Vol. 67
ANODIC OXIDATION OF K-METHYLANILIn’E AND S,K-DIiUETHYL-p-TQL‘CIDINE BY 2. GALUSAND RALPHN. ADAMS Department of Chemistry, University of Kansas, Lawrence, Kansas Received October 11, 1966 The anodic Oxidation of N-methylaniline was studied a t carbon paste and platinum electrodes using cyclic voltammetry and rotating disk techniques. N-INethylaniline oxidizes t o give N,N’-dimethylbenzidine as a primary product which is further oxidized t o the corresponding diquinoid. The anodic reaction is very siinilar t o that of i”\;,i”\;-dimethylaniline. However, if the para position of an 3-methylated aniline is blocked, as in N,N-dimethyl-p-toluidine, the reaction is entirely different. Here anodic oxidation of the para methyl group occurs leading to the corresponding aldehyde and acid.
Tbe anodic oxidation of K,N-dimethylaiiiline (DMA) was investigated in this Laboratory and shown to consist of a 2-electron charge transfer step followed by a chemical reaction to yield N,K’-tetramethylbenzidine (TLIB). This compound, being more easily oxidized than the parent DMA, undergoes further oxidation to the quinone diimine (TR4BOx). Further complicating side reactions of the ThIBOx with excess DMX were identified and the entire reaction scheme was studied in details1-3 It is t%erefore of considerable interest to investigate the anodic oxidation of monomethylaniline to see if it proceeds in similar fashion. Further, by blocking the para position, as in S,X-dimethyl-p-toluidine, it is possible to see if an ortho type coupling can occur to give a benzidine-like product or if some entirely new electrode reaction occurs. The following studies of the anodic oxidation of N-methylaniline (MA) and N,Ndimethyl-p-toluidine (DMPT) were carried out using single and multi sweep cyclic voltammetry as well as rotated disks. In addition some random tritium labeled electrolyses were investigated. Experimental details hare been presented Oxidation of MA.-The oxidation of MA was carried out over a wide pH range from 3 N K2S04to pH 9 buffer. The buffer solutions were Britton and Robinson and contained 1 M NazS04 to maintain reasonably constant ionic strength All polarograms were carried out with a controlled potential scanner. Over the entire pH range MA showed a single anodic wave with Ep/2 varying with pH as expected. The Ep/2 values a t Pt and carbon paste electrodes were very similar below pH 6-7. Above this pH range results on carbon paste showed considerable scatter. Below pH 5.5, the mean slope of the peak polarograms on Pt is 55 mv. and about 60 mv. for carbon paste electrodes. The family of curves in Fig. 1A shows the cyclic voltammetry of M A a t pI-1 2.4 a t a carbon paste electrode. The first anodic scan (1) shows only the oxidation wave for MA at ca. 0.7 v. us. s.c.e. Upon reversal of the scan two cathodic waves are obtained a t ca. $0.45 and 0.35 v. I n the subsequent anodic scans two anodic m-aves appeared at the corresponding potentials. Following the reasoning used in the DMA studies, the oxidation-reduction system at ca. 0.45 v. was pre( 1 ) T. hIizoeuch1 anif R. N. Pdains, J . A m Chent. SOC.,84, 20.58 (10G2). ( 2 ) Z. G d u s and R. N idams, %bid., 84, 2061 (1962) (‘3) Z Balus R. 32. White, F. S. Ronland, and R. N. Adams, %bzd., 84, 2065 (1962).
