CHEMISTRY ASSIGNMENT
MADE BY: ARPIT AGGARWAL ROLL NO.: DTU/2K13/B02/1205
ELECTROPLATING Electroplating Electroplating is a process that uses electrical current electrical current to reduce to reduce dissolved metal cations metal cations so that they form a coherent metal coating on anelectrode. anelectrode. The The term is also used for electrical oxidation electrical oxidation of anions anions onto a solid substrate, as in the formation silver chloride on silver wire to make silver/silver-chloride silver/silver-chloride electrodes. Electroplating electrodes. Electroplating is primarily used to change the surface properties of an object (e.g. abrasion (e.g. abrasion and wear resistance, corrosion resistance, corrosion protection, lubricity, protection, lubricity, aesthetic qualities, etc.), but may also be used to build up thickness on undersized undersized parts or to form objects by electroforming. electroforming. The process used in electroplating is called electrodeposition. It is analogous to a galvanic a galvanic cell acting cell acting in reverse. The reverse. The part to be plated is the cathode the cathode of the circuit. In one technique, the anode the anode is made of the metal to be plated on the part. Both components are immersed in asolution called an electrolyte an electrolyte containing one or more dissolved metal dissolved metal salts as well as other ions ions that permit the flow of electricity. A power A power supply supplies a direct a direct current to the anode, the anode, oxidizing oxidizing the metal atoms that comprise it and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode . Other electroplating processes may use a non-consumable non-consumable anode such as lead or carbon. In these techniques, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution. The most common form of electroplating electroplating is used for creating coins such as pennies, as pennies, which which are small zinc small zinc plates covered in a layer of copper. copper.
Process
ELECTROPLATING Electroplating Electroplating is a process that uses electrical current electrical current to reduce to reduce dissolved metal cations metal cations so that they form a coherent metal coating on anelectrode. anelectrode. The The term is also used for electrical oxidation electrical oxidation of anions anions onto a solid substrate, as in the formation silver chloride on silver wire to make silver/silver-chloride silver/silver-chloride electrodes. Electroplating electrodes. Electroplating is primarily used to change the surface properties of an object (e.g. abrasion (e.g. abrasion and wear resistance, corrosion resistance, corrosion protection, lubricity, protection, lubricity, aesthetic qualities, etc.), but may also be used to build up thickness on undersized undersized parts or to form objects by electroforming. electroforming. The process used in electroplating is called electrodeposition. It is analogous to a galvanic a galvanic cell acting cell acting in reverse. The reverse. The part to be plated is the cathode the cathode of the circuit. In one technique, the anode the anode is made of the metal to be plated on the part. Both components are immersed in asolution called an electrolyte an electrolyte containing one or more dissolved metal dissolved metal salts as well as other ions ions that permit the flow of electricity. A power A power supply supplies a direct a direct current to the anode, the anode, oxidizing oxidizing the metal atoms that comprise it and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current flowing through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode . Other electroplating processes may use a non-consumable non-consumable anode such as lead or carbon. In these techniques, ions of the metal to be plated must be periodically replenished in the bath as they are drawn out of the solution. The most common form of electroplating electroplating is used for creating coins such as pennies, as pennies, which which are small zinc small zinc plates covered in a layer of copper. copper.
Process
Electroplating of a metal (Me) with copper in a copper sulfate bath
The cations associate with the anions the anions in the solution. These cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, in an acid solution, copper solution, copper is oxidized at the anode to Cu 2+ by losing two electrons. The Cu2+ associates with the anion SO42- in the solution to form copper sulfate. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons. The result is the effective transfer of copper from the anode source to a plate covering the cathode. The plating is most commonly a single metallic element, metallic element, not not an alloy. an alloy. However, However, some alloys can be electrodeposited, notably brass notably brass andsolder. andsolder. Many plating baths include cyanides include cyanides of other metals (e.g., potassium (e.g., potassium cyanide) in cyanide) in addition to cyanides of the metal to be deposited. These free cyanides facilitate anode corrosion, corrosion, help to maintain a constant metal ion level and contribute to conductivity. Additionally, Additionally, non-metal chemicals such as carbonates as carbonates and phosphates and phosphates may be added to increase conductivity. When plating is not desired on certain areas of the substrate, stop-offs are applied to prevent the bath from coming in contact with the substrate. Typical stop-offs include tape, foil, lacquers, foil, lacquers, and waxes. and waxes.
