The mechanism and thermodynamics of transesterification of acetate-ester enolates in the gas phase

Haas, George W.; Giblin, Daryl; Gross, Michael L.

doi:10.1016/s0168-1176(97)83245-4

Cited by 16 publications

(26 citation statements)

References 84 publications

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“…Although Fourier transform mass spectrometry (FTMS) experiments on transesterifications between alcoholates and alkyl acetates in the gas phase indicated that transesterifications between alcohols and alkyl acetates proceed via the enolate mechanism in the gas phase [27], we thought it worthwhile to support these results by calculating the energetics of the relevant species in this process (Fig. 12).…”

Section: Theoretical Calculations On the Esteralcoholate Anion/enolatmentioning

confidence: 99%

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The base‐catalyzed, low‐temperature interesterification mechanism revisited

Dijkstra¹,

Tőke

Kolonits

et al. 2005

Eur. J. Lipid Sci. Technol.

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The base-catalyzed, low-temperature interesterification mechanism revisitedFor the base-catalyzed interesterification reaction carried out at low (,100 7C) temperatures, a number of mechanisms have been proposed in the literature. As these mechanisms are not in accordance with experimental observations, a novel mechanism will be proposed instead. This novel mechanism assumes that the reaction of a base (such as sodium methanolate) with the oil will eventually lead to the abstraction of an a-hydrogen from a fatty acid moiety and that the enolate anion thus formed acts as the catalytic intermediate. This enolate can re-abstract a proton from the hydroxyl group of a partial glyceride, whereupon the resulting alcoholate attacks the carbonyl group. This leads to a new ester and a glycerolate anion that then regenerates a new enolate anion. If the enolate anion reacts with methanol, this will lead to the formation of a fatty acid methyl ester and a glycerolate anion that again regenerates an enolate anion. Reaction with water leads to catalyst inactivation by converting the enolate anion to an unreactive fatty acid moiety (free fatty acid or soap) and a partial glyceride. Thermal inactivation of the enolate intermediate is assumed to be through the formation of catalytically inactive b-keto esters. The accelerating role of acetone is explained by assuming this compound to act as a highly mobile hydrogen transfer agent that facilitates the reaction between the glycerolate anion and the a-hydrogen atoms in fatty acid moieties. The above assumptions are independently supported by the observation that the addition of acetone-d 6 to an interesterifying reaction mixture leads to the almost quantitative incorporation of deuterium into the a-position of fatty acid moieties. Theoretical calculations on the enolate-alcohol system at PM3 level are also in agreement with the enolate mechanism.

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Section: Theoretical Calculations On the Esteralcoholate Anion/enolatmentioning

confidence: 99%

“…Moreover, evidence in favor of the enolate mechanism has been found for transesterification of "activated" b-keto esters in a liquid medium [26] and also for "nonactivated" alkyl acetates in the gas phase [27].…”

Section: Introductionmentioning

confidence: 99%

The base‐catalyzed, low‐temperature interesterification mechanism revisited

Dijkstra¹,

Tőke

Kolonits

et al. 2005

Eur. J. Lipid Sci. Technol.

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“…To describe the kinetic basicity of the surfaces further, we also considered a second model reaction, hept-1-ene isomerisation (Figure 1), which is supposed to be more sensitive to strong sites than MBOH conversion. Indeed, alkenes are less easily deprotonated than alcohols: the proton affinity of the conjugated bases is lower for alcohol (1563 kJ.mol -1 for methanol 14 ) than for alkene (1636 kJ.mol -1 for the propene 15 ). Therefore, the number of sites deprotonating alcohol should be higher than for alkene.…”

Section: Introductionmentioning

confidence: 99%

Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis

Petitjean

Costentin

Guesmi

et al. 2013

Phys. Chem. Chem. Phys.

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, et al.. Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2013, 15 (45), pp.19870-19878 isomerization and one alcohol conversion (hept-1-ene isomerization and 2-methyl-3-butyn-2-ol conversion). Catalysis experiments showed that carbon dioxide adsorption poisons the catalyst surface and the DRIFT-DFT combination showed that the nature of active sites in the two reactions differs. On the reverse, partial hydroxylation of the surface enhances activity for both reactions. Interestingly hept-1-ene isomerization gives a volcano curve for the conversion as a function of hydroxyl coverage.Calculations of the electronic structure of magnesium oxide surface show that neither Lewis basicity nor Brønstedt basicity of the surface defects (steps for example) are enhanced by hydroxylation.Meanwhile CO 2 adsorption followed by IR spectroscopy shows that (110) and (111) unstable planes are strongly basic and are stabilized by partial surface hydroxylation. That result could explain the volcano curve obtained for the evolution of alkene isomerisation as a function of hydroxyl coverage.present address :

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“…The determination of the number of replaceable hydrogens, such as OH, COOH, NH, or SH, by simple exchange of the sample in the presence of heavy water, D 2 O, is widely used in mass spectrometry. Although the incorporation of deuterium can be effectively carried out by a variety of preparative liquid and gas chromatographic procedures, the direct combination of LC with MS obviates the necessity of isolation of individual components.…”

mentioning

confidence: 99%

“…The chemical incorporation of deuterium followed by ionization has played a major role in the structural characterization of molecules, in gaining information on the mechanisms of chemical or biological reactions, and in the interpretation of mass spectra [1][2][3][4][5][6][7]. Many ionization methods have been used: Electron ionization (EI) [1], chemical ionization (CI) [8 -10], fast atom bombardment (FAB) [11][12][13], thermospray [14,15], particle beam [15], and electrospray ionization (ESI) [3, 16 -25].…”

mentioning

confidence: 99%

Mass spectral characterization of tetracyclines by electrospray ionization, H/D exchange, and multiple stage mass spectrometry

Kamel

Fouda

Brown

et al. 2002

J. Am. Soc. Mass Spectrom.

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Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N 2 and with ND 3 as the nebulizer gas. CID mass spectra were obtained forϪ , and produced by ESI using a Sciex API-III plus and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS N spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H 2 O (D 2 O) and NH 3 (ND 3 ) if there is a tertiary OH at C-6. The loss of H 2 O (D 2 O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH 3 (ND 3 ) initially. MS N experiments showed easily understandable losses of HDO, HN(CH 3 ) 2 , CH 3 Ϫ N¢CH 2 , and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH 3 (ND 3 ) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol. (J Am Soc Mass Spectrom 2002, 13, 543-557)

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The mechanism and thermodynamics of transesterification of acetate-ester enolates in the gas phase

Cited by 16 publications

References 84 publications

The base‐catalyzed, low‐temperature interesterification mechanism revisited

The base‐catalyzed, low‐temperature interesterification mechanism revisited

Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis

Mass spectral characterization of tetracyclines by electrospray ionization, H/D exchange, and multiple stage mass spectrometry

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