Kinetics of the Reaction between DNCB and Piperidine1
Adapted from reference 1 by J. M. McCormick
Last Update: November 4, 2008
Introduction
The reaction between DNCB (2,4-dinitrochlorobenzene) and piperidine will be examined using a literature procedure.1 Please refer to the handout for the necessary background, or to the on-line kinetics review for more information. While reference 1 gives several different ways to perform this experiment, you will be using the isolation method in real-time (method 1 in reference 1). Do not waste your time trying the other methods. Once you have determined the order of the reaction with respect to the reactants, you are to perform one, or both, of the following: 1) determine the activation energy of the reaction by measuring the variation of the rate constant with temperature using the Arrhenius equation, or 2) determine the rate constant for the reaction of 2,4-dinitrobromobenzene at 25 °C. If you select option 2, you should be aware that 2,4-dinitrobromobenzene is more expensive than DNCB and only the minimal amount to obtain the desired measurements should be used.
Experimental
Be sure that you are familiar with the hazards associated with DNCB and piperidine. using an Ocean Optics spectrometer to follow the change in the absorbance as a function of time. Review the set up and operation of the Ocean Optics spectrometer before coming to the laboratory. The reaction is to be run at a constant 25 °C.
Results
Determine the order with respect to each reactant, as described in method 1 of reference 1. In the results, include an example of each graph that you used to determine the order with respect to each reactant. If you determined the activation energy, show your Arrhenius plot. For the reaction of 2,4-dinitrobromobenzene, you will need to show a typical kinetics run and an integrated rate law graph to show how you determined the rate constant at one piperidine concentration.
Conclusions
Discuss whether your rate law is consistent with the proposed mechanism. Is the activation energy consistent with the proposed slow step in the mechanism? Discuss the results with 2,4-dinitrobromobenzene. Do they make sense in terms of bond strengths? Use the questions under “Questions and Further Thoughts” as a guide to further topics to address in your discussion.
1. Halpern, A. M. and McBane, G. C. Experimental Physical Chemistry, 3rd Ed.; W. H. Freeman: New York, 2006, p. 20.1-20-14.