Bodies consisting of pure cellulose. What are the chemical and physical properties of cellulose
home Cellulose – one of the most common natural polysaccharides, the main component and main structural material of plant cell walls. The cellulose content in cotton seed fibers is 95-99.5%, in bast fibers (flax, jute, ramie) 60-85%, in wood tissue (depending on the type of tree, 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 . Companions to cellulose 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 - a spatial polymer of an aromatic structure, silicon dioxide, resinous substances, etc.). Cellulose determines the mechanical strength of the cell membrane and plant tissue
generally. The distribution and orientation of cellulose fibers relative to the axis of the plant cell using wood as an example are shown in Fig. 1. The submicron organization of the cell wall is also presented there.
The wall of a mature wood cell, as a rule, includes a primary and secondary cell wall (Fig. 1). The latter contains three layers - outer, middle and inner. In the primary shell, natural cellulose fibers are arranged randomly and form a network structure ( dispersed texture
). The cellulose fibers in the secondary casing are oriented generally parallel to each other, which gives the plant material a high tensile strength. The degree of polymerization and crystallinity of cellulose in the secondary shell is higher than in the primary shell. In layer S 3 1 secondary shell (Fig. 1, In layer) the direction of the cellulose fibers is almost perpendicular to the axis of the cell, in the layer 4 2 (Fig. 1, In layer) they form an acute (5-30) angle with the cell axis. Fiber orientation in the layer 3 varies greatly and can differ even in adjacent tracheids. Thus, in spruce tracheids, the angle between the predominant orientation of cellulose fibers and the cell axis ranges from 30-60, and in the fibers of most hardwoods it is 50-80. Between layers R In layer 1 , In layer And In layer 2 , In layer 1 and In layer 3, transitional regions (lamellae) with a different microorientation of fibers than in the main layers of the secondary shell are observed.
Technical cellulose is a semi-finished fibrous product obtained by cleaning plant fibers from non-cellulose components. Cellulose is usually called by the type of raw material ( wood, cotton), method of extraction from wood ( sulfite, sulfate), as well as for its intended purpose ( viscose, acetate, etc.).
Receipt
1.Wood pulp production technology includes following operations: removing bark from wood (barking); obtaining wood chips; cooking wood 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 for 1.5-4 hours, and boiled at this temperature for 1-2 hours. Next, increase the temperature to 135-150°C and cook for 1-4 hours. In this case, all non-cellulosic components of wood (mainly lignin and hemicelluloses) become soluble, and de-lignified cellulose remains.
Sulfate method. Chips of any type of wood (as well as reed) are treated with cooking liquor, which is an aqueous solution of caustic soda and sodium sulfide (NaOH + Na 2 S). Within 2-3 hours, increase the temperature to 165-180°C and cook 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 linters. Receiving technology includes mechanical cleaning, alkaline cooking (in 1-4% aqueous NaOH solution at a temperature of 130-170°C) and bleaching. Electron micrographs of cotton cellulose fibers are shown in Fig. 2.
3. Bacterial cellulose synthesized by bacteria of the genus – one of the most common natural polysaccharides, the main component and main structural material of plant cell walls. The cellulose content in cotton seed fibers is 95-99.5%, in bast fibers (flax, jute, ramie) 60-85%, in wood tissue (depending on the type of tree, 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. 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 cellulose fibers formed increases proportionally with time. In this case, there is no noticeable increase in the transverse dimensions of microfibers (microfibrils). 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 bacterial cell. Therefore, on average, in each bacterial cell, 10 3 glucopyranose units are attached to the growing ends of insoluble cellulose fibers per second.
Microfibers of bacterial cellulose grow from both ends of the fibril to both at the same speed. Macromolecular chains inside microfibrils are arranged antiparallel. For other types of celluloses such data have not been obtained. An electron micrograph of bacterial cellulose fibers is shown in Fig. 3. It can be seen that the fibers have approximately the same length and cross-sectional area.
Currently, only two sources of cellulose are of industrial importance - cotton and wood pulp. Cotton is almost pure cellulose and does not require complex processing to become a starting material for man-made fibers and non-fiber plastics. After the long fibers used to make cotton fabrics are separated from the cotton seed, short hairs, or “lint” (cotton fluff), 10–15 mm long, remain. The lint is separated from the seed, heated under pressure for 2–6 hours with a 2.5–3% sodium hydroxide solution, then washed, bleached with chlorine, washed again and dried. The resulting product is 99% pure cellulose. The yield is 80% (wt.) lint, the rest being lignin, fats, waxes, pectates and seed husks. Wood pulp is usually made from the wood of coniferous trees. It contains 50–60% cellulose, 25–35% lignin and 10–15% hemicelluloses and non-cellulosic hydrocarbons. In the sulfite process, wood chips are boiled under pressure (about 0.5 MPa) at 140° C with sulfur dioxide and calcium bisulfite. In this case, lignins and hydrocarbons go into solution and cellulose remains. After washing and bleaching, the purified mass is cast into loose paper, similar to blotting paper, and dried. This mass consists of 88–97% cellulose and is quite suitable for chemical processing into viscose fiber and cellophane, as well as cellulose derivatives - esters and ethers.
