Cellulose formula. What are the chemical and physical properties of cellulose

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The composition of cellulose, like that of starch, is expressed by the formula (QHUiO5). The value of n in some types of cellulose reaches 40 thousand, and the relative molecular weight reaches several million. Its molecules have a linear (unbranched) structure, as a result of which cellulose easily forms fibers. Starch molecules have both linear and branched structures.

The composition of cellulose, like that of starch, is expressed by the formula (CjHioOJn. The n value in some types of cellulose reaches 40 thousand, and the molecular weight reaches several million. Its molecules have a linear (unbranched) structure, as a result of which cellulose easily forms fibers. Molecules the same starch have both linear and branched structure.

The composition of cellulose, as well as starch, is expressed by the formula (CgHioOsJn - The value of n in some types of cellulose reaches 40 thousand, and the molecular weight reaches several million. Its molecules have a linear (unbranched) structure, as a result of which cellulose easily forms fibers. Molecules the same starch have both linear and branched structure.

The composition of cellulose includes the remains of p - D-glucose molecules.

One of the most important reactions in terms of cellulose composition is the reaction involving acetolysis, in which acetic anhydride plays the same role as water in hydrolysis, and at the same time affects the acylation of cellulose fragments. In 1879, Fransimont treated cellulose with acetic anhydride and sulfuric acid to produce a derivative that Skraup and Koenig later identified as the crystalline octa-acetate of a disaccharide called cellobiose. The latter reduces Fehling's solution and is hydrolyzed with acid to form two moles of glucose. A similar hydrolysis is also carried out by emulsin, which characterizes the disaccharide as a p-glucoside.

The data given in table. 2.1, although it provides some information about the composition of cellulose, it is completely insufficient to assess the behavior of cellulose during processing.

This, in particular, was shown in experiments with radiocarbon, when C14 was quickly incorporated into cellulose. However, the mechanism for the synthesis of cellulose, like many other plant polysaccharides, has not yet been disclosed. The only polysaccharides that are synthesized outside organisms are only starch and glycogen.

In addition to basic (alpha) cellulose, paper pulp contains a number of hemi-celluloses - low molecular weight hydrocarbons such as hexosans, pentosans and uronic acids. The composition of cellulose also includes residual lignin (about 0 3%), the complete removal of which is impractical, since it has antioxidant properties.

The number of D-glucose residues in the cellulose molecule reaches several thousand, which corresponds to a mol. D-gluco-ea in cellulose is in a chair-like conformation and this excludes the possibility of spiralization of the polyglucoside chain; therefore, the cellulose molecule retains a strictly linear structure.

Observing changes in cell wall composition during development cotton fiber, found that maximum amount residues of galactose, mannose, rhamnose, arabinose, fu-cose, uronic acids and non-cellulosic glucose correspond to the end of the formation of the primary wall or the beginning of the formation of the secondary wall. Until the end of fiber development, only the absolute amounts of xylose and glucose residues, which are part of cellulose, increase.

Interesting data have been obtained on the biosynthesis of cellulose in cotton. With the introduction of glucose labeled with C14 in the first carbon atom, 44% of the radioactivity was found in cellulose, and the remaining 56% in other compounds. Thus, during the period of fiber formation, cellulose is the main compound in which glucose is incorporated.

Passing the resulting mass through a die - a small vessel made of a strong heat-resistant and corrosion-resistant material with a flat bottom, having up to several tens of thousands of small holes with a diameter from 0 04 to 1 mm - threads are obtained into a precipitation bath with a sulfuric acid solution. When interacting with sulfuric acid, the alkali is neutralized, and the viscose decomposes, splitting off carbon disulfide and forming shiny threads of cellulose, somewhat altered in composition. These threads are rayon fiber. The essence of the process of obtaining viscose fiber lies in the fact that first, insoluble cellulose for fiber spinning is transferred into a soluble state. Then it is again transferred to an insoluble state.

The composition of cellulose of various origins contains such functional groups as aldehyde, carboxyl, hydroxyl. Lignin also contains a significant number of functional groups, primarily meta-xyl and hydroxyl, some carbonyl groups and double bonds. Due to the peculiarities of their structure and composition, cellulose fibers have high tensile moduli and significant strength, along with sufficient flexibility due to the ribbon-like shape of the fibers. Fibers of softwood (coniferous) and hard (deciduous) wood exhibit different flexibility due to their equal thickness.

