Cellulose

Acid-catalyzed hydrolysis of glycosidic linkages constitutes a major reaction of carbohydrates

under sulfite cooking conditions. The depolymerization of dissolved

polysaccharides proceeds in acid sulfite processes mainly to the monosaccharide

level, while oligo- and polysaccharides are more frequent in bisulfite spent

liquors.

Protonation as the first step of the hydrolysis can take place either at the glycosidic

oxygen (dominant pathway) or at the ring oxygen (Scheme 4.46). The protonated

form releases the substituent in position C1 being converted into a carboniumoxonium

ion (e.g., pyranosyl cation), which is stabilized by resonance and exists in

a half-chair conformation. From the electron density distribution (Scheme 4.47) a

preferred localization of the positive charge at the C1 carbon can be observed,

where nucleophilic attack also occurs. The addition of water leads to a new reducing

end group. Hence, the formation of new reducing ends corresponds linearly

with the reduction of the molecular weight (degree of polymerization, DP) [25].

As a result of the acid hydrolysis, the DP is reduced, oligomers are formed, and –

depending on the severity of the conditions – the polymer can be degraded all the

way to the monomers. The reaction kinetics is in agreement with a pseudo-first

order rate law. The rate depends on acid concentration, temperature and the molecular

environment of the glycosidic bond [26].

O

OH

RO

HO

OH

O

RO

HO

OH

OH

O

O

HO

OH

OH

OR

O

HO

HO

OH

OH

OR

O

RO

HO

OH

OH

O

O

HO

OH

OH

OR

H

O

RO

HO

OH

OH

OH

O

OH

RO

HO

OH

- H+ + H+

+

+ other products

Slow

+

A

B 37

+

H2O

-H+

Scheme 4.46 Mechanisms of acid hydrolysis of cellulose [27]

416 4 Chemical Pulping Processes

Scheme 4.47 LUMO (right) and electron density distribution

(left) of the carbonium-oxonium ion intermediate (36).