The process of regenerating cellulose from a solution by adding acid to its concentrated copper-ammonium (i.e. containing copper sulfate and ammonium hydroxide) aqueous solution was described by the Englishman J. Mercer around 1844. But the first industrial application of this method, which marked the beginning of the copper-ammonium fiber industry, is attributed to E. Schweitzer (1857), and its further development is the merit of M. Kramer and I. Schlossberger (1858). And only in 1892 Cross, Bevin and Beadle in England invented a process for producing viscose fiber: a viscous (hence the name viscose) aqueous solution of cellulose was obtained after treating the cellulose first with a strong solution of caustic soda, which gave “soda cellulose”, and then with carbon disulfide (CS 2), resulting in soluble cellulose xanthate. By squeezing a stream of this "spinning" solution through a spinneret with a small round hole into an acid bath, the cellulose was regenerated in the form of rayon fiber. When the solution was squeezed into the same bath through a die with a narrow slit, a film called cellophane was obtained. J. Brandenberger, who worked on this technology in France from 1908 to 1912, was the first to patent a continuous process for making cellophane.
Chemical structure.
Despite the widespread industrial use of cellulose and its derivatives, the currently accepted chemical structural formula of cellulose was proposed (by W. Haworth) only in 1934. However, since 1913 its empirical formula C 6 H 10 O 5, determined from data, has been known quantitative analysis well washed and dried samples: 44.4% C, 6.2% H and 49.4% O. Thanks to the work of G. Staudinger and K. Freudenberg, it was also known that this is a long-chain polymer molecule consisting of those shown in Fig. 1 repeating glucosidic residues. Each unit has three hydroxyl groups - one primary (– CH 2 CH OH) and two secondary (> CH CH OH). By 1920, E. Fisher had established the structure of simple sugars, and in the same year, X-ray studies of cellulose first showed a clear diffraction pattern of its fibers. The X-ray diffraction pattern of cotton fiber shows a clear crystalline orientation, but flax fiber is even more ordered. When cellulose is regenerated into fiber form, crystallinity is largely lost. How easy it is to see in the light of achievements modern science, the structural chemistry of cellulose practically stood still from 1860 to 1920 for the reason that all this time the auxiliary scientific disciplines necessary to solve the problem remained in their infancy.
REGENERATED CELLULOSE
Viscose fiber and cellophane.
Both viscose fiber and cellophane are regenerated (from solution) cellulose. Purified natural cellulose is treated with an excess of concentrated sodium hydroxide; After removing the excess, the lumps are ground and the resulting mass is kept under carefully controlled conditions. With this “aging,” the length of the polymer chains decreases, which promotes subsequent dissolution. Then the crushed cellulose is mixed with carbon disulfide and the resulting xanthate is dissolved in a solution of sodium hydroxide to obtain “viscose” - a viscous solution. When viscose enters an aqueous acid solution, cellulose is regenerated from it. The simplified total reactions are:
Viscose fiber, obtained by squeezing viscose through small holes of a spinneret into an acid solution, is widely used for the manufacture of clothing, drapery and upholstery fabrics, as well as in technology. Significant quantities of viscose fiber are used for technical belts, tapes, filters and tire cord.
Cellophane.
Cellophane, obtained by squeezing viscose into an acid bath through a spinneret with a narrow slot, then passes through washing, bleaching and plasticizing baths, is passed through drying drums and wound into a roll. The surface of cellophane film is almost always coated with nitrocellulose, resin, some kind of wax or varnish to reduce the transmission of water vapor and provide the possibility of thermal sealing, since uncoated cellophane does not have the property of thermoplasticity. On modern production For this purpose, polymer coatings of the polyvinylidene chloride type are used, since they are less moisture permeable and provide a more durable connection during heat sealing.
Cellophane is widely used mainly in the packaging industry as a wrapping material for dry goods, food products, tobacco products, and also as a base for self-adhesive packaging tape.
Viscose sponge.
As well as forming a fiber or film, viscose can be blended with suitable fibrous and finely crystalline materials; After acid treatment and water leaching, this mixture is converted into a viscose sponge material (Fig. 2), which is used for packaging and thermal insulation.
