Solution Kinetics of an S_N2 Reaction

Jonathan Smith

Gustavus Adolphus College

Adapted from Experimental Physical Chemistry, 2^nd edition, Halpern 1997

 

We will study the kinetics of a reaction between 2,4-dinitrochlorobenzene (DNCB) and piperidine (Pip).  The kinetics of this reaction can be followed spectrophotometrically by taking advantage of the fact that some of the products and reactants have intense absorption features in the ultraviolet (UV) and the visible.  The background for this investigation is provided by Halpern² and by Bunnett et al ¹..

 

 

This reaction takes place in two steps:

 

 

If we assume that k₁ << k₂ then we can apply the steady state approximation. (See Ch. 25 p. 780 Atkins{Atkins})

Given that DNPP is a colored compound we can follow the reaction by using visible absorption and through Beer’s law we can get the [DNPP] at each time we record absorbance at a particular wavelength, l.

 

For DNPP:  e_{472 nm} = 360 dm³/mole cm

            e_{372 nm} = 17,000 dm³/mole cm

 

We will examine one of two simplifying cases:

1.      Simple second order: [Pip]₀ = 2 [DNCB]₀

Integrated rate law:

2.  Pseudo first order: [Pip]₀ >> [DNCB]₀

 

Each of these conditions permits determination of k₁ by using an appropriate plot vs. time and a linear regression.  Then we can examine the temperature dependence of rate which should obey an Arrhenius relationship permitting determination of the activation energy.

Method:

 

·        In preparation for this investigation review Simon and McQuarrie Ch. 26 section 26-1 through 26-5.  Compute concentrations of reactants necessary for both of the above cases.

·        Prepare stock solutions that will result in an appropriate absorption if we consider all of the DNCB converting into DNPP.  Stock solutions of 0.620 M Piperidine in absolute ethanol and 0.0104 M DNCB in absolute ethanol can be made up and diluted for your work.  For monitoring by absorption spectroscopy use the above e_{372 nm} to come up with a concentration that produces a ~1.0 absorbance at 372 nm in the products (assuming the process goes to completion).

·        Scan UV-Vis absorption spectrum of reactants.  We will use the Ocean Optics USB2000 sprectrometer.

See Ocean Optics Website for further information.  The program OIChem will be used for collecting absorption and kinetics data.

·        Mix reactants and set aside for 5-10 minutes and then record, store, and print absorption spectrum of product to determine wavelength for monitoring product.

·        Determine concentrations for simple second order and pseudo first order conditions and collect absorption data vs. time for each of these reactions at regular intervals for 15 minutes at room temperature.

·        If time permits carry out these kinetics runs at a second and third temperature and use Arrhenius equation to get at the activation barrier (E_a) for the rate determining first step.

·        Consider what effect changing from piperidine to another amine might have on the reaction.  Give two examples.

           

 

 

References

 

                1.             Bunnett, J. F.; Crockford, H. D. Journal of Chemical Education 1956, 33, 552.

2.        Halpern, A. M. Experimental Physical Chemistry, second ed.; Prentice Hall: Upper Saddle River, NJ, 1997.

Back to Kinetics and Thermodynamics Page

Created by Jonathan M. Smith

Updated November 7, 2004

Gustavus Adolphus College