A Spectrometric Study of the Oxidation
Michael P. Doyle Hope College Hollond, Michigan 49423
of
AICO~OIS by ~eriurn(W)
For the past three years we have employed experiments with cerium(1V) and alcohols in the second semester of the organic chemistry laboratory to introduce students to the principles of physical organic chemistry, the detection of a chemical intermediate, and the experimental determination of a reaction mechanism. The cerium(1V)-alcohol experiments reintroduce precise quantitative techniques in a laboratory devoted primarily to qualitative methods and exemplify the approach used in studying enzyme-catalyzed biochemical reactions. The use of cerium(1V) as a colorimetric reagent for the detection of alcohols has been known for over thirty years ( 1 ) . Cerium(1V) is orange-colored in 1 M aqueous solutions (A,. a t approximately 315 nm). When an alcohol and this reagent are mixed, the orange cerium(1V) color instantly deepens to dark red. Since the alcohol is colorless and since i t can be determined that the color change represents a shift in the A,, for cerium(1V) to a higher wavelength, the color change could reasonably be due to the formation of a complex between cerium(1V) and the alcohol (eqn. (1)) (2). Equilibrium between the alcohol, cerium(IV), and the complex is rapidly attained
n ROH
+ rn CeW)
f&
(orange)
Complex (red)
(1)
and measurement of the association or equilihrium constant (K,,) relatively simple (2). Ceric ammonium nitrate (CAN), Ce(NH&(NOda, the cerium reagent most commonly employed as a colorimetric reagent and in measurements of alcohol-complex equilibria, has six nitrate ions surrounding each cerium in a bidentate fashion (3). Association with an alcohol molecule would replace one of the Ce-0 bonds of the cerium nitrate with a new Ce-0 bond formed to the alcohol. Studies have also shown that cerium(1V) is a facile oxidant of alcohols and that the cerium oxidation of benzyl alcohols is a convenient method for the synthesis of henzaldehydes (4, 5). Not only is the deep red color observed when 1 M ceric ammonium nitrate and the alcohol are mixed, hut also a slow decrease in the intensity of this color with time. The end result is a colorless solution character~sticof cerium(II1). Such an observation suggests that the cerium(1V)-alcohol complex reacts by electron transfer to cerium, forming cerium(II1) and an organic product (eqn. (2)).
k
+
Complex Ce(IlI) Product (2) slow (red) (colorless) Simple observation of the progress of the reaction of ceric ammonium nitrate with alcohol, then, suggests a mechanism in which complex formation preceeds electron transfer to cerium(1V). Confirmation of this mechanism requires the measurement of the equilibrium law and equilibrium constant for complex formation and the determination of the rate law for alcohol oxidation. The procedure for the experimental determination of the equilihrium constant and of the number of alcohol molecules associated with one cerium is adopted from the original work of Young and Trahanovsky (2). The determination of the rate law and measurement of the rate constant for the oxidation of henzyl alcohol was taken from Young ( 6 ) .