Copper-ammonia fiber.
Regenerated cellulose fiber is produced in industrial scale also by dissolving cellulose in a concentrated copper-ammonia solution (CuSO 4 in NH 4 OH) and spinning the resulting solution into fiber in an acid precipitation bath. This fiber is called copper-ammonia fiber.
PROPERTIES OF CELLULOSE
Chemical properties.
As shown in Fig. 1, cellulose is a high-polymer carbohydrate consisting of glucosidic residues C 6 H 10 O 5 connected by ether bridges at position 1,4. The three hydroxyl groups in each glucopyranose unit can be esterified with organic agents such as a mixture of acids and acid anhydrides with a suitable catalyst such as sulfuric acid. Ethers can be formed by the action of concentrated sodium hydroxide leading to the formation of soda cellulose and subsequent reaction with an alkyl halide:
Reaction with ethylene or propylene oxide produces hydroxylated ethers:
The presence of these hydroxyl groups and the geometry of the macromolecule determine the strong polar mutual attraction neighboring links. The attractive forces are so strong that ordinary solvents are not able to break the chain and dissolve cellulose. These free hydroxyl groups are also responsible for the greater hygroscopicity of cellulose (Fig. 3). Esterification and etherization reduce hygroscopicity and increase solubility in common solvents.
Under the influence aqueous solution acids break oxygen bridges at the 1,4- position. Complete breakage of the chain produces glucose, a monosaccharide. The initial chain length depends on the origin of the cellulose. It is maximum in natural state and is reduced by the process of isolation, purification and conversion to derivatives ( cm. table).
Even mechanical shear, for example during abrasive grinding, leads to a decrease in chain length. When the length of the polymer chain decreases below a certain minimum value, the macroscopic physical properties cellulose.
Oxidizing agents affect cellulose without causing cleavage of the glucopyranose ring (Fig. 4). Subsequent action (in the presence of moisture, such as in climate testing) typically results in chain scission and an increase in the number of aldehyde-like end groups. Since aldehyde groups are easily oxidized to carboxyl groups, the content of carboxyl, which is practically absent in natural cellulose, increases sharply under conditions of atmospheric influences and oxidation.
Like all polymers, cellulose is destroyed under the influence of atmospheric factors as a result joint action oxygen, moisture, acidic components of air and sunlight. Important has the ultraviolet component of sunlight, and many good UV protection agents increase the life of cellulose derivative products. Acidic air components, such as nitrogen and sulfur oxides (and they are always present in atmospheric air industrial areas) accelerate decomposition, often with a stronger effect than sunlight. Thus, in England, it was noted that cotton samples tested for exposure to atmospheric conditions in winter, when there was practically no bright sunlight, degraded faster than in summer. The fact is that burning large quantities of coal and gas in winter led to an increase in the concentration of nitrogen and sulfur oxides in the air. Acid scavengers, antioxidants, and UV absorbers reduce the weathering sensitivity of cellulose. Substitution of free hydroxyl groups leads to a change in this sensitivity: cellulose nitrate degrades faster, and acetate and propionate - slower.
Physical properties.
Cellulose polymer chains are packed into long bundles, or fibers, in which, along with ordered, crystalline ones, there are also less ordered, amorphous sections (Fig. 5). The measured percentage of crystallinity depends on the type of cellulose as well as the method of measurement. According to X-ray data, it ranges from 70% (cotton) to 38–40% (viscose fiber). X-ray structural analysis provides information not only about the quantitative relationship between crystalline and amorphous material in the polymer, but also about the degree of fiber orientation caused by stretching or normal growth processes. The sharpness of diffraction rings characterizes the degree of crystallinity, and diffraction spots and their sharpness characterize the presence and degree of preferred orientation of crystallites. In a sample of recycled cellulose acetate produced by the dry-spinning process, both the degree of crystallinity and orientation are very small. In the triacetate sample, the degree of crystallinity is higher, but there is no preferred orientation. Heat treatment of triacetate at a temperature of 180–240°
Cellulose (fiber) is a plant polysaccharide, which is the most common organic substance on Earth.
1. Physical properties
This substance white, tasteless and odorless, insoluble in water, having a fibrous structure. Dissolves in an ammonia solution of copper (II) hydroxide - Schweitzer's reagent.
Video experiment “Dissolving cellulose in an ammonia solution of copper (II) hydroxide”
2. Being in nature
This biopolymer has great mechanical strength and acts as a supporting material for plants, forming a wall plant cells. Cellulose is found in large quantities in wood tissue (40-55%), flax fibers (60-85%) and cotton (95-98%). The main component of the membrane of plant cells. It is formed in plants during the process of photosynthesis.
