Organolithium reagents are organometallic compounds that contain carbon – lithium bonds. They are important reagents in organic synthesis, are used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers, they have been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C-Li bond is ionic. Owing to the polar nature of the C-Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form; these reagents are reactive, are sometimes pyrophoric. Studies of organolithium reagents began in the 1930 and were pioneered by Karl Ziegler, Georg Wittig, Henry Gilman.
In comparison with Grignard reagents, organolithium reagents can perform the same reactions with increased rates and higher yields, such as in the case of metalation. Since organolithium reagents have overtaken Grignard reagents in usage. Although simple alkyllithium species are represented as monomer RLi, they exist as aggregates or polymers; the degree of aggregation depends on the presence of other ligands. These structures have been elucidated by a variety of methods, notably 6Li, 7Li, 13C NMR spectroscopy and X-ray diffraction analysis. Computational chemistry supports these assignments; the relative electronegativities of carbon and lithium suggest that the C-Li bond will be polar. However, certain organolithium compounds possess properties such as solubility in nonpolar solvents that complicate the issue. While most data suggest the C-Li bond to be ionic, there has been debate as to whether a small covalent character exists in the C-Li bond. One estimate puts the percentage of ionic character of alkyllithium compounds at 80 to 88%.
In allyl lithium compounds, the lithium cation coordinates to the face of the carbon π bond in an η3 fashion instead of a localized, carbanionic center, allyllithiums are less aggregated than alkyllithiums. In aryllithium complexes, the lithium cation coordinates to a single carbanion center through a Li-C σ type bond. Like other species consisting of polar subunits, organolithium species aggregate. Formation of aggregates is influenced by electrostatic interactions, the coordination between lithium and surrounding solvent molecules or polar additives, steric effects. A basic building block toward constructing more complex structures is a carbanionic center interacting with a Li3 triangle in an η- 3 fashion. In simple alkyllithium reagents, these triangles aggregate to form tetrahedron or octahedron structures. For example, methyllithium and tert-butyllithium all exist in the tetramer 4. Methyllithium exists as tetramers in a cubane-type cluster in the solid state, with four lithium centers forming a tetrahedron.
Each methanide in the tetramer in methyllithium can have agostic interaction with lithium cations in adjacent tetramers. Ethyllithium and tert-butyllithium, on the other hand, do not exhibit this interaction, are thus soluble in non-polar hydrocarbon solvents. Another class of alkyllithium adopts hexameric structures, such as n-butyllithium and cyclohexanyllithium. Common lithium amides, e.g. lithium bisamide and lithium diisopropylamide, are subject to aggregation. Lithium amides adopt polymeric-ladder type structures in non-coordinating solvent in the solid state, they exist as dimers in ethereal solvents. In the presence of donating ligands, tri- or tetrameric lithium centers are formed. For example, LDA exists as dimers in THF; the structures of common lithium amides, such as lithium diisopropylamide and lithium hexamethyldisilazide have been extensively studied by Collum and coworkers using NMR spectroscopy. Another important class of reagents is silyllithiums, extensively used in the synthesis of organometallic complexes and polysilane dendrimers.
In the solid state, in contrast with alkyllithium reagents, most silyllithiums tend to form monomeric structures coordinated with solvent molecules such as THF, only a few silyllithiums have been characterized as higher aggregates. This difference can arise from the method of preparation of silyllithiums, the steric hindrance caused by the bulky alkyl substituents on silicon, the less polarized nature of Si-Li bonds; the addition of donating ligands, such as TMEDA and -sparteine, can displace coordinating solvent molecules in silyllithiums. Relying on the structural information of organolithium aggregates obtained in the solid state from crystal structures has certain limits, as it is possible for organolithium reagents to adopt different structures in reaction solution environment. In some cases the crystal structure of an organolithium species can be difficult to isolate. Therefore, studying the structures of organolithium reagents, the lithium-containing intermediates in solution form is useful in understanding the reactivity of these reagents.
NMR spectroscopy has emerged as a powerful tool for the studies of organolithium aggregates in solution. For alkyllithium species, C-Li J coupling can used to determine the number of lithium interacting with a carbanion center, whether these interactions are static or dynamic. Separate NMR signals can differentiate the presence of multiple aggregates from a common monomeric unit; the structures of organolithium compounds are affected by the presenc
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
A cubane-type cluster is an arrangement of atoms in a molecular structure that forms a cube. In the idealized case, the eight vertices are symmetry equivalent and the species has Oh symmetry; such a structure is illustrated by the hydrocarbon cubane. With chemical formula C8H8, cubane has carbon atoms at the corners of a cube and covalent bonds forming the edges. Most cubanes have more complicated structures with nonequivalent vertices, they may be macromolecular or supramolecular cluster compounds. Other compounds having different elements in the corners, various atoms or groups bonded to the corners are all part of this class of structures. Cubane clusters are common throughout bioinorganic chemistry. Ferredoxins containing iron–sulfur clusters are pervasive in nature; the four iron atoms and four sulfur atoms form an alternating arrangement at the corners. The whole cluster is anchored by coordination of the iron atoms with cysteine residues. In this way, each Fe center achieves tetrahedral coordination geometry.