Effects Electroplating Electroplating changes the chemical, physical, and mechanical properties of the workpiece. An example of a chemical change is when nickel plating improves corrosion resistance. An example of a physical change is a change in the outward appearance. An example of a mechanical change is a change in tensile in tensile strength or surface hardness surface hardness which is a required attribute in tooling industry.
Uses Electroplating is a useful process. It is widely used in various industries for coating metal objects with a thin layer of a different metal. The layer of metal deposited has some desired property, which the metal of the object lacks. For example chromium plating is done on many objects such as car parts, bath taps, kitchen gas burners, wheel rims and many others f or the fact that chromium is very corrosion resistant, and thus prolongs the life of the parts. Electroplating has wide usage in industries. It is also used in making unexpensive jewelry. Electroplating increases life of metal and prevents corrosion.
LI – ION BATTERIES A lithium-ion battery (sometimes Li-ion battery or LIB) is a member of a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. Li-ion batteries use anintercalated lithium compound as the electrode material, compared to the metallic lithium used in non-rechargeable lithium battery. Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of rechargeable battery forportable electronics, with one of the best energy densities, no memory effect (note, however, that a new study have shown signs of memory effect in the lithium-ion batteries which use LiFePO4 as the positive electrode), and only a slow loss of charge when not in use. Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle and aerospace applications. For example, Lithium-ion batteries are becoming a common replacement for the lead acid batteries that have been used historically for golf carts and utility vehicles. Instead of heavy lead plates and acid electrolyte, the trend is to use a lightweight lithium/carbon negative electrodes and lithium iron phosphate positive electrodes. Lithium-ion batteries can provide the same voltage as lead-acid batteries, so no modification to the vehicle's drive system is required. Chemistry, performance, cost and safety characteristics vary across LIB types. Handheld electronics mostly use LIBs based on lithium cobalt oxide (LiCoO 2), which offers high energy density, but presents safety risks, especially when damaged. Lithium iron phosphate(LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety. Such batteries are widely used for electric tools, medical equipment and other roles. NMC in particular is a leading contender for automotive applications. Lithium nickel cobalt aluminum oxide (NCA) and lithium titanate (LTO) are specialty designs aimed at particular niche roles. Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept pressurized. Because of this the testing standards for these batteries are more stringent than those for acid-electrolyte batteries, requiring both a broader range of test conditions and additional battery-specific tests. This is in response to reported accidents and failures, and there have been battery-related recalls by some companies.
Lithium-ion battery
Nokia Li-ion battery for powering a mobile phone
Specific energy
100 –265 W·h /kg (0.36 –0.95 MJ/kg)
Energy density
250 –730 W·h /L (0.90 –2.23 MJ/L)
Specific power
~250-~340 W/kg
Charge/discharge
80 –90%
efficiency Energy/consumer-price
2.5 W·h /US$
Self-discharge rate
8% at 21 °C 15% at 40 °C 31% at 60 °C (per month)
Cycle durability
Nominal cell voltage
400 –1200 cycles
NMC 3.6 / 3.7 V,LiFePO4 3.2 V
Electrochemistry
The participants in the electrochemical reactions in a lithium-ion battery are the negative and positive electrodes with the electrolyte providing a conductive medium for Lithium-ions to move between the electrodes. Both electrodes allow lithium ions to move in and out of their interiors. During insertion (or intercalation) ions move into the electrode. During the reverse process, extraction (ordeintercalation), ions move back out. When a lithium-ion based cell is discharging, the positive Lithium ion moves from the negative electrode (usually graphite) and enters the positive electrode (lithium containing compound). When the cell is charging, the reverse occurs. Useful work is performed when electrons flow through a closed external circuit. The following equations show one example of the chemistry, in units of moles, making it possible to use coefficient . The positive electrode half-reaction is:
The negative electrode half reaction is:
The overall reaction has its limits. Overdischarge supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction:
Overcharge up to 5.2 volts leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray diffraction:
In a lithium-ion battery the lithium ions are transported to and from the positive or negative electrodes by oxidizing the transition metal, cobalt (Co), in Li 1-xCoO 2 from Co3+ to Co4+ during charge, and reduced from Co4+ to Co3+ during discharge. The cobalt electrode reaction is only reversible for x < 0.5, limiting the depth of discharge allowable. This chemistry was used in the Li-ion cells developed by Sony in 1990. The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941 or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does
not consider the other materials that go into a lithium battery and that make lithium batteries many times heavier per unit of energy.