Wood consists of 50% cellulose, and cotton, flax, and hemp are almost pure cellulose.
Chitin (an analogue of cellulose) is the main component of the exoskeleton of arthropods and other invertebrates, as well as in the cell walls of fungi and bacteria.
3. Structure
Consists of β-glucose residues
4. Receipt
Obtained from wood
5. Application
Cellulose is used in the production of paper, artificial fibers, films, plastics, paints and varnishes, smokeless powder, explosives, solid rocket fuel, for the production of hydrolytic alcohol, etc.
· Production of acetate silk - artificial fiber, plexiglass, non-flammable film from cellulose acetate.
· Preparation of smokeless gunpowder from triacetylcellulose (pyroxylin).
· Preparation of collodion (thick film for medicine) and celluloid (production of films, toys) from cellulose diacetyl.
· Production of threads, ropes, paper.
Obtaining glucose ethyl alcohol(to obtain rubber)
The most important cellulose derivatives include:
- methylcellulose(cellulose methyl ethers) of the general formula
N( X= 1, 2 or 3);
- cellulose acetate(cellulose triacetate) – ester of cellulose and acetic acid
- nitrocellulose(cellulose nitrates) – cellulose nitrates:
N( X= 1, 2 or 3).
Hydrolysis
(C 6 H 10 O 5) n + nH 2 O t,H2SO4→ nC 6 H 12 O 6
glucose
Hydrolysis proceeds in stages:
(C 6 H 10 O 5) n → (C 6 H 10 O 5) m → xC 12 H 22 O 11 → n C 6 H 12 O 6 (
Note, m
starch dextrinmaltoseglucose
Video experiment “Acid hydrolysis of cellulose”
Esterification reactions
Cellulose is a polyhydric alcohol; there are three hydroxyl groups per unit cell of the polymer. In this regard, cellulose is characterized by esterification reactions (formation of esters). Reactions with nitric acid and acetic anhydride are of greatest practical importance. Cellulose does not produce a “silver mirror” reaction.
1. Nitration:
(C 6 H 7 O 2 (OH ) 3) n + 3 nHNO 3 H 2 SO4(conc.)→(C 6 H 7 O 2 (ONO 2 ) 3) n + 3 nH 2 O
pyroxylin
Video experiment “Preparation and properties of nitrocellulose”
Fully esterified fiber is known as gunpowder, which, after proper processing, turns into smokeless gunpowder. Depending on the nitration conditions, cellulose dinitrate can be obtained, which in technology is called colloxylin. It is also used in the manufacture of gunpowder and solid rocket propellants. In addition, celluloid is made from colloxylin.
2. Interaction with acetic acid:
(C 6 H 7 O 2 (OH) 3) n + 3nCH 3 COOH H2SO4( conc. .)→ (C 6 H 7 O 2 (OCOCH 3) 3) n + 3nH 2 O
When cellulose reacts with acetic anhydride in the presence of acetic and sulfuric acids, triacetylcellulose is formed.
Triacetylcellulose (or cellulose acetate) is a valuable product for the manufacture of flame retardant film andacetate silk. To do this, cellulose acetate is dissolved in a mixture of dichloromethane and ethanol, and this solution is forced through dies into a stream of warm air.
And the die itself schematically looks like this:
1 - spinning solution,
2 - die,
3 - fibers.
The solvent evaporates and the streams of solution turn into the finest threads of acetate silk.
Speaking about the use of cellulose, one cannot help but say that a large amount of cellulose is consumed for the production of various papers. Paper- This is a thin layer of fiber fibers, glued and pressed on a special paper-making machine.
5. If you grind pieces of filter paper (cellulose) soaked in concentrated sulfuric acid in a porcelain mortar and dilute the resulting slurry with water, and also neutralize the acid with alkali and, as in the case of starch, test the solution for reaction with copper (II) hydroxide, then the appearance of copper(I) oxide will be visible. That is, hydrolysis of cellulose occurred in the experiment. The hydrolysis process, like that of starch, occurs in steps until glucose is formed.
2. Depending on the concentration of nitric acid and other conditions, one, two or all three hydroxyl groups of each unit of the cellulose molecule enter into the esterification reaction, for example: n + 3nHNO3 → n + 3n H2O.
Application of cellulose.
Obtaining acetate fiber
68. Cellulose, its physical properties
Being in nature. Physical properties.
