of the Oxidation of Benzyl Alcohol Kinetics
Michael T. H. Liu
Un~vers~ty of Pr~nceEdward Island Charlottetown, Prince Edward Island, Canada
The present paper describes an experiment in kinetics that is suitable for an undergraduate laboratory course in physical chemistry. It has the advantage of being simple to carry out and of bringing out some points that are not often covered in conventional kimetics experiments. The experiment has been performed with very satisfactory results by a number of students. The reaction is the oxidation of benzyl alcohol by acid permanganate, with the formation of benzaldehyde; the stoichiometric equation is 2MnO.c
+ 5C6HsCH10H+ 6H
+
-
+ 5CeHsCH0 + 8H10
2MnP+
Further oxidation of benzaldehyde has been shown' to be insignificant. Experimental Procedure Two stock solutions are used: K M ~ O solution ~ at a concenM tration of 4 X 10-8 M, and a solution containing 2.23 X benzyl alcohol, 0.68 M of perchloric acid and 20% acetic acid. Tbe reaction was carried out by mixing 20 ml of KMnO, solution with 180 ml of the reaction mixture so that the concentration of benryl alrdml nnd I I ~ I ~ O C Qions I I were in ~ X C C S The reactions were carried out i n a rolr.tunt wrnpernturc bath rrrulatPd to +O.ISC. ' The two iolutim.i wrre brought to thc bath trmperrtture and then rapidly mixed. Aliquot samples were taken at different time intervals and estimated for residual permanganate ion by the iodometric method. The rate of oxidation of alcohol could a180 be followed spectrometrically by measuring the absorbmee as a function of time. The firshorder rate constants obtained by means of these two methods do not differ by more than three percent; however, the iodometric method was employed because it is more economically feasible for lhrge numbers of students.
Typical Results
The disappearance of permanganate ions is plotted in the figure in which a = initial concentration of permanganate ions; and x = amount of permanganate ions reacted. The plot of log (a - x) against time gives a good straight line a t all three temperatures and the pseudo-first-order rate constants kl were obtained graphically; the rate of disappearance of permanganate thus follows the firsborder rate law when the concentration of benzyl alcohol and hydrogen ions are in excess. The fact that the reaction is first-order with respect to benzyl alcohol and hydrogen ion is shown as follows. First, the concentrations of the hydrogen ions and permanganate ions are maintained constant and the alcohol concentration increased; the ratio kl/ [CaHr CHIOH] remains the same, thus showing the first-
I 10
40
60
80
TIME ( m i " )
Plot of loglo 10
- xl versus time.
Volume 48, Number lo, October 1971
/
703
order dependence on the alcohol. Similarly, with the concentrations of permanganate ion and alcohol held constant, the ratio k,/[H+] remains constant when hxdrogen ion concentration is varied; this shows firstorder dependence on hydrogen ion concentration. The data showing kt and ka (third order rate constant as defined below) for several starting mixtures a t 34.5'C are listed in Tables 1 and 2. The instructor may find it expedient to divide the students into groups, one of which +auld determine the dependence with respect to alcohol, the other with respect to hydrogen ions. Some results for reaction rates a t different temperatures are summarized in Table 3 in which the thirdorder rate constant for the reaction is
Activation enthalpy (AH*) and activation entropy (AS*) have been calculated by the method of least squares using Eyring's equation k T
ks h
log - = log -
Table 1.
Variation of Rote with Concentration of Benzyl Alcohol at 3 4 5 ° C
Table 2.
Variation of Rate with Concentration of Hydrogen Ions at 3 4 S ° C
Table 3. First-Order Rate Constant (k,) and Third-Order Rate Constant (ksl as a Function of Temuerature
AS* - AH* + ----2.303R 2.303RT
where k = reaction constant; ke = Boltzmann's constant; R = gas constant; h = Planck's constant; and T = absolute temperature. The heat of activation AH* for this reaction is found to be 8.92 kcal/mole, and the entropy of activation AS* is -31.2 eu. Conclusion
A great many textbooks have pointed out that the net equation does not always give an indication of the mechanism by which the reaction occurs. This experiment is an excellent example to illustrate this point. This reaction is unlikely to take place with the encounter of 2Mn04-, 6H+ and 5CBHSCH,0H; rather the reaction will take place through a sequence of simple steps. One possible mechanism is the hydride tramfer mechani~rn'.~ which satisfactorily accounts for the first-order dependence on hydrogen ions, permanganate ions and alcohol H+
+ Mn0,- +HMnOl
tion of the alcohol involves rupture of an or C-H bond as the rate determining step.s Also, the values for the heat of activation and entropy of activation are very similar to those for the oxidation of ethanolZ (AH* = 9.3 kcal/mole; AS* = -33 eu); this also suggests that the same mechanism is operating in both cases. When the student has completed this experiment, he should have a good understanding of reaction order, reaction mechanism, and the least squares treatment of data. The time required for a least squares calculation of the activation energy from data a t three temperatures is probably not warranted, unless this is the only such calculation the students will make in the laboratory course. Acknowledgment
fast
5Mn (V) +2Mn (11)
+ 3Mn (VII)
The fact that the rate of oxidation increases with increasing acidity suggests that the permanganic acid (formed by protonation of permanganate ion) plays an important role in this mechanism. The heat of activation and the entropy of activation suggest that oxida-
704
/
Journal of Chemical Education
The author wishes to thank his undergraduate colleagues, expecially, T. Ryan, D. Hooper, R. Taylor, E. Sheen, I. Chung, and K. Clark for trying out these experiments for classroom use. Also, he would like to thank Professor K. J. Laidler for valuable comments. 1 BANERJI K. K., AND NATE,P., Bull. Chem. Soc. Jap., 42, 2038 (1969). a BARTER, R. N., AND LITTLER, J. S., J. Chem. Soc. (B), 205 (1967). a BAKORE, G . v., AND NARAIN,s.,J. hem. Soc., 3419 (1963).
