Reaction Kinetics An experiment for biochemistry and organic chemistry laboratories Sheila Ewing Mount Holyoke College, South Hadley, MA 01075 Kinetic exveriments for the undergraduate oreanic or biochemistry iaboratory often have the drawbacks of &olving many manipulations of aliquots and the necessity of having a large block of time to complete the experiment. The combination of rushing to finish and the errors in titrating and pipetting keep the student toooccupied with detailsto analyze the data obtained or to obtain an understanding of the factors that affect rates. We have devised the following experimental procedure to examine the kinetics of carbamate decomposition (Fig. 1) and the effect of buffer catalysis upon the reaction. Since the reaction is followed bv visible svectroscoDv. .".the vressure of timing and taking aliquots is dispensed with. Buffers can be made ahead of time and four rate constants a t varying buffer concentrations can be measured in an hour. The absorbance change is -1 in the visible region and can be measured on either spec 20 or more sophisticated UV-VIS spectrophotometers. At the end of the experiment the student has enough data to allow examination of error analysis, standard deviation and also to calculate the effectiveness of buffer catalysis. This experiment is able to bring the concepts of rates of reaction, catalysis, error analysis, buffers, pH effects, and ionic strength effects into focus for the student. Description: The Study of Acid Catalysis of Carbamate Decom~osition Many important chemical reactions involw proton transfers. Making such transfers easier is one of the most im~ortnnt ways of increasing the rate of such reactions in the labbratory and it is also a major method used by enzymes in catalysis. Two types of acid catalysis are illustrated in Figure 2.l In A the rate is increased only by the increased concentration of H+, while in B the rate at a constant pH is increased by increasing the buffer concentration. Graph A is an example of specific acid catalysis while B is an example of general acid catalysis. Mechanisms of general acid and base catalysis have recentlv been reviewed bv Prof. W. P. J e n c k ~ . ~ The kffects of increaseb buffer concentration on the rate of decomvosition of a carbamate (Fia. 1) will he examined in the fnllo&ng experin~mt.This r e a r c ~ mis very similar to the reaction of co-enzsme biotin (Fir. d a v.s an indis- 3) "which . pensable role in numerous naturally occurring carboxylation
reaction^"^ Experimental The isocvanate stock solution is made bv saturatine 5 ml of dioxane-with fresh p-~Oz-phenylisoc;anate ( ~ a s t m a n P3185). The o ~ e n e dbottle of isocvanate should be stored under nitrogen to pre\.ent hydrolysis and decomposition. 'I'he reaction can be folluwrd hv visible svecrruscorw on the Beckman Spectro-20 or a more sophisticated insiiument. D-Nitroaniline has a maximum ahsor~tiona t a wavelength 380 nm in the visible range, but the absorption chan& is followed at 410 nm, on thesideofthe p-N02.aniline ahsorb-
ance, because this region is free of interfering absorption from the carbamate starting material. The reaction is pseudo first order, meaning the concentrations of the reactina. species are . such that un~~~~heconcentrntion d carbamate is changing with respect t
