HOH2C OH

OH

O

-OH

-ROHO

HOH2C OH

OH

O

O

HOH2C OH

OH

O CH2OH OH

O

HO OH

HO

RO

HO

HOH2C OH

OH

H

O

27 28 29

30 31

Endiol

BAR

R = cellulose chain

-

Scheme 4.14 Mechanism of the peeling reaction. BAR = benzilic acid rearrangement.

RO

HOH2C OH

O

H

OH

RO

HO

HOH2C OH

OH

O

RO

O

HOH2C

OH

OH

OH

RO

O

OH

OH

- H2O

O

O

OR

O

HOH2C

H

RO OH

O

OH

RO

HO

HOH2C

HO

OH

O

H

H

RO

HOH2C OH

O

H

O

RO

HOH2C OH

O

OH

OH

27 29

R = cellulose chain

-H2O

+

BAR

Metasaccharinic acid

Scheme 4.15 Major stopping reaction. BAR = benzilic acid rearrangement.

176 4 Chemical Pulping Processes

end groups are stabilized by conversion into the corresponding aldonic acid by

simple oxidation, or by conversion to metasaccharinic acid (41) or 2-hydroxy-2-

methyl-3-alkoxy-propanoic acid, respectively [65]. The activation energies for the

peeling and stopping processes have been estimated as 103 kJ mol–1 and

135 kJ mol–1, respectively [66]. This implies that the peeling reaction becomes less

pronounced with increasing temperature.

Oxidative Peeling

If keto or aldehyde groups are present along the cellulose chain, they generally

cause cleavage of the glycosidic bond in alkaline media by b-alkoxy elimination

(cf. Scheme 4.16). Therefore, oxidized groups in the C2, C3 or C6 position are

OR

O

HO

OH

O

O

OR

HO

OH

COOH

RO

O

OH

OH

O

HO

OR

O

HO

OH

O

OH

-

RO

O

OH

OH

HO

OH

reducing fragment

+

43 44 45

Scheme 4.16 b-elimination reaction at C2-keto units.

O

HO

O

OH

OR. RO

O

HO

OH

OH

OH

O

HO

OH

O

OR.

O

OH

OH

HO

O

RO +

reducing fragment 46 47

O

HO

OH

OH

OR.

RO

O

OH

OH

O

O

O

HO

OH

OH

OR.

RO

O

OH

OH

-O

O

O

O

OH

RO

OH

HO

O

HO

OH

OH

OR.

O

O

OH

RO

O

O

HO

OH

COOH

RO

OH

50 49 51

+

non-reducing fragment

-

Scheme 4.17 Cleavage of the glycosidic bond by

b-elimination at C3-keto and C6-aldehyde structures along

the cellulose chain [67].

4.2 Kraft Pulping Processes 177

considered as “weak links” or “hot spots” along the chain. Keto groups at C2 (42)

or an aldehyde at C6 (46) and the anomeric carbon are referred to as “active carbonyls”,

leading to a new reducing end group (43), which might undergo further

degradation reactions. A keto group at C3 (48) is considered as an “inactive carbonyl”,

since b-elimination eventually forms a non-reducing end (50) and a stable

acid (51) (cf. Scheme 4.17).