Influence of Substituents on the Rate of Hydrolysis

The glycosidic linkages between other sugar units than glucose are generally

more reactive. Figure 4.156 illustrates the relative rates of the hydrolysis of the

a- and b-anomers of the corresponding methyl glycosides. Ring strain also causes

a higher reactivity, in that furanoses react faster that pyranoses. A carboxyl group

in position C6 decreases the reactivity, whereas an aldehyde in the same position

increases the rate of hydrolysis considerably [28].

The relative rates for the monomers are not very representative for the polymeric

material as the nature of the aglycone influences the hydrolysis rate. In

addition, acid hydrolysis of cellulose depends not only on the chemical structure

as discussed above, but also greatly on its morphology (cf. Chapter 1). The accessibility

in this heterogeneous reaction is affected by the degree of crystallinity, and

also by provenience or pretreatment.

L-GlcA

D-Glc

D-Man

D-Gal

D-Xyl

L-Ara

L-Rha

Relative rate of hydrolysis

â-anomer

á-anomer

Cellulose

Mannan

Xylan

Galactan

0 1 2 3 4

Relative rate of acid hydrolysis

Fig. 4.156 Comparison of the relative rate of hydrolysis for

monomers (left) [29]and for the polymer (right) [30].

4.3 Sulfite Chemical Pulping 417