An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. The one-step mechanism is known as the E2 reaction, the two-step mechanism is known as the E1 reaction; the numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction and unimolecular respectively. In cases where the molecule is able to stabilize an anion but possesses a poor leaving group, a third type of reaction, E1CB, exists; the pyrolysis of xanthate and acetate esters proceed through an "internal" elimination mechanism, the Ei mechanism. In most organic elimination reactions, at least one hydrogen is lost to form the double bond: the unsaturation of the molecule increases, it is possible that a molecule undergoes reductive elimination, by which the valence of an atom in the molecule decreases by two, though this is more common in inorganic chemistry. An important class of elimination reactions is those involving alkyl halides, with good leaving groups, reacting with a Lewis base to form an alkene.
Elimination may be considered the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule or through Hofmann elimination if the carbon with the most substituted hydrogen is inaccessible. During the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination; the reaction involves a one-step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond. The specifics of the reaction are as follows: E2 is a single step elimination, with a single transition state, it is undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides and other compounds. The reaction rate is second order, because it's influenced by both the base; because the E2 mechanism results in the formation of a pi bond, the two leaving groups need to be antiperiplanar. An antiperiplanar transition state has staggered conformation with lower energy than a synperiplanar transition state, in eclipsed conformation with higher energy.
The reaction mechanism involving staggered conformation is more favorable for E2 reactions. E2 uses a strong base, it must be strong enough to remove a weakly acidic hydrogen. In order for the pi bond to be created, the hybridization of carbons needs to be lowered from sp3 to sp2; the C-H bond is weakened in the rate determining step and therefore a primary deuterium isotope effect much larger than 1 is observed. E2 competes with the SN2 reaction mechanism if the base can act as a nucleophile. An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium ethoxide in ethanol; the reaction products are isobutylene and potassium bromide. E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specifications It is a two-step process of elimination: ionization and deprotonation. Ionization: the carbon-halogen bond breaks to give a carbocation intermediate. Deprotonation of the carbocation.
E1 takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides. The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step, as known as the rate-determining step. Therefore, first-order kinetics apply; the reaction occurs in the complete absence of a base or the presence of only a weak base. E1 reactions are in competition with SN1 reactions because they share a common carbocationic intermediate. A secondary deuterium isotope effect of larger than 1 is observed. There is no antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol: Only reaction product A results from antiperiplanar elimination; the presence of product B is an indication. It is accompanied by carbocationic rearrangement reactions An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol. E1 eliminations happen with substituted alkyl halides for two main reasons.
Substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism. Substituted carbocations are more stable than methyl or primary substituted cations; such stability gives time for the two-step E1 mechanism to occur. If SN1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat. Specific features: 1. Rearrangement possible 2. Independent of concentration and basicity of base The reaction rate is influenced by the reactivity of halogens and bromide being favored. Fluoride is not a good leaving group, so eliminations with fluoride as the leaving group have slower rates than other halogens. There is a certain level of competition between the elimination reaction and nucleophilic substitution. More there are competitions between E2 and SN2 and between E1 and SN1. Substitution predominates and elimination occurs only during precise circumstances. Elimination is favored over substitution when steric hindrance around the α-carbon increases. A stronger base is used.
Temperature increases. Bases with steric bulk, are poor nucleophiles. In one study the kinetic isotope effect was determin
Simplified molecular-input line-entry system
The simplified molecular-input line-entry system is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules; the original SMILES specification was initiated in the 1980s. It has since been extended. In 2007, an open standard called. Other linear notations include the Wiswesser line notation, ROSDAL, SYBYL Line Notation; the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. Acknowledged for their parts in the early development were "Gilman Veith and Rose Russo and Albert Leo and Corwin Hansch for supporting the work, Arthur Weininger and Jeremy Scofield for assistance in programming the system." The Environmental Protection Agency funded the initial project to develop SMILES. It has since been modified and extended by others, most notably by Daylight Chemical Information Systems.
