3030
GEORGE
[CONTRIBUTION FROM
THE
T. E. G R A H A R I
AND
F.H. WESTHEIRIEK
VOl. bo
GEORGEHERBERT JONES LABORATORY OF THE UNIVERSITY OF C I ~ I C A L O ]
The Kinetics of the Chromic Acid Oxidation of Benzaldehyde BY GEORGET. E. GRAHAM' AND F. H. ~ ' E S T H E I R I E R ~ RECEIVED JANGARY 29, 1955 The rate of the chromic acid oxidation of benzaldehyde, in water at SO", increases with the first pvwcr of the conceiitratiou of benzaldehyde, with the first power of the concentration of acid chromate ion, and with a more complicated functiou of the concentration of acid. T h e data are consistent with a mechanism for the oxidation-reduction process which involves the chromic acid ester of hydrated benzaldehyde as intermediate. each measurement, the liquid in the Corex cell was returned to the flask in the thermostat bath by tipping the sealed container. The decrease in chromic acid concentration was followed spe~trophotometrically.6.~T h e experiment i n vacuum served to standardize those carried out under nitrogen. Rate Determination under Nitrogen.-The reaction mixture was maintained under prepurified nitrogen, and aliquots were delivered through a thermostated, jacketed pipet (Fig. 1) by pressure from a stream of nitrogen. Forty odd experiments were carried out uuder nitrogen in this manner; and only in one instance was the rate anomalous by coniparison with t h a t in vacuum. I n this one instance, the rate was high by a factor of 3.5. This anomalous result could not be duplicated, and the result was therefore discarded. I n all the rate determinations, the ionic strength was maintained a t 0.50 with sodium perchlorate. Effect of Oxygen.-Since benzaldehyde is rapidly autoxidized, the effect of oxygen on the rate was investigated. When air was present over the solution in a reaction flask, i t had very little effect upon the rate constant. When, however, oxygen was bubbled through a reaxtion mixture, the rate was increased by as much as a factor of three. Furthermore, when the stream of oxygen was cut off, and a stream Materials.-The chromic acid, and other standard reagents, were prepared as in previous s t u d i e ~ .Paragon ~ ~ ~ ~ ~ of nitrogen substituted, the rate only slowly returned to that which had obtained originally. I n one experiment, the rebrand "chlorine-free" benzaldehyde was distilled under vacuum, with purified nitrogen introduced through the capil- action proceeded under nitrogen with a rate constant of 1.87 inin.-' ( m ~ l e / l . ) - ~ . When the reaction was about 35'% lury "bleeder." The material was stored in the dark under nitrogen. Prior to each experiment, a sample of this benz- complete, a stream of oxygen was introduced, and the rate aldehyde was transferred, under nitrogen, to a separatory constant rose to 5.9 inin.-; (mole/l.) - 3 , U'hen nitrogen funnel, washed with alkali, saturated sodium sulfite and was again introduced, the rate constant gradually fell (at about 90% reaction) to 2.80 tnin.-l ( m ~ l e / l . ) - ~ . Althougli boiled distilled water. It was then dried over Drierite these experiments show that some precautions must be taken uiider nitrogen, and just prior to a n experiment, distilled at to obtain reproducible rates, they do not suggest t h a t the 10-6 mm. a t room temperature, I n all experiments (except chromic acid oxidation of benzaldeliyde in aqueous solution the rate determination in vacuum) this distilled aldehyde at 80" can be converted by oxygen into a free-radical chain was transferred under nitrogen to the kinetic reaction mixprocess. ture. Such aldehyde showed no peroxide by the starchEffect of Light.-The reaction I-ate was unaffccted b y the iodide test, and only a trace with the sensitive FeS04-hTH4light from a 150 watt lamp 1 ctn. from the reaction flask. CSS reagelit. Rate Determination in Vacuum.--A Corex Beckman cell T h e rate was also the same in a Beckman cell with the light on continuously, and in a parallel experiment where the rewas sealed t o a 250-cc. round-bottom flask by means of a long side-arm; 100 cc. of reaction mixture (without benz- action proceeded in the dark, except for four 5-second interaldehyde) was placed in the flask, degassed and frozen. vals when readings were taken. Product.-In the presence of a n excess of chromic acid, Then a known quantity of benzaldehyde was distilled into the flask a t mm. pressure, and the reaction vessel was peroxide-free benzaldehyde reacted under nitrogen t o consealed. T h e contents of the flask were melted, and the sume 98.57, of the calculated amount of chromic acid, and to flask introduced into the SO" thermostat. When the con- yield a 97.5% yield of crude benzoic acid which melted ( w i t h out recrystallization) a t 121-122'. tents c:tme to temperature, and at stated intervals thereafter, some of the solution was run into the Corex cell. T h e Results apparatus was so arranged t h a t the flask remained in the tliermostat bath while the Corex cell was in the Beckman Chromic Acid.-The reaction rate was measured spectrophotometer; the latter was thermostated by circuspectrophotometricallye~7as a function of the. Lttirig bath fluid through a special cell compartment. After
The rate of the chromic acid oxidation of isopropyl alcohol increases with the first power of the concentration of the alcohol, and the first power of the concentration of the acid chromate ion3 (as contrasted to the dichromate ion). The reaction induces the oxidation of manganous ion, and the induced oxidation is acconipanied by a diminution of reaction rate.4'5 These data have been interpreted to support the hypothesis that the reaction proceeds by abstracting a proton6 from a chromic acid ester7 of the alcohol. A comparable study now has been carried out for the chromic acid oxidation of benzaldehyde in aqueous solution; this work complements the investigation of Wiberg and Mi118 of the chromic acid oxidation of benzaldehyde and of substituted benzaldehydes in aqueous acetic acid solution. Experimental
.~ . ~ ~ _ _ _
( I ) Research Laboratories, U . S . Rubber C o . ( 2 ) Chemistry D e p a r t m e n t , Harvard University, Cambridge 38, 11Llss.
(3) F. H. Westheimer and A . S o v i c k , J . Chem. P h y x . , 11, 806 i1 i I 13). (4) 1%'. \%'utaiiabe a i i i i F. 11. \Vestheinier, ibid., 17, 61 (104H); J. 1Iainpton. A . T,en and 1'. 11. \Vcstlieimer, THISJ O U R N A L . 78, 306
. 13. Wcitliriincr,
Cltpiii
Rros., 45, 41!1 (l!449), and E r r a t a ,
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(6) N . Nicolaides :ind 1:. (I 9
11. Wcstheimcr, THISJ U I J K N A I . ,
11, 2 5
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( 7 ) I?, II(illnrvay, lf, Cohen and I?. 1%.Westheimer, ibid., 73, ( l ~ I , ? l ) , A . Leo and 1.' H . TVestheimer, ibid., 74, 4383 (1952). (81 I