The degree of extraction is the ratio of the mass of the pressed alkaline cellulose to the mass of the original cellulose. As a rule, alkaline cellulose is squeezed out to a threefold increase in weight relative to the weight of the original cellulose, which corresponds to its content of 29 - 31% a-cellulose, 16 - 17% NaOH and 54 - 57% water. With this composition of cellulose, about 7 - 8% NaOH (based on the mass of alkaline cellulose) is bound to cellulose in the form of a chemical or molecular compound, and about 9% is sorbed.

A complex carbohydrate from the group of polysaccharides that is part of the cell wall of plants is called cellulose or fiber. The substance was discovered in 1838 by the French chemist Anselm Payen. Cellulose formula - (C 6 H 10 O 5) n.

Structure

Despite the generality of characteristics, cellulose differs from another plant polysaccharide - starch. The cellulose molecule is a long, extremely unbranched chain of saccharides. Unlike starch, which consists of α-glucose residues, it includes many β-glucose residues that are linked to each other.

Due to the dense linear structure, the molecules form fibers.

Rice. 1. The structure of the cellulose molecule.

Cellulose has greater degree polymerization than starch.

Receiving

In industrial conditions, cellulose is boiled from wood (chips). For this, acidic or alkaline reagents are used. For example, sodium hydrogen sulfite, sodium hydroxide, lye.

As a result of cooking, cellulose is formed with an impurity organic compounds... To clean it, use an alkali solution.

Physical properties

Fiber is a tasteless, solid fibrous substance white... Cellulose is poorly soluble in water and organic solvents. Dissolves in Schweitzer's reagent - ammonia solution of copper (II) hydroxide.

The main physical properties:

• collapses at 200 ° C;
• burns at 275 ° С;
• self-igniting at 420 ° C;
• melts at 467 ° C.

In nature, cellulose is found in plants. It is formed during photosynthesis and has a structural function in plants. It is a food additive E460.

Rice. 2. The cell wall of plants.

Chemical properties

Due to the presence of three hydroxyl groups in one saccharide, fiber exhibits properties polyhydric alcohols and is capable of undergoing esterification reactions to form esters. When decomposed without access to oxygen, it decomposes into charcoal, water and volatile organic matter.

The main chemical properties of fiber are presented in the table.

Rice. 3. Pyroxylin.

Cellulose is mainly used for papermaking, as well as for the production of esters, alcohols, glucose.

What have we learned?

Cellulose or fiber is a polymer from the class of carbohydrates, consisting of β-glucose residues. It is a part of plant cell walls. It is a white, tasteless substance that forms fibers that are poorly soluble in water and organic solvents. Cellulose is isolated from wood by cooking. The compound enters into esterification and hydrolysis reactions and can decompose in the absence of air. When fully decomposed, forms water and carbon dioxide.

The soft part of plants and animals mainly contains cellulose. It is cellulose that makes plants flexible. Cellulose (fiber) - plant polysaccharide, which is the most abundant organic matter on the ground

Almost all green plants produce cellulose for their needs. It contains the same elements as sugar, namely: carbon, hydrogen and oxygen. These elements are found in air and water. Sugar is formed in the leaves and dissolves in the sap and spreads throughout the plant. Most of the sugar is used to promote plant growth and restoration work, the rest of the sugar is converted into cellulose. The plant uses it to create a shell for new cells.

Dissolving cellulose in Schweitzer's reagent

What is cellulose?

Cellulose is one of those natural products that is almost impossible to obtain artificially. But we use it in various fields. A person gets cellulose from plants even after they die off and there is no moisture in them. For example, wild cotton is one of the purest forms of natural cellulose that humans use to make clothing.

Cellulose is a part of plants used by humans as food - lettuce, celery, and bran. The human body is unable to digest cellulose, but it is useful as "roughage" in his diet. In the stomach of some animals, such as sheep, camels, there are bacteria that allow these animals to digest cellulose.

