Tautomerization of Acetylacetone Enol A Physical Organic Experiment in Kinetics and Thermodynamics Greg T. Spyridii and J. E. Meany1 Seattle University, Seattle. WA 98195
The concept of general acid, general base catalysis and its important applications to organic and biochemistry reactions are rarely discussed or demonstrated in undergraduate courses in organic or physical chemistry. Yet, in biocbemistry, for example, an understanding of such mechanisms is crucial to the appreciation of the mode of action of enzyme catalysts. The present experiment has allowed our students to study the catalysis of the tautomerization of acetylacetone enol by buffer components and by divalent metal ions. It also describes a method for carrying out thermodynamic measurements. An NMR method for determining the equilibrium constant of acetylacetone, 0-diketones, and P-ketoesters is described in Experiments in Physical Chemistry ( I ) . In that experiment, equilibrium constants are determined as a function of solvent polarity. We have found that an instructive complement to this experiment is a kinetic experiment using a recording, ultraviolet spectrophotometer. Our kinetic method is based on the relative positions of equilibrium between the keto and enol tautomers in solvents of varying polarities:
Most tautomerization reactions involving ketones must be monitored by halogen uptake. However, the tautomerization of acetylacetone enol can be monitored directly. Conjugation and intramolecular hydrogen bonding in the enol isomer cause the extent of enolization of 0-diketones and 0-ketoesters to be much greater than for other carbonyl systems. It is generally accepted that protic solvents of high dielectric constant, r , stabilize the more polar keto form and disrupt the intramolecular hydrogen bonding in the enol form. For example, a t 298 K acetylacetone is 96%enolized in CCId ( t = 2.23) (21,-80% enolized in p-dioxane ( 6 = 2.21) (3, 4 ) , and only 16% enolized in water ( r = 78.5% (5,6).The ultraviolet spectra of the acetylacetone system in dioxane and in water are shown in Figure 1. It is useful to compare these spectra with that of 3,3dimethyl-2,4-pentanedrione(I), 0
0
II
II
C6C\C/C\CH3
WAVELENGTH (nm) Figure 1.Comparative ultraviolet spectra of acelylicetone in H20(dashes) and in pdiaxane (solid line).
a compound that cannot form the conjugated enol. By comparison, this latter compound has a very small extinction coefficient ( 6 ) so we may assume that the extinction coefficient for the keto isomer of acetylacetone is also essentially negligible. Thus, the absorbance recorded in Figure 1is due
/\
CH,
CH, I
Author to whom correspondence should be addressed.
Volume 65 Number 5 May 1988
461
+
Figure 2. First-order plots for the acetate ion catalyzed tautomerization of acetylaunone enol. Symbols are O for 0.001 M acetate buffer.and 0 for 0.006 M acetate buffer.
entirely to the r -a* transition associated with acetylacetone enol. Given that the system is 16%enolized in water, the extinction coefficient for acetylacetone enol may he calculated: p27s o t nm = 13.000 M-' cm-' When a solution of acetylacetone in either 1,2-dimethoxyethane or p-dioxane is injected into an aqueous buffer, a shift in equilibrium in favor of the keto isomer results due to the increase in polarity of the solvent system. Thus, the reverse of reaction 1 can be monitored by the diminution of ahsorbance of acetylacetone en01a t 276 nm. Plots of In (At A,) vs. time (where A, is recorded as a function of time and A, is the ahsorbance after re-equilibration) as shown in Figure 2 show that the tautomerization of acetylacetone enol obeys good first-order kinetics. Since tautomerization is a reversible process, the observed rate constants, hob&, from the slopes of such plots refer to the sum of the first-order constants for the forward, kr, and the reverse, k,, processes:
kobd = kf
+ kr.
An interesting kinetic property of this reaction is its insensitivity toward caalysis by Lowry-Br$nsted acids but high degree of susceptibility toward general base catalysis (3, 7). This observation is to he expected since for a family of general acid-catalyzed reactions, the sensitivity of a given reaction toward acid catalysis is inversely related to the acidity of the substrate. Despite the lack of observed catalysis even by the hydronium ion, it has been shown that various transition metal ions powerfully and reversibly catalyze this reaction (3,8).It has been suggested that the transition state associated with the reversible tautomerization is stabilized by the complexation between the acetylacetone-acetylacetone enol system and these transition metal ions.
