Electrochemistry of Dihalogenated Nicotonic Acids in Aqueous and Aprotic Media

Journal of the Arkansas Academy of Science Volume 40 Article 20 1986 Electrochemistry of Dihalogenated Nicotonic Acids in Aqueous and Aprotic Media Ali U. Shaikh University of Arkansas at Little Rock Tappi A. Dixon University of Arkansas at Little Rock Lourie A. Battles University of Arkansas at Little Rock Frank L. Setliff University of Arkansas at Little Rock Follow this and additional works at: http://scholarworks.uark.edu/jaas Part of the Physical Chemistry Commons Recommended Citation Shaikh, Ali U.; Dixon, Tappi A.; Battles, Lourie A.; and Setliff, Frank L. (1986) "Electrochemistry of Dihalogenated Nicotonic Acids in Aqueous and Aprotic Media," Journal of the Arkansas Academy of Science: Vol. 40 , Article 20. Available at: http://scholarworks.uark.edu/jaas/vol40/iss1/20 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact , . Journal of the Arkansas Academy of Science, Vol. 40 [1986], Art. 20 ELECTROCHEMISTRY OF DIHALOGENATED NICOTINIC ACIDS IN AQUEOUS AND APROTIC MEDIA ALI U. SHAIKH, TAPPI A. DIXON, LOURIE A. BATTLES and FRANK L. SETLIFF Department of Chemistry University of Arkansas at Little Rock Little Rock, AR 72204 ABSTRACT The electrochemical reduction ofseveral 2,5- and 5,6- dihalonicotinic acids have been studied in dimethyl sulfoxide as well as in aqueous buffers of different pH. The polarographic half-wave potentials for the reduction of these compounds in both media are reported here. The compounds appear to reduce at the carboxyl group. The presence of halogen atoms on the pyridine ring facilitates reduction. INTRODUCTION Ifhe synthesis and spectroscopic (IR and NMR) characterization of eral 2,5- and 5,6- dihalonicotinic acids have been reported earlier tliff, 1970, 1972, 1973, 1976 and 1978). Because of the chemical lilarity with vitamin B (niacin), these compounds are of important logical significance. Since biological activities ofchemical agents are erally believed to occur via oxidation-reduction mechanism, it is jntialthat the redox properties of these compounds be determined >rder to understand the molecular basis ofsuch activities. No electromical studies ofnicotinic acid and its halogen derivatives have yet n reported. We have therefore undertaken the task of determining polarographic half-wave potentials for the reduction of these commds in both protic and aprotic media. In this report, the results obled in dimethyl sulfoxide and in aqueous buffers are presented. Amplitude = 25m V and Drop Time ¦ 1.0 sec. During potential scan in the negative direction (reduction), the solution was quiescent and the argon flow was diverted above the solution to keep atmospheric oxygen and moisture away. To the solution, about five mg of a particular compound was then added, stirred to dissolve completely, and the polarogram was taken under the same condition as above. The procedure was repeated for other compounds. RESULTS ANDDISCUSSION The technique ofDifferential Pulse Polarography (DPP) is superior to that of Direct Current Polarography (DCP) for the determination of half-wave potentials due to improvement of signal to noise ratio as a result of discrimination against capacitative current (Bond, 1984). Moreover, polarograms obtained by DPP show Gaussian peaks due to charge-transfer process, compared to sigmoidal curves in DCP, and EXPERIMENTAL gents: Dimethyl sulfoxide (DMSO) and tetrabutylammonium :hlorate (TBAP) were of analytical grade (Fisher Chemicals) and ; used without further purification. Aqueous buffers of various pH :prepared (Carmody, 1961) from analytical grade boric acid, citric !and trisodium phosphate (Fisher Chemicals). The aqueous reacmedia were 0.1M buffer solutions, whereas the aprotic medium a solution of 0.1M TBAP in DMSO, which was dehydrated (comcial TBAP and DMSO contain water) prior to use by passing over c alumnia (Woelm). I The half-wave potentials were determined by DifferenPulse Polarography using a three-electrode polarographic analyzer odel PAR 174A; Princeton Applied Research Corporation, nceton, NJ). The cathode, the counter-electrode and the reference ctrode were dropping mercury electrode (DME), platinum foiland urated calomel electrode (SCE) respectively. In aprotic medium, ,vever, a low-porosity calomel, filled with 0.40M tetraethylammonium oride to adjust voltage to 0.00 volt vs. SCE, was used as reference minimize water-leakage into the medium. The polarograms were orded on an X-Y recorder (Model 2200 Omnigraphic; Houston Injments, Austin, TX). The mercury drop-rate of the DME was conlied by a mechanical drop-timer (Princeton Applied Research Coration). The temperature of the reaction medium was maintained at Ipar Uis: Twenty ml of the solution was poured into the reaction ;1, purged with ultra-high purity argon for thirty minutes to remove )lved oxygen and a differential pulse polarogram of the medium kground) was taken using the followingconditions: Initial Poten= 0.00 volt vs. SCE, Potential Scan Rate = 5 mV/sec, Pulse P:edure: can be evaluated more precisely. In DPP, the half-wave potential (E 1/2), which is characteristic of a particular electroactive compound and is a measure of its ability to be either oxidized or reduced, can be related to the peak potential (Ep) of the polarogram by the equation: E1/2 = E p ± AE/2 ' ' ' -' signs refer to reduction where AE = pulse amplitude and + and and oxidation processes respectively. Figure 1 shows typical polarograms obtained by DPP in DMSO for nicotinic acid and two of its dihalogenated derivatives, namely, 5-chloro-6-iodo- and 6-chloro-5-iodo- nicotinic acids. Each polarogram displays two distinct peaks, indicating that the compounds are undergoing reduction in two steps. A previous report (Lund, 1983) showed that in acetonitrile, an aprotic medium, the electrochemical reduction of pyridine ring is very difficult. The two peaks, therefore, can be attributed to reduction of the carboxyl group. The half-wave potentials obtained for a number of dihalonicotin acids in both protic and aprotic media are shown in Table 1 Whi each compound showed two peaks in DMSO, additional peaks we observed in aqueous media, especially at a lower pH. This may indica that in aqueous media either the protonated nitrogen of the pyridi ring or halogen atoms on the ring are also reduced. The presence halogen atoms on the ring also have profound effect on the half-wa potential for the reduction of carboxyl group (see Figure 1 and Tab 1). Bo (...truncated)