J. R. Jones Battersea
College of Technology London, England
I
I
Kinetic Isotope Effects An experiment in physical organic chemistry
It is only in the case of hydrogen, where the mass differences are appreciable, that large kinetic isotope effects, frequently of the order of 6-10, can be observed. The actual magnitude of the effect can be used to yield information related to the nature of the transition state, as well as to predict likely mechanisms. A large number of organic compounds, such as the nitroparaffins, substituted acetylenes, and ketones possess weakly acidic character (pK, 15-25) and are able to donate a proton by ionization at a measurable rate. It therefore follows that the denterated compound with its lower zero point energy of vibration will donate a deuteron 6-10 times as slowly as the protonnted species, provided the ioni~at~ion process is the rate determining step. If the compound under investigation is only partially deuterated, then, under suitable conditions, the results will appear as consecutive reactions which can be analyzed separately. The Experiment
Acetone reacts with alkaline solutions of bromine according to the equation:
The reaction is zero order with respect to OBr- and first order with respect to the ketone and hydroxide ion catalyst. The rate determining step is the cleavage of the C-H (or G D ) bond:
+ OH-- k n CDrCOCI)z- + DHO
CDaCOCDa
Provided that there is an excess of hypobromite present and that the ketone concentration is very much lower than the hydroxide ion roncentration, the reaction follows firstiorder kinetics and the rate constant for the ionization of a single C- H (or C- D) hond is given by
where x is the hypobromite concantrnt.ion at time t and x, the final value. Reagent grade acetone is dried over anhydrous N a S 0 4 and distilled twice. Deuterated acetone (99.5%) can be supplied by CIBA Limited, Switzerland, or alternatively can be prepared by equilibrating acetone for a few days in the presence of a large excess of DzO containing one or two pellets of NaOH. The
Volume 44, Number I , Jonuory 1967
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and C=Ao-A+B.-B =
C,
- A o e h ''-D'&
where and log (C, - C ) = log (A.r'@+
B~oe-~o')
+
If therefore log(A B) or log(C, - C) is plotted against time the result is an initial curve which becomes linear as t.he more reactive component, acetone in this case, disappears and the expression for log(C, - C) becomes log B -20
I
5m
IMO
I
I 3600
2000
2MO
Ym
3600
Time. I.mnd.
A plot of the logarithm of optical density changer vr time. The upper curve *ummorizer data for o mixture of devterated and non-deuterated acetone. The lower curve summarizer the data, b y difference, for the nondeuteroted acetone. From there curves the values of ko and k~ con be salculoted.
deuterated acetone is separated by distillation and the procedure repeated. The hypobromite solution is prepared by adding a known concentration of stock bromine solution to the previously standardized NaOH solution, and the concentration of OBr- determined ~pectrophotom~trically. Its spectrum has an absorption peak a t 3300 A and an extinction coefficient of 300. The concentration of the CH3COCH8-CD8COCD3mixture (50:50) is approximately 0.0007 M, and the OBr- concentration (0.004 d l ) is sufficient to maintain the order at unity; the usual range of hydroxide concent,rationsis 0.01-0.08 M. The kinetics are followed by placing the OBr-OH- solution in a l-cm stoppered silica cell and allowing it to attain the specified temperature; the requisite amount of ketone solution is added rapidly from a measuring syringe and optical density measurements are taken at frequent intervals. The infinity reading is taken after more than 10 half-lives have elapsed. Results
If we denote the concentrations as [CH.COCHs] = A, [CDaCOCDa] = B, [Products]
and represent the reaction as
=
C
=
log(C,
- C) = log Ba - (k~t/2.303)
From the slope of the line both Bo and k~ may be determined, and using this information A may be calculated, i.e., A=C,-C-B
and then logloA against time results in values of A. and k ~ . In the present example the amount of ketone reacting is directly proportional to the change in hypobromite concentration, and as the latter is given by t,he optical density measurements at a wavelength of 3300 A, the term (C, - C)H+ D can be replaced by (O.D. O.D.,)H + where O.D. is the optical density value and H D are subscripts for the ketones. The figure represents the data obtained in a typical rum, the initial part showing both acetone and deuterated aretaonereacting, and t,hefinal part, deuterated a c e tone alone reacting as all the a.cet,one has been consumed. From the slope of this linear section kD may be obtained, and extrapolation to zero time allows changes in optical density values due solely t,o the deuterated acetone reaction to be obtained in the time region where both acetone and deuterated acetone are reacting. Subtract,ion of t,hese values from the corresponding values in t,he curved region allows a graph of log (O.D. - O.D,)H against time to be constructed and hence kH can be found. For the above example where [OH-] = 0.0259 M , kH = 9.24, k~ = 0.943 1mole-kin-', showing a kinetic isotope effect of 9.79 a t 25". These values can be compared ~ i t k~ h = 9.1, k~ = 0.93 and k ~ / k u= 9.8 obtained by Jones', and kH = 10.8, k~ = 1.44 and k ~ / k~ = 7.5 obtained by Pockerz.
+
' JONES,J. R., Trans. Pa~adaySoc., 61, 95 (1965). Pocnm, Y., Chem. and Ind., 1383 (19.59).
32 / lournol o f Chemical Education
