Dehydration of Carbohydrates to Aromatic Structures

Acid-catalyzed dehydration of carbohydrate monomers eventually leads to the formation

of hydroxymethylfurfural (47) from hexoses as the starting material

(Scheme 4.48). In a similar manner, the removal of 3 mol water from pentoses

results in the formation of furfural, which can be distilled off the pulping liquor.

The amount of furfural produced from degraded hemicelluloses during sulfite

pulping is sufficiently large to sustain commercial usage.

OH

HC OH

CH2OH

H O

HC

CH

HC

OH

HO

HC OH

CH2OH

O

HO CH

HC OH

CH2OH

HC OH

CH2OH

CHOH

C

CH

HC

HO

OH

OH OH

HC OH

CH2OH

H O

C

CH

HC

OH

-H2O

HC OH

CH2OH

H O

C

CH

CH

O O

HOH2C CHO

-H2O

O

OH

CH2OH

OH

CHOH

O

OH

CH2OH

CHO

-H2O

-H2O -H2O

-H2O

-H2O

O

OH

CH2OH

OH

CH2OH

OH

O

OH

CH2OH

OH

H

CHO

38 39 40 41

42 43 44 45

Scheme 4.48 Formation of hydroxymethylfurfural (HMF)

from glucose by acid-catalyzed dehydration.

Hydroxymethylfurfural (47) can be further converted to levulinic acid (c-oxo

pentanoic acid, 72) with 5-hydroxymethylfurfural as an intermediate (cf.

Scheme 4.49).

Furfural and its derivatives are highly prone to condensation reactions, either

with other furfural molecules or with lignin fragments present in the liquor

(Scheme 4.49). An example of such a Friedel-Kraft acylation-type condensation is

also shown in Scheme 4.49.

The formation of other aromatic compounds under acidic conditions has been

studied extensively by Theander et al. [36–38]. Scheme 4.50 shows aromatic compounds

resulting from the dehydration, degradation, and re-condensation of carbohydrates

under acidic solutions. The first step comprises the formation of anhydro-

sugars with intramolecular glycosidic linkages, resulting in elimination of

another water molecule from two hydroxyl groups (levoglucosan). The glycosidic

linkages are hydrolyzed, and further processes lead to aromatic and condensed

units.

4.3 Sulfite Chemical Pulping 419

O

HO

HO

OH

OH

O

CHO

O

OH

HO

HO

OH

OH

O