averaged. The concentration of the free amide remaining after complexation could then be calculated from the extinction coefficient and used to determine the apparent equilibrium constant. The equilibrium between free amide and the sec-butyllithium-amide complex was established within less than ca. 3 ms, which is the time required for the flowing solutions to move from the mixer to the observation chamber.Kinetic data were obtained by digitizing 100 transmittance measurements at a specified wavelength and time interval under rapid stoppedflow conditions. Plots of percent transmittance vs time were generated via computer using measured zero and infinity light readings. The data were stored and then processed according to whether the absorbance increased or decreased and to the appropriate rate law. Unweighted linear regression analysis of the treated data versus time yielded the pseudo-first-order rate constants which were typically reproducible within 5%.Freezing Point Depression Measurements and Data Analysis. The thermistor tube was hermetically sealed with a septum and connected to a vacuum line via a 2-ft stainless steel needle. Inside the tube was a glass stirring rod to which was attached a magnet and a 100-kQ thermistor.Two wire leads connected to the thermistor were threaded through the septum and connected to the Wheatstone bridge. As the temperature of the environment about the thermistor changes, its resistance changes in a nearly linear fashion within a certain temperature range. A Wheatstone bridge transforms this change in resistance into a voltage change which is amplified, digitized, and recorded on a PLAT0 V computer using the same system as described in the previous section.The thermistor was calibrated by immersing it in an ethanolwater/dry ice bath along with a precision calibrated thermometer from the National Bureau of Standards graduated in tenths of a degree Celsius. The digitized signal was measured as a function of temperature over the range of -5 to +7 "C, and all subsequent signal readings were transformed into temperatures with this calibration curve.Cyclohexane was introduced into the thermistor tube with the aid of a gas-tight calibrated syringe. It was then chilled and agitated by moving an external magnet up and down adjacent to the internal magnete. The temperature was measured as a function of time, giving an initial endotherm and supercooling, followed by an exotherm resulting from the heat of crystallization, followed by a plateau. Extrapolating the plateau back to the endotherm gives the temperature of the freezing point. Changes in freezing point depression and observable precipitation did occur after -30 min, so these experiments were carried out for no longer than 20 min.Computer Modeling. The fitting of a model to the data was done by deriving equations from the rate and equilibrium expressions defined by the scheme being tested. The initial concentrations of the reagent, substrate, and catalyst employed in the experiment being modeled were used in these equations and the results compared to the experimental observations.Acknowledgment. We a r e grateful t o the donors of t h e Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation for support.