I /
1 - 1 0 2
I 4
I 6
I
8
I
1
!
10
12
14
I
I
16
18
’
20
1
I
22
24
I 26
I 28
I 30
!
I
32
34
Time (min)
Figure 4. Chromatogram showing separation of hydrocarbon products from pyrolysis of phytane using the dual column consisting of a 5-ft packed column in front of a 50-ft SCOT column. The column temperature was programmed from 50 to 200 “C at a rate of 10 deg/min
The average pressure is equal to the outlet pressure (Pout) divided by the compressibility factor, j , where
packed column when used in the dual column has a higher efficiency than when used alone. The complete separation of the hydrocarbons from C1to CI8 cannot be performed with the SCOT column alone. The hydrocarbons C1,Cp,and C3appear as a single peak and even C4(butene) and Cs (pentene) are poorly resolved even when the initial temperature of the column was below ambient. However, with the dual column, good separation of the light hydrocarbon gases was obtained with the initial column temperature as high as 50 O C . A chromatogram obtained with the dual column for the separation of the products from the vapor phase pyrolysis of the phytane (2,6,10,14-tetramethylhexadecane, bp 323 “C) is shown in Figure 4. Here, CH,, CzH4,and C2H6 are separated to the base line, and even a trace of C2Hz is partially resolved. The apparatus and experimental procedure used in the pyrolysis experiment were described previously (10). In general, the use of dual columns can be applied to the analysis of many mixtures other than reported here. For example, a dual column consisting of the packed Porasil “B” and SCOT OV-101 columns yields symmetrical peaks for all aliphatic hydrocarbons. However, aromatic hydrocarbons form nonsymmetrical peaks with excessive tailing. This can be eliminated by using a dual column consisting of a 5-foot packed Porasil “C” (rather than Porasil “B”) column in series with the 50-foot SCOT OV-101 column. When two columns are joined to form a new dual column, it is desirable to choose the length of each column in such a manner as to correspond to the maximum efficiency for the particular flow rate used. The flow rate should be selected on the basis of resolution and analysis time.
We have previously shown (9) that an increase in the average pressure in the column decreases HETP, so that the 5-foot
RECEIVED for review May 22,1970. Accepted August 10,1970. This research was conducted under the McDonnell Douglas Independent Research and Development Program.
(9) J. Q.Walker, J. D. Kelley, and C. J. Wolf, Hydrocarbon Process., 47, (4) 288 (1968).
(10) D. L. Fanter, J. Q. Walker, and C. J. Wolf, ANAL.CHEM., 40, 2168 (1968).
Autocatalysis of the Kinetic Wave of Acetylacetone in Acetonitrile Solvent Thomas E. Neal and Royce W. Murray Department of Chemistry, University of North Carolina, Chapel Hill,N . C . 27514
THE ELECTROCHEMICAL REDUCTION of several aromatic 0diketones in DMSO solvent, has been investigated by Buchta and Evans ( I ) . The detailed experiments performed on one case, dibenzoylmethane, demonstrate a reduction to the radical anion followed by protonation by unreduced Pdiketone and coupling to form the pinacol. The pinacol subsequently, and on a relatively slow time scale, undergoes an autocatalyzed decomposition. Recent experiments in these laboratories on the reduction of acetylacetone in acetonitrile solvent have shown that this aliphatic P-diketone also exhibits an unusual electrochemical reaction but unlike that of the dibenzoylmethane case. Reduction waves for acetylacetone (1) R. C . Buchta and D. H. Evans, ANAL.CHEM., 40, 2181 (1968). 1654
have been reported in aqueous media (2-4), but other reports (5,6) note an inability to observe reduction in a variety of electrolytes. EXPERIMENTAL
Instrumentation. The solid-state potentiostat constructed for these experiments employed positive feedback control (7)
(2) G. Semerano and A. Chisini, Gazz. Chim. Itul., 66, 504 (1963). (3) A. Winkel and G . Proske, Ber., 69, 1917 (1936). (4) I. Tachi, Mem. Coll. Agr., Kyoto Imp. Unio., 42, 1 (1938). ( 5 ) H. Adkins and F. W. Cox, J . Amer. Chem. Soc., 60, 1151 ( 1938). (6) S. Harrison, Collect. Czech. Chem. Commun., 15, 818 (1950). (7) G. A. Lauer and R. A. Osteryoung, ANAL. CHEM., 38, 1106 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
Figure 2. Chronoamperometric reduction of 3.26mM acetylacetone 0.1M TEAP-acetonitrile, Hg electrode, 0.0149 cm2 area, potential step -1.8 to -2.26 V us. SCE Curve A . Experimental result Curve B. Predicted limiting long-time response, slope is 100/56 greater than Curve D Curve C. Illustration of Curve B corrected for spherical diffusion Curve D. Apparent limiting short time slope, corresponds cm2/sec for enol-acetylacetone to D = 1.3 X
