From d-glucosone From cellulose

Acid NaOH/air

0.04 M, 100 °C

4 h [189]

NaOH/O2

0.04 M, 95 °C

1bar, 5 min [187]

NaOH/N2

0.04 M, 95 °C

1bar, 5 min [187]

NaOH/air

18%, 25 °C

200 h [191]

NaOH/O2

0.5%, 100 °C

5 bar, 2 h [184]

Mannonic 11 18 47 15 27

Gluconic 2 5 5 2 3

Arabinonic 58 37 26 58 50

Ribonic 4 6 0 2 2

Erythronic 25 35 22 23 18

Two different pathways can form erythronic acid (11) (Scheme 7.16). The first

entails rearrangement of the glucosone to d- erythro –2,3-hexodiulose (10), followed

by an oxidative cleavage and loss of glycolic acid (12) [183]. In the second pathway,

erythronic acid (11) results from alkaline and oxidative degradation of the glucosone

(4) through arabinose (13) and arabinosone (14) as intermediates. In the

absence of dioxygen arabinose (13, Scheme 7.16) and arabinonic acid (8, Scheme

7.15), it may be formed by hydroxide ion attack at C1 and C2 respectively [185].

Minor amounts of 3-deoxy-pentonic acids (17, 18) are formed from an arabinose

intermediate (13), and the main pathway starts with a direct b-hydroxy-elimination

in the glucosone (4) followed by loss of the elements of carbon monoxide

from the intermediate 4-deoxy-d- glycero –2,3-hexodiulose (15) [187].

The yield of 3-deoxy-pentonic acids is lower in the presence of dioxygen [185],

and the formation of arabinonic and erythronic acid is particularly important.

Theander [185] stated that an attack of dioxygen to the glucosone (4, Scheme 7.17)

should give a hydroperoxide (20), which should further yield arabinonic acid (8)

and carbon dioxide. A similar attack at C3 could, via formation of a hydroperoxide

(21), result in the formation of an erythronic acid end-group (11) plus glyoxylic

acid (22).

About the same proportions of aldonic acids were produced from glucosone

and glucose treated with dioxygen and alkali [183], and cellobiose [190] and cellotriose

[192] yielded glucosyl- and cellobiosyl-arabinonic acids as the main products.

However, the presence of the substituted erythronic and mannonic acids was

also significant, especially at higher alkali concentrations. Malinen and Sjostrom

658 7Pulp Bleaching

R1 = -H for xylan

R = Polysaccharide chain

R1 = -CH2OH for cellulose and glucomannan

O

HO H

H OR

H OH

R1

O

H

+ OH-

CH2OH

O

O

H OR

H OH

R1

+

O2/OH-

COOH

H OR

H OH

R1

CH2OH

COOH

- HCOOH

+ OHH

O

H

H OR

H OH

R1

HO

O2/OH- O2/OH-

H O

O

H OR

H OH

R1

COOH

H OR

H OH

R1

H O

H

H OH

R1

OH + OH-

- ROH-

H O

H

H OH

R1

O

H

O

OH

H

H OH

R1

H O

+ OH-

- ROH

O

O

H

H OH

R1

O

H

H

HO O

H

H OH

R1

HO

H

H

HO O

H

H OH

R1

H

H

OH

HO O

H

H OH

R1

+ + H

10 D- erythro -2,3-hexodiulose

11 Erythronic acid

12 Glycolic acid

13 Arabinose

14 Arabinosone

15 4-Deoxy-D- glycero -2,3-hexodiulose

16 3-Deoxy-D- glycero -pentosulose

17 3-Deoxy-D- threo -pentonic acid

18 3-Deoxy-D- erythro -pentonic acid

19 3,4-Dihydroxybutyric acid

10 D- glycero -2,3-pentodiulose

11 Glyceric acid

13 Threose

14 Threosone

15 4-Deoxy-2,3-pentodiulose

16 3-Deoxy-Tetrosulose

17 2,4-Dihydroxybutyric acid

18 2,4-Dihydroxybutyric acid

19 2-Deoxy-glyceric acid

Scheme 7.16 Degradation pathways of the glucosone and

xylosone end-groups (adapted from Malinen [183] and

Theander [185]).

R = Cellulose chain

R1 = -CH2OH

O

HO H

H OR

H OH

R1

O

H

+

Дата добавления: 2015-10-21