John A. Landgrebe University of Kansas Lawrence
A Simple Kinetic lnvesiigaiion of an Organic Reaction Mechanism
Inherent in the trend toward a more mechanistic approach in the teaching of undergraduate organic chemistry is the desire to introduce mechanistic thinkimg into the laboratory experiments. Although it is realiied that simple preparative reactions are extremely valuable for teaching basic laboratory skills, many such experiments can be carried through successfully without compelling the student to consider the details of the transformations that are taking place. The following kinetic experiment, which was designed for an undergraduate organic laboratory section, allows the student to determine what factors affect the rate of a reaction1 and how this information can be rationalized in terms of a logical sequence of molecular events.= The hydrolysis of tertbutyl chloride was chosen for a number of reasons. The reaction, as far as it is covered in an undergraduate course, involves basically simple concepts. Students are generally introduced to alkyl halides and their reactions relatively early in the first semester of most organic chemistry courses. The reaction is very easy to follow and has a convenient halflifein a properly chosen solvent system. If it is desired, the tertbutyl chloride can be readily prepared by the student according to procedures which appear in nearly every organic laboratory manual. Measurement of the Rate
One of the primary features of the experiment is its fundamental simplicity. The entire experiment can be performed with only two 25-ml Erlenmeyer flasks, a 5-ml graduated cylinder (a pipet could be substituted), a 1-ml and a 5-ml graduated pipet, and a watch or clock with a second hand. The student should be provided with a 0.1 M solution of tertbutyl chloride in acetone, a 0.1 M solution of sodium hydroxide in water, and an aqueous-alcoholic solution of bromphenol blue. The reaction begins abruptly as soon as the aqueous solution is mixed with the acetone solution of tert butyl chloride. As long as hydroxide is in excess, the solution (with several drops of indicator added) will remain deep blue, but a t the instant the base is entirely neutralized, the indicator makes a sharp transition to yellow. (CHs)rCCI
+ HOH
-
(CH&COH
+ HCI
sodium hydroxide represents 10 mole % of the tert butyl chloride and the solvent system is 70% water and 30y0 acetone by volume. The time for 10% reaction under these conditions is sufficiently short that student laboratory time is used efficientlyallowing many variations of the experiment to be done in a 3-hr period. Because the rate of change of acidity becomes less as the reaction proceeds, it is undesirable to follow the reaction to a large percentage of completion. Under these circumstances the indicator does not change color sharply, but passes through several shades of green thus makimg the time measurement more difficult to reproduce. Procedure Pipet 3 ml of aO.l A4 mlution of lertbutyl chloride in acetone into a 25-ml Erlenmeyer flaak and place it on B piece of white paper. Pipet 0.3 ml (1-ml pipet) of the 0.1 M NaOH solution into the other 25-ml Erlenmeyer &ask followed by 6.7 ml of distilled (or deionized) water (small graduated cylinder or pipet). Add 3 drops of the bromphenol blue indicator solution. Note the time and pour the acetone solution into the water solution quickly, swirl for s. second and imrnedktely pour the solution back into the other flask. (This orocedure will insure drained.
The results can be tabulated in terms of the time for a particular per cent reaction or can be expressed as a standard rate constant by using equation (1).
The above expression can he easily derived from the integrated first order rate law, In ([RCl]o/[RCl]) = kt, by definingthe fractionof reaction as ([RCljo- [RCI])/ [RClIo. The derivation of equation (1) can be effectively given as an exercise to interested students who have had some physical chemistry background. I n addition it might be of interest to have several students measure the time and corresponding rate constant for a reaction carried to 20 or 30y0 completion in order to show that the rate constant is independent of initial hydroxide concentration.3 Reaction Order
A typical experinlent is indicated below in which the
The order of the reaction is most easily determined by noting the effect of a change in reactant concentration
R. G., 'Xinetics and MechanFROST,A. A,, AND PEARSON, ism," John Wiley and Sons, New York, 1953. STREITWIESER, A,, Jr., "Solvolytic Displacement Reactions," McGraw-Hill, Nea York, 1962.
Although the effectiveness of the experiment in no way depends upon the exact reproduction of literature values, thme values can be easily obtained for a variety of solvents and at various temperatures by using the tabulated data in the reference cited in footnote 2.
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on the time necessary for any given per cent reaction, e.g., the half life or tenth life.' Use the experimental procedure described previously, but before the two solutions are mixed, add 10 ml of a 70% water30% acetone solution to the Erlenmeyer flask which contains the aqueous sodium hydroxide. (The reaction is being carried out a t twice the total dilution that wan used in the previous experiment, but the relative amount of water and acetone in the solvent remains the same.)
Because the time for 10% reaction is independent of the substrate concentration, the student can readily show that the solvolysis is first order in tert-butyl chloride.
over the edge of the beaker provides s. convenient support.) Try the reaction a t least twice. Warm a large beaker of water to about 10' above room temperature and repeat the shove experiment twice.
The method described here is readily adapted to very large organic laboratory sections and in spite of its simplicity, provides results of sufficient accuracy that an approximate activation energy can be obtained. (Data a t room temperature is obtained from the first experimental procedure.) Figure 1 shows a student's Arrhenius plot. Solvent Polarity
A unimolecular solvolysis reaction provides an excellent opportunity to demonstrate the importance of solvation in decreasing the energy difference between a highly polar transition state and a slightly polar ground state! Various mixtures of acetone and water ranging from 80 to 60% water by volume will change the time for 10% reaction of tert-butyl chloride by a factor of approximately 10.
Figure 1. Student rewlh for hydrolysis of led-butyl chloride. 19.3 kcal/mole, solcvloted from slope.
E. =
Temperature Variation
Pipet 2 rnl of a 0.1 M acetone solution of tertbutyl chloride into one of the Erlenmeyer flanks and 0.2 ml of the 0.1 M NaOH solus brom~henolblue indition ~ l u 7.8 s ml of water and three d r o ~ of cato; into the other flask. (80% wster-20% ketone.) Carry out the reaction as previously described and carefully record the results. Design and carry out an experiment to measure the time for 10% reaction of tertbutyl chloride in a 60% water40% acetone solvent.
The rate determination carried out a t room temperature should be repeated a t two additional temperatures to illustrate in a quantitative fashion the marked temperature dependence of the rate constant.
Prepare a 0.1 M acetone solution of isapropyl chloride or secbutyl chloride and repest the first experimental procedure. After 5 min, heat the reaction mixture on a ateam bath,
Prepare a water bath from a large beaker and adjust the temperature (crushed ice) to about 10" below room temperature. Before you mix the two solutions (see general procedure above), allow the temperature of the Erlenmeyer flasks to equilibrate in the water bath for about 5 min. (A piece of copper wire wound around the neck of the flasks and hooked
The entire experiment is designed for flexibility. If time is limited, portions of the experiment can be omitted without loss of continuity. Pertinent questions and discussion material are left to the discretion of the instructor.
4
See FROSTAND PEARSON (footnote l), p. 40,
568
/
Journal of Chemical Education
Structural Variations in the Substrate
See STREITWISER(f00tnote 2), p. 43.
