Alkaline degradation of monosaccharides V: Kinetics of the alkaline isomerization and degradation of monosaccharides

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Abstract
A new kinetic model for the alkaline isomerization and degradation of monosaccharides is presented which includes the interconversion of D‐glucose, D‐fructose, D‐mannose and D‐psicose. Computer simulations using this model fit the experimental data and allow the determination of all relevant reaction rate constants. Additionally, it has been established that, for the alkaline degradation of either D‐fructose, D‐glucose or D‐mannose, substantial amounts of acidic products, i.e. ∼65% and ∼20%, are formed via D‐fructose and D‐psicose, respectively.The influence of some reaction parameters on the pseudo‐first‐order rate constants involved in the kinetic model has been investigated. The enolization of monosaccharides appears to be the rate‐limiting step in both the isomerization and the degradation reactions. The enolization rate is markedly enhanced at higher HO− concentration or by the addition of calcium(II). Direct retro‐aldolization of D‐fructose and D‐psicose occurs at [HO−] > 10−2 M or in the presence of calcium(II), leading to an additional increase in the degradation rate towards lactic acid. The shift of the isomerization equilibrium by borate towards D‐fructose and D‐psicose is accompanied by a decrease of the degradation rate constants, due to stabilization of the monosaccharides as their borate esters.

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