Wood Chemistry

Essential of Carbohydrate

Chemistry

Carbohydrates

Photosynthesis is probably the most fundamental of all life

processes and provides a means of converting inorganic

carbon, in the form of carbon dioxide, into carbohydrates

and then into other organic compounds.

The early photosynthetic bacteria first appeared about

3,000 million years ago, and were joined much later by

blue-green algae (2,000 million years ago) and the first

vascular land plants (400 million years ago).

Carbohydrates

Together these organisms now produce

approximately 14 1010 tones of organic matter

every year according to the process shown in

CO2 H2Oh

Photosyntheticorganism

Carbohydrates O2

Carbohydrates

Definitions

The name carbohydrate was originally derived from the general formula Cx(H2O)y formally to hydrates of carbon, but this type of

sample definition does not cover the broad class of carbohydrates.

The sugars in a plant usually function as a source of energy while polysaccharides, such as starch, fulfill the need for the storage of reserve food or they contribute mechanical strength to the plant cell.

CarbohydratesA variety of carbohydrates are included as essential building elements in natural compounds performing vital functions in living organisms.

Carbohydrates may be classified into following three large groups

Monosaccharide

Oligosaccharides

Polysaccharides

Monosaccharide is the simple sugars. Oligosaccharides

Oligosaccharide consist of several monosaccharide residues joined together by glycosidic linkages, namely di-, tri-, tetra-saccharide. The name oligosaccharide is usually restricted to the group of carbohydrates in which the number of monosaccharide units less than 10.

Polysaccharides Polysaccharides are complex molecules composed of a

large number of monosaccharide unit joined together by glycosidic linkages.

Some Terms Used in Carbohydrate Chemistry Aglycone

The nonsugar part of a glycoside; usually an alcohol, phenol or amine.

Aldoses Monosaccharide contain aldehyde function.

Aldopentose A five-carbon sugar with an aldehyde or cyclic

hemiacetal functional group. Aldohexose

A six-carbon sugar with an aldehyde group or cyclic hemiacetal group.

Some Terms Used in Carbohydrate Chemistry Ketoses

Monosaccharide contain keto function.

Furanose Five-membered cyclic sugar from furan.

Pyranose

Six-membered cyclic sugar from pyran.

Septanose

Seven-membered cyclic sugar.

Glycosans

Intramolecular glycoside

GlycosansAnhydro sugars are from sugars by the elimination of water from a pair of hydroxyl groups. Glycosan are strictly intramolecular gycosides.

GlycosansAnhydro sugars are from sugars by the elimination of water from a pair of hydroxyl groups. Glycosan are strictly intramolecular gycosides.

Some Terms Used in Carbohydrate Chemistry

- or -Glucoside Acetal derived from the cyclic hemiacetal from - or -glucoside

glucose. - or -Glycoside

General term for the acetal derivative of any suger. Anomers

Sugar isomers differing only in the configuration of the hemiacetal carbon atom; they are designated asor .

Epimers Sugars that differ only in the configuration at 1 carbon .

Mutarotation Change in optical rotation as a fresh solution stands.

anomers(epimers)

anomers(epimers)

Configuration of Monosaccharide

Isomers Compounds which have the same molecular

formula but differ in some way in the arrangement of atoms.

Type of Isomers Structural isomers

Stereoisomer

Structural isomers

Isomers in which the bonding arrangement of atoms differ; they include chain isomers, position isomers, and functional group isomers Chain isomers (Sketal isomers)

Butane Isobutane

Position isomers

1-Propanol 2-Propanol

Functional group isomers

Propionic acid Methyl acetate

Stereoisomer Isomers in which the bounding of the atoms in the same but the

spatial arrangements of the atoms differ; they include geometric isomers and optical isomers, etc. Geometric isomers

cis-2-butene trans-s-butane

Optical isomers

Enantiomers vs. Diastereoisomers

DiastereomersIf a compound contains two chiral atoms, it may exist in four stereoisomeric forms. Since the configuration at each chiral carbon may be either R or S, there are four stereochemical possibilities: RR, SS, RS, and SR. The RR and SS stereoisomers are enantiomers. The RS and SR stereoisomers are also enantiomers. The RR stereoisomer is a diastereomer of both the RS and the SR stereoisomers. The SS stereoisomer is a diastereomer of both the RS and SR stereoisomers.

Configuration of Aldoses

For many system which n chiral carbon atoms, there are 2n stereoisomers, composed 2n-1

enatiomeric (mirror-image) pairs.

An aldotetrose is a four-carbon sugar that has two chiral centers. There are 22 = 4 possible stereoisomers, or two D, L pairs of enatiomers called Erythrose and Threose.