Acid precipitation of cellulose

Cellulose is a valuable raw material

Cellulose is a valuable raw material from which a person obtains various products. Cotton, 99.8% cellulose, is great example what a person can produce from cellulose fiber. If cotton is treated with a mixture of nitric and sulfuric acid, we get pyroxylin, which is an explosive.

After various chemical treatment of cellulose, other products can be obtained from it. Among them: a base for photographic film, additives for varnishes, viscose fibers for the production of fabrics, cellophane and other plastic materials. Cellulose is also used in papermaking.

Structure.

The molecular formula of cellulose is (-C 6 H 10 O 5 -) n, like that of starch. Cellulose is also a natural polymer. Its macromolecular is composed of many residues of glucose molecules. The question may arise: why starch and cellulose are substances with the same molecular formula- have different properties?

When considering synthetic polymers, we have already found out that their properties depend on the number of elementary units and their structure. The same provision applies to natural polymers. It turns out that cellulose has a much higher degree of polymerization than starch. In addition, comparing the structures of these natural polymers, it was found that cellulose macromolecules, unlike starch, consist of residues of the b-glucose molecule and have only a linear structure. Cellulose macromolecules are arranged in one direction and form fibers (flax, cotton, hemp).

Each residue of the glucose molecule contains three hydroxyl groups.

Physical properties .

Cellulose is a fibrous substance. It does not melt and does not pass into a vaporous state: when heated to about 350 ° C, cellulose decomposes - charred. Cellulose is insoluble neither in water nor in most other inorganic and organic solvents.

The inability of cellulose to dissolve in water is an unexpected property for a substance containing three hydroxyl groups for every six carbon atoms. It is well known that polyhydroxyl compounds are readily soluble in water. The insolubility of cellulose is explained by the fact that its fibers are, as it were, "bundles" of parallel threadlike molecules linked by a multitude of hydrogen bonds, which are formed as a result of the interaction of hydroxyl groups. The solvent cannot penetrate inside such a "beam", and therefore, there is no separation of molecules from each other.

The solvent for cellulose is Schweitzer's reagent - a solution of copper (II) hydroxide with ammonia, with which it simultaneously interacts. Concentrated acids (sulfuric, phosphoric) and a concentrated solution of zinc chloride also dissolve cellulose, but at the same time its partial decomposition (hydrolysis) occurs, accompanied by a decrease in molecular weight.

Chemical properties .

The chemical properties of cellulose are primarily determined by the presence of hydroxyl groups. Acting with metallic sodium, cellulose alcoholate n can be obtained. Under the influence of concentrated aqueous solutions alkalis, the so-called mercyrization occurs - the partial formation of cellulose alcoholates, leading to swelling of the fiber and an increase in its susceptibility to dyes. As a result of oxidation, a number of carbonyl and carboxyl groups appear in the cellulose macromolecule. Under the influence of strong oxidants, the disintegration of the macromolecule occurs. The hydroxyl groups of cellulose are capable of alkylation and acylation, giving simple and esters.

One of the most characteristic properties of cellulose is the ability, in the presence of acids, to undergo hydrolysis to form glucose. Similar to starch, cellulose hydrolysis proceeds stepwise. In summary, this process can be depicted as follows:

(C 6 H 10 O 5) n + nH 2 O H2SO4_ nC 6 H 12 O 6

Since there are hydroxyl groups in cellulose molecules, it is characterized by esterification reactions. Of these, the reactions of cellulose with nitric acid and acetic anhydride are of practical importance.

When cellulose interacts with nitric acid in the presence of concentrated sulfuric acid, depending on the conditions, dinitrocellulose and trinitrocellulose are formed, which are esters:

When cellulose interacts with acetic anhydride (in the presence of acetic and sulfuric acids), triacetyl cellulose or diacetyl cellulose is obtained:

The cellulose is burning. This produces carbon monoxide (IV) and water.

When wood is heated without air access, cellulose and other substances decompose. This produces charcoal, methane, methyl alcohol, acetic acid, acetone and other products.

Receiving.

