Hammett Correlations in the 1H NMR Spectra of Some N-Arlydihalonicotinamides

Journal of the Arkansas Academy of Science Volume 46 Article 11 1992 Hammett Correlations in the 1H NMR Spectra of Some N-Arlydihalonicotinamides Frank L. Setliff University of Arkansas at Little Rock Nikhil G. Soman University of Arkansas at Little Rock Jody Z. Caldwell University of Arkansas at Little Rock Debra L. Rogers 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 Setliff, Frank L.; Soman, Nikhil G.; Caldwell, Jody Z.; and Rogers, Debra L. (1992) "Hammett Correlations in the 1H NMR Spectra of Some N-Arlydihalonicotinamides," Journal of the Arkansas Academy of Science: Vol. 46 , Article 11. Available at: http://scholarworks.uark.edu/jaas/vol46/iss1/11 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. 46 [1992], Art. 11 FRANK L.SETUFF, NIKHILG. SOMAN, JODY Z. CALDWELL and DEBRA L.ROGERS Department of Chemistry University of Arkansas at Little Rock LittleRock, AR72204 ABSTRACT Excellent linear correlations of amide proton chemical shifts (S^h) (> n DMSO-d6) with Hammett substituent constants (5) for a series of 4-substituted anilides of four dihalonicotinic acid systems were observed. Dihalonicotinanilides with chlorine in the pyridine 2 position exhibited a more positive slope in a Hammett plot of S^H vs a R- where R is the substituent in the 4 position of the benzene ring. This observation is explained in terms of the inductive effect of chlorine which results in a slightly more acidic amide proton, which in turn causes an enhanced hydrogen bonding tendency to solvent. Four disubstituted anilides were also prepared, and the 5NH of these derivatives correlated well with the additive value of the 6R 's of the two substituents. - INTRODUCTION The Hammett equation (Jaffe, 1953) has been used for many years to assess the electronic effects ofsubtituents (R) through an aryl system on a reaction site (Y). Substituents located in the 2- position with respect to Y chloro- and S-bromo-2-chloronicotinic acid (Setliff and Caldwell, 1991) and of 2,6-dichloro- and 5,6-dichloronicotinic acid (Setliff and Soman, 1992). These compounds correspond to amide series IV, III,Iand IIas depicted in Figure 1. The R substituents are designated in Table 1 by letters. Having available such a closely related series of compounds we reasoned that there might be noticeable trends in their 1H NMR spectra R are normally not studied to avoid any complication by steric effects. The equation normally takes the form: logk = poR + log k o where k is the rate constant of the reaction being studied with different R substituents present, and ko is the corresponding rate constant of the unsubstituted compound (R = H). The substituent constant Or is a value determined from a standard reaction and is characteristic of the nature and position of the substituent. Ifthe substituent is electron withdrawing o~r has a value > 0, and a value < 0 indicates an electron donating substituent. The reaction constant p is characteristic of a given reaction and denotes the sensitivity of the reaction to substituent effects. Ithas a positive value if the reaction is enhanced by electron withdrawing substituents, and a negative value ifthe reaction is facilitated by electron releasing groups. Thus, insight into the polar nature of the transition state of the rate controlling step of a reaction may be obtained. A value for p may be determined graphically by plotting log k vs. the known value for Or and calculating the slope of the line. There have been many successful as well as unsuccessful attempts to correlate properties other than reaction rates with the Hammett Or values (Kxncr, 1988). Properties such as ultraviolet and infrared absorption frequencies and intensities as well as biological activities have met with • • xTwn J !.u i itt i only limitedjsuccess. However, correlations ofriff 'H NMR data with substituent constants have generally proved more rewarding (Ewing, 1978). Mostrelevant to this study is the reported correlation ofsubstituent effects in a series of substituted acetanilides and phenylureas. (Giffney and O'Connor, 1975). Figure 1. Dihalonicotinamide Systems and their Hammett 1H NMR Correleation Equations. Table 1. Substituent Constants and Amide Proton chemical Shifts (S^h ppm) for the four Dihalonictinamide Series (Fig. 1) *jih zi Bar !•¦ «™ izz aarlaa zv H 0.0 10. Ct 10 10.70 10.53 OCH, 3 -0.2a 10.53 10.43 10.it 10.41 CH3 -0.14 10 io.4« io.« 10.45 10.75 10.5* 10.77 10. S r h. Published by Arkansas Academy of Science, 1992 'mr z ««ri«a b. RESULTS ANDDISCUSSION 72 'm ¦•ri«« C£Kl f. We have reported previously the preparation and complete structural characterization of several N-(4-substituted pheny Damides of5-bromo-6- „ K Br o.aa io.S3 io.es lo.ts 10.(4 COCH, 0.47 11.02 10. a2 11.04 10. ¦ C» 0.53 11. Ot 10. •< MO, 0.S1 11.30 11.04 11.31 11.04 och a ch -o.2« 10.55 10.40 3 ' Proceedings Arkansas Academy of Science, Vol. 46, 1992 72 Journal of the Arkansas Academy of Science, Vol. 46 [1992], Art. 11 which could be related to transmission of electronic effects through the benzene and/or pyridine ring. Inthis regard we were able to correlate the amide proton chemical shifts of the R-substituted Naryldihalonicotinamides within all four dihaloamide series with the standard Hammett Or values. Chemical shift1, values (8NH) and Or values (Exner, 1988) are summarized in Table and the excellent linear correlations are shown in Figures 2 and 3. Results ofthe linear regression analysis of these data are summarized in Figure 1 together with the correlation equation in slope intercept form. The slope of the line is interpreted as the Hammett p value. sensitivity to the transmission of electron density by R groups in the 4position of the benzene ring. Thus, in the Hammett context, this greater sensitivity should predict a p value (slope) of greater magnitude. In cases where there are more than one substituent on a benzene ring operating on a reaction center, the effects of the substituents may be generally shown to be additive (Exner, 1988). In order to test our model for additive substituent effects we prepared the N-(3-chloro-4methoxyphenyl)dihalonicotinamides HIj and IVj,and the N-(4-bromo-3- methylphenyl)-dihalonicotinamides mk (...truncated)