Figure 1. Optically transparent thin layer cell spectra 0.1M TEAP-acetonitrile, 1000-lpi Au minigrid electrode, 1.83 cmz area, cell thickness 0.0125 cm Curve A . 3.78mM acetylacetone Curve B. 90 seconds after reduction of 3.78mM acetylacetone Curve C. 3.78mM acetylacetonate
enol-Hacac
and exhibited a double-layer charging time of less than 0.1 msec for a potential step applied to a 0.2 cm*electrode in 0.1M tetraethylammonium perchlorate-acetonitrile solution. Data were recorded oscilloscopically. Spectra were obtained using a Cary Model 14 spectrophotometer. Hanging Hg drop electrodes were of conventional design, and the optically transparent (Au minigrid) thin layer cell previously described (8, 9) was used with several modifications for operational convenience (10). Potentials are referred to an aqueous SCE-NaC1 electrode. Chemicals. Eastman Spectrograde acetonitrile solvent was used without further treatment. Eastman tetraethylammonium perchlorate was recrystallized twice from water and dried under vacuum at 40 "Cand then 80 "C. Eastman acetylacetone was distilled before use. RESULTS AND DISCUSSION
Acetylacetone is reduced in acetonitrile in a well formed but completely irreversible wave; in potential sweep experiments Epeak = -2.0 V (on Au) and -2.2 V ES. SCE (on Hg). The ultraviolet spectrum of the reaction product on Au, obtained in an optically transparent thin layer cell, is quantitatively that of acetylacetonate, as shown in Figure 1. Acetylacetone exists as a 56:44 enol-keto mixture in acetonitrile solvent ( I 1 , 1 2 ) , and it is logical to expect that the hydrogen reduction process indicated by the spectral result occurs through the enol form with a keto -+ enol tautomeric shift permitting exhaustive conversion. (8) R. W. Murray, W. R. Heineman. and G. W. ODom, ANAL. 'CHEM.,39, 1666 (1967). (9) W. R. Heineman. J. N. Burnett. and R. W. Murray, ibid.,40. 1974 (1968). (10) T. E. Neal, Ph.D. Thesis, University of North Carolina, Chapel Hill, N. C., 1970. (11) G. S . Hammond, W. G . Borduin, and G. A. Guter, J . Amer. Chem. Soc., 81, 4682 (1959). (12) A. S . N. Murthy, A. Balasubramanian, and C. N. R. Rao, C m . J. Clzem., 40, 2267 (1962).
+ e-
-+
+
1/2H2 acac-
(1)
Hydrogen reduction from the acidic form is analogous to that observed (13-15) for other weak acids in this solvent. Potential step thin layer coulometry of 3.7mM acetylacetone, carried out simultaneously with the experiment of Figure 1, gives an n-value of 0.84 for the reduction of total acetylacetone. This value, near unity, is consistent with Reaction 1 and exhaustive keto --t enol conversion. (The value of n is less than the ideal 1.00 largely because of the background electrolysis, which commences at about - 1.8 V on Au in the solvent used and which, through experiments in highly purified solvent, is demonstrably in part base-producing and acetylacetone-consuming reduction of impurity water. If no correction is made for background charge, n is 0.920.95.) Experiments performed under semi-infinite diffusion conditions reveal that the reduction of acetylacetone is not a diffusion-controlled process on time scales shorter than that of the thin layer electrolysis. Potential step chronoamperometry on a mercury drop electrode is illustrated by Figure 2. (No background electrolysis interference is experienced on Hg.) Simple linear Cottrell i-t-1!2 behavior is clearly lacking. Potential sweep results also reflect kinetic control; i,/c1'2 decreases by factors of 2.2 and 2.0 over a sweep range of 0.5 to 5.7 V/sec on Hg (16) for 1.0 and 4.9mM acetylacetone, respectively, and by a factor of 1.3 over a sweep range of 0.5 to 20 V/sec on Au for 2.7mM P-diketone. These data qualitatively suggest kinetic control of the acetylacetone reduction by a preceding chemical reaction. This is postulated to be: keto-Hacac F? enol-Hacac
(2)
(13) J . F. Coetzee and I. M. Kolthoff, J . Amer. Chem. SOC.,79, 6110 (1957). (14) P. J. Elving and M. S. Spritzer, T u l u ~ ~ t12, a , 1243 (1965). (15) I. M. Kolthoff, J. Polurogr. Soc., 10, 22 (1965). (16) L. K. Hiller, Jr., Ph.D. Thesis, University of North Carolina, Chapel Hill, N. C., 1966.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
1655
Table I. Half Lives for Tautomeric Equilibration of l.OmM Acetylacetone in Acetonitrile Ill?
t112
(enol-. keto) 6-7 min
(keto -, enol) 4-5 min
