Study On The Tautomerism Of β-Keto Esters By Gas Chromatography

The tautomerism of β-keto esters is a typical system of enol-keto tautomerism, which has been thoroughly studied in liquid phase, yet less in gas phase, for lacking of effective techniques. The ability of separation of tautomers by gas chromatography (GC) has been reported, and the mass spectrometer (MS) has been proved to be one of the most informative and practical method for studying tautomers in gas phase. So, gas chromatography connected with mass spectrometer is very suitable for the study of tautomerism in gas phase. In this paper, the tautomers of 11 β-keto esters were separated with a HP-5 capillary column. The Kinetics and thermodynamics parameters of the tautomerism of several β -keto esters (methyl acetoacetate, ethyl acetoacetate, isobutyl acetoacetate, methyl 3~oxo pentanoate) on the stationary phase of the HP-5 column were discussed. The main fragmentation mechanism of the tautomers of some β -keto esters were also discussed. The main results of study in this paper were summarized below:1. Two chromatographic peaks were obtained during the GC analysis of these 11 β-keto esters on a HP-5 capillary column. The two chromatographic peaks possess of resemble mass spectrums. The peaks were identified by MS and Deuterium-Labeling. The enol tautomers elute fast from the HP-5 column than the keto tautomer. The main reason leading to the thorough separation of the enol-keto tautomers is not the polar interaction of tautomers with the column.2. When ethyl acetoacetate, methyl 3-oxo pentanoate and various alcohols were injected into GC together, the transesterfication occurred , which provides a novel method to obtain on-line β-keto esters.3. In-depth discussion of the mass spectrums of enol-keto tautomers, obtained in this way, were made:(1) The enol molecular ions are more stable than the keto molecular ions, according to the relative abundance of the corresponding molecular ions.(2) The keto tautomers are readily to undergo McLafferty rearrangement. The first kind McLafferty rearrangement is not noticeable and is affected by the steric effect of the group R. The second kind McLafferty rearrangement is quite characteristic, which was more readily to undergo for the enol tautomers thanfor the keto tautoerms.(3) The bond between C2-C3 of the keto tautomers is readily to take hemolytic cleavage. The fragment ion arising from this way, is often the base peak in their mass spectrums. The bond between C2-C3 of the enol tautomers is also readily to take cleavage, which maybe be induced by the loss of a electron from the carbonyl group and also maybe by the loss of a electron from the double bond under El, followed by the emigration of the double bond. The relative abundance araised from this cleavage is not very high, which is always not the base peak in its mass spectrum.(5) The remarkable difference in the fragmentations between enol-keto molecular ions is that the enol forms are readily to lose neutral alcohols. The presence of the lose of neutral alcohols can be used to judge the form of tautomers.4. GC as chemical reactor, some kinetics and thermodynamics pertaining to the separation of several 3- keto esters on the HP-5 capillary column were calculated, applying the chromatographic equation available.Under 41 kPa column pressure and different isothermal condition, the activation energy for the tautomerism of methyl acetoacetate on the HP-5 capillary was exacted as 27. 5 kj mol"1, and 32. 1 kj mol"1 for ethyl acetoacetate, 35. 1 kj mol"1 for methyl 3~oxo pentanoate, 42. 7 kj mol"1 for isobutyl acetoacetate. The enthalpy and the entropy changes ( ts.H, AS) pertaining to the tautomersim on the column were -5. 00 kj moT2, -10. 92 J K~J mol"' for methyl acetoacetate, -4. 10 kj mol"1,-8. 60 J K"1 mol"1 for ethyl acetoacetate, -3. 98 kj mol"1,-7. 90 J K^mol"' for isobutyl acetoacetate and -4.39 kj mol"1,-9. 01 J K"1 mol"1 for methyl 3-oxo pentanoate.The study showed the tautomerim was pseudo first-order on the HP-5 column for methyl acetoacetate, ethyl acetoacetate, isobutyl acetoacetate and methyl 3-oxo pentanoate. The kinetics and thermodynamics parameters pertaining to the tautomerism of P -keto esters on the stationary phase are some correlative to its molecular bulk.