The Polarography of Quaternary . Ammonium Lompounds A
A
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BURNETT C. SOUTHWORTH1 and ROBERT OSTERYOUNG2 Rensselaer Polytechnic Institute, Troy, N. Y. KENNETH D. FLEISCHER and FREDERICK C. NACHOD Sterling- Winthrop Research Institufe, Rensselaer, N. Y.
b The products of electrolytic reduction for several representative quaternary ammonium compounds have been identified. On the basis of the products of electrolysis, two types of reaction become evident. In the first type, simple aliphatic quaternaries, the quaternary ion acquires an electron to form a neutral entity. This entity subsequently reacts with water to liberate hydrogen and form the quaternary hydroxide. In the second type of reaction, the neutral entity may break down into a tertiary amine and a hydrocarbon or two molecules may rearrange to form a bis compound. This type of reaction i s found in those compounds having either an aromatic or a pyridinium group, respectively. Any reasonable polarographic procedure for the analysis of quaternary ammonium compounds i s limited to those quaternaries whose reduction potentials are less negative than -1.9 volts. At more negative poteniials the solution travels up inside the capillary of the dropping mercury electrode to cause large irregular current levels. The effects of this unique situation are discussed and a hypothesis i s advanced to relate the high polarographic waves observed to this phenomenon.
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after Smith ( I S ) established the fact that a neutral ammonium configuration can exist as an amalgam, McCoy and Moore (9, 10) demonstrated that a tetramethylammonium ion can also form a neutral configuration in a n amalgam. These authors electrolyzed an alcoholic solution of tetramethylammonium chloride a t a mercury cathode a t -34" C. A stable amalgam was formed which appeared as silverwhite granules which were only slightly soluble in mercury. Other phases were thought to exist but were not isolated. These granules reacted violently HORTLT
Present address, Sterling-Winthrop Research Institute, Rensselaer, N . Y . 2 Present address, Atomics International, Canoga Park, Calif.
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ANALYTICAL CHEMISTRY
with water to form the quaternary hydroxide, hydrogen, and finely dispersed mercury. If the amalgam were allowed to warm, i t slowly decomposed into trimethylamine, methane, and ethylene. Other amines and quaternary ammonium compounds were studied but only monomethylamine hydrochloride was found to form a stable amalgam. Schluback (11, 12) studied the electrolysis of tetramethyl- and tetraethylammonium iodides in liquid ammonia. He reported that their behavior was similar to that of the alkali metals in that typical blue "schlieren" were formed. The starting material could be regenerated by treatment with iodine. When an electrolyzed solution of tetraethylammonium iodide was alloned to stand, the neutral quaternary molecule decomposed into triethylamine and butane. If the quaternary compound contains certain aromatic groups, its reduction potential is sufficiently lor5 that reduction may proceed a t electrodes other than the mercury. Emmert ( I , 2, 3) was able to reduce quaternary coinpounds containing the brnzyl or pyridinium groups a t lead or platinum cathodes. He reported the formation of toluene from the bmzyl group (plus a tertiary amine). Pyridinium compounds formed bis compounds a t the 4-position. Finkelstein el al. (4, using platinum or aluminum cathodes, electrolyzed conipounds containing benzyl, allyl, cinnaniyl, and other groups with analogous results. (The benzyl group yields toluene, the allyl group yields propene, etc.) I n nonaqueous systems, the benzyl radical forms dibenzyl rather than toluene. The extremely high current levcls found in the polarograms of quaternary ammonium compounds were notcd by von Stackelberg and Stracke (IC-) 11ho suggested that hydrogen n avcs are catalyzed by these compounds. Iofa et al. (6) stated that the quateinary ammonium compound serves onlj- to lower the water overvoltage on mercury. Referring to the polarography of aliphatic compounds, van Rysselberghe and LIcGee (14) postulated thc de-
composition of the quaternary into a tertiary amine and a hydrocarbon. Although this is true of a quaternary compound containing an aromatic group as mentioned above, little or no amine can be detected at the conclusion of an electrolysis of an aliphatic compound. On the basis of the large pseudocapacity exhibited by the reaction, Grahame (6) has postulated the reaction:
+ e-- -+
(CH3)rN+
Hg
(CH34N(Hg)
This mechanism is supported by the work of McCoy and Moore and also Schluback. APPARATUS AND REAGENTS
Polarograms were recorded on a Sargent hlodcl XXI Polarograph. A modified H cell was employed, thc reference electrode (saturated calomel electrode) being separated from the cathode compartnicnt by a section isolated by glass frits and filled with the supporting electrolyte. The dropping mercury electrode was constructed of marine barometer tubing 2 feet in length. l17ith the height of tlie mercury column a t i o em. the drop time in 0.1X tetra-n-butylammo~iiumhydroxide varied from 10.3 seconds a t 0 volt to 5.55 seconds a t -2.0 volts to 0.63 second a t 2.9 volts. The value of m was 1.067 mg. per second. All measurements were made a t 25" i 1' C. For observation of the diopping inercury electrode a Bausch & Lomb TriSimplex micro projector (Cat. KO. 3263-59) nas used. A. 1OX l m s was used to project the image of the drop and capillary on a white screen a t a distance of about 2 feet. ~Iacroelectrol~-scs n ere c-onductcd in cells having a nicrcury pool cathode and platinum gauze anode either in the same compartment (cell 1) or separated by a very fine glass frit (cell 2). The solution was purged tontinuously m-ith nitrogen. Gaseous or volatilc products of the clectrolmis nere trapped i n a suitable medium such as a gas buret or boric acid solution for subsequent analysis. The cathode potential was measured os. a Beckman saturated calomel microelectrode. The quantity of electricity passed was measured with a Leeds & Korthrup coulometer (constant current method) or an Clectromethods, Ltd.,
acetic acid. Calculate the result as a neutralization equivalent ( N E.). This method has a n accuracy of about 1%. HYDROHALIDE SALT. Dissolve a weighed sample in 5 ml. of anhydrous dimethylformamide Add a drop of thymolphthalein indicator and deaerate the solution with nitrogen from which traces of carbon dioxide and water have been removed with Ascarite and Anhydrone. Titrate the solution to a blue end point with 0.OlM lithium methoside or tetrabutylammonium hydroside in a benzene-methanol solution. If the amount of hydrohalide salt present is greater than O.l%, subject the compound to further purification. Other reagents used were common laboratory itcms. If available, ACS reagent grade compounds were used. Otherwise, purified grades were used with caution.
integrating motor Type 913 (constant potential method). The source of direct current n-as either the Leeds &: Northrup coulometer or a Heathkit Model PS 3 power supply. Reagents. T h e technique of preparation and yuiification of quaternary ammonium compounds need not be discussed except that estreme care should be cvrcised to exclude any trace of aminc in the final product. The matcrials uscd are given in Table I. The melting points reported were obtained according to the specifications of the Unitzd States Pharmacopeia. To characterize the identity and purity of the rpagents three tests were used : FREE AMINE. Weigh slightly less than a milliequivalent of the quaternary ammoniuin compound and dissolve it in about 10 ml. of glacial acetic acid. Add a drop of crystal violet indicator and titrate the solution to a green end point u
