Kinetics of the Substitution Tropylium by

Acetonitrile in (C7H7)Mo(CO)3BF4

In this experiment, you will study the kinetics of the substitution of the cycloheptatrienyl cation or tropylium cation (C7H7+) by acetonitrile (CH3CN) in the (C7H7)Mo(CO)3BF4 complex. You will monitor the kinetics of the substitution reaction using infrared spectroscopy and will perform the experiments at several temperatures.

Background Information

The study of how a reaction occurs of often of great interest to chemists because once one knows how a reaction proceeds, it can often be modified to encourage or discourage certain reaction pathways. By studying the rate of a reaction under several conditions, i.e., different concentrations of reactants, different temperatures, different pressures, one can often obtain insight into the key steps of a reaction and therefore deduce the most likely mechanism of the reaction.

In this experiment, you will study the reaction of CH3CN with the cationic (C7H7)Mo(CO)3+ complex for which the overall reaction is given below by reaction 1.

(C7H7)Mo(CO)3+ + 3CH3CN → (CH3CN)3Mo(CO)3 + C7H7+ (1)

A complete kinetic analysis of this reaction would require that you examine the effect of the concentrations of both reactants on the rate of the reaction and that you vary the temperature at which the reaction is performed. The variation of reactant concentrations would allow you to determine the rate law for the reactions, which entails determining the orders of each of the reactants in the rate law. The variation of the temperature at which the reaction is performed would allow you to determine the activation parameters for the reaction. For a reaction involving two reactants A and B, the general form of the rate law would be as follows, where x and y represent the reaction orders of A and B, respectively.

Rate = −k[A]x[B]y

Typically to simplify the determination of the orders for each reactant, the experiments are performed with all but one of the reactants in a large excess of concentration, such that the concentrations of these reactants are essentially constant throughout the course of the reaction. For two reactants A and B, this would involve a set of experiments in which reactant A is in excess and a separate series of experiments in which reactant B is in excess.

In this experiment, you will not have time to perform all of the experiments to determine the reaction orders of both reactants CH3CN and (C7H7)Mo(CO)3+ separately. The order of the CH3CN for reaction 1 is known to be first order and you will be performing your kinetic experiments with pure CH3CN, which means that this reactant will be in large excess. The general form of the rate law for reaction 1 would then be as follows.

Rate = −k[(C7H7)Mo(CO)3+]x[CH3CN] = −kobs[(C7H7)Mo(CO)3+]x

Since the concentration of CH3CN is essentially constant over the course of the reaction, an observed rate constant, kobs, can be defined as being equal to the true rate constant multiplied by the concentration of CH3CN, i.e., kobs = k[CH3CN]. In this experiment, you will determine the reaction order of the Mo complex, which will be either zero, first, or second order, by monitoring the concentration of the Mo complex as a function of time using infrared spectroscopy and making the appropriate plots based on the possible integrated rate equations. You will perform the reaction at several temperatures in the range of 30 - 50 °C and determine kobs at each temperature. From this data, you will be able to determine the activation parameters for the reaction.

Experimental Procedures

You will be using the (C7H7)Mo(CO)3BF4 complex that you have synthesized in order to study the kinetics of its reaction with CH3CN. The reaction will be performed in 25 mL of neat CH3CN and the concentration of the Mo complex should be approximately 2 – 4 mM. You will make up the solution in a 14/20 50 mL round bottom flask with a stir bar and a septum. The CH3CN needs to be purged with nitrogen before preparing the solution and the reaction should be performed under a static nitrogen atmosphere, i.e., have a bubbler in the septum but not flowing N2. You will be performing the reaction at several temperatures between 30 – 50 °C using a constant temperature water bath setup, which consists of a 1000 mL beaker with water, a heater coil, and a temperature controller setup. A picture of the experimental setup is shown below. Approximately 15 – 30 minutes is needed for the water bath to reach a steady temperature. On the days that you will be running the lower temperature kinetic experiments, you need to either come in earlier or stay later than the scheduled lab time to allow for enough total time for complete the experiment.