Electrolytes[edit] The cell voltages given in the Electrochemistry section are larger than the potential at which aqueous solutions will electrolyze. Given lithium metal's high reactivity to water, nonaqueous or aprotic solutions are used. Liquid electrolytes in lithium-ion batteries consist of lithium salts, such as LiPF 6, LiBF 4 or LiClO 4 in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate. A liquid electrolyte acts as a carrier between the positive and negative electrodes when current flows through an external circuit. Typical conductivities of liquid electrolyte at room temperature (20 °C (68 °F)) are in the range of 10 mS/cm (1 S/m), increasing by approximately 30 –40% at 40 °C (104 °F) and decreasing slightly at 0 °C (32 °F). Organic solvents easily decompose on the negative electrodes during charge. When appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial charging and forms a solid layer called the solid electrolyte interphase (SEI), which is electrically insulating yet provides significant ionic conductivity. The interphase prevents further decomposition of the electrolyte after the second charge. For example, ethylene carbonate is decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable interface. Composite electrolytes based on POE (poly(oxyethylene)) developed by Syzdek et al., provide a relatively stable interface. It can be either solid (high molecular weight) and be applied in dry Lipolymer cells, or liquid (low molecular weight) and be applied in regular Li-ion cells. Room temperature ionic liquids (RTILs) are another approach to limiting the flammability and volatility of organic electrolytes.
Charge and discharge During discharge, lithium ions Li + carry the current from the negative to the positive electrode, through the nonaqueous electrolyte and separator diaphragm. During charging, an external electrical power source (the charging circuit) applies an overvoltage (a higher voltage but of the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction. The lithium ions then migrate from the positive to the negative electrode, where they become embedded in the porous electrode material in a process known as intercalation.
Uses Li-ion batteries provide lightweight, high energy density power sources for a variety of devices. To power larger devices, such as electric cars, connecting many small batteries in a parallel circuit is more effective and more efficient than connecting a single large battery. Such devices include:
Portable devices: these include mobile phones and smartphones, laptops and tablets, digital cameras and camcorders, electronic cigarettes, handheld game consoles andtorches (flashlights). Power tools: Li-ion batteries are used in tools such as cordless drills, sanders, saws and a variety of garden equipment including whipper-snippers and hedge trimmers. Electric vehicles: Because of their light weight Li-ion batteries are used for energy storage for many electric vehicles for everything from electric cars to Pedelecs, from hybrid vehicles to advanced electric wheelchairs, from radio-controlled models and model aircraft to the Mars Curiosity rover.
Li-ion batteries are used in telecommunications applications. Secondary non-aqueous lithium batteries provide reliable backup power to load equipment located in a network environment of a typical telecommunications service provider. Li-ion batteries compliant with specific technical criteria are recommended for deployment in the Outside Plant (OSP) at locations such as Controlled Environmental Vaults (CEVs), Electronic Equipment Enclosures (EEEs), and huts, and in uncontrolled structures such as cabinets. In such applications, li-ion battery users require detailed, battery-specific hazardous material information, plus appropriate fire-fighting procedures, to meet regulatory requirements and to protect employees and surrounding equipment.
PRIMARY CELL A primary cell is a battery that is designed to be used once and discarded, and not recharged with electricity and reused like a secondary cell (rechargeable battery). In general, the electrochemical reaction occurring in the cell is not reversible, rendering the cell unrechargeable. As a primary cell is used, chemical reactions in the battery use up the chemicals that generate the power; when they are gone, the battery stops producing electricity and is useless. In contrast, in a secondary cell, the reaction can be reversed by running a current into the cell with a battery charger to recharge it, regenerating the chemical reactants. Primary cells are made in a range of standard sizes to power small household appliances.
Terminology Anode and cathode The plate that carries the positive terminal (usually carbon) is termed the anode and the plate that carries the negative terminal (usually zinc) is termed the cathode. This is the reverse of the terminology used in an electrolytic cell. The reason is that the terms are related to the passage of electric current through the electrolyte, not the external circuit Inside the cell the anode is the electrode where chemical oxidation occurs, as it donates electrons to the circuit. The cathode is defined as the electrode where chemical reduction occurs, as it accepts electrons from the circuit. Outside the cell, different terminology is used. Since the anode accepts electrons from the electrolyte, it becomes negatively charged and is therefore connected to the terminal marked "−" on the outside of the cell. The cathode, meanwhile, donates electrons to the electrolyte, so it becomes positively charged and is therefore connected to the terminal marked "+" on the outside of the cell. Old textbooks sometimes contain different terminology that can cause confusion to modern readers. For example, a 1911 textbook by Ayrton and Mather describes the electrodes as the "positive plate" and "negative plate" .
