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 deter- mination of a reaction mechanism. The cerium(1V)-alco- hol experiments reintroduce precise quantitative tech- niques in a laboratory devoted primarily to qualitative methods and exemplify the approach used in studying en- zyme-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 solu- tions (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 color- less 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

f& n ROH + rn CeW) Complex (1)

(orange) (red)

and measurement of the association or equilihrium con- stant (K,,) relatively simple (2).

Ceric ammonium nitrate (CAN), Ce(NH&(NOda, the cerium reagent most commonly employed as a colorimet- ric reagent and in measurements of alcohol-complex equi- libria, has six nitrate ions surrounding each cerium in a bidentate fashion (3). Association with an alcohol mole- cule 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 oxi- dant of alcohols and that the cerium oxidation of benzyl alcohols is a convenient method for the synthesis of henz- aldehydes (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~stic of 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

slow (2)

(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 de- termination of the rate law for alcohol oxidation. The pro- cedure for the experimental determination of the equilih- rium constant and of the number of alcohol molecules as- sociated 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 ) .

Michael P. Doyle Hope College

Hollond, Michigan 49423

Experimental

A Spectrometric Study of the Oxidation

of AICO~OIS by ~eriurn(W)

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 L C 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 spec- trophotometrically at 520 nm using a Bauseh and Lomb Spec- tronic 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 pipet- ted into an Erlenmeyer flask followed by 1.00 ml of the benzyl al- cohol stock solution and 6.00 ml of acetonitrile. The time of addi- tion 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. Temper- ature variation during these readings should be less than one de- gree. If a decrease in the absorbance of the c&ium(N)-alcohol so- lution 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 solu- tion 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 vol- ume 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

K.. Ce(W + aROH + Ce(IV)(ROH), (3)

Ce(1V) and Ce(N) (ROH), has been derived (7)

where Ac is the difference in extinction coefficients be- tween Ce(IV) and Ce(IV)(ROH), and Beer's law is as- sumed. The form of this equation, y = m x n + b, is such 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 inter- cept, determined from the linear plot, are then used to calculate the equilihrium constant, K,,, since

intercept slope = K,,

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 tem- perature 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~dly mixed in thr constant temperature bath: the time of add~trun is 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 mea- surements can be obtained by recording the time at which half of the aliquot is removed. After reaching room temperature, the ali- quot 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 com- plete (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%.aqueous acetonitrile solution that is 095 M in nitric acid. .... ~ ~ ~ ~ ~ . ~ ~ ~ - . ~

The rate of solwnt oxidation, although mearural,le, is consider- ably slower than that of alcohol oxidat~un. The solvent oxidation is usually followed for the same rime period as the slruhul uxida-

Treatment of Data. The rate of reduction of cerium(1V) is assumed to he dependent only on [Ce(IV)lm and [ROWn

-d[Ce(N)' = k[Ce(IV)]"[ROH]" dt

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 rewrit- ten as

where k' = k[ROHIn. This pseudo rate expression is used to determine the order of the reaction in cerium(1V). The oxidations a t two alcohol concentrations are used to deter- mine 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 ex-

nression and eauilibrium constant for the cerium(1V)-hen- zyl alcohol systkm can he performed within one 3-hr labo- ratory period. Additional time may he required if the ex- periment 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 ceri- um(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 stu- dent-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

Enlhalov of Reactlon for CeriumWI-Alcohol Comolex Formaiion

ROH 25DK... lit.'2' 2s Keo, exp. A& kcal/molc Methanol 0.52 0.51 7.5 f 1.0 Ethanol 0.74 0.72 8.3 * 1.0 1-Kerannl 1.59 1.5 6.6 .L 1.0

(K, , in H2S04 u O), and the effect of temperature in in- creasing the value of the association constant. These proj- ects utilized variations of the procedure given in the ex- perimental 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 oh- tained.

Students were divided into groups of three for the de- termination 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 d was sufficient for this Dart of the exneri- ment. ~Lient- repor ted results 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 valueof 2.0 x M-'secc1(6).

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 increas- ing 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 equilihri- um and rate expressions of a reaction that can he oh- served visually provides insight into the stepwise mecha- nism of a relatively complex chemical process. The indi- vidualized 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 in- volved in the rate-limiting step of a two-electron oxida- tion, 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 Amer 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 \."".,. (6) Young. L. B.. "Ccrium(N) Oxidation of Organic Compounds," PhD Thaais, Iowa

StateUoivemity, 1968. (I) Adon, M. J . , J Chem. Sm., 1811 11957).

132 / Journalof Chemical Education