sumed to be that of N,?j’-dimethylbenzidine (DMB) and its corresponding diquinoid DMBOx. Therefore DMB was prepared by standard methods and subjected to cyclic voltammetry under identical conditions. The system of almost reversible cyclic polarograms seen in Fig. 1B resulted. There is very little doubt that the DRIB-DMBOx system is present after electrolysis of MA. The magnitude of the currents for the DMBDMBOx system obtained in electrolysis of MA (Fig. 1A) is small since DRIBOx reacts rapidly with excess MA as indicated below. The oxidation-reduction system a t ea. $0.35 v. was assumed to result from coupling of DMBOx and excess MA Accordingly, DMB was oxidized in the presence of excess MA, care being taken to keep the potential below that at which M A oxidizes. The family of polarograms in Fig. 1C resulted. Kote especially that on the first anodic scan (I), only a wave for DlliIIB oxidation is obtained. On the reverse (cathodic going) cycle both cathodic waves are observed and on the second and subsequent anodic scans the semi-reversible oxidation-reduction couple at $0.35 v. is clearly evident. It is clear then that the compound formed which gives rise to the oxidation-reduction system a t $0.35 v. is formed by interaction between DMBOx and hL4. It is fairly clear then that DMB is formed in M A anodic oxidation and that DMB, being more easily oxidized than the parent MA, is found finally as DMBOx. I n this respect the oxidation of MA is almost identical with that of dimethylaniline. However, the secondary reaction betveeii DMBOx and MA is much faster and occurs even in acidic media whereas this reaction was only prevalent in alkaline media with dimethylaniliiie. It remained to be decided if the DMB arises from a free radical coupling of two i\lA molecules which had lost one electron each, or by the dipositive species interacting with an unoxidized molecule of MA. This question can be decided by studying the order of the reaction w-ith respect to MA and the number of electrons in the charge transfer step. The reaction order was determined by studies at a rotated disk of carbon paste. The rotation speed was varied between 0 and 30 r.p.s. Figure 2 shows some typical results. Here the current vs. the square root of rotation speed (iV1/z)is plotted for MA oxidation at various applied potentials. At low Eapp(0.87 v.) the current is almost independent of rotation speed, Le., the current is practically limited by the charge transfer process. At the highest potential, 0.95 v., the current increases with rotation speed but is still not linear m-ith #/’. Such behavior is typical of a reaction controlled both
ANODICOXIDATIONOF S-METHYLANILINE
April, 1963
863
E v s SCE ( v . ~ (4)
0.6
017
0;8
0.4
0,s
0.2 I
0.p
CURRENT
0x1D
Fig. 1A.-Cyclic polarograms of MA.
E v s SCE OF’
Oi
Of
013
tv.1, O*P
0.1
0.0 1
CURRENT
Fig. 1B.-Cyclic
polarograms of DMB-DMBQx.
values were calculated for 3 and 21 r.p.s. and found to bv mass transfer arid rate of electrochemical reaction. yhis case has been treated extensively by L e ~ i c h . ~ (4) V. G. Levioh, “Fiziko-khimicheskaya gidrorlinamika (Physiooby the method The reaction Order, Chemical Hyarodynamics).” p. 77, Godudarstvennoe, Izdatel’stvo Fizikoused for DMA previously (see ref. 2 , eq. 8). The 6matematicheskoi Literatury, N ~ S C O V , 1959. mj
2. GALUSAND RALPHN.ADAMS
864
T'ol. 67
E vs SCE w. OI6
015
014
0.2 I
013
0.I I
0.0 I
polarograms of mixture of DXBOx and excess XIA.
Fig. 1C.-Cyclic
The results are shown in Table I. (BnJ is difficult to evaluate for P t a t slow scan rates since the peak polarograms are dra~wiiout (ie., (Ep - Ep/a) is difficult to evaluate). At the higher scan rates the mean value is about 0.83. The result,s are excellent a t carbon paste obwith a mean value of 0.91 The values of (,ha) tained strongly suggest n = 2 assuming a reasonable value for p of ca. 0.5, Higher values of n, are not' excluded by this evaluation. TARLE I KIXUETIC PMMMETERS FOR M A OXIUATION~
--.-
Scan rate (v./rnin.)
1
2
3
4
J
6
5 ' / 2 .
Fig. 2.-Oxidation
of MA a t rotated diEk electrode.
have the values 3.39 x and 1.28 X em., respectively. The corresponding fluxes a t these speeds were calculated from the currents of Fig. 2 . D values were evaluated from peak current polarograms with nT = 2 electrons. The kinematic viscosity was assumed to be 0.01 cm.2/sec. The value of na at 0.93 v. was found to be 0.87; at 0.95 v. m was 0.85. This is fairly coiiclusive evidence that one molecule of MA is iiivolvcd in thr charge transfer step. The quantity (pn,), the product of the anodic transfer coefficient by the number of electrons in the charge transfer step, was next evaluated from the peak polarograms using the well knoxn relations for an irreversible process given by Matsuda and Ayabe5 (6) H. l [ a t s u d a and Y . Ayabe, Z . Elektrochem., 59, 494 (1955).
pns.-.--
7.--log
Pt
C.P.
KbO
~_-
C.P.
Pt
0,00083 .. 0.88 .... ,0033 0.73 .91 -15,Y ,0083 .81 .92 -17.1 -17.1 .0167 .82 .92 ,033 .85 .90 -17.6 a In sulfuric acid-sulfate buffer pH 1.32.
