8) (Fig. 12.8). In a separate example (Scheme 12.5), the conversion of the IR band from the carbonate carbonyl group in (9) to the IR band of the carbamide carbonyl group in (10) can be monitored to assure the reaction completion (Fig. 12.9). Based on FTIR analysis, the reaction time course can be analyzed by integrating peak areas of the IR bands from the starting resin and the product. From the point of view of kinetics, the side reaction product formation can be excluded if the pseudo first order rates of the starting material consumption and the product formation are identical. 

TTie classification of kinetic methods proposed by Pardue [18] is adopted in the software philosophy. TTie defined objective of measurement in the system is to obtain the best regression fit to a minimum of 10 data points, taken over either a fixed time (i.e. the maximum time for slow reactions) or variable time (for reactions complete in less than 34 min, which is the maximum practical observation time). In an analytical system generating information at the rate of SO datum points per second, with reactions being monitored for up to 2040 s, effective data-reduction is of prime importance. To reduce this large quantity of analytical data to more manageable proportions, an algorithm was devised to optimize the time-base of the measurements for each individual specimen. 

FIA is a fixed-time analytical methodology, since neither physical equilibrium (homogenization of a portion of the flow) nor chemical equilibrium (reaction completeness) has been attained by the time the signal is detected. The operational timing must be highly reproducible, because the measurements are made under non-steady-state conditions, so that small changes may give rise to serious errors in the results obtained. 

Reaction time is extremely important in avoiding the side reaction illustrated in Eq. (1.1), where the nitroalkane product reacts with nitrite anion and any nitrite ester, formed as byproduct, to give a pseudonitrole. The reaction of sodium nitrite with alkyl halides is much faster than this competing nitrosation side reaction, even so, prompt work-up on reaction completion is essential for obtaining good yields. 

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