SECONDARY BATTERIES A rechargeable battery, storage battery, or accumulator is a type of electrical battery. It comprises one or more electrochemical cells, and is a type of energy accumulator used for electrochemical energy storage. It is technically known as a secondary cell because itselectrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilize an electrical distribution network. Several different combinations of chemicals are commonly used, including: lead –acid, nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer). Rechargeable batteries have a lower total cost of use and environmental impact than disposable batteries. Some rechargeable battery types are available in the same sizes as common consumer disposable types. Rechargeable batteries have a higher initial cost but can be recharged inexpensively and reused many times.
Charging and discharging
A solar-powered charger for rechargeable AA batteries
During charging, the positive active material is oxidized, producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute the current flow in the external circuit. The electrolyte may serve as a simple buffer for internal ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or it may be an active participant in the electrochemical reaction, as in lead –acidcells. The energy used to charge rechargeable batteries usually comes from a battery charger using AC mains electricity, although some are equipped to use a vehicle's 12-volt DC power outlet. Regardless, to store energy in a secondary cell, it has to be connected to a DC voltage source. The negative terminal of the cell has to be connected to the negative terminal of the voltage source and the positive terminal of the voltage source with the positive terminal of the battery. Further, the voltage output of the source must be higher than that of the battery, but not much higher: the greater the difference between the power source and the battery's voltage capacity, the faster the charging process, but also the greater the risk of overcharging and damaging the battery. Chargers take from a few minutes to several hours to charge a battery. Slow "dumb" chargers without voltage or temperature-sensing capabilities will charge at a low rate, typically taking 14 hours or more to reach a full charge. Rapid chargers can typically charge cells in two to five hours, depending on the model, with the fastest taking as little as fifteen minutes. Fast chargers must have multiple ways of detecting when a cell reaches full charge (change in terminal voltage, temperature, etc.) to stop charging before harmful overcharging or overheating occurs. The fastest chargers often incorporate cooling fans to keep the cells from overheating.
Diagram of the charging of a secondary cell battery.
Battery charging and discharging rates are often discussed by referencing a "C" rate of current. The C rate is that which would theoretically fully charge or discharge the battery in one hour. For example, trickle charging might be performed at C/20 (or a "20 hour" rate), while typical charging and discharging may occur at C/2 (two hours for full capacity). The available capacity of electrochemical cells varies depending on the discharge rate. Some energy is lost in the internal resistance of cell components (plates, electrolyte, interconnections), and the rate of discharge is limited by the speed at which chemicals in the cell can move about. For lead-acid cells, the relationship between time and discharge rate is described by Peukert's law; a lead-acid cell that can no longer sustain a usable terminal voltage at a high current may still have usable capacity, if discharged at a much lower rate. Data sheets for rechargeable cells often list the discharge capacity on 8-hour or 20-hour or other stated time; cells for uninterruptible power supply systems may be rated at 15 minute discharge. Battery manufacturers' technical notes often refer to VPC; this is volts per cell, and refers to the individual secondary cells that make up the battery. (This is typically in reference to 12-volt leadacid batteries.) For example, to charge a 12 V battery (containing 6 cells of 2 V each) at 2.3 VPC requires a voltage of 13.8 V across the battery's terminals. Non-rechargeable alkaline and zinc –carbon cells output 1.5V when new, but this voltage drops with use. Most NiMH AA and AAA cells are rated at 1.2 V, but have a flatter discharge curve than alkalines and can usually be used in equipment designed to use alkaline batteries.
FUEL CELL A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a continuous source of fuel and oxygen/air to sustain the chemical reaction whereas in a battery the chemicals present in the battery react with each other to generate an emf. Fuel cells can produce electricity continuously for as long as these inputs are supplied. The first fuel cells were invented in 1838. The first commercial use of f uel cells came more than a century later in NASA space programs to generate power for probes, satellites and space capsules. Since then, fuel cells have been used in many other applications. Fuel cells are used for primary and backup power for commercial, industrial and residential buildings and in remote or inaccessible areas. They are also used to power fuel-cell vehicles, including forklifts, automobiles, buses, airplanes, boats, motorcycles and submarines. There are many types of fuel cells, but they all consist of an anode, a cathode and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct currentelectricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use followed by the difference in startup time ranging from 1 sec for PEMFC to 10 min for SOFC. Fuel cells come in a variety of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to increase the voltage and meet an application's requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very sm all amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40 –60%, or up to 85% efficient in cogeneration if waste heat is captured for use. The fuel cell market is growing, and Pike Research has estimated that the stationary fuel cell market will reach 50 GW by 2020.