D, L System of Carbohydrate Configuration Nomenclature Compounds are assigned to the D-family or the L-family

according to the projection of the OH group at the

lowest chiral carbon atom.

If the OH group projects to the right in a plane

projection structure, the compound is in the D-family. If

this OH projects to the left, the substances is in the L-

family.

D-Glyceraldehyde L-Glyceraldehyde

D-Erythrose D-Threose

Fischer Projection Formulas for Acyclic Forms of D-aldoses

Mutarotation

On dissolution of sugars

in water, the optical

rotation of the solution

changes continuously

until an equilibrium is

reached. This

phenomenon, termed

Mutarotation.

Conformation of the Six-membered Ring Sysem

Chair Boat

Skew boat Half-chair

4C1

1C4

Monosubstituted molecules the substituent favors the equatorial position

Stable chair conformation of -D-glucopyranose and -D-fructopyranoseStrong hydrogen-bonding interactions

More stable

Conformation of the Five-membered Ring System

Envelope conformation Twist conformation

Pseudoequatorial carbon-hydrogen bonds

Monosaccharide Most of the monosaccharides occur as glycosides and as

units in oligosaccharide and polysaccharide and only comparatively few of them are present free in plants.

D-Glucose is the most abundant monosaccharide in nature. It occurs in a free state in many plants, especially in fruits and can be prepared from cellulose and starch by acidic or enzymic hydrolysis.

D-Glucose -D-GlucoseFischer-Tollens projectionHaworth perspective formulas

: -CH2OH and -OH at the same side

Monosaccharide D-Mannose and D-galactose,

which are aldohexoses, are important components in hemicellulose.

The most comment aldopentose, abundant members of the hemicellulose, are D-xylose and L-arabinose. D-Ribose is a constituent of nucleosides.

D-mannose D-galactose

D-xylose L-arabinose

D-ribose

galactose: galactose:

Monosaccharide No tetroses or triose have been

detected free in plants, but D-erythrose

4-phosphate is an important

intermediate in many transformation,

and D-glyceraldehyde and

dihydroxyacetone are essential

components in cellular metabolism.

D-erythrose 4-phosphate

D-glyceraldehyde

dihydroxyacetone

Monosaccharide Deoxysugar

L-rhmnose (6-deoxy-L-mannose) occurs as a constituent in gum polysaccharides and traces of it are present in hemicellulose (xylan)

D-Fructose, which represents the only abundant ketose in plants, is present both free and in a combined state. Compositae and Gramineae families store polymers of D-fructose such as insulin, as serve material rather than starch.

L-rhmnose

Monosaccharide Derivatives

In principal, the sugar derivation are formed

by:

Reaction of the free carbonyl or the anomeric

hydroxyl at C-1.

Reaction of the hydroxyl groups at other positions.

Glycosides Sugars react as hemiacetals with hydroxyl compounds,

such as alcohols and phenols forming glycosides.

The glycosides are easily hydrolyzed by aqueous acids

to free sugars but they are fairly stable toward alkali.

Acetals

Acetal formation

aldehyde Cyclic acetal

Ethers

Etherification is often used in the determination of structure and types of

linkages between sugars in oligo and polysaccharides.

Ethers are very stable against both acids and bases.

Carbohydrates can be converted into ethers by treatment with an alkyl

halide in the presence of base (Williamson ether synthesis)

Ag2O

CH3I

Cellulose Etherification

Cellulose etherification: by treating alkali cellulose with

Alkyl or aryl halides (or sulfate)

Alkene oxides

Unsaturated compounds activated by electron-attracting group

Cell-OH + OH -

Cell-O + H2O-

Cell-O + R-Cl Cell-OR + Cl- -

Anhydro sugars Anhydro sugars are formed from sugars by the elimination of

water from a pair of hydroxyl groups

Glucosans are strictly intramolecular glycosides. Its anhydro

linkage is readily by action of acids, sometimes also by bases.

Anhydro sugars

Epoxides

Ethers are derived only from alcoholic

hydroxyls and the hydroxyl group in the

anomeric center does not participate.

Epoxides are formed when the sugar molecule

contains both a good leaving group and a

suitable located ionized hydroxyl group. (SN2

reaction).

Anhydro sugars The ring size of epoxides can vary from three- to six-

membered rings.

Oxiranes (three membered derivatives) intermediates are probably formed during alkaline hydrolysis of polysaccharides such as cellulose and starch.

Formation of methyl 3,4-anhydro--D-galactopyranosides

Esters

Hydroxyl groups of sugars can form esters both with

organic and inorganic acids.

The phosphate esters, such as D-glucose 6-

phosphate, are important natural products and key

intermediates in the biosynthesis and bioconversion

of various carbohydrates.

