Chemical Properties of Wood

Properties of wood play a predominant role in its utilization. Knowledge of the chemical properties of wood is very essential for understanding the principles and techniques in different domains of chemical utilization of wood.

Generally, wood is taken as an inert substance but technically it can be oxidized or converted into various products under suitable treatments.

The chemical components of wood vary from species to species. But there is approximately 50% carbon, 6% hydrogen, 44% oxygen, and trace amounts of several metal ions.

Wood is comprised of two types of substances:

• Wood Substances – They are insoluble in neutral solvents and non-volatile with steam. Also known as cell wall substance.
• Extraneous Substances – They are extractable by neutral solvents like benzene, ether, or by steam distillation.

Wood Substance

It consists of cellulose, other polysaccharides (hemicellulose and pectin), and lignin substances.

1. Cellulose

It is an organic compound. and is the skeleton of the cell wall. It constitutes about 40 to 50 % of wood. Cellulose has no taste, is odorless, insoluble in water and most organic solvents, and is biodegradable.

2. Other polysaccharides

They comprise 10 to 35 % of cell walls.

• Hemi-cellulose: The amount of hemicellulose is usually between 25 and 35%. It is around 20% in deciduous trees but near 30% in conifers. Hemicelluloses include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan.
• Pectin: It usually forms 1-2% and consists of a complex set of polysaccharides that are present in most primary cell walls. Particularly abundant in the non-woody parts of terrestrial plants. It helps to bind cells together. May undergo chemical changes during the subsequent stages of cell development.

3. Lignin

It accounts for 20 to 35% of the wood. It is an encrusting material of plant tissues and is built of phenyl propane units. It is easily oxidizable, and soluble in hot alkali and bisulfites.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin
components. It is an aromatic compound and does not appear in a free state but rather combines with other constituents of wood such as cellulose or hemicellulose or simple sugars.

Extraneous substances

They are not part of the cell wall and are found in the lumen of cells. Most of them are removed either by steam distillation or by extraction with neutral solvents like benzene, ether, acetone, alcohol, gasoline, and water.
The presence of extractives is a problem sometimes in wood like corrosion caused in
metal equipment, however, some factors like durability, color, odor, taste, inflammability, toxicity, density, etc. are also correlated.

Extraneous substances can be organic or inorganic.

1. Inorganic extractives

They are either purely inorganic salts, including free silica, or salts of organic acids, and those form ash on burning. Ash content and composition vary with the place, growing condition, and the season of the year.

The metallic radicles are primarily calcium, potassium, and magnesium, and sometimes aluminum, iron, manganese, sodium, lead, and zinc also can be found in trace amounts.

2. Organic extractives

• Essential Oils – These are volatile, odoriferous liquids that occur in many species of wood and their content is below 1% (sandalwood has 8%). They may evaporate when coming in contact with air.
  They are a comprised form of hydrocarbons, acids, phenols, alcohol, oxides, ether, ketones, etc. These are used in perfumes, soaps, confectionary medicines, etc.
• Wood resins – They occur in intercellular passages, cell walls, and the interior of cells. They are comprised of resin acids and fatty acids, along with other chemicals.

• Dyes and tans – They do not have a fixed chemical composition. In general, they are composed of carbon, hydrogen, and oxygen. Obtained from barks, roots, leaves, flowers, wood, and other parts of plants that contain tanning and coloring matters. Tans are used for tanning the skins of animals, while dyeing is used for coloring.
• Carbohydrate components – Many soluble carbohydrates are found in the cell and their proportion ranges from 0.5 to 5%. Their presence makes plants susceptible to insect attack. Examples: sandalwood oil, eucalyptus oils, etc.
• Cyclitols – They are sweet like sugar, but neither contain any carbonyl group nor do they ferment.
• Other extraneous materials – Fatty oils (in Tilia sps.), and alkaloids (in Rubiaceae, Leguminosae).