Chemistry Everyday for Everyone edited by
products of chemistry
George B. Kauffman California State University–Fresno Fresno, CA 93740
Chem-Is-Tree Dana M. Barry Center for Advanced Materials Processing (CAMP), Clarkson University, Potsdam, NY 13699 A tree is a woody plant that contains chemicals and undergoes chemical reactions. The three main parts of a tree are the crown, the stem, and the roots. The crown includes foliage, twigs, and branches, which are really extensions of the main stem. The stem, called the trunk or bole, furnishes lumber, provides mechanical support of the crown, and conducts water and mineral nutrients upward and food downward. The roots, usually underground and as extensive as the crown, anchor the trunk in the ground, absorb nutrients and transport them to the stem, and store reserve food. Chemical Make-Up Wood is obtained from the stems, roots, and branches of trees. It is not a homogeneous material with a uniform structure but instead a group of tissues composed of different kinds of cells that perform specific functions in the living plant. Xylem is the principal water-conducting tissue, phloem is the food-conducting tissue, and the cambium is a layer of cells between the phloem and xylem that is responsible for generating new cells and for secondary growth. Wood’s major component is the organic compound cellulose (C6H 10O5 )n, a polymer of glucose (see Fig. 1). It is classified as a complex homopolysaccharide because it results from combining 9 or more identical monosaccharide units. It is a colorless, virtually odorless and tasteless, combustible solid. Closely associated with the cellulose is a phenylpropane polymer of amorphous structure called lignin, which can be separated from cellulose by a chemical reaction at high temperature. The cellulosic outer layer of trees is bark. The bark of certain species (oak, hemlock, and others) provides tannic acid, and that of fir trees provides the chemical quercitin used in medicine. The bark from the oak species Quercus suber is in the form of cork, which is extremely light and impervious to water. Hemicellulose can make up as much as 30% of the dry weight of wood. In contrast to cellulose, it is a heteropolysaccharide, containing monosaccharide residues in addition to glucose, which is the most abundant one. Like cellulose, most hemicelluloses function as supporting materials in a tree’s cell walls. Other chemical substances such as volatile oils, fatty acids, and aromatic compounds are also present in wood. Some are described below. Volatile oils, which may contain terpenes and related materials, paraffins, and aromatic compounds, are responsible for the odor associated with fresh wood. Pine oil, a combustible liquid having a piny odor and containing tertiary and secondary terpene alcohols, is in the wood of species such as Pinus palustris. Camphor, a ketone used in medicine, occurs naturally in the wood of the camphor tree, Cinnamomum camphora. In addition, colored constituents provide esthetic value to wood, and certain phenolic compounds confer resistance to fungal and insect attack.
Figure 1. A white birch has cellulose in its stems and branches. A structure of cellulose is shown on the next page.
Like most leaves, leaves of trees such as maples contain the green pigment chlorophyll and chemicals classified as carotenoids (yellow and orange colors) and flavonoids (red, yellow, blue, orange, and ivory colors) that are responsible for a tree’s beautiful fall colors (Fig. 2). Trees and other plants need water and certain essential elements to grow and reproduce. In addition to carbon, oxygen, and hydrogen, the building blocks for all organic compounds, nitrogen is required for the protein and nucleotide components, phosphorus is used for energy metabolism and nucleic acids, and a variety of mineral elements participate in enzymatic reactions and serve other functions common to all living cells.
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Chemistry Everyday for Everyone
OH O HO
OH
O OH
O HO
OH
O OH
O HO
OH
O OH
O HO
OH
O OH
cellulose
O HO
OH
O O HO
OH
O OH
O
Figure 2. A maple tree displaying beautiful fall colors due to carotenes. The structure of zeaxanthin, a common carotenoid pigment in maple leaves, is illustrated on the right.
OH
HO
zeanxanthin
chlorophyll
N
N Mg
N
N
H H 2C H O
O
O O
Figure 3. A weeping willow contains chlorophyll. The structure of chlorophyll is shown on the right.
