chemistry of wood constituents 2

8/10/2019 Chemistry of Wood Constituents 2

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Chemistry of the wood constituents

1. Carbohydrates Cellulose is the world's most abundant and

important biopolymer. It has been estimated that globally 1 a 11

tons of this macromolecular substance aresynthesized and destroyed each year.

Although the technical utilization of cellulosehas a long history, the understanding of itschemistry and structure is relatively new.


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Today, the chemical structure of cellulose has beenclarified in detail, but its supramolecular state andits polymer properties are not yet fullyunderstood.

Cellulose is a polydispersed linearhomopolysaccharide consisting of beta-D-glucopyranose moieties (in 4C 1 conformation)linked together by (1-4)-glycosidic bonds .


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Because of the strong tendency for intra- andintermolecular hydrogen bonding, bundles ofcellulose molecules aggregate to microfibrils,which form either highly ordered (crystalline) orless ordered (amorphous) regions. Thesemicrofibrils pass through several crystallineregions (about 60 nm in length).

As a result of further aggregation of microfibrils,"fiber wall cellulose" with a high degree ofcrystallinity (60%-75%) is formed.

This also means that cellulose is relatively inert

during chemical treatments but also that it issoluble only in a few solvents. The most commoncellulose solvents are cupriethylenediamine(CEO) and cadmiumethylenediamine (Cadoxen).


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Besides cellulose, the other major naturallyoccurring carbohydrate based polymers,hemicelluloses, are heteropolysaccharides and areclearly less welldefined than cellulose.

Their building units are hexoses (D-glucose, D-mannose, and D-galactose), pentoses (0- xylose,

L-arabinose, and 0- arabinose), or deoxyhexoses(L-rhamnose or 6-deoxy-Lman nose and rare L-fucose or 6-deoxy-L-galactose).

Softwoods and hardwoods differ not only in thecontent of total hemicelluloses but also in thepercentages of individual hemicelluloseconstituents.


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Lignin Lignin is an amorphous polymer with a chemical structure

that distinctly differs from the other macromolecularconstituents of wood.

It is also characteristic that, unlike wood carbohydrates, thechemical structure of lignin is irregular in the sense thatdifferent structural elements (phenylpropane units) are notlinked to each other in any systematical order.

In general, lignins are roughly classified into three majorgroups: softwood, hardwood, and grass lignins.

There are several industrially based technical lignins thatare byproducts of the chemical pulping.

Kraft lignin (or sulfate lignin), alkali lignin (or soda lignin),and lignosulfonates are derived from kraft, soda-AQ, andsulfite pulping of wood, respectively. It is also known thatthere are many characteristic differences between all theselignins.


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However, in the following, the chemical structure oflignins present in intact softwood and hardwood willbe emphasized.

According to a widely accepted concept confirmedoriginally by numerous studies of lignin labeled withradioactive carbon (14C), lignin can be defined as apolyphenolic material arising primarily from enzymic

dehydrogenative polymerization of three phenyl-propanoid units (p-hydroxycinnamyl alcohols). This biosynthesis process comprises various oxidative

coupling reactions of the resonance stabilized phenoxy

radicals obtained from these precursors leading to theformation of a randomly cross-linked macromolecule. Although the precursors of the p-hydroxycinnamyl

alcohol type are practically the only building units of alllignins.


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trans-Coniferyl alcoholtrans-Sinapyl alcohol trans-p-Coumaryl alcohol


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The structural building blocks of lignin are joinedtogether by ether linkages (C-OC) and carbon-carbon bonds (C-C).

Of these inter-unit linkages, the former onesdominate with the most prominent type in bothsoftwood and hardwood lignins.

Detailed knowledge about the characteristics ofthese linkages is of great theoretical interest, forexample, for a better understanding of thedegradation reactions of lignin in technical

processes, such as delignification. In addition, numerous miscellaneous linkages and

minor structures are known. It is also evident thatthe frequency of these groups can vary according

to the morphological location of lignin.


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Functional groups of native lignin

Functional group Softwood lignin Hardwood lignin

Phenolic hydroxyl 20-30 10-20

Aliphatic hydroxyla 115-120 110-115

Methoxyl 90-95 140-160

Carbonyl 20 15


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Lignin-hemicellulose bonds

The close association between lignin andcarbohydrate components in wood stronglysuggests the existence of chemical linkagesbetween these constituents.

This question has been a subject of much debateand many intensive studies. It has been evident for a long time that physical

and chemical interactions (i.e., hydrogen bonds,

van der Waals' forces, and chemical bonding)occur between lignin and carbohydrates.

But it has been difficult to definitively verify theprecise type and amount of chemical linkages


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The nature of the different bonds between ligninand carbohydrates is highly complex and far frombeing completely understood.

However, it is now generally accepted that lignin ischemically linked at least with a part of woodhemicelluloses, although there are even indicationsof lignin and cellulose bonds.

The terms "lignin-polysaccharide complex" (LPC) or"lignin-carbohydrate complex" (LCC) are generallyused for describing the covalently bondedaggregates of lignin and hemicelluloses.

Although the more accurate term "lignin-hemicellulose complex" (LHC) can be used as well. Chemical bonds have been reported between lignin

and practically all the hemicellulose constituents.