In 2007, an open standard called "OpenSMILES" was developed by the Blue Obelisk open-source chemistry community. Other'linear' notations include the Wiswesser Line Notation, ROSDAL and SLN. In July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is considered to have the advantage of being more human-readable than InChI; the term SMILES refers to a line notation for encoding molecular structures and specific instances should be called SMILES strings. However, the term SMILES is commonly used to refer to both a single SMILES string and a number of SMILES strings; the terms "canonical" and "isomeric" can lead to some confusion when applied to SMILES. The terms are not mutually exclusive. A number of valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol. Algorithms have been developed to generate the same SMILES string for a given molecule; this SMILES is unique for each structure, although dependent on the canonicalization algorithm used to generate it, is termed the canonical SMILES.
These algorithms first convert the SMILES to an internal representation of the molecular structure. Various algorithms for generating canonical SMILES have been developed and include those by Daylight Chemical Information Systems, OpenEye Scientific Software, MEDIT, Chemical Computing Group, MolSoft LLC, the Chemistry Development Kit. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database; the original paper that described the CANGEN algorithm claimed to generate unique SMILES strings for graphs representing molecules, but the algorithm fails for a number of simple cases and cannot be considered a correct method for representing a graph canonically. There is no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, double bond geometry; these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES.
A notable feature of these rules is. The term isomeric SMILES is applied to SMILES in which isotopes are specified. In terms of a graph-based computational procedure, SMILES is a string obtained by printing the symbol nodes encountered in a depth-first tree traversal of a chemical graph; the chemical graph is first trimmed to remove hydrogen atoms and cycles are broken to turn it into a spanning tree. Where cycles have been broken, numeric suffix labels are included to indicate the connected nodes. Parentheses are used to indicate points of branching on the tree; the resultant SMILES form depends on the choices: of the bonds chosen to break cycles, of the starting atom used for the depth-first traversal, of the order in which branches are listed when encountered. Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, have no formal charge, have the number of hydrogens attached implied by the SMILES valence model, are the normal isotopes, are not chiral centers.
All other elements must be enclosed in brackets, have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or. Hydrogen may be written as a separate atom; when brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1 by the sign + for a positive charge or by - for a negative charge. For example, for ammonium. If there is more than one charge, it is written as digit.
Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. The prefix di- is of Greek origin, meaning "two". If a diatomic molecule consists of two atoms of the same element, such as hydrogen or oxygen it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide or nitric oxide, the molecule is said to be heteronuclear; the only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure are the gases hydrogen, oxygen and chlorine. The noble gases are gases at STP, but they are monatomic; the homonuclear diatomic gases and noble gases together are called "elemental gases" or "molecular gases", to distinguish them from other gases that are chemical compounds. At elevated temperatures, the halogens bromine and iodine form diatomic gases. All halogens have been observed as diatomic molecules, except for astatine, uncertain; the mnemonics BrINClHOF, pronounced "Brinklehof", HONClBrIF, pronounced "Honkelbrif", HOFBrINCl have been coined to aid recall of the list of diatomic elements.
Other elements form diatomic molecules when evaporated, but these diatomic species repolymerize when cooled. Heating elemental phosphorus gives diphosphorus, P2. Sulfur vapor is disulfur. Dilithium is known in the gas phase. Ditungsten and dimolybdenum form with sextuple bonds in the gas phase; the bond in a homonuclear diatomic molecule is non-polar. Dirubidium is diatomic. All other diatomic molecules are chemical compounds of two different elements. Many elements can combine to form heteronuclear diatomic molecules, depending on temperature and pressure; some examples include, gases carbon monoxide, nitric oxide, hydrogen chloride. Many 1:1 binary compounds are not considered diatomic because they are polymeric at room temperature, but they form diatomic molecules when evaporated, for example gaseous MgO, SiO, many others. Hundreds of diatomic molecules have been identified in the environment of the Earth, in the laboratory, in interstellar space. About 99% of the Earth's atmosphere is composed of two species of diatomic molecules: nitrogen and oxygen.
