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Kinetics and Mechanism of Dehydrochlorination ofN-Aryl 2-Oxo-2-phenylaminoethanehydrazonoyl Chloridesand their Mass Spectra

 

作者: Ahmad S. Shawali,  

 

期刊: Journal of Chemical Research, Synopses  (RSC Available online 1997)
卷期: Volume 0, issue 8  

页码: 268-269

 

ISSN:0308-2342

 

年代: 1997

 

DOI:10.1039/a608117g

 

出版商: RSC

 

数据来源: RSC

 

摘要:

PhNHCOC Cl N NHC6H4X PhNHCOC NNC6H4X + – N N N N PhNHCO C6H4X C6H4X CONHPh + PhNHCOCONHNHC6H4X H2O –HCl 2 3 4 a X = 4-MeO b X = 4-Me c X = H d X = 4-Cl e X = 3-Cl f X = 4-EtOCO g X = 3-NO2 h X = MeCO i X = 4-NO2 1 PhNHCOC Cl NNHC6H4X + Et3N PhNHCOC Cl NNHC6H4X + Et3NH + Ka (fast) PhNHCOC Cl NNC6H4X PhNHCOC NNC6H4X + Cl– + – k1 (slow) PhNHCOC NNC6H4X + Cl– + – – – 268 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 268–269 J. Chem. Research (M), 1997, 1870–1881 Kinetics and Mechanism of Dehydrochlorination of N-Aryl 2-Oxo-2-phenylaminoethanehydrazonoyl Chlorides and their Mass Spectra Ahmad S.Shawali,* Nehal M. Elwan and Ahmad M. Awad† Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt The dehydrochlorination mechanism of the title compounds in solution and inside a mass spectrometer is studied. A number of studies on the chemistry of N-aryl 2-oxo-2- phenylaminoethanehydrazonoyl chlorides 1 have been reported in the literature,1,2 However, the kinetics of the base catalysed dehydrochlorination of such compounds have not yet been reported. We now report the kinetics and mechanism of such a process.In addition, the mass spectra of 1 were also studied to compare their unimolecular reactions in the mass spectrometer to those that would occur under normal solution conditions. A series of compounds 1a–i were prepared by reported methods.3 The structures of the new derivatives 1a and 1h were substantiated by both elemental and spectral (IR, 1H NMR and mass) analyses.Treatment of 1 with triethylamine in 1,4-dioxane–water (4:1 v/v) at 25 °C gave a mixture of the corresponding oxanilic hydrazide 3 and 1,4-diaryl-1,2,4,5- tetrazine 4. Such products are undoubtedly formed via the addition of water to the nitrilium imide 2 and by a headto- tail dimerization of 2 which is generated in situ by the action of triethylamine on 1 (Scheme 1). The kinetics of the formation of 2 from 1 were carried out under pseudo-first order conditions by keeping an excess (Å10 or greater) of triethylamine over 1 in 1,4-dioxane–water (4:1 v/v) at 25�0.1 °C with an ionic strength of 0.1.The reactions were followed potentiometrically up to 90% reaction extent by monitoring the increase in the chloride ion concentration using an ion selective electrode. The ionic strength was maintained at 0.1 in all the kinetic runs by the addition of the appropriate volume of sodium nitrate solution (5 M) in the same solvent system.The pseudo-first-order rate constant, k0, was computed from the linear (r2a0.990) least squares plot of log (Cl µCt) vs. time where Cl and Ct are the concentrations of the chloride liberated at infinite time and time t, respectively. The second-order rate constant, k2, was determined from the relation k2=k0/[Et3N]. In all cases, the plots of k0 vs. [Et3N] have zero intercepts indicating that the uncatalysed reaction is negligible under the reaction conditions employed.The second-order rate constant, k2, gave an excellent correlation with the substituent constant sµx . The equation of the regression line is: log k2=0.37sµx µ0.77; where r=0.940 and s=�0.09. A mechanism compatible with the small r2 value (0.37) is presented in Scheme 2. According to this suggested mechanism, it can be shown that k2=Kak1 and in turn