462
Journal of Chemical Education
A ACCH, +
CH&
clt
fasf
This experiment will give students experience in making kinetic measurements on a relatively rapid reaction. (The reaction is, of course, much slower if the spectrophotometer cell compartment is thermostated, for example, to 5 "C.) The reaction follows excellent first-order kinetics (see Fig. 2), and, because of its relatively~. rapid rate, many kinetic runs can be made in a relatively short period of time. Stu-. dent pairsmay he assigned to carry out specific series of runs. The dooled data from the class mav be used to illustrate the &$nsted catalysis law and the existence of catalysis by transition metal ions. Finally, since c ~ ~ ' , , , is known in water, the thermodynamic parameters, AG, AH, and AS may be evaluated hv determining the absorbance of solutions of known concentrations of theacetylacetone system as a function of temperature. Stock Solutions and Buffers A. 0.03 M acetylacetonein 1,2-dimethoxyethane2. B. 0.300 M NaCI. C. 0.01 M buffers (0.005 M both in acid and conjugate base), made up in 0.300 M NaCI. Suggested buffers: chloracetate (pH = 2.9), formate (pH = 3.8), acetate (pH = 4.6), malonate (HA-/A2-, pH = 6.5). D. 0.1 M solutions of zinc, cobalt, nickel, and cadmium nitrates in 0.002 M acetate buffer, pH = 4.6 and p = 0.305 M. The metal nitrates should be of analytical or reagent grade. Methods and Equipment Speetrophotometer. An ultraviolet, recording spectrophotometer, preferably with a thermostated cell compartment. If the cell compartment cannot he temperature controlled, an external constant temoerature bath mav he used to incubate the samde cuvette at25.0°~.orior . to the kine& run. The kinetic runs are raiidenoueh " chat any temperaturechangebetween t l w kinetic zeroand equilibrium should ha negligit,le. Pipers and S ~ r i n g e s One . 3-mL volumetric pipet and one 10-rl. Hamilton syringe. Mixing Paddle. A hexagonally or octagonally shaped piece of rubber, cut from aseptum, can serve as a "mixing paddle". (SeeFig. 3.) The oaddle should beshout 0.5cm wideso that it can fit into the samolecell without rubhine aeainst the sides. It may. be . olaced ah& 1 cm from the end of tKe earnilton syringe needle. Arapid up and down motion simultaneous with the injection of the acetylacetone solution causes the mixing of the substrate at kinetic zero. Initiation of the Reaction. Kinetic runs may be initiated by injecting and mixing 10 pL of the stock acetylacetone solution (solution A) into 3 mL of reaction solutions C or D. ~
~
~
2The solvent of choice here is 12dimethoxyethane since p dioxane is toxic and possibly carcinogenic. For complete toxicity information,see ref 10. Note that I,?-dimethoxyethane is fiammable and should not be handled near open flames. Acetyiacetone should be distilled prior to use (bp 133-135 "Cat 760 torr).
Table 1. Reaction Mixtures for General-Base-Catalysis Studles
Table 2.
Reactlon Solutions for Metal-lon-Catalysis Studles
Run
mL buffer mL 0.305 M NaCl [A-1 (M)
1, 2 2 8 0,001
3.4 4 6 0.002
5, 6 6 4 0.003
7.8 8 2 0.004
9, 10 10 0 0.005
mL 0.002 M acelate buffer g = 0.305 M mL 0.100 M M2+in 0.002 M acelate buffer g = 0.305 M (Souions D) [MZtl (M)
1.2 8
3.4 6
5.6 4
7.8 2
9. 10
2
4
6
8
10
0.02
0.04
0.06
0.08
0.10
0
Determination of the Br#nsted Coefficient for Base Catalysis Each student pair working on the project may determine ka- for a different huffer. (See stock solutions and suggested buffers.) The class may pool their data to illustrate the application of the Bransted catalysis law: log kn- = B log Kh constant, where Kh is the base dissociation for the reaction: A- + Hz0 = HA + OH-. A plot of log kh- VS. log Kh constructed by the students will allow the calculation of the B3nsted coefficient, 0, which may be compared with the value given in the literature (9)for this reaction. In order to evaluate koH- it is suggested that the malonate buffers (pK,% = 6.2) and or even diethylmalonate buffers (pK.2 = 7.0) he used to determine valuesof k' = ko ko~-[OH-].For eachaf these buffers the hydroxide ion concentration may he calculated from pH measurements. The value k o ~ may he evaluated from the slope of the straight line obtained by plotting k'vs. [OH-].
SYRINGE
+
GUVETTE
+
MIXING PADDLE
Determination of Catalytic Coefficients for Transition Metal Ions Each student oair mav determine k ... w + for a different metal ion. Theclass may pool thesererultr and theorder ofcatalyticeffecti~~eness for the metal ims studied may be compared to that reporced in refs 3 and 8. In the presence of metal ions and acetate buffers the observed rate constants may be represented by
Figure 3. Hamilton syringe with mixing paddle used to initiate kinetic runs.
Suggested Experiments
+
kohd = ko koAc.[OAc-]
Determination of General-Base-Catalytic Coefficients Since the tautomerizatiun ofacetylacetone e n d ir a general-basecatalyzed prwera, the observed rate constants consist of a catalytic component ans