Ideally you should perform the reaction at four different temperatures, but three different temperatures is the minimum number that needs to be performed. You will monitor the concentration of the Mo complex using infrared spectroscopy and in particular you will be monitoring the carbonyl stretching region (2200 – 1800 cm-1). The round bottom flask needs to be kept in the water bath for the entire reaction period. You will remove small samples at timed intervals using a plastic syringe with a needle and will acquire the infrared spectrum of the sample at room temperature using the NaCl “infrared kinetics solution cell”. For each temperature, the time recorded for the sample is the time at which it is removed from the round bottom flask. The reaction is still occurring at room temperature, however the rate will be much slower than at the higher temperatures. You should generally be taking samples at 5 – 15 minute intervals over a time period of several hours.

The kinetics cell used in these experiments is very expensive and there is a very specific procedure to be used to fill, empty and clean the cell, that is different from the procedure you used with the other infrared solution cells. The detailed procedure is given in a handout in the “OBS 108 Lab Procedures” binder that is kept above the instructor desk in the lab. For filling the cell, involves using two syringes, one with the liquid sample and an empty one, to draw the liquid through the cell using suction rather than pushing the liquid through the cell. For emptying the cell, you will use one syringe to remove the liquid via suction and then you will draw air through the cell several times until the cell is ready to fill with the next liquid sample.

Report and Results/Discussion Topics

This is not meant to be an exhaustive list of topics you should discuss in your report, but below are some key topics that you should discuss in your report for this experiment. You will be using the Journal of the American Chemical Society communication template (on Angel) to write the report for this experiment. Two kinetics communications from a recent issue of the Journal of the American Chemical Society have been uploaded to Angel for you to use as examples of the report style. The final printed report should be single sided.

The reaction of (C7H7)Mo(CO)3BF4 complex with CH3CN has been studied and reported in the chemical literature. You need to perform a literature search and find the paper in which this was reported in order to be able to compare your results to the literature results.

For one of the temperatures at which you performed your experiment, you should include a stacked plot displaying the infrared spectra in the carbonyl stretching region from the subtracted spectra with several times shown over the range of the experiment to illustrate the decrease of the carbonyl peaks with time.

To determine the reaction order of the (C7H7)Mo(CO)3BF4 complex, use the spectral data from one of the temperatures you studied and make the appropriate kinetic plots for zero, first and second order. Present the three plots you made and discuss what you conclude from these plots. Once you determine the reaction order for the (C7H7)Mo(CO)3BF4 complex, give the overall rate law for the reaction.

Given below are three possible mechanisms for the reaction. Predict what the rate law would be for each of these mechanisms. Using your knowledge of the experimental rate law, determine the most likely mechanism for the reaction.

Concerted Mechanism

(C7H7)Mo(CO)3+ + 3CH3CN → (CH3CN)3Mo(CO)3 + C7H7+ fast

Polyolefin Dissociation Followed by Concerted Addition

(C7H7)Mo(CO)3+ → Mo(CO)3 + C7H7+ slow

Mo(CO)3 + 3CH3CN → (CH3CN)3Mo(CO)3 fast

Polyolefin Dissociation With Stepwise Addition

(η7-C7H7)Mo(CO)3+ + CH3CN → (η4-C7H7)(CH3CN)Mo(CO)3+ slow

(η4-C7H7)(CH3CN)Mo(CO)3+ + CH3CN → (η2-C7H7)(CH3CN)2Mo(CO)3+ fast

(η2-C7H7)(CH3CN)2Mo(CO)3+ + CH3CN → (CH3CN)3Mo(CO)3 + C7H7+ fast

Present the values of kobs at each of the temperatures you performed the reaction at and for each kobs, calculate the true rate constant (k) using the “concentration” of neat CH3CN.

To compare your true rate constant values to the literature, you will need to make an Arrhenius plot with the literature data because the literature rate constants (labeled k2 in the literature article) may not have been measured at the exact temperatures you used. Using the literature Arrhenius equation, determine the literature k values at the actual temperatures you used and compare to your k values. Comment on any deviations.

Using your experimental rate constants (k), make an Arrhenius plot to determine the activation energy (Ea) and the pre-exponential factor (A), and make an Eyring plot to determine the enthalpy of activation (D‡H) and the entropy of activation (D‡S). Make the same plots for the literature data and compare your values to the literature values.