Experimental Procedure for Measurement of the Equilibrium Constant for Alcohol-Cerium (IV) Complex Formation A 50.0-ml solution of aooroximatelv 0.067 M ceric ammonium .. nitrate in I 6 .M nitric ?ad is prepared from stuck iuluttons of 1-2 .M erric nmmonium nitrate and cancentrared nrtrtc a c d The I-:! .%I CAT solution rs itahle over a perrod LC appmximarrlv two weeks; more dilute solutions are relatively unstable and should not be stored for longer than four hours. A solution of 1.00-2.00 M benzyl alcohol in acetonitrile (50.0ml) is also prepared. A 3.00-ml volume of the 0.067 M CAN solution is accuratelv . oi. petted into an Erlenmeyer flask and diluted by adding 7.00 ml of acetonitrile. The resulting solution is mixed thoroughly and transferred to the spectrometer cell. Analyses are performed spectrophotometrically at 520 nm using a Bauseh and Lomb Spectronic 20 spectrophotorneter. The absorbance of the CAN solution without alcohol (Ao) is measured with a solution composed of 70% acetonitrile-30%aqueous nitric acid as the reference. A 3.00-ml volume of the 0.067 M CAN solution is again pipetted into an Erlenmeyer flask followed by 1.00 ml of the benzyl alcohol stock solution and 6.00ml of acetonitrile. The time of addition and temperature of the solution is noted. The solution is thoroughly mixed, transferred to the spectrometer cell, and the absorbance is recorded at 520 nm (AT).Absorbance readings are taken at 1.0, 2.0, and 3.0 min after the time of addition. Temperature variation during these readings should be less than one degree. If a decrease in the absorbance of the c&ium(N)-alcoholsolution is observed during the time that the readmgs are taken, the absorbance value (AT) used in calculating the equilibrium constant is determined by extrapolating the absorbance to zero time by assuming that the absorbance decreases linearly with time. The procedure is repeated using the same amount of CAN solution with 2.00, 3.00, 4.00, and 5.00 ml of the alcohol solution; the amount of added acetanitrile is decreased accordingly. Far each solution examined, the volume of alcohol stack solution, total volume of cerium-alcohol solution, concentration of alcohol in the cerium-alcohol solution, l/[ROH], [ROHI2, 1/[ROHIZ,AT, AT Ao, and 1/Ar - Ao (ljA.4) is tabulated. Treatment of Data. The general equilihrium expression (eqn. (3)) is assumed. An equation relating the change in absorbance (AA) with the change in concentration of Ce(W
K.. + aROH + Ce(IV)(ROH),
(3)
Ce(1V) and C e ( N ) (ROH), has been derived (7)
where Ac is the difference in extinction coefficients between Ce(IV) and Ce(IV)(ROH), and Beer's law is asb, is such sumed. The form of this equation, y = m x n that a plot of 1/AA versus [ROH]" will give a straight line when the value of n corresponds to the number of alcohol molecules complexed to one cerium. The slope and intercept, determined from the linear plot, are then used to calculate the equilihrium constant, K,,, since intercept = K,, slope Procedure for Determinhg the Rare Expressron and Rate Constant lor the Oxidation of Benzyl Alcohol Dy Cerium ( 1 V) A 25.0-ml solution of 0.020 M ceric ammonium nitrate in 3.80 M nitric acid is prepared from stack solutions of CAN and nitric
+
Volume51, Number 2, February 1974
/
131
acid. Additionally, 75 ml of an acetanitrile solution that is 0.10 M in benzyl alcohol; 0.20 M in benzyl alcohol; or without henzyl alcohol is prepared. These solutions are placed in a constant temperature bath at 55 1" far 5 min. A 1WO-ml beaker containing water and heated by a Bunsen hurner serves adequately as a temperature bath. The CAN solution is added to the acetonitrile solution. and the resulting solution is rap~dlymixed in thr constant temperature bath: the time of add~trunis recorded. At appropriate intervals. usually 5 min, 5.W-ml nliquots are removed, transferred to an Er. lenmeyer flask, and cooled ta mom temperature. The time at which each aliquot is removed is recorded; consistent time measurements can be obtained by recording the time at which half of the aliquot is removed. After reaching room temperature, the aliquot is transferred to the spectrometer cell and the absorbance measured at 460 nm. The absorbance measurement can be made within 2-3 min after the aliquot is'removed. The rate of reaction is followed until the absorbance readings are less than 0.15. A final absorbance reading can be taken when the reaction is complete (greater than five half-lives) to ensure that no colored species are produced in the reaction. The reference solution for absorbance readings is s 75%.aqueousacetonitrile solution that is 0...9. 5 M in ~ nitric ~ acid. ~ ~
~
~
.
~
~
~
-
.