-18.5 -18.8 -18.9 -18.9 -18.5
On the basis of the above data we propose the electrochemical oxidation of RIA proceeds according to
-
HNCHa
(1)
i8miH,
CH3 g
-m
i
H
r
CH
-
CHs N
-m
+ 2Hf
H
8
(2)
2H' + 2+e (3)
AXODICOXIDBTIOSOF 3-METHYLAXILINE
April, 1963
In actual fact IIRIB was not identified positively. The identification via a double rotated disk technique, which was successful with DMh (see ref. 2 ) , was not possible in the present study since the secondary reaction of DMBOx with ?VIA was so rapid. The product of this secondary reaction interferes with the double disk method. The subsequent reaction of DMBOx and RIA was investigated in more detail by classical poteiitiometry. X solution of DNBOx was prepared by oxidizing a known amount of DMB with bromine cia a brornatebromide reaction in acid solution. Care was taken to add an amount of bromine which mould oxidize only about one half of the known quantity of DRIB (Le,, no excess bromine was present at the end of the reaction). Yext, this solution of DMBOx was titrated potentiometrically with MA solution using a carbon paste indicator electrode us. s.c.e. Figure 3 shows a typical potentiometric curve. In solutions from 0.1 to 1.5 ,V HzS04,the ratio of MA to DlCIBOx was found to be very close to 2. Thus, it can be postulated that the oxidation-reduction system which develops at ea. +0.33 v. is that of a compound with the generic formula
0.60
865
1
I
\.-.0.2
0.8
1.2
1.6
L
2.0
T'ol. 1LIA. (ml.).
titration of DRlBOx with M A
Fig. 3.-Potentiometric
1
d,e (+)
E
VS
SCE
(v.J.
__
,
I
DP~IBO~ This material is oxidizable and forms a semi-reversible oxidation-reduction system. The exact structure of the coupling product is somewhat speculative. At higher pH's, the ratio of RL4/DMBOx is less than 2 indicating partial reduction of DRlBOx prior to reaction. The latter point is borne out by examination of the tritium content of random labeled MA after usual oxidation procedures. The activity was always lower than expected on the basis of the postulated reactions at high pH's. Either deamination reactions occur a t high pH's or rapid exchange of the tritium occurs. Finally, a t low pH's, when the formation of the DRlBOx-MA compound is relakively slow, the rate constants for MA oxidation were determined from the peak polarogram behavior using the expression of RIatsuda and Ayabe
I
/ Fig. 4.-Anodic oxidation of DMPT: 1, carbon paste electrode: 2, platinum electrode; 3, D J I P T plus p-dimethylaniinobenzaldehyde.
DMA log Kho = -19.8 where Kbo is the rate constant evaluated at E = 0 (i.e.> os. H t = 0 ) . I n Table I, the rate constants (actually log (Kbo X f R e d ) ) are seen to be independent of scan rate and have the mean value - 18.7 on carbon paste. The reaction appears slightly more rapid a t Pt with a mean log Kb" = - 17.3. The apparent activation energies were calculated by varying the temperature between 25.2 and (56.3'. The product pn, was practically constant over the temperature interval. h plot of log Kbo us. 1/T gives an apparent activation energy of 30.8 kcal. This is quite a bit smaller than that found for DNA under similar conditions. Rate constants evaluated a t other thaii the equilibrium potential cannot be compared for different substances. Howver, since the hypothetical equilibrium potentials for DMA and M A can be supposed to be quite similar, it is possible to make ai1 approximate correlation of the anodic oxidation rates of DMX2 and RIA. Thus, in the same pH region wn-eh a w
?VIA
log Kbo = -17.2
Oxidation of N,N-Dimethyl-p-toluidine.-Since some 20% ortho-para coupling was shown to exist in the oxition of DM9,3 a study of N,N-dimethyl-p-toluidine (DMPT) was of interest to see if similar coupling would occur or whether the blocking of the para position would result in an entirely new electrode reaction. It was found, indeed, that the oxidation of DNIPT is quite different. During cyclic scanning of DRIPT in solutioiis from 3 N HzSO4 to pH 11 buffer, a t both carbon paste and Pt electrodes, no reversible electroactive systems were 1.2 v. This alone is eviformed in the region 0 to dence that the o,o'-dimethyl-N,K'-tetramethylbenzidine is not formed. Instead only anodic waves were found in DMPT oxidatioii. Two waves were always seen with even a small third anodic wave occurring in the pH iiiterval 2-6. The slopes of these waves were not very repro-
+
z. GALTXAND RALPHs.AD4;LIS
SG6
ducible and the value of Ep/2 varied with scan rate. The peculiar behavior was not caused by film formation on the electrode since repeat scans were quite reproducible. However, quantitative measurements of pn, were not attempted. The nature of the various waves in D N P T oxidatioii can be explained qualitatively. Since no reversible oxidation-reduction systems were observed it is reasonable to assume the para CH:, group is oxidized. In alkaline solution, where 2 waves are observed, tlie ratio of peak currents was about 2 : 1. This is seen in Fig. 4. Curve 1 mas obtained on C paste and curve 2 on Pt. By comparison of the peak currents for oxidation of DMA and DbfPT (assuming D D ~ I=A DDIIPT)and taking into account the ratio of the apparent transfer coefficients a reasonable value for n of the D J I P T oxidation is 4 electrons. Thus, it is most likely that the first wave corresponds to oxidation to aldehyde. The second wave ( 3 electrons) is then an oxidation to the corresponding acid.
