Hammett Correlations of Half-Wave Reduction Potentials in a Series of N-(Aryl substituted)-Dichloronicotinamides

Journal of the Arkansas Academy of Science Volume 48 Article 27 1994 Hammett Correlations of Half-Wave Reduction Potentials in a Series of N-(Aryl substituted)Dichloronicotinamides Cecil C. Persons University of Arkansas at Little Rock Ali U. Shaikh University of Arkansas at Little Rock Julie Shiflett 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 Organic Chemistry Commons Recommended Citation Persons, Cecil C.; Shaikh, Ali U.; Shiflett, Julie; and Setliff, Frank L. (1994) "Hammett Correlations of Half-Wave Reduction Potentials in a Series of N-(Aryl substituted)-Dichloronicotinamides," Journal of the Arkansas Academy of Science: Vol. 48 , Article 27. Available at: http://scholarworks.uark.edu/jaas/vol48/iss1/27 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. 48 [1994], Art. 27 Hammett Correlations of Half-Wave Reduction Potentials in a Series of N-(Arylsubstituted) JMchloronicotinamides Cecil C. Persons, AliU. Shaikh, Julie Shiflett and Frank L.Setliff Department of Chemistry University of Arkansas at Little Rock Little Rock, AR 72204 Abstract Excellent correlations of Hammett substituent constants (a R) of a series of N-(R-substituted aryl) -2,6-, 2,5-, and 5,6dichloronicotinamides with polarographic half-wave potentials were observed. Although the correlations demonstrate that all three series of amides experience comparable sensitivity to the R groups at the carbonyl reduction site, the relative ease of reduction varies according to the chlorine substitution pattern on the pyridine ring. These differences are suggested to be due to combinations of mesomeric, inductive, and field effects which operate differently in the three systems. Correlation analysis also revealed that Hammett heteroatomic replacement constants previously determined by NMR studies are valid in the present polarographic studies. Introduction In connection with our interest in the preparation of dihalonicotinic acids and their derivatives as potential agricultural agents (Setliff and Soman, 1992), we recently reported the excellent correlation of Hammett substituent constants (aR) with the amide proton chemical shift in several series of N- (R- substituted aryl) -dihalonicotinamides (Setliff et al., 1992). From the correlation equations obtained therefrom, we were subsequently able to determine Hammett pyridyl 3-aza and 4-aza replacement constants by measuring the amide proton chemical shifts of several N- (pyridyl substituted) -dihalonicotinamides (Setliff et al., 1993). This paper describes an extension of the study of structural and electronic effects in these systems by the use of polarography. The objectives of this research are as follows: 1) to investigate the electronic effects of R groups upon the half-wave reduction potentials in three series of N-(R-substituted-aryl)dichloronicotinamides, and to determine ifHammett correlations exist, 2) if such correlations exist, to test the validity of the aza replacement constants previously determined by the NMR studies, and 3) To determine the relative reduction ease of the three systems as dictated by the substitution pattern of the chlorine atoms on the pyridine ring. Materials and Methods The preparation of the amides has been described previously (Setliff and Soman, 1992). Samples used had been recrystallized several times and had sharp melting Polarographic reductions were conducted in anhydrous dimethylsulfoxide (DMSO) containing 0.01 M tetrabutylammmonium perchlorate (TBAP) as supporting electrolyte. A three-electrode polarographic analyzer, Model PAR 174 A (Princeton Applied Research Corporation, Princeton, NJ) with a dropping mercury electrode, and a drop-timer was used in conjunction with an X-Y recorder, Model 2200 (Houston Instruments, Austin, TX). The reference electrode was a specially prepared low-porosity calomel electrode (Fisher Scientific, Pittsburgh, PA) filled with a 0.40 M solution of tetraethylammonium chloride in order to adjust the potential to 0.00 volt vs. the saturated calomel electrode (SCE). The counter electrode was a platinum wire. Exactly 10.00 mL of the TBAP/DMSO solution was poured into the electrochemical cell and was purged with dry nitrogen for about 30 min to remove dissolved oxygen. The nitrogen flow was then diverted above the solution, and the potential of the dropping mercury electrode was scanned between 0.00 volt and -2.5 volt vs. the SCE at a rate of 5 mV/sec to obtain the d. c. background polarogram. Approximately 1- 2 mg of the test compound was then added, the solution purged again with nitrogen for 5 min to assure complete dissolution, and the potential was scanned as above. Polarograms of all samples were obtained using this procedure. The half-wave potentials and the number of electrons involved in the reduction were determined from the direct current polarograms using the relationship: E = E1/2 + (0.0591/n)log[(i d i)/i]volts at 25°C where E]/2 is the half-wave potential, n the number of electrons, and id the diffusion current. E and iare the corresponding values of the potential and current at any - points. 130 Published by Arkansas Academy of Science, 1994 130 Journal of the Arkansas Academy of Science, Vol. 48 [1994], Art. 27 stage in the rising portion of the curve. E1/2 and n were calculated by a least squares analysis of data using the above equation. The plots of E!/ 2 vs. GR were done using an Axumlinear regression program available from Trimetrix, Inc., Seattle, WA. Table 2. Hammett Substituent Constants (o"R) and Halfwave Potential Values (E 1/2 ) for the three Dichloronicotinamide Series (Fig. 1). Cpd. d. R O-CH 3 CH3 H F e. P-O-CH3 a. b. Results and Discussion c. The compounds studied are described in Fig. 1 and Table 1, and the half-wave reduction potentials together with the Hammett substituent constants (Exner, 1988) are shown in Table 2. Our calculations revealed a one electron reduction in each case. The excellent linear correlations of E1/2 with aR in all three series are depicted in Fig. 2. Equations of the lines in slope intercept form are represented below with the correlation coefficients shown in parentheses. - E1/2 = 0.176 aR 1.79 E1/2 = 0.161 a R 1.71 E1/2 = 0.164 ct r 1.65 Series I Series II Series III X I(2,6-Series) (...truncated)