The equilibrium constants for ring-chain tautomerism in a series of 8-aroyl-1 -naphthoic acids in dioxan have been determined by an i.r. spectroscopic method and have been correlated with the substituent constants cI and cRo using a modified Hammett equation. The observed pK, values, which have been measured in 80% (w/w) 2-methoxyethanol-water, have been corrected for these tautomeric equilibria to give the true pKaT value. The effect of substitution on the latter values have been assessed using the Hammett equation. The equilibrium constants for ring-chain tautomerism in a series of 8-acyl-1 -naphthoic acids have been determined by using the observed pK, and estimated true pKaT values in 80% (w/w) 2-methoxyethanol-water. These equilibria constants have been correlated with steric substituent constants €, . The ' cis ' and ' trans ' conformers of methyl 8-aroyl-and 8-acyl-1 -naphthoates has been detected by i.r. spectroscopy and the carbonyl stretching frequencies of the ester carbonyl groups have been assigned. The results are related to the polar, resonance, and steric effects of the substituents.IT has been suggested by Lansbury that all 8-aroyl-and 8-acyl-l-naphthoic acids may exist in solution as ringchain tautomeric equilibria, in which the ring form predominates. An early study 3 investigated the structure of 8-benzoyl-l-naphthoic acid and deduced that in solution the acid was very largely, if not completely, in the ring form. Examinations4j5 of the reactions of 8-formyl-and 8-acetyl-l-naphthoic acids (as well as for 5-formyl-4-phenanthroic acid 677) found their reactivity
The equilibrium constants for ring-chain tautomerism in a series of cis-3-acylacrylic acids have been determined by using the observed pK, and true pK, (pKST) values in 80% (w/w) 2-methoxyethanol-water. The latter were estimated from the relation between pK, values of a series of cis-and rrans-3-substituted acrylic acids. The results are discussed.EARLIER investigations 2-4 demonstrated the occurrence of ring-chain tautomerism in cis-3-acylacrylic acids, (I). and ( 11), as shown in equation (1). No quantitative studies of these equilibria appear to have been madeRecently, quantitative investigations of this tautornerism in 2-benzoylbenzoic 596 and cis-3-aroylacrylic acids have been described. The qualitative evidence 2-4 regarding the cis-3-acylacrylic acids indicates that these compounds exist predominantly as the ring isomer. The equilibrium constants, Kc, are given by the relation (2).
Ke = aRinglaChain(2) The observed pK, can be related to the true pK,T by the relation (3). If a reliable estimate of pKaT can be P K T = pKalog (Ke + 1)(3) made, K , can be found by measuring the observed pK,.In the present study, we have investigated the ionisation of a series of cis-and trans-3-substituted
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