The natural abundance of hydrogen in the Earth's atmosphere is only of the order of parts per million, but H2 is the most abundant diatomic molecule in the universe. The interstellar medium is, dominated by hydrogen atoms. Diatomic elements played an important role in the elucidation of the concepts of element and molecule in the 19th century, because some of the most common elements, such as hydrogen and nitrogen, occur as diatomic molecules. John Dalton's original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would have the simplest atomic ratios with respect to one another. For example, Dalton assumed water's formula to be HO, giving the atomic weight of oxygen as eight times that of hydrogen, instead of the modern value of about 16; as a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century. As early as 1805, Gay-Lussac and von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen, by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules.
However, these results were ignored until 1860 due to the belief that atoms of one element would have no chemical affinity toward atoms of the same element, partly due to apparent exceptions to Avogadro's law that were not explained until in terms of dissociating molecules. At the 1860 Karlsruhe Congress on atomic weights, Cannizzaro resurrected Avogadro's ideas and used them to produce a consistent table of atomic weights, which agree with modern values; these weights were an important prerequisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer. Diatomic molecules are in their lowest or ground state, which conventionally is known as the X state; when a gas of diatomic molecules is bombarded by energetic electrons, some of the molecules may be excited to higher electronic states, as occurs, for example, in the natural aurora. Such excitation can occur when the gas absorbs light or other electromagnetic radiation; the excited states are unstable and relax back to the ground state.
Over various short time scales after the excitation, transitions occur from higher to lower electronic states and to the ground state, in each transition results a photon is emitted. This emission is known as fluorescence. Successively higher electronic states are conventionally named A, B, C, etc.. The excitation energy must be greater than or equal to the energy of the electronic state in order for the excitation to occur. In quantum theory, an electronic state of a diatomic molecule is represented by 2 S + 1 Λ ( v
Acetaldehyde is an organic chemical compound with the formula CH3CHO, sometimes abbreviated by chemists as MeCHO. It is one of the most important aldehydes, occurring in nature and being produced on a large scale in industry. Acetaldehyde occurs in coffee and ripe fruit, is produced by plants, it is produced by the partial oxidation of ethanol by the liver enzyme alcohol dehydrogenase and is a contributing cause of hangover after alcohol consumption. Pathways of exposure include air, land, or groundwater, as well as drink and smoke. Consumption of disulfiram inhibits acetaldehyde dehydrogenase, the enzyme responsible for the metabolism of acetaldehyde, thereby causing it to build up in the body; the International Agency for Research on Cancer has listed acetaldehyde as a Group 1 carcinogen. Acetaldehyde is "one of the most found air toxins with cancer risk greater than one in a million". Acetaldehyde was first observed by the Swedish pharmacist/chemist Carl Wilhelm Scheele. In 1835, Liebig named it "aldehyde".
In 2003, global production was about 1 million tonnes. Before 1962, ethanol and acetylene were the major sources of acetaldehyde. Since ethylene is the dominant feedstock; the main method of production is the oxidation of ethylene by the Wacker process, which involves oxidation of ethylene using a homogeneous palladium/copper system: 2 CH2=CH2 + O2 → 2 CH3CHOIn the 1970s, the world capacity of the Wacker-Hoechst direct oxidation process exceeded 2 million tonnes annually. Smaller quantities can be prepared by the partial oxidation of ethanol in an exothermic reaction; this process is conducted over a silver catalyst at about 500–650 °C. CH3CH2OH + 1⁄2 O2 → CH3CHO + H2OThis method is one of the oldest routes for the industrial preparation of acetaldehyde. Prior to the Wacker process and the availability of cheap ethylene, acetaldehyde was produced by the hydration of acetylene; this reaction is catalyzed by mercury salts: C2H2 + Hg2+ + H2O → CH3CHO + HgThe mechanism involves the intermediacy of vinyl alcohol, which tautomerizes to acetaldehyde.
