:‘i 0
a
0
4
v)
w
0”
I I I 100 200 300 400 500 AVERAGE MOLECULAR WEIGHT
1
i 600
Figure 2. Oxidation of monoaromatic petroleum concentrates
excellent yield is obtained from npropylbenzene. sec-Butylbenzene gives quantitatively 1 mole of acid per mole of sample, and even in the case of neicosylbenzene, 0.84 mole of acid per inole of sample is obtained. The oxidation of polysubstituted and fused ring monoaromatic systems, more typical of the heavier monoaroniatic types found in petroleum concentrates, was also investigated. The moles of acetic acid found, per mole of sample, were: mesitylene, 2.15; Tetralin, 0.02; 6-methyltctmlin, 0.69. Both mesitylene and 6-methyltetralin give significant yields of acetic acid, cvcn though the methyl groups present arc attached to the aromatic ring. This acetic acid, as well as the small amounts of acid obtained from toluene, ethylbenzene, and isopropylbenzene (Table 11), has been shown to arise from destruction of the aromatic ring system ( 1 ) . Although ring oxidation occurs in polyalkylatcd systems to give a lessthan-quantitative yield of acetic acid, the 707G yield, based on ring oxidation, from mesitylene and 6-methyltetralin is comparable t o that obtained from the higher-molecular-weight monoalkylbenzenw-for example, n-eicosylbmzcnc. The occurrence of both ring and side-chain ohidation in an unknown alkylbcnzene should therefore provide a minimum value (70 to 85%) for the total number of methyl groups present, whether on the side chain or attached to the aromatic ring. Application to Monoaromatic Petroleum Fractions. T h e results obtained from a series of monoaiomatic petroleum fractions separated b y a combination of molecular distillation and chromatographic techniques are given in Table 111. T h e number of moles of acetic acid obtained per mole of sample is shown
as a function of molecular weight in Figure 2. h systematic increase in the number of methyl groups per molecule with increasing molecular weight is observed. On the basis of a 70 to 85% yield of acetic acid per methyl group present, this increase is equivalent to one methyl group for each 10 to 12 additional carbon atoms. In a series of high-molecular-weight monoaroniatic petroleum concentrates, the aromatic ring would be expected to be highly substituted, increasing molecular weight being due to increased length of the principal side chain rather than increased degree of ring substitution (4). Accordingly, any increase in acetic acid found a t higher molecular weights should be due to increased side-chain branching. The data of Table I11 indicate that the extent of chain branching with increasing chain length is rather limited, the average structure being consistent either with minor branching in a single sidechain type, or n i t h a mixture of sidechain types in which unbranched side chains predominate.
ACKNOWLEDGMENT
The authors are indebted to W. W. Hargrove for critical preliminary experiments and to J. 11. Xartin, Jr., for the ultraviolet analyses. They also acknowledge the helpful comments of hf. J. O’Neal. LITERATURE CITED
(1) Brandenberger, S. G., Dvoretzky, I., Division of Organic Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. (2) Eisenbraun, E. J., McElvain, S. ?VI., Aycock, B. F., J . Ani. Chem. SOC. 76, 607 (1954). 13) Ginger. L. G.. J . B i d . Chem. 156, 452 (i944). (4) Hood, A,, Clerc, R. J., O’Kertl, 11.J., J . Inst. Petrol. 45, 168 (1959). ( 5 ) Jurecek, &I., Soncek, >I., Churacek, J., Renger, F., Z. a n d . Chem. 165, 109 (1958). (6) Kirsten, IT7., Stcnhagen, E., Acta Chem. Scand. 6, 682 (1952). (7) Kuhn, R ,Roth, H., Ber. deut. chem. Ges. 66, 1274 (1933). (8) llarvel, C. S., Rands, R. D., J . Am. Chem. SOC.72, 2642 (1950). (9) O’Seal. 11. ,J., TTier, T. P., AXAL. ’CHEM. 23.830 11951) (10) Tashinian, T-. H., Baker, 11. J., Koch, C. IT.,Ibzd , 28, 1304 (1956). (11) Zbinovskv, IT.,Burris, R . I€., Ibid., 26,208 (1953). I
Table
II. Oxidation of Monoalkylbenzenes
Acetic Acid, Moles/11oIe of Sample Compound Found Expected Toluene 0 17 0.00 Ethylbenzene 0 09 0.00 Isopropylbenzene 0 14 0 00 n-Propylbenzene 0 90 1 00 n-Butylbenzene 1 01 1 00 sec-Butylbenzene 0 98 1 00 n-Heptylbenzene 0 88 loo n-Eicosylbenzene 0 84 1 00
-
\
RECEIYEDfor review July 22, 1960. Accepted Kovember 21, 1960. Presented before the Division of Organic Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Correct ion Table 111. Oxidation of Monoaromatic Petroleum Concentrates
Sample Source East Texas
Carbon So.
Rangea 14-16 20-24
...
TTest Texas
13-28 15-333 19-39 23-47 26-54
Acetic Acid Found, Av. Xoles/ Mol. Mole K t . * Sample 190 1.64 300 2 20 390 2 84 257 2 07 310 2 40 357 2 61 480 3 22 560 3 55
West TexasEllenberger 27-33 390 2 59 a Determined from mass spectrometric carbon number distribution (9). * Determined ebullioscopically.
The Graphite Electrode: An improved Technique for Voltammetry and Chronapotentiometry In this article by P. J. Elving and D. L. Smith [ANAL. CHEM. 32, 1849 (1960)], the designations of the two sets of curves in Figure 3 should be reversed -i.e., the curyes in acetate solution should be indicated as D , E, and F , respectively, and those in potassium chloride solution should be designated as -4, B, and C, respectively. The subhead in the left column, page 1853, should read Voltammetric Analysis instead of Volumetric Analysis.
VOL. 33, NO. 3, MARCH 1961
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