1. Cellulose, or fiber, is part of plants, forming cell walls in them.
2. This is where its name comes from (from the Latin “cellulum” - cell).
3. Cellulose gives plants the necessary strength and elasticity and is, as it were, their skeleton.
4. Cotton fibers contain up to 98% cellulose.
5. Flax and hemp fibers are also mainly composed of cellulose; in wood it is about 50%.
6. Paper and cotton fabrics are products made from cellulose.
7. Particularly pure examples of cellulose are cotton wool obtained from purified cotton and filter (un-glued) paper.
8. Selected from natural materials Cellulose is a solid fibrous substance that is insoluble in either water or common organic solvents.
Cellulose structure:
1) cellulose, like starch, is a natural polymer;
2) these substances even have the same structural units in composition - residues of glucose molecules, the same molecular formula (C6H10O5)n;
3) the n value of cellulose is usually higher than that of starch: its average molecular weight reaches several million;
4) the main difference between starch and cellulose is in the structure of their molecules.
Finding cellulose in nature.
1. In natural fibers, cellulose macromolecules are located in one direction: they are oriented along the fiber axis.
2. The numerous hydrogen bonds that arise between the hydroxyl groups of macromolecules determine the high strength of these fibers.
What are the chemical and physical properties of cellulose
In the process of spinning cotton, flax, etc., these elementary fibers are woven into longer threads.
4. This is explained by the fact that the macromolecules in it, although they have a linear structure, are located more randomly and are not oriented in one direction.
The construction of starch and cellulose macromolecules from different cyclic forms of glucose significantly affects their properties:
1) starch is an important human food product; cellulose cannot be used for this purpose;
2) the reason is that enzymes that promote starch hydrolysis do not act on the bonds between cellulose residues.
69. Chemical properties of cellulose and its application
1. From everyday life it is known that cellulose burns well.
2. When wood is heated without air access, thermal decomposition of cellulose occurs. This produces volatile organic compounds, water and charcoal.
3. Among the organic products of wood decomposition are methyl alcohol, acetic acid, and acetone.
4. Cellulose macromolecules consist of units similar to those that form starch; it undergoes hydrolysis, and the product of its hydrolysis, like starch, will be glucose.
5. If you grind pieces of filter paper (cellulose) soaked in concentrated sulfuric acid in a porcelain mortar and dilute the resulting slurry with water, and also neutralize the acid with alkali and, as in the case of starch, test the solution for reaction with copper (II) hydroxide, then the appearance of copper(I) oxide will be visible.
69. Chemical properties of cellulose and its application
That is, hydrolysis of cellulose occurred in the experiment. The hydrolysis process, like that of starch, occurs in steps until glucose is formed.
6. In total, the hydrolysis of cellulose can be expressed by the same equation as the hydrolysis of starch: (C6H10O5)n + nH2O = nC6H12O6.
7. Structural units of cellulose (C6H10O5)n contain hydroxyl groups.
8. Due to these groups, cellulose can produce ethers and esters.
9. Cellulose nitrates are of great importance.
Features of cellulose nitrate ethers.
1. They are obtained by treating cellulose with nitric acid in the presence of sulfuric acid.
2. Depending on the concentration of nitric acid and other conditions, one, two or all three hydroxyl groups of each unit of the cellulose molecule enter into the esterification reaction, for example: n + 3nHNO3 -> n + 3n H2O.
A common property of cellulose nitrates is their extreme flammability.
Cellulose trinitrate, called pyroxylin, is a highly explosive substance. It is used to produce smokeless powder.
Cellulose acetate esters – cellulose diacetate and triacetate – are also very important. Cellulose diacetate and triacetate appearance similar to cellulose.
Application of cellulose.
1. Due to its mechanical strength, wood is used in construction.
2. Various types of carpentry products are made from it.
3. In the form of fibrous materials (cotton, flax) it is used for the manufacture of threads, fabrics, ropes.
4. Cellulose isolated from wood (freed from accompanying substances) is used to make paper.
O.A. Noskova, M.S. Fedoseev
Wood chemistry
And synthetic polymers
PART 2
Approved
Editorial and Publishing Council of the University
as lecture notes
Publishing house
Perm State Technical University
Reviewers:
Ph.D. tech. sciences D.R. Nagimov
(CJSC "Karbokam");
Ph.D. tech. sciences, prof. F.H. Khakimova
(Perm State Technical University)
Noskova, O.A.
N84 Chemistry of wood and synthetic polymers: lecture notes: in 2 hours / O.A. Noskova, M.S. Fedoseev. – Perm: Perm Publishing House. state tech. University, 2007. – Part 2. – 53 p.
ISBN 978-5-88151-795-3
Information is provided regarding the chemical structure and properties of the main components of wood (cellulose, hemicelluloses, lignin and extractives). The chemical reactions of these components that occur during the chemical processing of wood or during the chemical modification of cellulose are considered. Also given general information about cooking processes.
