The Primary Deuterium Isotope Effect in the Base-Catalyzed Enolization of Acetone John Henderson Jackson Community College, Jackson, MI 49201 For several years 1 have used a set of three experiments by Waddington and Meany' that study the general base-caralyeed enulization of acetone by following the disappearance 20. They determine the kinetic of iodine usine-a Soectronic . order of the reaction with respect t o both acetone and iodine and the catalytic constant for its general base catalysis by hydrogen phosphate ion. Recently I have added a fourth experiment in which students measure the primary deuterium isotope effect for the reaction. Student results give an isotope effect of about 4. An isotope effect for this particular reaction hasnot been reported, hut this is reasonably close to the value of 6.5 reported2 for the acid-catalyzed enolization of acetone and to the value of 5.5 revorted3 for the hasecatalyzed enolization of a beta-diketone. One additional solution is needed and students do the two kinetic runs specified in the table in addition to the 14 required in the experiment of Waddington and Meany'. The solution is 2.72 M acetone-ds with 0.2 M NaC1, which can be made by diluting 5 mL of acetone-ds (Aldrich99.5 atom%D) to 25 mL with 0.25 M NaC1. I include a question in the experiment on why the acetone-ds is introduced in an NaCl solution rather than in a ~ h o s o h a t ebuffer solution as acetone is in all of the other runs. 'The exchange of the deuteriums that takes place rapidly in the buffer also takes place in the NaCl solution, although a t a much slower rate, so ideally this solution should be made uv immediately before use. However, making i t up 2 hours iiadvance seems to cause no problems. The two additional kinetic runs are initiated by adding the ~~
~
'
~
Waddington, M. D.: Meany, J. E. J. Chem. Educ. 1978, 55, 60. 2Toullec,J.; Dubois, J. E. J. Am. Chem. Sac. 1974, 96, 3524. Riley, T.; Long, F. A. J. Am. Chem. Sac. 1962, 84, 522.
Solutions Used In the Experlment* Phosphate
Run
Bulter (mL)
Triiadide (mL)
NaCl (mL)
15
4.0
16
5
1.0 to
1.O 0.0
n
Acetone (mL)
1.0~ 1.05
if used together with Waddlngfon and Msany1the title would be "Table 5. Solutlonr Used in Experiment 4." b ~ c e t ~h ne phosphatebuffer. "Acetoneds in NaCl.
acetone solution to the mixture of the other solutions rather than by adding the triiodide solution as in the runs described bv Waddineton and Meany'. For the acetone-ds reaction (run 16) this is to prevent formation of protium acetone before the reaction is initiated. The same order of addition of reagents is followed in run 15 so that the conditions will he as identical as possible between the two runs. The concentration of acetnne-d~in run 16 is four times that of acetone-h6 in run 15. This c¢ration effect on the rate is just about cancelled by the isotope effect of about 4 so the rates of the two runs are almost the same. This requires the students to deal with the difference between rates and rate constants to determine the isotope effect. A c o w of the student instructions for this experiment is available from the author. Acknowledgment I am indebted to the University of Michigan School of Education and especially to its dean, Carl Berger, for the facilities and support, both financial and moral, provided to me.
Volume 65
Number 4
April 1988
349