Types of fuel cells; design Fuel cells come in many varieties; however, they all work in the same general manner. They are made up of three adjacent segments: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load. At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.
The most important design features in a fuel cell are
The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.
The fuel that is used. The most common fuel is hydrogen.
The anode catalyst breaks down the fuel into electrons and ions. The anode catalyst is usually made up of very fine platinum powder. The cathode catalyst turns the ions into the waste chemicals like water or carbon dioxide. The cathode catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:
Activation loss Ohmic loss (voltage drop due to resistance of the cell components and interconnections) Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).
To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits to yield higher voltage, and parallel-channel of configurations allow a higher current to be supplied. Such a design is called a fuel cell stack. The cell surface area can be increased, to allow stronger current from each cell. In the stack, reactant gases must be distributed uniformly over all of the cells to maximize the power output.
CELLULOSE AND ITS DERIVATIVES Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fiber is 90%, that of wood is 40 –50% and that of dried hemp is approximately 45%. Cellulose is mainly used to produce paperboard and paper. Smaller quantities are converted into a wide variety of derivative products such as cellophane and rayon. Conversion of cellulose from energy crops into biofuels such as cellulosic ethanol is under investigation as an alternative fuel source. Cellulose for industrial use is mainly obtained from wood pulp and cotton. Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms that live in their guts, such as Trichonympha. Humans can digest cellulose to some extent, but it mainly acts as a hydrophilic bulking agent forfeces and is often referred to as a "dietary fiber".
Cellulose
Identifiers
CAS number
9004-34-6
UNII
SMD1X3XO9M
EC-number
232-674-9
ChEMBL
CHEMBL1201676
Properties
Molecular formula
(C6H10O5)n
Appearance
white powder
Density
1.5 g/cm3
Melting point
decomposes
Solubility in water
none
Structure and properties
Cellulose has no taste, is odorless, is hydrophilic with the contact angle of 20 –30, is insoluble in water and most organic solvents, ischiral and is biodegradable. It can be broken down chemically into its glucose units by treating it with concentrated acids at high temperature. Cellulose is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds. This linkage motif contrasts with that for α(1→4)-glycosidic bonds present in starch, glycogen, and other carbohydrates. Cellulose is a straight chain polymer: unlike starch, no coiling or branching occurs, and the molecule adopts an extended and rather stiff rod-like conformation, aided by the equatorial conformation of the glucose residues. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighbor chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. This confers tensile strength in cell walls, where cellulose microfibrils are meshed into a polysaccharide matrix.
A triple strand of cellulose showing the hydrogen bonds (cyan lines) between glucose strands
Compared to starch, cellulose is also much more crystalline. Whereas starch undergoes a crystalline to amorphous transition when heated beyond 60 –70 °C in water (as in cooking), cellulose requires a temperature of 320 °C and pressure of 25 MPa to become amorphous in water. Several different crystalline structures of cellulose are known, corresponding to the location of hydrogen bonds between and within strands. Natural cellulose is cellulose I, with structures Iα and Iβ. Cellulose produced by bacteria and algae is enriched in I α while cellulose of higher plants consists mainly of Iβ. Cellulose in regenerated cellulose fibers is cellulose II. The conversion of cellulose I to cellulose II is irreversible, suggesting that cellulose I is metastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV. Many properties of cellulose depend on its chain length or degree of polymerization, the number of glucose units that make up one polymer molecule. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibers as well as bacterial cellulose have chain lengths ranging from 800 to 10,000 units. Molecules with very small chain length resulting from the breakdown of cellulose are known as cellodextrins; in contrast to long-chain cellulose, cellodextrins are typically soluble in water and organic solvents. Plant-derived cellulose is usually found in a mixture with hemicellulose, lignin, pectin and other substances, while bacterial cellulose is quite pure, has a much higher water content and higher tensile strength due to higher chain lengths.
Cellulose is soluble in Schweizer's reagent, cupriethylenediamine (CED), cadmiumethylenediamine (Cadoxen), Nmethylmorpholine N-oxide, and lithium chloride /dimethylformamide. This is used in the production of regenerated celluloses (such as viscose and cellophane) from dissolving pulp. Cellulose is also soluble in many kinds ofionic liquids. Cellulose consists of crystalline and amorphous regions. By treating it with strong acid, the amorphous regions can be broken up, thereby producing nanocrystalline cellulose, a novel material with many desirable properties. Recently, nanocrystalline cellulose was used as the filler phase in bio-based polymer matrices to produce nanocomposites with superior thermal and mechanical properties.