Some clusters arise via dimerization of square-shaped precursors. Many synthetic analogues are known including heterometallic derivatives. Several alkyllithium compounds exist as clusters in solution tetramers, with the formula 4; the individual RLi molecules are not observed. The four lithium atoms and the carbon from each alkyl group bonded to them occupy alternating vertices of the cube, with the additional atoms of the alkyl groups projecting off their respective corners. Octaazacubane is a hypothetical allotrope of nitrogen with formula N8. Like the carbon-based cubane compounds, octaazacubane is predicted to be unstable due to angle strain at the corners, it does not enjoy the kinetic stability seen for its organic analogues
In organic chemistry, an alkane, or paraffin, is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. Alkanes have the general chemical formula CnH2n+2; the alkanes range in complexity from the simplest case of methane, where n = 1, to arbitrarily large and complex molecules, like pentacontane or 6-ethyl-2-methyl-5- octane, an isomer of tetradecane. IUPAC defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, therefore consisting of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote any saturated hydrocarbon, including those that are either monocyclic or polycyclic, despite their having a different general formula. In an alkane, each carbon atom is sp3-hybridized with 4 sigma bonds, each hydrogen atom is joined to one of the carbon atoms; the longest series of linked carbon atoms in a molecule is known as its carbon skeleton or carbon backbone.
The number of carbon atoms may be considered as the size of the alkane. One group of the higher alkanes are waxes, solids at standard ambient temperature and pressure, for which the number of carbon atoms in the carbon backbone is greater than about 17. With their repeated –CH2 units, the alkanes constitute a homologous series of organic compounds in which the members differ in molecular mass by multiples of 14.03 u. Alkanes are not reactive and have little biological activity, they can be viewed as molecular trees upon which can be hung the more active/reactive functional groups of biological molecules. The alkanes have two main commercial sources: natural gas. An alkyl group abbreviated with the symbol R, is a functional group that, like an alkane, consists of single-bonded carbon and hydrogen atoms connected acyclically—for example, a methyl or ethyl group. Saturated hydrocarbons are hydrocarbons having only single covalent bonds between their carbons, they can be: linear wherein the carbon atoms are joined in a snake-like structure branched wherein the carbon backbone splits off in one or more directions cyclic wherein the carbon backbone is linked so as to form a loop.
According to the definition by IUPAC, the former two are alkanes, whereas the third group is called cycloalkanes. Saturated hydrocarbons can combine any of the linear and branching structures. Alkanes are the acyclic ones, corresponding to k = 0. Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers; the simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer; however the chain of carbon atoms may be branched at one or more points. The number of possible isomers increases with the number of carbon atoms. For example, for acyclic alkanes: C1: methane only C2: ethane only C3: propane only C4: 2 isomers: n-butane and isobutane C5: 3 isomers: pentane and neopentane C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane C12: 355 isomers C32: 27,711,253,769 isomers C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. In addition to the alkane isomers, the chain of carbon atoms may form one or more loops; such compounds are called cycloalkanes. Stereoisomers and cyclic compounds are excluded; the IUPAC nomenclature for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane". In 1866, August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine, -one, -une, for the hydrocarbons CnH2n+2, CnH2n, CnH2n−2, CnH2n−4, CnH2n−6. Now, the first three name hydrocarbons with single and triple bonds, it is impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets.
Numbers in the name, referring to which carbon a group is attached to, should be as low as possible so that 1- is implied and omitted from names of organic compounds with only one side-group. Symmetric compounds will have two ways of arriving at the same name. Straight-chain alkanes are sometimes indicated by the prefix "n -". Although this is not necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g. n-hexane or 2- or 3-met
Polybutadiene is a synthetic rubber. Polybutadiene rubber is a polymer formed from the polymerization of the monomer 1,3-butadiene. Polybutadiene has a high resistance to wear and is used in the manufacture of tires, which consumes about 70% of the production. Another 25% is used as an additive to improve the toughness of plastics such as polystyrene and acrylonitrile butadiene styrene. Polybutadiene rubber accounted for about a quarter of total global consumption of synthetic rubbers in 2012, it is used to manufacture golf balls, various elastic objects and to coat or encapsulate electronic assemblies, offering high electrical resistivity. 1,3-Butadiene is an organic compound, a simple conjugated diene hydrocarbon. Polybutadiene forms by linking many 1,3-butadiene monomers to make a much longer polymer chain molecule. In terms of the connectivity of the polymer chain, butadiene can polymerize in three different ways, called cis and vinyl; the cis and trans forms arise by connecting the butadiene molecules end-to-end, so-called 1,4-polymerisation.
