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Hexenuronic acids are only formed under alkaline pulping conditions (cf. reaction
in kraft pulping). However, in two-stage sulfite processes with a first stage at a
higher pH, their reactions in the subsequent acidic stage must be considered.
According to Teleman et al. [40], hexenuronic acids are degraded under acidic conditions
to 5-formyl furancarbonic acid (61) according to Scheme 4.51.
O OXyl
OH
O
HO
OH
-XylOH
O
OH
O
O
OH
H
OH2
+
O
OH
O O
HO
O
O
O
HO OH
O
H
H
H
O
O
HO O
H
O
H
H
O
O
HO
O
O
HO O
H2-HCO O 2H
-H2O
-H2O
-H2O
55 56
60 59
+
+
Scheme 4.51 Formation of 5-formylfurancarboxylic acid from hexenuronic acid [40].
Formation of Sulfur-Containing Carbohydrates
The limited stability of sulfur-containing carbohydrate derivatives renders their
isolation and identification rather difficult. (It must be borne in mind that the
reaction mechanisms presented in this chapter are based on investigations carried
out mainly during the period 1940–1960; no recent data are available to verify the
much-esteemed work of that period with more recent analytical approaches.)
Stable products from the reaction of sulfite with both reducing and nonreducing
[41]carbohydrate model compounds were obtained only with sulfite solutions
of higher pH (e.g., pH 6) [42]. Adler [43] crystallized a sulfonic acid derived
from glucose under the same pH.
Theander [44]showed the formation of such products to proceed in a similar
manner as shown for a xylose in Scheme 4.52. Sulfonic acid 66 was formed via
oxidation, and 65 by rearrangement of 4-sulfo-3-deoxy-glucosone 64, which in
turn was formed from the dicarbonyl intermediate 62 [45]. The formation of 64
from 63 was also shown to proceed [46].
4.3 Sulfite Chemical Pulping 421
HSO3
-
HO
HO
HOH2C
HO
OH
O
H
H
HO
HOH2C
O
OH
O
H
HOH2C
OH
OH
O
H
HOH2C
O
OH
O
H
HO3S
HOH2C O
OH
O
OH
HO3S
HOH2C
HO
OH
O
OH
HO3S
HSO3
-
38 62 63
Scheme 4.52 Possible mechanism of sulfonic acid formation from glucose [45].
In a later account, Yllner [47]studied the reaction of xylose with neutral sulfite
solution and isolated a,d-dihydroxy-c-sulfo-valeric acid. The reaction mechanism
proposed for the formation of this sulfo-sugar acid is similar to the peeling process
under alkaline conditions. Neutral sulfite systems seem to be “alkaline”
enough to promote the initial rearrangement and formation of the intermediate
3-deoxy-d- glycero -pentos-2-ulose, which reacts with the hydrogen sulfite at position
C4, a benzylic acid rearrangement (BAR) finally yields the stable acid (70) (cf.
Scheme 4.53).
HO
OH
O
H
OH
HO
HO
OH
OH
O
HO
O
OH
OH
HO OH
HO
HO
OH
O
H
H
HO
OH
O
H
O
HO3S
OH
O
OH
OH
HSO3
-
HO3S
OH
O
H
O
-H2O
BAR
A
69 70
B
Scheme 4.53 Possible formation of sulfonic acids from xylose
intermediates under neutral sulfite conditions.
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