Question

5. Professor Karl von Fildenshmirtzen is studying a transition metal ligand exchange reaction as follows below and is attempting to determine whether or not the mechanism is associative or dissociative (RDS rate-determining step): Dissociative X RDS + Associative L RDS X + a. Describe how Professor Fildenshmirtzen may distinguish between these two mechanisms using an Eyring plot. b. The AS for the reaction was determined to be negative. Which mechanism is operative?

Answer 1

a. Erying plots are observed during the experiments in enzyme catalyzed reactions. Where it is the plot of ln(k/T) vs T^-1. From this we can determine $\Delta$H$\ddagger$ and $\Delta$S$\ddagger$  which increases or decreases with temperature.Here (a) plot for the dissociative mechanism observed during above substitution reaction and that of plot (b) is for the associative mechanism.

FIHH 6/7) AS * = negative. 45*= positive (6)

b. The $\Delta$S$\ddagger$ for the reaction was determined to be negative, so mechanism should be preferably associative. Because negative entropy of activation implies a more ordered transition state and vice versa.