Derivatives The hydroxyl groups (-OH) of cellulose can be partially or fully reacted with various reagents to afford derivatives with useful properties like mainly cellulose esters and celluloseethers (-OR). In principle, though not always in current industrial practice, cellulosic polymers are renewable resources. Ester derivatives include: Cellulose ester
Organic esters
Reagent
Organic acids
Example
Reagent
Group R
Cellulose acetate
Acetic acid and acetic H or -(C=O)CH3 anhydride
Cellulose triacetate
Acetic acid and acetic -(C=O)CH 3 anhydride
Cellulose propionate
Propanoic acid
H or -(C=O)CH2CH3
Cellulose acetate propionate (CAP)
Acetic acid and propanoic acid
H or -(C=O)CH3 or (C=O)CH2CH3
Cellulose acetate butyrate Acetic acid (CAB) and butyric acid
H or -(C=O)CH3 or (C=O)CH2CH2CH3
Inorganic Inorganic Nitrocellulose (cellulose esters acids nitrate)
Cellulose sulfate
Nitric acid or another powerful nitrating agent
H or -NO2
Sulfuric acid or another powerful sulfuring agent
H or -SO3H
The cellulose acetate and cellulose triacetate are film- and fiber-forming materials that find a variety of uses. The nitrocellulose was initially used as an explosive and was an early film forming material. With camphor, nitrocellulose gives celluloid. Ether derivatives include:
Cellulose ethers
Alkyl
Reagent
Example
Reagent
Water E Group R = H solubili Application numb or ty er
Halogenoalka Methylcellulos Chlorometh -CH3 nes e ane
Ethylcellulose
Ethyl methyl cellulose
Chloroethan -CH2CH3 e
Chlorometh -CH3 or ane and CH2CH3 chloroethan
Cold water soluble
E461
A commercial thermoplast ic used in Water coatings, insolub inks, E462 le binders, and controlledrelease drug tablets
E465
e
Hydroxyal Epoxides kyl
Hydroxyethyl Ethylene cellulose oxide
Cold/h Gelling and ot -CH2CH2OH thickening water agent soluble
Hydroxypropy l Propylene cellulose(HP oxide C)
Cold CH2CH(OH) water CH3 soluble
Chlorometh Hydroxyethyl ane and -CH3 or methyl ethylene CH2CH2OH cellulose oxide
E463
Cold Production water of cellulose soluble films
Viscosity Hydroxypropy Chlorometh modifier, -CH3 or Cold l methyl ane and gelling, CH2CH(OH) water E464 cellulose (HP propylene foaming CH3 soluble MC) oxide and binding agent
Chloroethan Ethyl e and hydroxyethyl CH2CH3 or — ethylene cellulose CH2CH2OH oxide
Carboxymeth Halogenated Carboxyal yl Chloroaceti carboxylic -CH2COOH kyl cellulose (CM c acid acids C)
E467
Often used Cold/H as its sodium s ot E466 water alt, sodium soluble carboxymet hyl cellulose
(NaCMC) The sodium carboxymethyl cellulose can be cross-linked to give the croscarmellose sodium (E468) for use as a disintegrant in pharmaceutical formulations.
GREEN SOLVENTS Solvents define a major part of the environmental performance of processes in chemical industry and also impact on cost, safety and health issues. The idea of ―green‖
solvents expresses the goal to minimize the environmental impact resulting from the use of solvents in chemical production. Here the question is raised of how to measure how ―green‖ a solvent is. We propose a comprehensive framework for the environmental assessment of solvents that covers major aspects of the environmental performance of solvents in chemical production, as well as important health and safety issues. The framework combines the assessment of substance-specific hazards with the quantification of emissions and resource use over the full life-cycle of a solvent. The proposed framework is demonstrated on 26 organic solvents. Results show that simple alcohols (methanol, ethanol) or alkanes (heptane, hexane) are environmentally preferable solvents, whereas the use of dioxane, acetonitrile, acids,formaldehyde, and tetrahydrofuran is not recommendable from an environmental perspective. Additionally, a case study is presented in which the framework is applied for the assessment of various alcohol –water or pure alcohol mixtures used for solvolysis of p-methoxybenzoyl chloride. The results of this case study indicate that methanol –water or ethanol –water mixtures are environmentally favourable compared to pure alcohol or propanol –water mixtures. The two applications demonstrate that the presented framework is a useful instrument to select green solvents or environmentally sound solvent mixtures for processes in chemical industry. The same framework can also be used for a comprehensive assessment of new solvent technologies as soon as the present lack of data can be overcome.
