Mechanisms of Acid Degradation Reactions of Wood Hemicelluloses

The treatment of polysaccharides with aqueous acidic compounds results in the

occurrence of two types of reaction – glycosidic hydrolysis and dehydration. Extensive

studies have been conducted on this issue, and although some aspects remain

unclear, the general principles are well understood. The mechanism of glycosidic

bond cleavage has been reviewed in several articles on homogeneous [21,22]and

heterogeneous reactions [23]. It is agreed that hydrolytic cleavage proceeds

through initial protonation of one of the hemiacetal-oxygen atoms to form a conjugated

acid. In principle, two mechanisms are possible. Protonation of the glycosidic

oxygen atom forms a conjugated acid, followed by fission of the exocyclic

C1–O bond to give a cyclic carbenium ion, which most probably exists in the halfchair

conformation having C-2, C-1, O and C-5 in a plane [24]. After reaction with

water, the protonated reducing sugar and subsequently the reducing sugar is

formed.

The alternative mechanism involves protonation of the ring oxygen atom to

form the conjugated acid, followed by ring opening to give an acyclic carbenium

ion. Again, after the addition of water the protonated hemiacetal is hydrolyzed to

the free sugar [25]. The findings that methyl d-glucosides are anomerized in fully

deuterated methanolic methanesulfonic acid with complete exchange with the solvent

strengthens the mechanism with the cyclic carbenium ion intermediate [26].

It has been shown that the hydrolysis rate is highly controlled by the rigidity of

the glycone ring. Thus, the hydrolysis rate of the furanosides is faster as compared

to the pyranosides and substitution on the ring further decreases the hydrolysis

rate. The rates in dilute acid for b-methylpyranosides and b-(1–4) disaccharides

are listed in Tab. 4.41.

The major rate-controlling factors are steric diequatorial intramolecular interactions

within the glycone. Based on hydrolysis experiments with cellotrioses-1-14C,

it was observed that the two glycosidic bonds within the molecule behave differently.

The nonreducing end of the cellotriose molecule contains an unsubstituted

d-glucopyranose residue which enables a faster hydrolysis rate as compared to

that of the bond at the reducing end of the molecule containing more bulky residues

[28]. Such residues would also be expected to influence the hydrolysis of

4.2 Kraft Pulping Processes 327

Tab. 4.41 Relative hydrolysis rates of methyl-b-pyranosides and

(1–4)-.-linked disaccharides (according to Harris [27]).

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