the overall reaction constant r2 will be the algebraic sum of ra and r1. These two reaction constants are expected to have opposite algebraic signs, ra being positive whereas r1 being negative.Such opposite effects would thus lead to a small r2. This conclusion was substantiated by measuring the acid dissociation constants, pKa, of the related hydrazone series 5a–i in 1,4-dioxane–water (4:1 v/v) at 25�0.1 °C and ionic strength of 0.1. The choice of 5 was based on the similarity of the inductive effects of the CN and Cl groups (sI values of CN and Cl are 0.60 and 0.47, respectively).5 PhNHCOC(CN)�NNHC6H4X PhC(Br)�NNHC6H4X 5 6 PhC(Cl)�NC6H4X 7 The pKa data of 5a–i also showed an excellent correlation with sx.The equation of the regression line is: log Ka=1.18sx µµ0.87 where r=0.996 and s=�0.01. Substitution of the values of ra (1.18) and r2 (0.37) in the equation r2=ra+r1 gives a r1 value of µ0.81 which compares favourably with that (µ0.63) reported for the heterolysis of the C·Br bond of N-aryl benzenecarbohydrazonoyl bromides 6 in the same solvent system and at the same temperature.7 Furthermore, a comparison of the value of r1 (µ0.81) obtained in this work with that reported for the heterolysis of the C·Cl bond in the imidoyl chlorides 7 (r7=µ2.75) indicates that the transmission factor pp=(r1/r7) of the substituent effects for the trivalent anionic nitrogen bridge is 0.29.This small efficiency of anionic nitrogen to transmit substituent effects indicates that the changes in Ka are dominant in the studied reaction, that is the changes in k2 (=Kak1) are mainly due to changes in Ka.*To receive any correspondence. †Abstracted from the MSc thesis of A. M. Awad, University of Cairo, 1995. Scheme 1 Scheme 2PhNHCOC Cl N PhNHCOC NNHAr –H• –HCl NHAr + + PhNHCOC NNHAr + –Cl [M]+ [M–HCl]+ –PhNCO HC NNAr + –PhNCO ArN + [M–Cl]+ –PhNCO HC NNHAr + –HCN ArNH + . J. CHEM. RESEARCH (S), 1997 269 The mass spectra show that the molecular ions of 1 seem to undergo fragmentation via the two possible routes outlined in Scheme 3.The data indicate that the ions [MµCl]+ are considerably less abundant than [MµHCl]+ indicating that the formation of the latter is more favourable than the formation of the former azocarbocation. Furthermore, the data reveal that the abundance of the [MµHCl]+, which formally corresponds to the nitrilium imide seems to depend on the nature of the substituent of the N-aryl of the hydrazone moiety which is increased by electron-withdrawing substituents.Techniques used: Potentiometry, spectrophotometry, correlation analysis, mass spectrometry References: 12 Fig. 1: Plots of k2 for the base catalysed dehydrochlorination of 1a–i and pkas for the acid ionization of 5a–i against the substituent constant sx Table 1: Melting points and spectral data of the new compounds 1a,h,4c and 5f,h Table 2: Second-order rate constantss, k2, for dehydrochlorination of 1a–i and acid dissociation constants, pKa, of 5a–i in 1,4-dioxane– water (4:1 v/v) at 25 °C and ionic strength of 0.1 Table 3: The principal peaks in the mass spectra of the hydrazonoyl chlorides 1a–i Received, 2nd December 1996; Accepted, 18th April 1997 Paper E/6/08117G References cited in this synopsis 1 A. S. Shawali, Chem. Rev., 1993, 93, 2731. 2 A. S. Shawali and C. Parkanyi, J. Heterocycl. Chem., 1980, 17, 883. 3 A. S. Shawali and A. Osman, Tetrahedron, 1971, 27, 2517. 5 S. Y. Hong and J. E. Baldwin, Tetrahedron, 1965, 24, 3787. 7 F. L. Scott, M. Cashman and A. F. Hegarty, J. Chem. Soc., B, 1971, 1607. 8 A. F. Hegarty, J. D. Cronin and F. L. Scott, J. Chem. Soc., Perkin Trans. 2, 1975,

 



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