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BENZYL ETHERS

SoftwoodsS

oftwoods

BENZYL ESTERS PHENYL GL YCOSIDES

Softwoods and hardwoods Softwoods and hardwoods


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Extractives Extractives comprise an extraordinarily large

number of diverse substances (i.e., severalthousands of individual compounds), mainly withlow molecular masses.

By a broad definition, these extractives are either

soluble in neutral organic solvents [e.g., diethylether, methyl tert-butyl ether, petroleum ether,dichloromethane, acetone, ethanol, methanol,hexane, toluene, and tetrahydrofuran (THF)] orwater.

Thus, these substances may be both lipophilic andhydrophilic and are regarded as nonstructural

wood constituents.


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The term "resin" is often used as a collectivename for the lipophilic extractives (with the

exception of phenolic substances), which canbe extracted from a wood sample by nonpolarorganic solvents but are insoluble in water.

The extractives impart color, odor, and tasteto wood, and some of them can be the energysource (fats and waxes) of the biologicalfunctions in the wood cells.

Most resin components protect the woodagainst microbiological damage or insectattacks.


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The composition of extractives varies widelyfrom species to species, and the total amount

of extractives in a given species depends ongrowth conditions. For example, the typical content of

extractives in Scots pine Norway spruce andsilver birch is, respectively, in the range of 2.5to 4.5, 1.0 to 2.0, and 1.0 to 3.5% of the wooddry solids.


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Classification of organic extractives in woods

Aliphatic and alicyclic compounds Phenolic compounds Other compounds

Terpenes and terpenoids Simple phenols Sugars

(including resin acids and Stilbenes Cyclitols

steroids) Lignans Tropolones

Esters of fatty acids Isoflavones Amino acids

(fats and waxes) Condensed tannins Alkaloids

Fatty acids and alcohols Flavonoids Coumarins

Alkanes Hydrolyzable tannins Quinones


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Classification of the main terpene structural types in woodytissues

Name Number of Molecular

(C 1o H16 ) units formula

Monoterpenes 1 C10H16

Sesq u ite rpenes 1.5 C15H24

Diterpenes 2 C2oH32

Triterpenes 3 C30H48

Polyterpenes >4 > C 4oH64

Ch i l f d id


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Chemical structure of some common terpenes and terpenoids.

MONOTERPENES AND MONOTERPENOIDS

(-)- Limonene3-Carene

SESQUITERPENES AND SESQUITERPENOIDS

Longifolenea-Cadinola-MuuroleneFarnesene


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Monoterpenes and monoterpenoids are volatilecompounds and contribute substantially to the odorof wood.

Wood contains a large variety of aromatic extractivesreaching from simple phenols to complex polyphe-nols and their related compounds . It is characteristic for polyphenols that they often

are colored compounds which are accumulatedabundantly in the heartwood of many species andonly small amounts are present in sapwood . Some of them are probably degradation products ofcompounds that can be hydrolyzed during extractionor steam distillation . This kind of extractives also has fungicidal propertiesand thus protect the tree against microbiologicalattack.


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Examples of fatty alcohols and fatty acidsin wood

FATTY ALCOHOLS, Arachinol or eicosanol (C 20). behenol or docosanol (C 22 ). and

lignocerol or tetracosanol (C 24)FATTY ACIDS

Saturated acids: lauric or dodecanoic acid (Cd. myristic ortetradecanoic acid (C 14 ). palmitic or hexadecanoic acid (C 16 ).stearic or octadecanoic acid (CIS)' arachidic or eicosanic acid(C20), behenic or docosanoic acid (C 22), and lignoceric ortetracosanoic acid (C 24 ). and unsaturated acids.' oleic or cis-9-

octadecenoic acid (C 181 (ge)), linoleic or cis. cis-9, 12-octadecadienoic acid (C 182 (ge 12e))' linolenic or cis.cis.cis-9, 12,15-octadecatrienoic acid (C 183 (ge.12e15e)), pinolenic orcis.cis.cis-5,9, 12-octadecatrienoic acid (C 183 (5ege 12e))' andeicosatrienoic or cis. cis, cis-5, 11, 14-eicosatrienoic acid(C

203(5e,11 e14e))

I g i t


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Inorganic components In woods from temperate zones, elements other than

carbon, hydrogen, oxygen, and nitrogen make up between 0.1% and 0.5% of the dry solids in wood, whereas those fromtropical and subtropical regions make up even to 5%.

In practice, the total amount of wood inorganics is measuredas ash, which is the residue obtained after the propercombustion of the organic matter of a wood sample.

The ash contains mainly different metal oxides and averagevalues for the ash content of commercial softwoods and hard-woods are generally in the range of 0.3% to 1.5% of the wooddry solids.

There is also a significant dependence of the ash content andcomposition on the environmental conditions (e.g., sitefertility and climate) under which the tree has grown and, onthe other hand, on the location within the tree.


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Approximative concentration levels of various elements a in the drystemwood of softwoods and hardwoods

Range, Elements

ppm

400-1000 K Ca

100-400 Mg P

10-100 F Na Si S Mn Fe Zn Ba

1-10 B AI Ti Cu Ge Se Rb Sr y Nb

Ru Pd Cd Te P

0.1-1 Cr Ni Br Rh Ag Sn Cs Ta Os

chemistry of wood constituents 2

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