PTC In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase in the absence of the phase-transfer catalyst. The catalyst functions like a detergent for solubilizing the salts into the organic phase. Phase-transfer catalysis refers to the acceleration of the reaction upon the addition of the phase-transfer catalyst. By using a PTC process, one can achieve faster reactions, obtain higher conversions or yields, make fewer byproducts, eliminate the need for expensive or dangerous solvents that will dissolve all the reactants in one phase, eliminate the need for expensive raw materials and/or minimize waste problems. Phase-transfer catalysts are especially useful ingreen chemistry — by allowing the use of water, the need for organic solvents is reduced. Contrary to common perception, PTC is not limited to systems with hydrophilic and hydrophobic reactants. PTC is sometimes employed in liquid/solid and liquid/gas reactions. As the name implies, one or more of the reactants are transported into a second phase which contains both reactants.
Types of phase-transfer catalysts Phase-transfer catalysts for anionic reactants are often quaternary ammonium and phosphonium salts. Typical catalysts include benzyltrimethylammonium chloride and hexadecyltributylphosphonium bromide. For example, the nucleophilic aliphatic substitution reaction of an aqueous sodium cyanide solution with an ethereal solution of 1-bromooctane does not readily occur. The 1bromooctane is poorly soluble in the aqueous cyanide solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile: C8H17Br(org) + NaCN(aq) → C8H17CN(org) + NaBr(aq) (catalyzed by a R4P+Cl− PTC) Via the quaternary phosphonium cation, cyanide ions are "ferried" from the aqueous phase into the organic phase. Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as tetra-n-butylammonium bromide ormethyltrioctylammonium chloride in benzene/water systems. An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. In the research lab, crown ethers are used for this purpose. Polyethylene glycolsare more commonly used in practical applications. These ligands encapsulate alkali metal
cations (typically Na+ and K+), affording large lipophilic cations. These polyethers have ahydrophilic "interiors" containing the ion and a hydrophobic exterior.
Applications PTC is widely exploited industrially. Polyester polymers for example are prepared from acid chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTCcatalyzed alkylation of phosphothioates. One of the more complex applications of PTC involves asymmetric alkylations, which are catalyzed by chiral quaternary ammonium salts derived from cinchona alkaloids.
DNA Deoxyribonucleic acid (DNA) is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA is a nucleic acid; alongside proteins andcarbohydrates, nucleic acids compose the three major macromolecules essential for all known forms of life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are known as polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of a nitrogen-containing nucleobase— either guanine (G), adenine (A), thymine (T), or cytosine (C)—as well as amonosaccharide sugar called deoxyribose and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternatingsugar-phosphate backbone. According to base pairing rules (A with T and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. DNA is well-suited for biological information storage. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information. Biological information is replicated as the two strands are separated. A significant portion of DNA (more than 98% for humans) is non-coding, meaning that these sections do not serve a function of encoding proteins. The two strands of DNA run in opposite directions to each other and are therefore anti-parallel, one backbone being 3′ (three prime) and the other 5′ (five prime). This refers to the direction the 3rd and 5th carbon on the sugar molecule is facing. Attached to each sugar is one of four types of nucleobases (informally, bases). It is the sequence of these four nucleobases along the backbone that encodes biological information. Under the genetic code, RNA strands are translated to specify the sequence of amino acids within proteins. These RNA strands are initially created using DNA strands as a template in a process called transcription. Within cells, DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process ofDNA replication, providing each cell its own complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast, prokaryotes(bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. Scientists use DNA as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity. The unique material properties of DNA have
made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. The obsolete synonym "desoxyribonucleic acid" may occasionally be encountered, for example, in pre-1953 genetics.
Properties
Chemical structure of DNA. Hydrogen bondsshown as dotted lines.
