Kinetics and Mechanism of the Oxidation of Benzyl Alcohol and Benzaldehyde by Aqueous Sodium Dichromate

Transcription

1 Kinetics and Mechanism of the Oxidation of Benzyl Alcohol and Benzaldehyde by Aqueous Sodium Dichromate DONALD G. LEE AND UDO A. SPITZER The Department of Chemistry, University of Regina, Regina, Saskatchewan S4S 0A2 Received May'6, 1975 DONALD G. LEE and UDO A. SPITZER. Can. J. Chem. 53, 3709 (1975). A kinetic study of the oxidation of a series of substituted benzyl alcohols and benzaldehydes by neutral aqueous sodium dichromate has been completed. The aldehydes are more resistant to oxidation because the reaction mechanism presumably involves hydration prior to oxidation, and under the conditions employed the extent of this hydration is suppressed. These conclusions are consistent with the activation parameters which indicate that the difference in oxidation rate is due to a more negative entropy of activation for the aldehydes. DONALD G. LEE et UDO A. SPITZER. Can. J. Chem. 53,3709 (1975). On a complete une etude cinetique de I'oxydation d'une serie d'alcools benzyliques substitues et de benzaldehvdes - Dar du dichromate de sodium aaueux en milieu neutre. Les aldehvdes sont plus resistants a I'oxydation db au fait que le mkanisme de reaction implique probablement une hydratation avant I'oxydation et dans les conditions utilisks I'hydratation est supprim&. Ces conclusions sont en accord avec les parametres &activation qui indiquent que les differences entre les vitesses de reaction sont dbes a une entropie qui est plus negative pour I'activation des aldehydes. [Traduit par le journal] Introduction The oxidation of aromatic alcohols to the corresponding aldehydes by aqueous sodium dichromate at elevated temperatures (- 100 "C) is a reaction that may be used for synthetic preparations that are difficult to accomplish using other oxidants (1). For example, Wakselman et al. have recently described the preparation of 7-coumarincarbaldehyde in 84% yield by treatment of the corresponding alcohol with aqueous sodium dichromate at reflux temperature (2). The reaction is of particular interest because it is known that under acidic conditions (which are usually employed for chromic acid oxidations) primary alcohols are oxidized by chromium(v1) to give substantial amounts of carboxylic acids or esters (3, 4) unless the aldehyde can be distilled from the reaction mixture as it forms (5, 6). However, neutral chromium(vi) solutions do not oxidize aromatic aldehides at an appreciable rate until considerably higher temperatures ( C) are attained (1). In the present work we have attempted to increase our quantitative understanding of these reactions by studying the kinetics of the oxidation of several substituted bennl alcohols and benzaldehydes by aqueous sodiim dichromate. Experimental The kinetic analyses were performed using a pseudo first-order approach where the substrate concentrations were in excess and the chromium(v1) concentration was monitored spectrophotometrically. In all cases good pseudo first-order kinetics were maintained for more than two half-lives of the reaction (Fig. 1). The rate data were obtained in the following manner. The required amount of alcohol was weighed into a flask, 20 ml of phosphate buffer was added, and the flask was stoppered and placed in a thermostated bath. Then 1.00 ml of 0.1 M chromium(v1) solution was added and the timer started after the solution had been thoroughly mixed. One ml aliquots were withdrawn at intervals, quenched by dilution with cold water to 10.0 ml, centrifuged, and analyzed spectrophotometrically for chromium(v1). Since the oxidation of the aldehydes could only be studied at temperatures above 150 "C it was necessary to use a sealed reactor for these experiments. The reaction vessel, which has previously been described (7), was charged with the reactants at room temperature, sealed, purged with nitrogen, and brought to a pressure of 320 p.s.i. of N,. The reactor was then heated to and stabilized at the desired temperature. This required about 1 h of heating; however, since the reactions were always pseudo first-order the rates could be followed by withdrawing samples when the temperature had stabilized and taking the time of the first sample as zero. The pseudo first-order rate constants were obtained from the slopes of plots of the logarithm of the concentration of HCr04- against time, and then converted to second-order rate constants through division by the concentration of the substrate.

2 3710 CAN. J. CHEM. VOL TIME (MIN ) FIG. 1. Pseudo first-order rate plots for the oxidation of benzyl alcohol at 95.9 "C (upper) and the oxidation of benzaldehyde at 170 "C (lower). The substituted benzyl alcohols and benzaldehydes were all obtained commercially and purified prior to use. Benzyl alcohol a,a-d2 was prepared by the lithium aluminum deuteride reduction of benzoic acid (8). I I I log [ALCOHOL] FIG. 2. Dependence of rate constants upon benzyl alcohol concentration. Slope = 1.09, r = Fm. 3. Hammett plot for the aqueous dichromate oxidation of benzyl alcohols. Slope = -0.61, r = Results and Discussions Oxidation of Benzyl Alcohol chromate esters. A similar effect was noted in ~h~ aqueous dichromate oxidation of benzyl the aqueous sodium dichromate oxidations when alcohol appears to be similar in several respects various mixtures of dioxane and water were used. to chromic acid oxidations (9). Some of the Because of these several similarities it is not common features are: the reaction is firstmorder that parallel mechanisms prevail for in oxidant (Fig. 1) and in benzyl alcohol (Fig. 2); the two reactions. Consequently we have assuma primary isotope is observed when the ed, in analogy with chromic acid oxidations, that a-hydrogens are replaced by deuterium (Table chromium(v1) esters are also involved as inter- 1); a negative Hammett value is obtained mediates in the oxidation of alcohols by aqueous (Fig. 3); and similar activation parameters sodium dichromate. (See Scheme 1). (AH* = kcal/mole and AS* = 25 + [I] HCr0,- + +CH20H e +CH20Cr03- + H20 2 e.u.) are obtained (10). Furthermore, chromic [2] 4CH20Cr CHO + HCr03- acid oxidations of alcohols are known to exhibit SCHEME I rate increases when non-polar organic co-solvents are used (11). These increases, caused by the The unfavorable entropy of activation may, decrease in solvent polarity, have been attributed at least partially, be due to formation of a cyclic to an equilibrium shift favoring the formation of transition state such as 1.