Designed for students of specialty 240406 “Technology of chemical wood processing”.
UDC 630*813. + 541.6 + 547.458.8
ISBN 978-5-88151-795-3 © State Educational Institution of Higher Professional Education
"Perm State
Technical University", 2007
| Introduction……………………………………………………………………………………… | ……5 | |||
| 1. Chemistry of cellulose……………………………………………………….. | …….6 | |||
| 1.1. Chemical structure of cellulose………………………………….. | .…..6 | |||
| 1.2. Chemical reactions of cellulose…………………………………….. | .……8 | |||
| 1.3. Effect of alkali solutions on cellulose…………………………… | …..10 | |||
| 1.3.1. Alkaline cellulose…………………………………………. | .…10 | |||
| 1.3.2. Swelling and solubility of industrial cellulose in alkali solutions…………………………………………………………………… | .…11 | |||
| 1.4. Oxidation of cellulose……………………………………………………………….. | .…13 | |||
| 1.4.1. General information about cellulose oxidation. Oxycellulose… | .…13 | |||
| 1.4.2. The main directions of oxidative reactions…………… | .…14 | |||
1.4.3. Properties of oxycellulose……………………………………… Chemical properties of cellulose. |
.…15 | |||
| 1.5. Cellulose esters…………………………………………. | .…15 | |||
| 1.5.1. General information about the preparation of cellulose esters. | .…15 | |||
| 1.5.2. Cellulose nitrates……………………………………………………………… | .…16 | |||
| 1.5.3. Cellulose xanthates…………………………………….. | .…17 | |||
| 1.5.4. Cellulose acetates……………………………………………………………… | .…19 | |||
| 1.6. Cellulose ethers……………………………………………………………… | .…20 | |||
| 2. Chemistry of hemicelluloses……………………………………………………… | .…21 | |||
| 2.1. General concepts about hemicelluloses and their properties…………………. | .…21 | |||
| .2.2. Pentosans…………………………………………………………….. | .…22 | |||
| 2.3. Hexosans………………………………………………………………………………… | …..23 | |||
| 2.4. Uronic acids……………………………………………………. | .…25 | |||
| 2.5. Pectic substances…………………………………………………………………… | .…25 | |||
| 2.6. Hydrolysis of polysaccharides…………………………………………….. | .…26 | |||
| 2.6.1. General concepts about the hydrolysis of polysaccharides…………………. | .…26 | |||
| 2.6.2. Hydrolysis of wood polysaccharides with dilute mineral acids………………………………………………………….. | …27 | |||
| 2.6.3. Hydrolysis of wood polysaccharides with concentrated mineral acids………………………………………………………. | …28 | |||
| 3. Chemistry of lignin…………………………………………………………….. | …29 | |||
| 3.1. Structural units of lignin………………………………………. | …29 | |||
| 3.2. Methods for lignin isolation……………………………………………………………… | …30 | |||
| 3.3. Chemical structure of lignin…………………………………………… | …32 | |||
| 3.3.1. Functional groups of lignin………………….……………..32 | ||||
| 3.3.2. The main types of bonds between the structural units of lignin……………………………………………………………….35 | ||||
| 3.4. Chemical bonds lignin with polysaccharides……………………….. | ..36 | |||
| 3.5. Chemical reactions of lignin………………………………………….. | ….39 | |||
| 3.5.1. general characteristics chemical reactions lignin……….. | ..39 | |||
| 3.5.2. Reactions of elementary units…………………………………… | ..40 | |||
| 3.5.3. Macromolecular reactions………………………………….. | ..42 | |||
| 4. Extractive substances…………………………………………………………………… | ..47 | |||
| 4.1. General information……………………………………………………………………………… | ..47 | |||
| 4.2. Classification of extractive substances……………………………………………………… | ..48 | |||
| 4.3. Hydrophobic extractives………………………………. | ..48 | |||
| 4.4. Hydrophilic extractives……………………………… | ..50 | |||
| 5. General concepts about cooking processes…………………………………. | ..51 | |||
| Bibliography……………………………………………………………. | ..53 | |||
Introduction
Wood chemistry is a branch of technical chemistry that studies the chemical composition of wood; chemistry of formation, structure and chemical properties of the substances that make up dead wood tissue; methods for isolating and analyzing these substances, as well as chemical essence natural and technological processes processing of wood and its individual components.
The first part of the lecture notes “Chemistry of Wood and Synthetic Polymers,” published in 2002, addresses issues related to the anatomy of wood, the structure of the cell membrane, chemical composition wood, physical and physico-chemical properties of wood.
