As a model system for hydrolysis and dehydration of xylo-oligomers and xylan, static quan­tum mechanical calculations were conducted on the decomposition of xylobiose. As with xylose, acid-catalyzed reactions were studied by computing the energy barriers for proto — nated xylobiose in vacuum. Because of the large size of this molecule, highly accurate CBS calculations could not be conducted. Instead DFT [B3LYP/6-31(d, p)] calculations were con­ducted. This level of theory could underestimate (33-37) reaction barriers by 5 kcal mol-1, but is sufficient for a semi-quantitative comparison of the barriers for different reaction pathways. Initial molecular structures of neutral xylobiose, the protonated reactants, and the transition states used the corresponding structures from the study of xylose (17, 18) discussed above. Likewise, analogous reaction mechanisms were studied for xylobiose. For example, Reactions (9.5) and (9.6) show the first steps in the decomposition of xylobiose protonated O2 and O3 on the non-reducing end to form a furanyl ring or the precursor to formic acid. Reaction (9.5) exhibits a xylobiose dehydration mechanism similar to a monomer xylose (Figure 9.1). The degradation of the xylobiose is initiated when O2 on the non-reducing end is protonated. The protonated hydroxyl group (i. e., H2O) leaves the sugar ring forming a carbocation, which reorganizes to form an uncharged five-member ring leaving the positive charge outside the ring structure. A water molecule will then likely hydrolyze the ether linkage to break the dimer into a furan ring and an intact xylose molecule. The further dehydration of the furan ring structure leads to the formation of furfural. Likewise, the reaction could also initiate by protonation of O3 on the non­reducing end of xylobiose in a similar mechanism to the xylose monomer. However, here the
product with the open structure will dissociate into a xylose ester and a four-carbon cation

Protonation at O2 on the reducing end of xylobiose results in the reaction shown in (9.7), while protonation at O3 on the reducing end produces an unstable structure, which without a barrier, transfers its proton to O5 on the non-reducing end. The product from Reaction

(9.7)

might then dehydrate through reactions similar to those shown in Figure 9.1 to form a furfural molecule and a xylose molecule.

(9.7)

Proton addition to O2

In addition to the dehydration reaction shown in Reactions (9.5)-(9.7), the hydrolysis reaction to form two xylose molecules was considered. For this reaction, a proton is added to the ether linkage, which decomposes to a xylose molecule and an oxonium ion as shown in Reaction (9.8). In aqueous solution, the oxonium cation willbe quicklyhydrolyzed to form xylose as was discussed earlier. A comparison of the barrier for this process to the barriers for dehydration reactions shown, (9.5)-(9.7), was used to determine which reaction pathways are most likely. Importantly, no reaction barrier could be found for the hydrolysis reaction

(9.7) . If a proton was added to the ether linkage, the molecule decomposed without a barrier into xylose and the oxonium ion shown in Reaction (9.8).These calculationswere conducted
several times and always led to the same result. On the other hand, the reaction barriers for the dehydration reactions were found to be significantly larger and were consistent with the barriers for the dehydration reactions of xylose monomer. Using B3LYP/6-311G(d, p), barriers of 17.8 and 19.8 kcal mol-1 were obtained for Reactions (9.5) and (9.6). These are close to the barriers calculated for similar reactions for neat xylose (Table 9.1). Figures 9.5 and

9.2 compare the calculated molecular geometries for the transition states for dehydration of xylose and xylobiose protonated at O2 and O3. Notice that the bond lengths at the reacting centers are similar for xylose and xylobiose. The barrier calculated for Reaction

97

(9.7) is lower (4.7 kcal mol 1), but is still significantly higher than the barrier for Reaction

(9.8)

.

The absence of a barrier for Reaction (9.8) suggests that this process is kinetically favored over the dehydration reactions (9.5)-(9.7). This is not surprising, because, as mentioned above, Reaction (9.8) results in the formation of a relatively stable oxonium. Figure 9.7 shows a comparison of calculated reaction barriers for xylobiose protonated at the O2 and O3 sites and protonated on the linker oxygen atom. These calculations were conducted in vacuum, and as we have shown in our calculations of xylose, the barrier in aqueous solution is likely to be higher due to the endothermicity of the proton transferring from the solvent water molecules to the sugar oxygen atoms. However, since ethers have higher proton affinities than alcohols (44), the increase in the barrier due to solvation should be less for hydrolysis reactions (9.8), than for dehydration reactions, (9.5)-(9.7). Our calcu­lations suggest that the barrier for hydrolysis of xylobiose should be significantly lower than the barriers for dehydration reactions and that the kinetics of hydrolysis, consequently, should dominate. Because the ether linkage is identical in other xylo-oligomers, this re­sult further suggests that for all xylo-oligomers, hydrolysis should dominate. Loss of xy­lose due to dehydration reactions should only result from the dehydration of xylose itself, not from dehydration reactions of the xylo-oligomers when there is only acid in the so­lution. The accelerate destruction of xylose and xylotriose molecules with the addition of

inorganic salts (9) is likely due to a catalytic function, which lowers the barrier of dehydration reactions.