The reaction is conducted at 90–95 °C, the acetaldehyde formed is separated from water and mercury and cooled to 25–30 °C. In the wet oxidation process, iron sulfate is used to reoxidize the mercury back to the mercury salt; the resulting iron sulfate is oxidized in a separate reactor with nitric acid. Traditionally, acetaldehyde was produced by the partial dehydrogenation of ethanol: CH3CH2OH → CH3CHO + H2In this endothermic process, ethanol vapor is passed at 260–290 °C over a copper-based catalyst; the process was once attractive because of the value of the hydrogen coproduct, but in modern times is not economically viable. The hydroformylation of methanol with catalysts like cobalt, nickel, or iron salts produces acetaldehyde, although this process is of no industrial importance. Noncompetitive, acetaldehyde arises from synthesis gas with modest selectivity. Like many other carbonyl compounds, acetaldehyde tautomerizes to give an enol: CH3CH=O ⇌ CH2=CHOH ∆H298,g = +42.7 kJ/molThe equilibrium constant is 6×10−7 at room temperature, thus that the relative amount of the enol form in a sample of acetaldehyde is small.
At room temperature, acetaldehyde is more stable than vinyl alcohol by 42.7 kJ/mol: Overall the keto-enol tautomerization occurs but is catalyzed by acids. Photo-induced keto-enol tautomerization is viable under stratospheric conditions; this photo-tautomerization is relevant to the earth's atmosphere, because vinyl alcohol is thought to be a precursor to carboxylic acids in the atmosphere. Acetaldehyde is a common electrophile in organic synthesis. In condensation reactions, acetaldehyde is prochiral, it is used as a source of the "CH3C+H" synthon in aldol and related condensation reactions. Grignard reagents and organolithium compounds react with MeCHO to give hydroxyethyl derivatives. In one of the more spectacular condensation reactions, three equivalents of formaldehyde add to MeCHO to give pentaerythritol, C4. In a Strecker reaction, acetaldehyde condenses with cyanide and ammonia to give, after hydrolysis, the amino acid alanine. Acetaldehyde can condense with amines to yield imines; these imines can be used to direct subsequent reactions like an aldol condensation.
It is a building block in the synthesis of heterocyclic compounds. In one example, it converts, to 5-ethyl-2-methylpyridine. Three molecules of acetaldehyde condense to form "paraldehyde", a cyclic trimer containing C-O single bonds. Condensation of four molecules of acetaldehyde give the cyclic molecule metaldehyde. Paraldehyde can be produced in good yields. Metaldehyde is only obtained in a few percent yield and with cooling using HBr rather than H2SO4 as the catalyst. At -40 °C in the presence of acid catalysts, polyacetaldehyde is produced. Acetaldehyde forms a stable acetal upon reaction with ethanol under conditions that favor dehydration; the product, CH3CH2, is formally named 1,1-diethoxyethane but is referred to as "acetal". This can cause confusion as "acetal" is more used to describe compounds with the functional groups RCH2 or RR'C2 rather than referring to
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
Poly is a water-soluble synthetic polymer. It has the idealized formula n, it is used in papermaking, a variety of coatings. It is odorless, it is sometimes supplied as solutions in water. Polyvinyl acetals: Polyvinyl acetals are prepared by reacting aldehydes with polyvinyl alcohol. Polyvinyl butyral and polyvinyl formal are examples of this family of polymers, they are prepared from polyvinyl alcohol by reaction with butyraldehyde and formaldehyde, respectively. Preparation of polyvinyl butyral is the largest use for polyvinyl alcohol in the U. S. and Western Europe. Polyvinyl alcohol is used as an emulsion polymerization aid, as protective colloid, to make polyvinyl acetate dispersions; this is the largest market application in China. In Japan its major use is vinylon fiber production. Other uses of polyvinyl alcohol include: Injection moulding of soluble containers for active release of detergents and agrichemicals Paper adhesive with boric acid in spiral tube winding and solid board production Thickener, modifier, in polyvinyl acetate glues Textile sizing agent Paper coatings, release liner As a water-soluble film useful for packaging.
