Reactions of Polysaccharides

Objective: a general understanding of what happens to polysacccharides when exposed to aqueous acid or base.

Important Concepts

Effect of Acids and Bases.

Factors Stabilizing/Destabilizing Glycosidic Bonds.

Acid Hydrolysis of Cellulose---Leveling-Off Degree of Polymerization.

Reactions in Base---Swelling and Peeling.

 

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Acid Hydrolysis---a water splitting reaction which in the case of polysaccharides results in the breaking of glycosidic bonds. Hydrolysis decreases the degree of polymerization of polysaccharides. All polysaccharides do not hydrolyze at the same rate and several factors influence the rate of acid hydrolysis of polysaccharides. The Table below shows the hydrolysis rates of several glycosides, and this is followed by a listing of the major factors influencing hydrolysis rates.

 

Relative Acid Hydrolysis Rates of Several Glycosides

Methyl-Beta-D- pyranoside	Rate	Beta-D-(1-4)-Linked Disaccharide	Rate	Cellulose Oligosaccharide	Rate
Glucose	1.0	Cellobiose	1.5	Cellobiose	3.5
Galactose	3.0	Glucopyranosyl-Mannose	1.5	Cellotriose	2.1
Mannose	4.8	Pseudocellobiuronic Acid	1.5	Cellotetraose	1.7
Xylose	5.8	Cellobiuronic Acid	0.05	Cellopentaose	1.5
		Mannobiose	2.7	Cellohexaose	1.4
		Lactose (Galp--Glc)	3.6	Cellulose	1.0
		Xylobiose	11

 

Factors Influencing the Acid Hydrolysis Rates of Polysaccharides (Glycosidic Bond Cleavage)

1. Type of Glycosidic Bond (carbohydrate involved and stereochemistry). Beta glycosidic bonds are more stable than alpha, and beta-linked glucosides are the most stable glycosidic bonds. Hexopyranosides are more stable than pentopyranosides (compare cellobiose and xylobiose), and furanosides form relatively weak glycosidic bonds.

2. Location of the Glycosidic Bond within the polysaccharide chain and the length of the polysaccharide. The rate is higher at the end of a polysaccharide due to increased flexibility.

3. Inductive Effect (carboxyl group at C-5). Uronic acids have strong glycosidic bonds due to the presence of the carboxyl group.

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How does this all pertain to wood? Consider the softwood glucuronoxylan. This has a xylan backbone with uronic acid and arabinofuranoside side groups. Upon exposure to acid, the arabinofuranoside linkages would cleave quite rapidly followed by the xylan backbone. The most resistant glycosidic bond would be the uronic acid linkage to xylose.

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Cellulose Acid Hydrolysis. There are several factors that stabilize cellulose to acid hydrolysis:

• Crystallinity---acid cannot rapidly penetrate the crystalline zones. Therefore the crystalline regions are somewhat resistant to acid hydrolysis.
• Type of glycosidic bond---a beta-linked glucopyranoside is relatively stable.
• The acidic breakdown of cellulose involves two stages. The first stage is removal of the amorphous regions as this portion of the cellulose polymer is readily accessible by acid. Removal of the amorphous zones only results in a net increase in cellulose crystallinity in the remaining cellulose. Cellulose with the amorphous zones removed is termed hydrocellulose.
• The plot below shows the acid hydrolysis rates of various celluloses. Note that treated celluloses are more rapidly hydrolyzed than untreated. The mercerized and rayon samples are Cellulose II. The plot suggests that Cellulose II is more susceptible to acid than Cellulose I. Why is this the case when Cellulose II has an extra hydrogen bond between planes? The crystallite of Cellulose II is smaller than Cellulose I. If one were to take equal amounts of Cellulose I and Cellulose II and determine total crystallite surface area, the total surface area of Cellulose II would be greater. Therefore Cellulose II has more area which acid can attack. If the rates were corrected for surface area, the data would show that Cellulose II is indeed a bit more stable than Cellulose I.

• Leveling-Off Degree of Polymerization (LODP). If one were to monitor the degree of polymerization of cellulose samples during exposure to acid, one would see a rapid drop followed by a leveling off as shown in the Figure below. This leveling off is independent of acid type or concentration but is source dependent. The degree of polymerization at the leveling-off point is related to the size of the crystallite. As can be seen from the Figure, rayon has a smaller LODP than cotton or wood cellulose (and hence a smaller crystallite).

Cellulose Source	LODP	% Hydrocellulose
Ramie	300-400	90-94
Wood Pulp	200-300	90-94
Milled Wood Pulp	80-100	88-92
Rayon	40-100	95-98

REACTIONS OF CELLULOSE AND HEMICELLULOSE WITH ALKALI (BASE)

1. AT TEMPERATURES BELOW 100°C.

• Inter- and Intra-crystalline swelling of cellulose, and swelling and partial dissolution of hemicelluloses. Can get dissolution without degradation.
• Alkali-catalyzed oxidation, causing random degradation.
• Peeling reactions: a stepwise degradation from the reducing end. Contributes to the polysaccharide yield losses of the kraft pulping process. Reaction also consumes alkali.
• Stopping reactions: forms an alkali stable end-group which prevents further peeling.
• Removal of hemicellulose acetate groups.

2. AT TEMPERATURES ABOVE 100°C.

• A. Reactions B-E above.
• B. Alkaline hydrolysis, causing random degradation.

3. PEELING/STOPPING REACTIONS.

• Require the reducing end group.
• Will take off 1-3, 1-4, and 1-6 linkages. 1-2 linkages are stable, offer protection. The products of both the stopping and peeling reactions are saccharinic acids. These compounds consume alkali in the kraft pulping process.
• About 50-80 units cleaved off before the stopping reaction occurs (which prevents further peeling). The peeling and stopping reactions are in competition with one another but the peeling reaction has a faster rate.
• Softwood xylans are more stable than hardwood xylans, due to the presence of the (1-3)-alpha-L-arabinofuranosyl side chains and that there are twice as many glucuronic acid side chains (compared to hardwood xylan) which help to stabilize from peeling.

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