The second part of the lecture notes “Chemistry of Wood and Synthetic Polymers” discusses issues related to the chemical structure and properties of the main components of wood (cellulose, hemicelluloses, lignin).
The lecture notes provide general information about cooking processes, i.e. on the production of technical cellulose, which is used in the production of paper and cardboard. As a result chemical transformations Technical cellulose is obtained from its derivatives - ethers and esters, from which artificial fibers (viscose, acetate), films (film, photo, packaging films), plastics, varnishes, and adhesives are produced. This part of the summary also briefly discusses the production and properties of cellulose ethers, which are widely used in industry.
Chemistry of cellulose
Chemical structure of cellulose
Cellulose is one of the most important natural polymers. This is the main component of plant tissues. Natural cellulose is found in large quantities in cotton, flax and other fibrous plants, from which natural textile cellulose fibers are obtained. Cotton fibers are almost pure cellulose (95–99%). More important source industrial production Woody plants serve as cellulose (technical cellulose). In wood of various tree species mass fraction cellulose averages 40–50%.
Cellulose is a polysaccharide, the macromolecules of which are built from residues D-glucose (β units -D-anhydroglucopyranose), connected by β-glycosidic bonds 1–4:
Cellulose is a linear homopolymer (homopolysaccharide) belonging to heterochain polymers (polyacetals). It is a stereoregular polymer in which the cellobiose residue serves as a stereo repeating unit. The total formula of cellulose can be represented as (C6H10O5) P or [C6H7O2 (OH)3] P. Each monomer unit contains three alcohol hydroxyl groups, of which one is primary – CH2OH and two (at C2 and C3) are secondary – CHOH–.
The end links are different from the rest of the chain links. One terminal link (conditionally right - non-reducing) has an additional free secondary alcohol hydroxyl (at C4). The other terminal link (conditionally left - reducing) contains free glycosidic (hemiacetal) hydroxyl (in C1 ) and, therefore, can exist in two tautomeric forms - cyclic (coluacetal) and open (aldehyde):
The terminal aldehyde group gives cellulose its reducing (reducing) ability. For example, cellulose can reduce copper from Cu2+ to Cu+:
Amount of copper recovered ( copper number) serves as a qualitative characteristic of the length of cellulose chains and shows its degree of oxidative and hydrolytic destruction.
Natural cellulose has a high degree of polymerization (DP): wood - 5000-10000 and above, cotton - 14000-20000. When isolated from plant tissues, cellulose is somewhat destroyed. Technical wood pulp has a DP of about 1000–2000. The DP of cellulose is determined mainly by the viscometric method, using some complex bases as solvents: copper-ammonia reagent (OH)2, cupriethylenediamine (OH)2, cadmiumethylenediamine (cadoxene) (OH)2, etc.
Cellulose isolated from plants is always polydisperse, i.e. contains macromolecules of various lengths. The degree of cellulose polydispersity (molecular heterogeneity) is determined by fractionation methods, i.e. separating a cellulose sample into fractions with a certain molecular weight. The properties of a cellulose sample (mechanical strength, solubility) depend on the average DP and the degree of polydispersity.
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Date of publication: 2015-11-01; Read: 1100 | Page copyright infringement
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Structure, properties, functions of polysaccharides (homo- and heteropolysaccharides).
POLYSACCHARIDES- these are high molecular weight substances ( polymers), consisting of large quantity monosaccharides. Based on their composition, they are divided into homopolysaccharides and heteropolysaccharides.
Homopolysaccharides– polymers consisting from monosaccharides of one type . For example, glycogen and starch are built only from molecules of α-glucose (α-D-glucopyranose); the monomer of fiber (cellulose) is also β-glucose.
Starch. This reserve polysaccharide plants. The monomer of starch is α-glucose. Leftovers glucose V starch molecule in linear sections are interconnected α-1,4-glycosidic , and at branch points – α-1,6-glycosidic bonds .
Starch is a mixture of two homopolysaccharides: linear - amylose (10-30%) and branched – amylopectin (70-90%).
Glycogen. This is the main one reserve polysaccharide human and animal tissues. The glycogen molecule has approximately 2 times more branched structure than starch amylopectin. Glycogen monomer is α-glucose . In the glycogen molecule, glucose residues in linear regions are interconnected α-1,4-glycosidic , and at branch points – α-1,6-glycosidic bonds .
Cellulose. This is the most common structural plant homopolysaccharide. IN linear fiber molecule monomers β-glucose interconnected β-1,4-glycosidic bonds . Fiber is not digestible in the human body, but, due to its rigidity, irritates the mucous membrane of the gastrointestinal tract, thereby enhances peristalsis and stimulates the secretion of digestive juices, promotes the formation of feces.