3 LEE AND SPITZER: OXIDATION BY SODIUM DICHROMATE TABLE 1. Isotope effects on the oxidation of benzyl alcohol by aqueous sodium dichromate* Substrate [Alcohol] (M) k, x ~o~(m-'s-' ) t k~lko Benzyl alcohol Benzyl alcohol Benzyl alcohol a,a-d, Benzyl alcohol a,u-d, *[Cr(VI)] = 5.8 x 10-3M, T = 96.4% The ph was adjusted to 5.52 at 25"C, th~p04-i = M, IHP04Z-] = M.?The second-order rate constants were obtained by division of the pseudo first-order constants with the concentration of alcohol present. TABLE 2. Rate constants for the oxidation of benzyl alcohol* PH~ [HzPO4-] (M) [HP042-] (M) kl x lo5 (s-')$ log kz *[Cr(VI)] = 5.18 x 10-3 M, [alcohol] = M, T = 96 C. ph was measured at 25 OC.!k i s a pseudo first-order rate constant. 5kz = kllialcohol]. The negative Hammett p value (-0.6) is probably a consequence of the fact that the slow step involves development of a carbonyl which would be destabilized by electron-withdrawing substituents. The magnitude of the p value is less than that observed for the corresponding chromic acid oxidations, thus suggesting that the carbonyl is less fully developed under neutral conditions. The slight increase in rate that is observed as the ph is decreased (Table 2) is difficult to account for quantitatively and may, in fact, be due to experimental uncertainties since the ph was measured at 25 "C while the reactions were actually carried out at 96 "C(7). 1 The Oxidation of Benzaldehyde The rate of the oxidation of benzaldehyde shows a first-order dependence upon both the oxidant (Fig. 1) and the substrate (Fig. 4), but requires a much higher temperature than the oxidation of benzyl alcohol (Table 3). FIG. 4. Dependence of rate upon benzaldehyde concentration. Slope = 0.9 f 0.1, r =

4 3712 CAN. J. CHEM. VOL TABLE 3. Rate constants for the oxidation of benzaldehyde* P H ~ 1H2PO4-I (M) [Hpo42-1 (M) kl x lo4 (s-l) log kz *[Cr(Vl)I = 5.45 x M, [benzaldehyde] = , T = 170 "C.?pH was measured at 25 OC. Skz = kl/[benzaldehydel. I expected increase in the rate of [6] (Scheme 2) FIG. 5. Hammett plot for the aqueous dichromate oxidation of benzaldehyde. Slope = 1.1, r = The Hammett p value is positive for the oxidation of substituted benzaldehydes (Fig. 5) in contrast to the negative p value observed for the oxidation of the corresponding benzyl alcohols. A similar observation for the oxidation of aldehydes by chromic acid (12) has been shown to be due to a pre-oxidative hydration step [3]. The suppression of such a hydration step at high temperatures (13) offers an explanation as to why aldehydes are more resistant to oxidation under the conditions required for oxidation by aqueous K [31 RCHO + H20 + RCH(OH)2 sodium dichromate. Because HCr0,- is not a vigorous oxidant, high temperatures must be used to obtain a reasonable reaction rate; however, independent studies have shown that the extent of carbonyl hydration decreases as temperature increases (13, 14). Consequently the with temperature would be partly nulified by a concurrent decrease in K,, and it is thus necessary to use higher temperatures for aldehyde oxidations than for alcohol oxidations. It is for this reason that aqueous sodium dichromate is particularly useful for the oxidation of alcohols to aldehydes (1). When the response of the rate of reaction to temperature changes was studied the activation parameters were found to be AH * = 13.2 f 0.4 kcal/mole and AS* = - 30 f 4 e.u. This highly unfavorable entropy of activation is probably due to a combination of three factors: (i) the transition state is likely cyclic as in 2, (ii) the formation of an aldehyde hydrate which precedes the rate determining step would have a negative entropy (14), and (iii) it is likely that the esterification step would also exhibit a negative entropy (15). A further interesting fact that emerges from a consideration of the activation parameters is that the difference in the rates of oxidation of benzyl alcohol and benzaldehyde is due mainly to a difference in the entropy of activation, which is in turn likely related to the additional hydration step that occurs only in the aldehyde oxidation mechanism. The authors are grateful to the National Research Council of Canada and the Saskatchewan Research Council for financial assistance.

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