A sample of almost pure cellulose is cotton wool obtained from refined cotton. The bulk of cellulose is isolated from wood, in which it is contained along with other substances. The most common method of obtaining cellulose in our country is the so-called sulfite. According to this method, shredded wood in the presence of a solution of calcium hydrosulfite Ca (HSO 3) 2 or sodium hydrosulfite NaHSO 3 is heated in autoclaves at a pressure of 0.5-0.6 MPa and a temperature of 150 ° C. In this case, all other substances are destroyed, and cellulose is released in a relatively pure form. It is washed with water, dried and sent for further processing, mostly for the production of paper.

Application.

Cellulose has been used by humans since very ancient times. At first, wood was used as a combustible and construction material; then cotton, linen and other fibers were used as textile raw materials. The first industrial methods of chemical processing of wood arose in connection with the development of the paper industry.

Paper is a thin layer of fiber that is pressed and glued to create mechanical strength, smooth surface, to prevent ink bleeding. Initially, for the manufacture of paper, plant materials were used, from which the necessary fibers could be obtained purely mechanically, stalks of rice (the so-called rice paper), cotton, and worn-out fabrics were also used. However, with the development of book printing, the listed sources of raw materials became insufficient to meet the growing demand for paper. Especially a lot of paper is consumed for printing newspapers, and the issue of quality (whiteness, strength, durability) for newsprint does not matter. Knowing that wood is about 50% fiber, they began to add crushed wood to the paper pulp. Such paper is fragile and turns yellow quickly (especially in the light).

To improve the quality of wood pulp additives, different ways chemical processing of wood, allowing to obtain from it more or less pure cellulose, freed from accompanying substances - lignin, resins and others. For the isolation of cellulose, several methods have been proposed, of which we will consider sulfite.

According to the sulfite method, crushed wood is “boiled” under pressure with calcium hydrosulfite. In this case, the accompanying substances dissolve, and the cellulose freed from impurities is separated by filtration. The resulting sulphite liquor is waste in the paper industry. However, due to the fact that they contain, along with other substances, fermentable monosaccharides, they are used as a raw material for the production of ethyl alcohol (the so-called hydrolysis alcohol).

Cellulose is used not only as a raw material in paper production, but also for further chemical processing. The most important are ethers and esters of cellulose. So, when cellulose is exposed to a mixture of nitric and sulfuric acids, cellulose nitrates are obtained. They are all flammable and explosive. Maximum number of residues nitric acid that can be introduced into cellulose is equal to three for each glucose unit:

N HNO3_ n

The product of complete esterification - cellulose trinitrate (trinitrocellulose) - should contain, in accordance with the formula, 14.1% nitrogen. In practice, a product with a slightly lower nitrogen content (12.5 / 13.5%), known in the art as pyroxeline, is obtained. When treated with ether, pyroxylin gelatinizes; after evaporation of the solvent, a compact mass remains. The finely cut pieces of this mass are smokeless powder.

Nitration products, containing about 10% nitrogen, correspond in composition to cellulose dinitrate: in technology, such a product is known as colloxylin. When a mixture of alcohol and ether acts on it, a viscous solution is formed, the so-called collodion, used in medicine. If camphor is added to such a solution (0.4 tsp of camphor for 1 tsp of colloxylin) and the solvent is evaporated, then a transparent flexible film - celluloid - will remain. Historically, this is the first famous type plastics. Since the last century, celluloid has been widely used as a convenient thermoplastic material for the production of many products (toys, haberdashery, etc.). Especially important is the use of celluloid in the production of film and nitro-lacquers. A serious drawback of this material is its flammability, therefore, celluloid is now increasingly being replaced by other materials, in particular cellulose acetates.

Cellulose- one of the most common natural polysaccharides, the main constituent and the main structural material of plant cell walls. The cellulose content in the fibers of cotton seeds is 95-99.5%, in bast fibers (flax, jute, ramie) 60-85%, in wood tissues (depending on the tree species, its age, growing conditions) 30-55%, in green leaves , grass, lower plants 10-25%. Almost in an individual state, cellulose is found in bacteria of the genus Acetobacter... Cellulose satellites in the cell walls of most plants are other structural polysaccharides that differ in structure and are called hemicelluloses- xylan, mannan, galactan, araban, etc. (see section "Hemicelluloses"), as well as non-carbohydrate substances (lignin is a spatial polymer of aromatic structure, silicon dioxide, resinous substances, etc.).

