Decomposition of Short-Chain Dicarboxylic Acid Esters during


Decomposition of Short-Chain Dicarboxylic Acid Esters during Separation on Polyester Gas Chromatography Media. Table I. Proportion of Methyl EstersĀ ...
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Decomposition of Short-Chain Dicarboxylic Acid Esters during Separation on Polyester Gas Chromatography Media SIR: T h e osidative fission of polyethylenic fatty acids may yield a wide range of dicarboxylic acids which can be a n n l y z ~ dconwniently by gas chroniatography of the methyl epters. Studies in this laboratory arc being carried out with the hon-,ologous scxries oxalic acid to azc,laic acid. Initial analyses carried out on gas-liquid chromatography columns employing silicone grease as the partition mcdium were satisfactory, hut whcn polyestw media n-ere em1 1 l o ~ xthe I two l o w s t nipmbers of the 'cries failed to give quantitatiw results. This rcvelat'ion is of extreme import'ance in determining the structure of polyctlij,lcnic fatty a d s , \\-hiell may yield :I high proportion of these particular tlicmboxylic acids on osidative fission, :ind may also apply to Linalyses of other strong organic acids. 'The problem was studied initially with n known mixturc of dimethyl malonate, climethy1 succinate, and methyl undecanoate, the latter serving as a stable standard. The apparatus was a I'odbielniak Chromacon, fit,ted with two caolunins. One was 1 / 4 inch in outside diameter, 10 feet in length, and packed vith 307, Craig succinate-butanediol polyester on 60- to SO-mesh Chromasorb. The other \VIS 5/16 inch in outside c!ianieter, 8 fect in length, and packed ivith 307, Don--Corning silicone grease on 40- t o 60-mesh Chromasorb. The curier gas v a s hydrogen and the flow was adjusted where possible t o keep t h r rctent'ion times similar a t various tcrnperatures. The results (Table I) show that the proportion of dimethyl malonate declined rapidly with increasing teniperat'iire when the polyester column was employed, but remained virtually constant on the silicone grease column. The ratio of dimethyl succinate t o methyl undecanoate remained relatively constant on the polyester column, despite the longer retention time of t'hc dimethyl succinat'e compared to dimethyl

Table I.

Proportion of Methyl Esters Unchanged by Area (Uncorrected)

Temp., "C.

a

Polyester Columna Silicone Grease C o l u i c Succinate Malonate Succinate IIalonate 0.71 0.63 0 78 140 0.98 0 86 0 .i B 0.40 0.93 1.50 0 90 0.70 0.22 0.92 160 0 90 0 70 0.11 0.86 1i0 0.02 0.87 180 0.00 0.84 190 Value for undecanoate \VAS 1.00 at all temperatures listed.

malonate, but the slight decrease may indicate some reaction. Dimethyl oxalate also shoJved appreciable decomposition, but as the retention time is short the results were less noticeable. Ethyl esters of oxalic and malonic acids decompose readily under the same conditions on polyester media. As the proportion of dimethyl malonate passing through the polyester column declined, a number of illdefined peaks appeared near the injection point. When nitrogm was the cariier gas, severe negative deflections appeared in this region, indicating the presence of polar compounds. Use of a l/r-inch column, 10 feet in length, and packed nifh 10% LAC-3-R-728 polyester (succinate-diethylene glycol), did not significantly alter the proportion of oxalic and malonic esters decomposed, hut did give much better resolution of the decomposition product peaks. Tentative identifications based on retention values indicated the presence of carbon dioxide, ethyl alcohol, and acetic acid in the case of diethyl malonate, and the presence of carbon dioxide, methanol, and acetic acid with dimethyl malonate. X number of other peaks could not readily be identified. Oxalic esters showed peaks corresponding t o carbon dioxide and the alcohol, but formic acid could not be identified with certainty. Methyl esters of weaker long-chain aliphatic acids interact n i t h polyester

partition media a t somewhat higher temperatures (S), while the polyesters themselves may also be unstable (1). Because oxalic and malonic esters are thermally stable, the initial step is probably interaction with the polyester with liberation of the alcohol and the free acid, the latter then undergoing thermal decomposition t o give carbon dioxide and a monobasic acid ( 2 ) . The possibility that the copper tubing employed in the columns caused this reaction was eliminated when stainless steel tubing packed with polyester mpdia gave similar results. Thermal degradation of a poll ethylene glycol partition medium has recently been reported to give misleading analytical results through interaction of alcohol samples with the formic acid ix-oduced (4). LITERATURE CITED

(1) Fulco, A. J., Mead, J. F., J . Bid. Chem. 234, 1411 (1959). (2) Karrer, P., "Organic Chemistry," p. 264, Elsevier, New York, 1947. (3) Lipsky, S. R., Landowne, R. -4., Godet, M. R., Biochim. et Biophys. Acta 31, 336 (1959). (4) Weurman, C., Dhont, J., Nature 184, 1480 (1959). R. G. ACKMAN 31. A. BAKNERNIAN F. -4. VANDESHEUVEL Fisheries Research Board of Canada Technology Station Halifax, S o v a Scotia, Canada

Spectrophotometric Determination of Azide with Ferric Perchlorate SIR: -1 colorimetric determination of azide was first attempted by Labruto and Rantlisi ( 2 ) . based on the reaction of ferric chloride with a dilute solution of the azide. Standard solutions were prepared bj- distilling hydrazoic acid, formed from the reaction of sulfuric acid on R known quantity of

sodium azide, into a ferric chloride solution. A method recently developed by Roberson and Austin (4) involves essentially the same technique, except t h a t ferric nitrate solution is used. The method presented describes a direct determination of azide with ferric perchlorate, which allows for the

determination of a larger concentration of azide and eliminates the distillation procedure. Ferric perchlorate has the added advantage that it will not precipitate lead or barium from the corresponding azide salts. The procedure involves measuring the ferric azide complex a t a wave length of 454 VOL. 32, NO. 9, AUGUST 1960

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