Pectic substances- polysaccharides, the monomer of which is D- galacturonic acid , the residues of which are connected by α-1,4-glycosidic bonds. Contained in fruits and vegetables, they are characterized by gelation in the presence of organic acids, which is used in the food industry (jelly, marmalade).
Heteropolysaccharides(mucopolysaccharides, glycosaminoglycans) – polymers consisting from monosaccharides various types . By structure they represent
straight chains built from repeating disaccharide residues , which necessarily include amino sugar (glucosamine or galactosamine) and hexuronic acids (glucuronic or iduronic).
Physical and chemical properties of cellulose
They are jelly-like substances that perform a number of functions, including: protective (mucus), structural, are the basis of the intercellular substance.
In the body, heteropolysaccharides are not found in a free state, but are always associated with proteins (glycoproteins and proteoglycans) or lipids (glycolipids).
Based on their structure and properties, they are divided into acidic and neutral.
ACID HETEROPOLYSACHARIDES:
They contain hexuronic or sulfuric acid. Representatives:
Hyaluronic acidis the main structural component of the intercellular substance capable of binding water (“biological cement”) . Solutions of hyaluronic acid have a high viscosity, therefore they serve as a barrier to the penetration of microorganisms, participate in the regulation of water metabolism, and are the main part of the intercellular substance).
Chondroitin sulfates are structural components cartilage, ligaments, tendons, bones, heart valves.
Heparin – anticoagulant (prevents blood clotting), has an anti-inflammatory effect, activator of a number of enzymes.
NEUTRAL HETEROPOLYSACHARIDES: are part of glycoproteins in blood serum, mucins in saliva, urine, etc., built from amino sugars and sialic acids. Neutral GPs are part of the plural. enzymes and hormones.
SIALIC ACIDS - a combination of neuraminic acid with acetic or amino acid - glycine, are part of cell membranes and biological fluids. Sialic acids are determined for the diagnosis of systemic diseases (rheumatism, systemic lupus erythematosus).
home (C 6 H 10 O 5) n – a natural polymer, a polysaccharide consisting of β-glucose residues, the molecules have a linear structure. Each residue of a glucose molecule contains three hydroxyl groups, so it exhibits the properties of a polyhydric alcohol.
Physical properties
Cellulose is a fibrous substance, insoluble either in water or in ordinary organic solvents, and is hygroscopic. Has great mechanical and chemical strength.
1. Cellulose, or fiber, is part of plants, forming cell walls in them.
2. This is where its name comes from (from the Latin “cellulum” - cell).
3. Cellulose gives plants the necessary strength and elasticity and is, as it were, their skeleton.
4. Cotton fibers contain up to 98% cellulose.
5. Flax and hemp fibers are also mainly composed of cellulose; in wood it is about 50%.
6. Paper and cotton fabrics are products made from cellulose.
7. Particularly pure examples of cellulose are cotton wool obtained from purified cotton and filter (un-glued) paper.
8. Cellulose, isolated from natural materials, is a solid fibrous substance that is insoluble in either water or ordinary organic solvents.
Chemical properties
1. Cellulose is a polysaccharide that undergoes hydrolysis to form glucose:
(C 6 H 10 O 5) n + nH 2 O → nC 6 H 12 O 6
2. Cellulose is a polyhydric alcohol that undergoes esterification reactions to form esters
(C 6 H 7 O 2 (OH) 3) n + 3nCH 3 COOH → 3nH 2 O + (C 6 H 7 O 2 (OCOCH 3) 3) n
cellulose triacetate
Cellulose acetates are artificial polymers used in the production of silk acetate, film (film), and varnishes.
Application
The uses of cellulose are very diverse. Paper, fabrics, varnishes, films are obtained from it. explosives, artificial silk (acetate, viscose), plastics (celluloid), glucose and much more.
Finding cellulose in nature.
1. In natural fibers, cellulose macromolecules are located in one direction: they are oriented along the fiber axis.
2. The numerous hydrogen bonds that arise between the hydroxyl groups of macromolecules determine the high strength of these fibers.
3. In the process of spinning cotton, flax, etc., these elementary fibers are woven into longer threads.
4. This is explained by the fact that the macromolecules in it, although they have a linear structure, are located more randomly and are not oriented in one direction.
The construction of starch and cellulose macromolecules from different cyclic forms of glucose significantly affects their properties:
1) starch is an important human food product; cellulose cannot be used for this purpose;
2) the reason is that enzymes that promote starch hydrolysis do not act on the bonds between cellulose residues.