The Isolation and Pseudo-Order Techniques

The isolation experimental design can be illustrated with the rate equation v = kc%CB, for which we wish to determine the reaction orders a and b. We can set Cb Ca, thus establishing pseudo-oth-order kinetics, and determine a, for example, by use of the integrated rate equations, experimentally following Ca as a function of time. By this technique we isolate reactant A for study. Having determined a, we may reverse the system and isolate B by setting Ca Cb and thus determine b. 

More commonly a combination of techniques is used as illustrated by the following study. The reaction is the acylation of allylamine by rrans-cinnamic anhydride, Eq. (2-34). 

The initial anhydride concentration was about 3 x 10 M, and the amine concentration was much larger than this. The reaction was followed spectrophoto-metrically, and good first-order kinetics were observed hence, the reaction is first-order with respect to cinnamic anhydride. It was not convenient analytically to use the isolation technique to determine the order with respect to allylamine, because it is easier to observe the cinnamoyl group spectrophotometrically than to follow the loss of amine. Therefore, the preceding experiment was repeated at several amine concentrations, and from the first-order plots the pseudo-first-order rate constants were determined. These data are shown in Table 2-1. Letting A represent 

More complicated behavior is shown by the acylation of -butylamine with N-/rom-cinnamoylimidazole, 1. 

The isolation technique showed that the reaction is first-order with respect to cin-namoylimidazole, but treatment of the pseudo-first-order rate constants revealed that the reaction is not first-order in amine, because the ratio k Jc is not constant, as shown in Table 2-2. The last column in Table 2-2 indicates that a reasonable constant is obtained by dividing by the square of the amine concentration hence the reaction is second-order in amine. For the system described in Table 2-2, we therefore find that the reaction is overall third-order, with the rate equation 

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Use cf Integrated Equations 24 The Isolation and Pseudo-Order Techniques Initial Rate Method 28 Fractional Time Methods 29... 

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