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Chemistry Everyday for Everyone Chemical Reactions
Interesting Products From Trees
Like all green plants, trees rely on photosynthesis in their leaves to form carbohydrates, a source of energy for metabolism and growth. Here carbon dioxide reacts with water in the presence of sunlight and chlorophyll (see Fig. 3). The general reaction is:
Vanilla flavorings in frozen desserts and baked goods need not be natural vanilla. Vanillin is an artificial vanilla flavor that can be made from lignin in wood pulp (1). Pulverized forms of wood pulp have been used as fillers in foods and pharmaceuticals. Their use is limited because the highly fibrous form feels uncomfortable in one’s mouth. This problem is overcome by reducing the wood pulp fibers to colloidal microcrystalline cellulose, which is used in cookie fillings and whipped toppings and to control the formation of ice crystals. The pharmaceutical industry uses microcrystalline cellulose to improve the stability of drugs in tablet form. Rayon, which is used in felts, blankets, and nonwoven fabrics, and cellophane, which is used in wrapping, are made from wood pulp by the viscose process. This process makes regenerated cellulose (such as rayon) by first converting cellulose to the soluble xanthate, which can be spun into fibers. Then, by treatment with acid, it is reconverted to cellulose. Much of the world’s natural rubber supply comes from the Hevea brasiliensis tree. The tree is tapped and a saplike substance called latex is collected. Latex includes water as well as globules of rubber hydrocarbon coated with protein. Rubber latex can be concentrated by evaporation or centrifugation. It can be preserved by the addition of ammonia and coagulated with acetic or formic acid. Natural rubber is used in many items such as cements, adhesives, vehicle tires, footwear, and electric insulation.
6 CO 2 + 6 H2O + 672 kcal → C 6H 12O6 + 6 O2↑ Food is stored in a tree primarily in the form of starch. It may also be stored as fat in seeds and some fruits and as protein in seeds. Energy and building materials are made available from the stored materials by chemical reactions. Starch molecules are hydrolyzed to form a soluble and transportable sugar. In a similar fashion proteins are converted to amino acids. Resistance of Wood to Solvents and Chemicals At ordinary temperatures, wood is essentially unattacked by neutral organic solvents and cold water, although water dissolves small amounts of extractive components of the wood. The amount of material dissolved by water increases as the water temperature increases. The presence of hot water leads to an increase in acidity caused by hydrolysis of acetyl groups to acetic acid. Wood is somewhat resistant to dilute acids at ordinary temperature, but more concentrated acids or dilute mineral acids at higher temperatures attack wood with hydrolysis of the polysaccharides. Solutions of strong bases attack carbohydrates and dissolve a considerable quantity of wood. Wooden boards exposed to the weather without a protective coating tend to develop roughness and a blue-gray color on the surface. This is due to oxidation of cellulose and lignin under the influence of light and moisture. Under suitable conditions, wood is disintegrated by decay because of the growth of fungi in the tissue. The burning of wood provides heat and light energy, water vapor, carbon products, and more. Pyrolysis, in the thermal decomposition of wood, yields noncombustible and combustible gases and vapors as well as charcoal. Pulping Process Many manufacturers of cellulosic products begin with cellulose in the form of pulp. Pulping employs chemical and mechanical methods that emphasize the importance of energy efficiency, chemical recovery, and measures to avoid pollution. In mechanical pulping where a stone grinds wood billets and in thermochemical and mechanical wood chip refining, yields are high and the amount of waste that causes pollution is relatively low. Chemical methods are used to free the cellulose fibers more fully. Chemical pulp includes soda process pulp, sulfite process pulp (which is mostly from spruce and other coniferous woods), and sulfate process pulp, which is mainly from softwoods.
Literature Cited 1. Hocking, M. B. J. Chem. Educ. 1997, 74, 1055–1059.
For Further Reading Agosta, W. C. “Medicines and Drugs from Plants”; J. Chem. Educ. 1997, 74, 857–860. Alkema, J.; Seager, S. L. “The Chemical Pigments of Plants”; J. Chem. Educ. 1982, 59, 183–186. Emsley, J. The Consumer’s Good Chemical Guide; W. H. Freeman & Spectrum: New York, 1994. Goheen, D. W. “Chemicals from Wood and Biomass, Parts I and II”; J. Chem. Educ. 1981, 58, 465, 544. Hocking, M. B. “Vanillin: Synthetic Flavoring for Spent Sulfite Liquor”; J. Chem. Educ. 1997, 74, 1055–1059. Kimbrough, D. R. “Supermarket Column Chromatography of Leaf Pigments”; J. Chem. Educ. 1992, 69, 987–988. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Wiley: New York, 1982; Vol. 20. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1993; Vol. 5. Sax, N.; Lewis, R. Hawley’s Condensed Chemical Dictionary, 11th ed.; Van Nostrand Reinhold: New York, 1987. Sjöström, E. Wood Chemistry Fundamentals and Applications; Academic: New York, 1981. Snyder, C. The Extraordinary Chemistry of Ordinary Things, 2nd ed.; Wiley: New York, 1995. Starr, C.; Taggart, R. Biology: the Unity and Diversity of Life, 7th ed.; Wadsworth: Belmont, CA, 1995. Wilson, J. D.; Hamilton, K. J. “Wood Cellulose as a Chemical Feedstock for the Cellulose Esters Industry”; J. Chem. Educ. 1986, 63, 49.
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