Cellulose determines the mechanical strength of the cell wall and plant tissue as a whole. The distribution and orientation of cellulose fibers in relation to the axis of the plant cell are shown in Fig. 1 for the example of wood. The submicron organization of the cell wall is also presented there.

The wall of a mature wood cell, as a rule, includes primary and secondary membranes (Fig. 1). The latter contains three layers - outer, middle and inner.

In the primary shell, natural cellulose fibers are randomly arranged and form a network structure ( dispersed texture). The cellulose fibers in the secondary casing are oriented mainly parallel to each other, which results in a high tensile strength of the plant material. The degree of polymerization and crystallinity of cellulose in the secondary shell is higher than in the primary one.

In the layer S 1 secondary shell (Fig. 1, 3 ) the direction of the cellulose fibers is almost perpendicular to the cell axis, in the layer S 2 (Fig. 1, 4 ) they form an acute (5-30) angle with the cell axis. Orientation of fibers in a layer S 3 is highly variable and may differ even in adjacent tracheids. Thus, in the spruce tracheids, the angle between the preferred orientation of cellulose fibers and the cell axis ranges from 30-60, and in the fibers of most deciduous species - 50-80. Between layers R and S 1 , S 1 and S 2 , S 2 and S 3, there are transition regions (lamellae) with a different microorientation of fibers than in the main layers of the secondary shell.

Technical cellulose is a fibrous semi-finished product, obtained by cleaning plant fibers from non-cellulose components. Cellulose is usually called by the type of feedstock ( wood, cotton), the method of extraction from wood ( sulfite, sulfate), as well as by purpose ( viscose, acetate, etc.).

Receiving

1.Wood pulp production technology includes the following operations: removing bark from wood (debarking); obtaining wood chips; cooking chips (in industry, cooking is carried out using the sulfate or sulfite method); sorting; bleaching; drying; cutting.

Sulfite method. Spruce wood is treated with an aqueous solution of calcium, magnesium, sodium or ammonium bisulfite, then the temperature is raised to 105-110C within 1.5-4 hours, and boiled at this temperature for 1-2 hours. Then the temperature is raised to 135-150C and cooked for 1-4 hours. In this case, all non-cellulosic components of wood (mainly lignin and hemicelluloses) pass into a soluble state, and de-ligninized cellulose remains.

Sulfate method. Chips of any wood species (as well as reed) are treated with cooking liquor, which is an aqueous solution of sodium hydroxide and sodium sulfide (NaOH + Na 2 S). Within 2-3 hours, the temperature is raised to 165-180C and cooked at this temperature for 1-4 hours. The non-cellulose components converted into a soluble state are removed from the reaction mixture, and cellulose, purified from impurities, remains.

2.Cotton pulp obtained from cotton lint. Production technology includes mechanical cleaning, alkaline cooking (in 1-4% aqueous NaOH solution at a temperature of 130-170С) and bleaching. Electron micrographs of cotton cellulose fibers are shown in Fig. 2.

3. Bacterial cellulose synthesized by bacteria of the genus Acetobacter... The resulting bacterial cellulose has a high molecular weight and a narrow molecular weight distribution.

The narrow molecular weight distribution is explained as follows. Since carbohydrate enters the bacterial cell evenly, average length of the resulting cellulose fibers increases proportionally with time. In this case, a noticeable increase in the transverse dimensions of microfibers (microfibrils) does not occur. The average growth rate of bacterial cellulose fibers is ~ 0.1 μm / min, which corresponds to the polymerization of 10 7 -10 8 glucose residues per hour per one bacterial cell. Consequently, on average, in each bacterial cell, 10 3 glucopyranose units are attached to the growing ends of insoluble cellulose fibers per second.

The microfibers of bacterial cellulose grow from both ends of the fibril to both ends at the same rate. Macromolecular chains inside microfibrils are located antiparallel. For other types of cellulose, such data were not obtained. An electron micrograph of bacterial cellulose fibers is shown in Fig. 3. The fibers are seen to have approximately the same length and cross-sectional area.