Radical Reactions in Crystalline Carbohydrates

Radical Reactions in Crystalline Carbohydrates Crystalline carbohydrates have been widely studied, both by e.s.r. spectroscopy265-281 and by product analysis.25,26,47 54 4 282-300 Despite 

In nonaromatic systems, ionization usually plays a major role, as compared to excitation.301 Whereas, in liquids or highly disordered solids, the electron can be solvated,302 no, or very low yields of, solvated electrons are observed in solid carbohydrates, even at low temperatures.275 This implies that the electron may return to the positive hole (reaction 295). However, it cannot be excluded that ion-molecule reactions (reaction 194) precede this reaction, and that recombination occurs with the resulting ion N+, instead of with the parent ion M+ (reaction 196). Process 194 has been studied with simple alcohols in the gas phase.303,304 

The reaction scheme as given by reactions 199-202 demands that O-deuterated alcohols shall give high yields of HD. This has, in fact, been found.305 However crystalline, O-deuterated carbohydrates yield296 H2 almost exclusively. Thus, this concept appears to be invalid for crystalline carbohydrates, although, in both cases, hydrogen is the major product [G(H2) = 3-5], 

It could then be argued that the electron rapidly returns to the primary ion, affording an excited state (reaction 195). However, the alcohol models (liquid and gas phase) show that at least the lowest, electronically excited state predominantly splits the O-H bond or eliminates H2, to give the corresponding carbonyl compounds. With O-deuterated alcohols, these processes lead80 to HD ( 75%). Obviously, the mechanistic concepts developed for the liquid alcohols cannot be applied directly to solid carbohydrates. 

Despite the present poor understanding of the primary processes, the radiolysis of carbohydrates is a unique way to generate carbohydrate radicals within the carbohydrate matrix. It has enabled some interesting chain reactions to be detected (see Sect. IV,1). Many radical reactions observed in aqueous solution are also observed in the solid state. In addition, there are reactions that are more pronounced in the solid state, and their products can be studied more readily than in the liquid state. 

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