Головна |
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 4.14Mechanism 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 4.15Major stopping reaction. BAR = benzilic acid rearrangement.
1764 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 4.16b-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 4.17Cleavage 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).
Parameter unit Axial Radial | Parameter Unit Conditions | Effect of Impregnation on the Uniformity of Delignification | Numerical Solution of the Diffusion Model | Phenolic Subunits | Reaction Path A | Reaction Path B | Reaction Path C | Residual Lignin Structure (see Section 4.2.5) | Reactions of Carbohydrates |