Esters

1: D-glucose 1-phosphate

2: 2-O-acetyl--D-xylopyranosides

3: 3-O-acetyl--D-xylopyranosides

4: -D-galactopyranose 4-sulfate

5: cis-Inositol

inositol: inositol:

Oligo- and Polysaccharides

More than 500 oligosaccharides are known today,

most of them occurring as free natural substances.

Oligosaccharides are also obtained by partial acidic

or enzymic hydrolysis of polysaccharides.

Disaccharides can be considered to be glycosides in

which the aglycon part is another monosaccharide.

Oligosaccharides

Disaccharides are called reducing or nonreducing, depending on

whether one or both reducing groups are involved in the

formation of the glycosidic linkage.

Disaccharides

Cellobiose

Maltose

Sucrose

Polysaccharides

Polysaccharides are the most abundant constituents of living

matter.

The chain molecules can be either linear or branched, a fact

that markedly affects the physical properties of the

polysaccharides.

Reaction of Carbohydrates

Oxidation

By mild oxidants, aldoses are oxidized to aldonic acid or to

corresponding aldonic acid end groups, whereas ketoses

are resistant.

Stronger acids, convert aldoses to dicarboxylic acids,

termed aldaric acid.

Aldonic and aldaric acids occur in acidic solution mainly in

the form of lactones, which are intramolecular esters.

aldaric acidaldaric acid

D-glucose D-gluconic acid

D-glucaric acid

Preparation of D-glucuronic acid from D-glucose

Examples of Neutral Oxidation Products of Aldoses and Ketoses

The neutral oxidation products of carbohydrates include dialdose,

aldosuloses, and glycodiuloses.

They are important intermediates in the synthesis of carbohydrates and

are prepared by chemical or enzymic oxidation of hydroxyl groups in the

free aldoses or ketoses or their protected derivatives.

D-galactose can be selective oxidized to meso-galacto-hexodialdose by galactose oxidase

D-glucose is oxidized by pyranose-2-oxidase to arabino-hexos-2-ulose

L-sorbose is oxidized by pyranose-2-oxidase to threo-2,5-hexodiulose

Periodate Oxidation of 1,4--D-glucan

1-OH groups are oxidized to formaldehyde, 2-OH to higher aldehyde, and 3-OH to ketones.

-Hydroxyaldehydes are oxidized to formic acid and an aldehyde. Useful for structural studies.

Reduction Aldoses and ketoses can be reduced to alditols by

various agents for which purpose sodium borohydride is very useful.

Only one product is formed from aldoses, whereas ketoses give rise to two diastereoisomers because of a new asymmetric center.

**

Addition and Condensation Reaction of Carbonyl Groups Addition reaction of carbonyl groups severed as valuable tools

for structural studies of carbohydrates.

Hydroxylamine, hydrazine, and phenylhydrazine react with

carbonyl groups to yield oximes and hydrazones.

phenylosazone

Kiliani Reaction Cyanide ions react reversibly with sugars to yield

cyanohydrins.

Because of the formation of a hydroxyl group in place of the aldehyde group, a new asymmetric center is generated.

Catalytic hydrogenation of the cyanohydrins gives the corresponding aldoses, and the kiliani reaction thus opens the possibility for chain lengthening of aldoses.

Kiliani Reaction

Formation of epimeric -hydroxysulfonic acids from D-xylose in the presence of hydrogen sulfite ions

The equilibrium of this reaction depends on the configuration of the sugar

Mannose and xylose form more stable bisulfite addition products than glucose, and ketoses (fructose) show almost negligible affinity toward hydrogen sulfite ions

The Influence of Acid Mechanism of the acid-catalyzed hydrolysis of methyl -

D-glucopyranoside to D-glucose

Reactions of Sugars in the Presence of Concentrated Mineral Acids

R = H : furfuralR = CH2OH: hydroxymethylfurfural

- and -angelica lactones

prolonged heating time hydroxymethylfurfural decompose to form - and -angelica lactones

The Influence of Alkali

In alkaline solutions

aldoses and ketoses

undergo rearrangements.

Lobry de Bruyn-Alberda

van Ekenstein

transformation.

Enolization to an 1,2-enediol

D-mannoseD-glucose

D-fructose D-allulose

Allulose Allulose

Endwise Alkaline Degradation (Peeling reaction)

isomerization enediol formation alkoxy elimination

tautomerization benzilic acid rearrangement

Termination of the Peeling Reaction

-hydroxy elimination

In kraft pulping, the cellulose molecules are subjected to this endwise peeling, which results in a loss of about fifty glucose units from a single molecule.