Peeling Reactions Starting from the Reducing End-Groups

The peeling removes the terminal anhydro-sugar unit, generating a new reducing

end-group until a competitive stopping reaction sets in forming a stable saccharinic

acid end-group (see Section 4.2.4.2, Carbohydrate reactions).

In studying the oxidative alkaline peeling reaction of cellulose by using cello-oligosaccharides

and hydrocellulose, Malinen and Sjostrom ([190, 192]) found in

addition to the “normal” alkaline peeling products [isosaccharinic acid (27,

Scheme 7.18) and lactic acid (32)], large amounts of 3,4-dihydroxybutyric acid

(28), glycolic acid (33), 3-deoxy-pentonic acid (17, 18, Scheme 7.16), formic acid

(34) and glyceric acid (35). The formation of the two isomeric glucoisosaccharinic

acids (e.g., 27) by alkaline treatment of cellulose is much depressed in the presence

of dioxygen [185], and the 4-deoxy-d- glycero –2,3-hexodiulose (26) is instead

fragmented to 3,4-dihydroxybutyric acid (28) and glycolic acid (33). These are

formed via oxidative cleavage of 4-deoxy-d- glycero –2,3-hexodiulose (26), which can

660 7Pulp Bleaching

H OH

HO H

H OR

H OH

R1

CH2OH

O

HO H

H OR

H OH

R1

H O

R1 = -H for xylan

R = Polysaccharide chain

R1 = -CH2OH for cellulose / glucomannan

23 Glucose end-group

24 Mannose end-group

25 Fructose end-group

26 4-Deoxy-D- glycero -2,3-hexodiulose

27 Isosaccharinic acid

28 3,4-Dihydroxybutyric acid

29 Glycolic acid

30 Dihydroxyacetone and glyceraldehyd

31 Methyl glyoxyl

32 Lactic acid

33 Glycolic acid

34 Formic acid

35 Glyceric acid

23 Xylose end-group

25 Xylulose end-group

26 4-Deoxy-2,3-pentodiulose

27 Xyloisosaccharinic acid

28 2-Deoxy-glyceric acid

29 Glycolic acid

30 Dihydroxyacetone and glycolaldehyd

31 Methyl glyoxyl

32 Lactic acid

33 Glycolic acid

34 Formic acid

35 Glyceric acid

- ROH

+ OH-

CH2OH

O

OH

H

H OH

R1

CH2OH

O

O

H

H OH

R1

COOH

HO

H

H OH

R1

+ OH-

+ O2/OH CH2OH

H

COOH

H

H OH

R1

H

+

COOH

CH2OH

HO H

HO H

H OH

H OH

CH2OH

H O

OH

H

H O

R1

CH2OH

O

CH2OH

OH

CH2

COOH

OH

CH2OH

COOH

CH2OH

HCOOH

+ OH-

O2/OH-

O2/OH H

O

+

O

CH3

H O

+ OH-

H

COOH

OH

CH2OH

H

Scheme 7.18 Peeling reactions of polysaccharides during

alkaline and oxidative alkaline conditions (redrawn from

Ref. [183]).

also rearrange to isosaccharinic acids (27) or cleave to yield glyceraldehyde (30)

[183]. Glyceraldehyde is further converted to lactic (32), glycolic (33) and glyceric

(35) acids.

Malinen and Sjostrom [192] reported that the extent of the peeling reaction for

cello-oligosaccharides was very low and that stabilization proceeded quickly. However,

the stabilization of hydrocellulose – that is, the formation of aldonic acid

end-groups – was less extensive, and peeling resulted in a loss of 10–50 sugar

units, depending on the reaction.

The peeling reactions of xylan and glucomannan that take place under alkaline

conditions have been described in detail (see Section 4.2.4.2, Carbohydrate reactions).

In the presence of dioxygen, the peeling of xylan is more extensive than in

alkali alone, and greater than that of cellulose and glucomannan. However, in the

absence of dioxygen the degradation rate is lower for xylan than for cellulose and

glucomannan [192,193,195]. 2,4-Dihydroxy-butyric acid (17, 18, Scheme 7.16), 2-

deoxy-glyceric acid (28, Scheme 7.18), glycolic acid (33), glyceric acid (35), xyloisosaccharinic

acid (27), lactic acid (32) and formic acid (34) are the main peeling

products of xylan, which are analogous to the peeling products of cellulose.

The xylan chains are partly substituted with 4- O -methyl-glucuronic acid units at

C2 [196], which prevent migration of the carbonyl group to the b-position relative

to the glycosidic bond constraining b-elimination (see Section 4.2.4.2, Carbohydrate

reactions; specific reactions of xylan). Model studies with aldobiuronic acid

7.3 Oxygen Delignification 661

[194,197] revealed that, under alkaline conditions at 80 °C, the degradation rate

was rapid but much slower than that of xylobiose. Under dioxygen alkali conditions,

aldobiuronic acid degraded almost as fast as xylobiose, suggesting that the

substituent at C2 has a low retarding effect on the peeling reaction. The arabinose

substituent at C3 position of softwood xylan is easily cleaved by b-elimination

through the peeling process, and the chain is partly stabilized to xylometasaccharinic

acid end-groups [198].

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