1-01. 67
Support of this mechanism is afforded by adding p-dimethylaminobenzaldehyde to a solution of DMPT and examining the second wave. Curve 3 of Fig. 4 shows the polarogram obtained at pH 6.8 where 5.0 X X aldehyde was added to 4 X M DMPT. The 2nd wave quite clearly is due to the presence of the aldehyde formed from tlie oxidation during the 1st wave. Summary.-The aiiodic oxidation pathways of Nmethyl and N-dimethylaniline have been shown to be quite similar. Both lead to the formation of N-substituted benzidines. However, with the para position blocked, as in N,N-dimethyl-p-toluidine, oxidation of the para methyl substituent occurs. Acknowledgments.-It is a pleasure to acknowledge the support of this work by the Atomic Energy Commission through contract AT( 11-1)-686. We are indebted t o A. Rogers for technical assistance.
THE INVESTIGATIOS OF THE KINETICS OF MODERATELY RAPID ELECTRODE REACTIOSS USIKG ROTATISG DISK ELECTRODES1 B Y z.GALL-sA S D RALPHN.XDAMS Department of Cheinzstry, Cniversnty of Kansas, Lawrence, Kansas Received October ib, 1961 The capabilities and limitations of rotated disk electrodes for the measurement of heterogeneous rate constants (k,) have been examined in detail. T'alues of ks for the oxidation-reduction sJatems Fe(II1)-Fe(IT), Fe( C N ) G - ~ - Fcn')6-4, ~( M n O ~ - - ~ l n O ~ and - , Ce(1V)-Ce( 111) were determined a t platinum and carbon paste rotated disks as a function of snlution environment. The results for moderately rapid charge transfer rates are in good agreement with existing data. Further correlations of ks and the homogeneous exchange rate are possible uia recent theoretical interpretations of Marcus.
The rotating disk electrode (r.d.e.) is uniquely suited to the determination of the heterogeneous rate constants for moderately rapid electron transfer (charge transfer) processes. As was shown by Levich,* the mass transport rate to the disk surface is independent of the distance from the axis of revolution. Thus, the concentration of the electroactive species a t the electrode surface (C") is everywhere equal on the surface and tlie thickness of the diffusion layer is given by Levich as
m.t. rate can be regulated via the disk rotation velocity. One might think it would be possible to determine very fast c.t. rates by increasing rotation velocity to very high values. TTith high rotation velocities, the fluid flow at the r.d.e. becomes turbulent and relations based 011 laminar m.t. are no longer valid. According to Levichjs the m.t. a t smooth and well centered r.d.e.'s ceases to be laminar at values of the Reynolds number (Re) between lo4 and 105. The dimensionless parameter Re is given by r2w
Re = V
where D = diffusion coefficient,, cm.2/sec. y = kinetic viscosity, cm2/sec. w =
angular velocity of disk given by w = 2nN where N = revolutione per sec.
It follows that the rate of an electrochemical reaction at the surface of the r.d.e. is equal at all points on tlie surface (disregarding micro heterogeneity, i.e., active sites on the surface). To investigate tlie kinetics of an electrode reaction, mass transport (m.t.) should be fast in comparison to the charge transfer (c.t.) rate. With the r.d.e., the (1) Based on material presented a t Symposium on Elertrode Processes, Division of Physical Chemistry., 142nd Xational Meeting of t h e American Chemical Society, Atlantic City, W. J., Sept., 1962. ( 2 ) V. G. Levich, "Physioochemicsl Hydrodynamics," Prentice-Hall, Inc., Englewood Cliffs, S. J., 1962.
where r = radius of r.d.e. in cm.
?;on-turbulent conditions are favored by using ail electrode of small radius (this includes tlie working plus lion-working shield radius). On the other hand, the size of the disk working surface must be large enough to allow one to neglect edge effects. In the present work good results were obtained with disk diameters of 2 mm. (working surface). Smaller disks have been described by Vielstich and Jahn,4a Azzim and Riddif~rd,~b and Frumkin and Teodoradse.5 (3) Ref. 2 , p. 86. (4) (a) W, Vielstirh and D. Jahn. Z. Eleklrochem., 64, 43 (19G'l);
(b)
S.h z i m and .4.C. Riddiford, Anal, Chem., 34, 1023 (1962). ( 5 ) A . S. Frunikin and G. Teodoradso, Z. Eleklr, c h e m . , 62, 231 (19,58).