An example is the envelope containing laundry detergent in "liqui-tabs". Feminine hygiene and adult incontinence products as a biodegradable plastic backing sheet. Carbon dioxide barrier in polyethylene terephthalate bottles As a film used in the water transfer printing process As a form release because materials such as epoxy do not stick to it Movie practical effect and children's play putty or slime when combined with borax Used in eye drops and hard contact lens solution as a lubricant PVA fiber, as reinforcement in concrete Raw material to polyvinyl nitrate an ester of nitric acid and polyvinyl alcohol; as a surfactant for the formation of polymer encapsulated nanobeads Used in protective chemical-resistant gloves Used as a fixative for specimen collection stool samples When doped with iodine, PVA can be used to polarize light As an embolization agent in medical procedures Carotid phantoms for use as synthetic vessels in Doppler flow testing Used in 3D printing as support structure that can be dissolved away.
PVA is used in freshwater sport fishing. PVA moulded capsules and small bags made from PVA are filled with dry or oil based bait and attached to the hook, or the baited hook is placed inside the bag and cast into the water; when the bag lands on the lake or river bottom it dissolves in water, leaving the hook bait surrounded by ground bait, pellets etc. Anglers use string made of PVA for the purpose of making temporary attachments. For example, holding a length of line in a coil, that might otherwise tangle while the cast is made. Unlike most vinyl polymers, PVA is not prepared by polymerization of the corresponding monomer; the monomer, vinyl alcohol, is unstable with respect to acetaldehyde. PVA instead is prepared by first polymerizing vinyl acetate, the resulting polyvinylacetate is converted to the PVA. Other precursor polymers are sometimes used, with formate, chloroacetate groups instead of acetate; the conversion of the polyesters is conducted by base-catalysed transesterification with ethanol: n + C2H5OH → n + C2H5OAcThe properties of the polymer depend on the amount of residual ester groups.
Worldwide consumption of polyvinyl alcohol was over one million metric tons in 2006. Larger producers include Kuraray and Sekisui Specialty Chemicals but mainland China has installed a number of large production facilities in the past decade and accounts for 45% of world capacity; the North Korean-manufacture fiber Vinalon is produced from polyvinyl alcohol. Despite its inferior properties as a clothing fiber, it is produced for self-sufficiency reasons, because no oil is required to produce it. PVA is an atactic material. In terms of microstructure, it is composed of 1,3-diol linkages but a few percent of 1,2-diols occur, depending on the conditions for the polymerization of the vinyl ester precursor. Polyvinyl alcohol has excellent film forming and adhesive properties, it is resistant to oil and solvents. It has flexibility, as well as high oxygen and aroma barrier properties; however these properties are dependent on humidity, in other words, with higher humidity more water is absorbed. The water, which acts as a plasticiser, will reduce its tensile strength, but increase its elongation and tear strength.
PVA has a melting point of 230 °C and 180–190 °C for the hydrolysed and hydrolysed grades, respectively. It decomposes above 200 °C as it can undergo pyrolysis at high temperatures. PVA is close to incompressible; the Poisson's ratio is between 0.42 and 0.48. Gohsenol Kuraray Poval Mowiol Selvol Exceval Polyviol Sinopac Elvanol PVA is nontoxic, it biodegrades and solutions containing up to 5% PVA are nontoxic to fish. Polyvinyl acetate Vinyl acetate Polymerization Polyvinyl nitrate MSDS "Slime" recipe Forming PVA layers in PET bottles