The rate of solwnt oxidation, although mearural,le, is considerably slower than that of alcohol oxidat~un.The solvent oxidation is usually followed for the same rime period as t h e slruhul uxidaTreatment of Data. The rate of reduction of cerium(1V) is assumed to he dependent only on [Ce(IV)lm and [ROWn
-d[Ce(N)' dt
=
k[Ce(IV)]"[ROH]"
where m and n are integers. Since the concentration of benzyl alcohol used in this experiment is much greater than that of cerium(IV), the rate expression can he rewritten as
where k' = k[ROHIn. This pseudo rate expression is used t o determine the order of the reaction in cerium(1V). The oxidations a t two alcohol concentrations are used to determine the order in alcohol. By applying the correction for solvent oxidation, the rate constant, k, is obtained. Results and Discussion The experimental determination of the equilibrium exnression and eauilibrium constant for the cerium(1V)-henzyl alcohol systkm can he performed within one 3-hr laboratory period. Additional time may he required if the experiment is to be repeated. Students were not informed of the expected results prior to the experiment. The results could, with few exceptions, he explained by 1:l cerium(N)-alcohol complex formation with an equilibrium constant that approximated the value in the literature (0.75 l/mole) (2). Alcohols other than henzyl alcohol have been k e d without appreciable differences between student-obtained and literature-reported values. In individualized projects students examined the effect of alcohol structure on K,, values (K,, for 3" ROH > 2" > lo),the effect of t h e cerium ligand on complex formation
132
/ Journalof Chemical Education
Enlhalov of Reactlon for CeriumWI-AlcoholComolex Formaiion ROH 25DK... lit.'2' 2 s Keo, exp. A& kcal/molc 0.52 0.51 7.5 f 1.0 Methanol 0.74 0.72 8.3 * 1.0 Ethanol 1-Kerannl 1.59 1.5 6.6 .L 1.0
(K,, in H2S04 u O), and the effect of temperature in increasing the value of the association constant. These projects utilized variations of the procedure given in the experimental section. The data presented in the table from the determination of the enthalpies of cerium(N)-alcohol complex formation indicate the quality of the results ohtained. Students were divided into groups of three for the determination of the rate expression and rate constant for the oxidation of benzyl alcohol. Two students from each group obtained the rate constants, k',.for oxidation .of benzvl alcohol a t different concentrations: one student determined the rate constant for solvent oxidation. One lahoratorv ~ e r i o dwas sufficient for this Dart of the exneriment. ~ L i e n t - r e p o r t e dresults confirmed the first-order dependence on both benzyl alcohol and cerium(N) and provided rate constants (k), determined from the results of each group, that were consistent with the reported M-'secc1(6). valueof 2.0 x Individualized nroiects utilizine kinetic techniaues have been used to o h t s n Hdditional in7ormation conceming the oxidation of alcohols bv cerium(1V). The effect of increasing solvent polarity and acid strength on increasing the rate of oxidation was examined. The activation energies for selected alcohol oxidations were obtained (for example, methanol, 15 kcal/mole, benzyl alcohol, 20 kcal/mole, and p-chlorohenzyl alcohol, 21 kcal/mole). Rate constants for the oxidation of substituted benzyl alcohols provided a Hammett pvalue of -2 when log kx/kH was plotted against .T+, a value comparable to the literature value (6). The basic experimental determination of the equilihrium and rate expressions of a reaction that can he ohserved visually provides insight into the stepwise mechanism of a relatively complex chemical process. The individualized projects reinforce and add to the information ohtained in the basic experiment. Since cerium(N) is a one-electron oxidant (4) and only one cerium(N) is involved in the rate-limiting step of a two-electron oxidation, the basic experiment, only begins to describe the mechanism of the reaction; mechanistic steps following the rate-limiting step can he implied by individualized experiments or by a discussion of the known mechanism of oxidation. Literature Cited (I) Duke. P.R.,andSmith,G.F..lnd. EM. Chem.Anol.Ed., 12.201 (1940). (2) young. L.. Brewate.. and Trahanovaky. waiter S., J A m e r ChmL soe, 91.
5060
, ,C,a, ~.""",.
13) Beineke,T.A.,andDelgaudio,J.,Inorg Chem., 1,715(1968). 14) Wibor~,K. B.. "Oxidation in Organic Cherniatri." Pan A, Academic Prcna. New York, 1965. 15) 'IYahsnovsky, W. S.. Young, L. B., and B m , G . L., J Org, Chom., 32. 3865
\."".,. ,,wm
B.."Ccrium(N) Oxidation of Organic Compounds," StateUoivemity, 1968. (I) Adon, M. J . , J Chem. Sm.,181111957). (6) Young. L.
PhD Thaais, Iowa