DNA is a long polymer made from repeating units called nucleotides. DNA was first identified and isolated by Friedrich Miescher and the double helix structure of DNA was first discovered by James Watson and Francis Crick. The structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 ångströms(3.4 nanometres) and a radius of 10 ångströms (1.0 nanometres). According to another study, when measured in a particular solution, the DNA chain measured 22 to 26 ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit measured 3.3 Å (0.33 nm) long. Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides. For instance, the largest human chromosome, chromosome number 1, consists of approximately 220 million base pairs and is 85 nm long. In living organisms DNA does not usually exist as a single molecule, but instead as a pair of molecules that are held tightly together. These two long strands entwine like vines, in the shape of a double helix. The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a nucleobase, which interacts with the other DNA strand in the helix. A nucleobase linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. A polymer comprising multiple linked nucleotides (as in DNA) is called apolynucleotide. The backbone of the DNA strand is made from alternating phosphate and sugar residues. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that formphosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their
direction in the other strand: the strands are antiparallel. The asymmetric ends of DNA strands are called the 5′ (five prime) and 3′ (three prime) ends, with the 5′ end having a terminal phosphate group and the 3′ end a terminal hydroxyl group. One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar ribose in RNA.
A section of DNA. The bases lie horizontally between the two spiraling strands. [13](animated version).
The DNA double helix is stabilized primarily by two f orces: hydrogen bonds between nucleotides and base-stacking interactions among aromaticnucleobases. In the aqueous environment of the cell, the conjugated π bonds of nucleotide bases align perpendicular to the axis of the DNA molecule, minimizing their interaction with the solvation shell and therefore, the Gibbs free energy. The four bases found in DNA are adenine(abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.
RNA Ribonucleic acid (RNA) is a ubiquitous family of large biological molecules that perform multiple vital roles in the coding, decoding,regulation, and expression of genes. Together with DNA, RNA comprises the nucleic acids, which, along with proteins, constitute the three major macromolecules essential for all known forms of life. Like DNA, RNA is assembled as a chain of nucleotides, but is usually single-stranded. Cellular organisms use messenger RNA (mRNA) to convey genetic information (often notated using the letters G, A, U, and C for the nucleotides guanine, adenine, uracil and cytosine) that directs synthesis of specific proteins, while many viruses encode their genetic information using an RNA genome. Some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function whereby mRNA molecules direct the assembly of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome, where ribosomal RNA (rRNA) links amino acids together to form proteins.
Comparison with DNA
Three-dimensional representation of the 50S ribosomal subunit. RNA is in ochre, protein in blue. The active site is in the middle (red).
The chemical structure of RNA is very similar to that of DNA, but differs in three main ways:
Unlike double-stranded DNA, RNA is a single-stranded molecule in many of its biological roles and has a much shorter chain of nucleotides. However, RNA can, by complementary base pairing, form intrastrand double helixes, as in tRNA. While DNA contains deoxyribose, RNA contains ribose (in deoxyribose there is no hydroxyl group attached to the pentose ring in the 2'position). These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. The complementary base to adenine is not thymine, as it is in DNA, but rather uracil, which is an unmethylated form of thymine.
Like DNA, most biologically active RNAs, including mRNA, tRNA, rRNA, snRNAs, and other noncoding RNAs, contain self-complementary sequences that allow parts of the RNA to fold and pair with itself to form double helices. Analysis of these RNAs has revealed that they are highly structured. Unlike DNA, their structures do not consist of long double helices but rather collections of short helices packed together into structures akin to proteins. In this fashion, RNAs can achieve chemical catalysis, like enzymes. For instance, determination of the structure of the ribosome—an enzyme that catalyzes peptide bond formation—revealed that its active site is composed entirely of RNA.
Structure
Watson-Crick base pairs in a siRNA(hydrogen atoms are not shown)
Each nucleotide in RNA contains a ribose sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general,adenine (A), cytosine (C), guanine (G), or uracil (U). Adenine and guanine are purines, cytosine and uracil are pyrimidines. A phosphategroup is attached to the 3' position of one ribose and the 5' position of the next. The phosphate groups have a negative charge each at physiological pH, making RNA a charged molecule (polyanion). The bases form hydrogen bonds between cytosine and guanine, between adenine and uracil and between guanine and uracil. However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge, or the GNRA tetraloop that has a guanine – adenine base-pair.
Chemical structure of RNA
An important structural feature of RNA that distinguishes it from DNA is the presence of a hydroxylgroup at the 2' position of the ribose sugar. The presence of this functional group causes the helix to adopt the A-form geometry rather than the B-form most commonly observed in DNA. This results in a very deep and narrow major groove and a shallow and wide minor groove. A second consequence of the presence of the 2' -hydroxyl group is that in conformationally flexible regions of an RNA molecule (that is, not involved in formation of a double helix), it can chemically attack the adjacent phosphodiester bond to cleave the backbone.
Secondary structure of a telomerase RNA.
