Spectroscopic Studies of Keto–enol Equilibria: Part 1. Solvent Effects

Abstract

Solvent effects on the keto-enol equilibria of ethyl acetoacetate, acetylacetone, ethyl cyclopentanone-2-carboxylate, and methyl 4-metliylcyclopentane-1,2-dione-3,4,5-tricarboxylate have been st~idied by ultraviolet spectroscopy. The extent of enolization is mainly determined by the stabilization of the keto form by local association with polar or protondonating solvent molecules, just as in the case of n -+ T* transitions and infrared stretching frequencies. Solvent effects on infrared spectra reveal useful infornlation regarding the characteristic Ireq~iencies of the tautomers.Keto-en01 tautomerism has been investigated extensively by ultraviolet (I), infrared (2), and n.m.r. (3) spectroscopy. Keto-en01 equilibria are extrenlely solvent sensitive and the proportion of en01 form is found to be much greater in non-polar solvents such a s cyclohexane than in polar solvents such as alcohol or water. This is probably due to the stabilization of tlie lteto for111 by polar solvents through local association. Recently in this laboratory we have investigated the solvent effects on the n -+ a" transitions of several cliromophores and have evaluated contributions from different factors (4). These studies clearly indicate that solvent effects on the n -+ a" transitions are nlainly controllecl by local association between the solute and the solvent nlolecules and tliat the magnitude of tlie solvent blue shifts are determined by the polarity and/or proton-donating ability of the solvents. Studies of the solvent effects on infrared group frequencies have also shown tliat local association is an important factor in the determination of the solvent shifts (5). I t was consiclered interesting to sj~ste~natically investigate the lteto-en01 equilibria of a few systems in several solvents and correlate the observed effects with the solvent effects on the 7% -+ a:Qransitions and the stretching vibration frequencies of carbonyl groups. We have now studied the lteto-en01 equilibria of ethyl acetoacetate, acetylacetone, ethyl cyclopentanone-2-carboxy!ate, and methyl 4-~nethylcyclopentane-l,2-dione-3,4,5-tricarboxylate (I) in solvents of varying degree of polarity or/and proton-donating ability. EXPERIMENTALCommercial samples of ethyl acetoacetate and acetylacetone mere ~ised after suitable purification. Ethyl cyclopentanone-2-carboxylate was prepared by the procedure described in Orgaltic Syntheses (6) and methyl 4-metl~ylc):clopentane-l,2-dione-3,4,5-tricarbosylate was prepared according to the method of Ch~lang and M a (7). All the solvents were purified and fractionated before use.Ultraviolet absorption spectra were recorded with a MiIger U\,ispek, model H 700, spectrophotometer. Thc infrared spectra were recorded with a I'erkin-Elmer infrared spectrometer, model 137, with sodium chloride optics.Percentage of the en01 form was calculated from the extinction coefficieilt in the ultraviolet spectrum using the formula 1 0 0 (~~/~~) , where 6, is the observed molar extinction coefficient and s, the assumed value for the chromo...

Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

“…[24]), mark the n→π* electronic transition in the C=O group. This would indicate an internal proton transfer from the −OH group in the orto position to the nitrogen in the thiadiazole ring and the formation of the keto form of FABT, compare Fig.…”

The acidity and tautomerism of β-diketones in aqueous solution

“…[8] demonstrate that P-diketones are somewhat less sensitive to variation in the R substituent of the acyl functional group than are the less acidic P-keto esters and amides Interpolation in these correlation equations can be used to predict pK: for some related species. Equation [5] and pKF= 10.60 for methyl acetoacetate (7: R = OCH,) (14) predicts pK: = 12.9 for dimethyl malonate (9: R = OCH,). Equation [6] and P K~ = 10.28 for methyl benzoylacetate (8: R = OCH,) (14) lead to pKF = 13.0 for this same malonate diester.…”

“…For symmetrical diketones, two R groups are changed simultaneously and, while there is only one en01 spe- cies, the effects of the two R groups upon the stability of 2 will probably be quite different. For unsymmetrical P-diketones, there is the additional problem that the experimentally determined KE contains contributions from two structurally isomeric enols (5 and 6) and the likelihood that the equilibrium ratio of [5]/ [6] will also be dependent upon R' and R ' .…”

Electronic effects on keto–enol tautomerism of p-substituted aryl-1,3-diketone malonates

“…Keto-enol tautomerism in solution has been extensively investigated by a wide variety of spectroscopic methods, NMR [5e12], UV absorption [13], infrared spectroscopy [5,7,13,14] or HPLC [15]. In most 1,3-diketones, the enol form is in greater proportion.…”