James 0. Schreck'
Georgia Institute of Technology Atlonta
I I
Determining a Reaction Rate Constant An organic laboratory experiment
M o s t experiments in a typical undergraduate organic laboratory deal with syntheses of compounds and reactions of a class of compounds. Even though these are basic to the teaching of laboratory techniques, most students are concerned with carrying out the "recipe," and do not consider the actual reactions that are occurring. After using these experiments a number of times, many teachers in charge of these laboratories usually like to introduce more thought-provoking experiments. I n addition, a time factor is involved; it is necessary that these experiments be of such length that they can be completed in one or two laboratory periods. This experiment in kinetics involves the determination of the rate constant for the reaction of methyl iodide and potassium terk butoxide as shown in the following equation:
The student can set up his apparatus and obtain his data in the generally required three-hour laboratory period. This nucleophilic displacement of iodide ion by tert butoxide is a typical 5 ~ reaction. 2 I t can be demonstrated experimentally that this reaction follows second-order kinetics by determining the effect of concentrations of halide and base on the rate of the reaction; or it can be shown by a graphical method. The reaction can be followed experimentally by measuring the amount of base remaining a t any time. One of the advantages of this experiment is that it is readily adaptable to the laboratory since all the apparatus needed is commonly available. The reagents required are tert-butyl alcohol, methyl iodide, about 0.50 M potassium tert-butoxide, and about 0.025 A6 perchloric acid. The latter two reagents should be prepared and standardized beforehand by the laboratory attendant. It is not necessary that the tertbutyl alcohol be redistilled if it melts at about 25', but opened bottles of methyl iodide are decomposed by oxidation forming a faint violet color characteristic of free iodine. Therefore, the methyl iodide should be redistilled and stored according to Fieser.=
added to the tbutyl alcohol, and the flask is stoppered and shaken. Both flasks are immersed in a constant temperature baths to equilibrate for 10-15 min; then 15 ml of the potassium t-butoxide is pipetted into the flask containing t-butyl alcohol and methyl iodide. Zero time is recorded as the point where the pipet is one-half empty. The reaction flask is stoppered, shaken to insure mixing and returned immediately to the constant temperature bath. The entire operation can be accomplished in less than 40 sec. At appropriate intervals (see the fig.) 10-ml aliquots are withdrawn and pipetted into a 200-ml Erlenmeyer flask containing about 20 rnl of ice water to quench the reaction. Each aliquot is then titwted to the phenolphthalein endpoint with standardized 0.025 M perchloric acid (about 100 ml of acid is required for all titrations). The reaction is about 80% completed in 120 min. Data obtained a t 24.5' is shown in the figure. This experiment was successfully performed as a lecture demonstration in Professor Jack Hine's kinetics course a t Georgia Institute of Technology. The comments of Professor Hine in the preparation of this paper are gratefully acknowledged. 'FIESER, L. F., "Experiments in Organic Chemistry," 3rd ed., D. C. Heath and Co., Boston, 1957, p. 304. 8 A simple bath constructed from large beakers filled with enough water to cover the flasks is adequate and usually maintains the tem~eraturewithin 0.5' for the reouired time. Cantrolled constant temperature baths would he more desirable.
Experirnenl
Approximately 20 ml of standardized potassium t-butoxide is placed in a 100-ml volumetric flask, and 160 ml of t-butyl alcohol is pipetted into a 250-ml volumetric flask. One ml of redistilled methyl iodide is then
1 Army Research Office (Durham) Postdoctoral Fellow, 196465. Present address: Depsstment of Chemistry, Louisiana State University, Baton Rouge.
TIME (MIN.1
Graphical treatment of doto, o = [t-BuOKlo, b = [Mello.
Volume 43, Number 3, March 1966
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