The properties of the resulting isomeric forms of polybutadiene differ. For example, "high cis"-polybutadiene has a high elasticity and is popular, whereas the so-called "high trans" is a plastic crystal with few useful applications; the vinyl content of polybutadiene is no more than a few percent. In addition to these three kinds of connectivity, polybutadienes differ in terms of their branching and molecular weights; the trans double bonds formed during polymerization allow the polymer chain to stay rather straight, allowing sections of polymer chains to align to form microcrystalline regions in the material. The cis double bonds cause a bend in the polymer chain, preventing polymer chains from aligning to form crystalline regions, which results in larger regions of amorphous polymer, it has been found that a substantial percentage of cis double bond configurations in the polymer will result in a material with flexible elastomer qualities. In free radical polymerization, both cis and trans double bonds will form in percentages that depend on temperature.
The catalysts influence. The catalyst used in the production determines the type of polybutadiene product; this type is characterized by a small proportion of vinyl. It is manufactured using Ziegler–Natta catalysts based on transition metals. Depending on the metal used, the properties vary slightly. Using cobalt gives branched molecules, resulting in a low viscosity material, ease of use, but its mechanical strength is low. Neodymium gives a higher percentage of 98 % cis. Other less used catalysts include titanium. Using an alkyllithium as the catalyst produces a polybutadiene called "low cis" which contains 36% cis, 59% trans and 10% vinyl. Despite its high liquid-glass transition, low cis polybutadiene is used in tire manufacturing and is blended with other tire polymers it can be advantageously used as an additive in plastics due to its low contents of gels. In 1980, researchers from the Japanese company Zeon discovered that high-vinyl polybutadiene, despite having a high liquid-glass transition, could be advantageously used in combination with high cis in tires.
This material is produced with an alkyllithium catalyst. Polybutadiene can be produced with more than 90% trans using catalysts similar to those of high cis: neodymium, nickel; this material is a plastic crystal which melts at about 80 °C. It was used for the outer layer of golf balls. Today it is only used industrially, but companies like Ube are investigating other possible applications; the use of metallocene catalysts to polymerize butadiene is being explored by Japanese researchers. The benefits seem to be a higher degree of control both in the distribution of molecular mass and the proportion of cis/trans/vinyl; as of 2006, no manufacturer produces "metallocene polybutadiene" on a commercial basis. 1,3-butadiene is copolymerized with other types of monomers such as styrene and acrylonitrile to form rubbers or plastics with various qualities. The most common form is styrene-butadiene copolymer, a commodity material for car tires, it is used in block copolymers and tough thermoplastics such as ABS plastic.
This way a copolymer material can be made with good stiffness and toughness. Because the chains have a double bond in each and every repeat unit, the material is sensitive to ozone cracking; the annual production of polybutadiene is 2.1 million tons. This makes it the second most produced synthetic rubber behind styrene-butadiene rubber. Polybutadiene is used in various parts of automobile tires; the polybutadiene is used in the sidewall of truck tires, this helps to improve fatigue to failure life due to the continuous flexing during run. As a result, tires will not blow out in extreme service conditions, it is used in the tread portion of giant truck tires to improve the abrasion, i.e. less wearing, to run the tire comparatively cool, since the internal heat comes out quickly. Both parts are formed by extrusion, its main competitors in this application are natural rubber. Polybutadiene has the advantage compared to SBR in its lower liquid-glass transition temperature, which gives it a high resistance to wear and a low rolling resistance.
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The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure has a higher boiling point than when that liquid is at atmospheric pressure. For example, water at 93.4 °C at 1,905 metres altitude. For a given pressure, different liquids will boil at different temperatures; the normal boiling point of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere. At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid; the standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar.
The heat of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure. Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules located near the liquid's edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid. A saturated liquid contains as much thermal energy. Saturation temperature means boiling point; the saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase. The liquid can be said to be saturated with thermal energy. Any addition of thermal energy results in a phase transition. If the pressure in a system remains constant, a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy is removed.
A liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied. The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the surrounding environmental pressure. Thus, the boiling point is dependent on the pressure. Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa, or the IUPAC standard pressure of 100.000 kPa. At higher elevations, where the atmospheric pressure is much lower, the boiling point is lower; the boiling point increases with increased pressure up to the critical point, where the gas and liquid properties become identical. The boiling point cannot be increased beyond the critical point; the boiling point decreases with decreasing pressure until the triple point is reached. The boiling point cannot be reduced below the triple point. If the heat of vaporization and the vapor pressure of a liquid at a certain temperature are known, the boiling point can be calculated by using the Clausius–Clapeyron equation, thus: T B = − 1, where: T B is the boiling point at the pressure of interest, R is the ideal gas constant, P is the vapour pressure of the liquid at the pressure of interest, P 0 is some pressure where the corresponding T 0 is known, Δ H vap is the heat of vaporization of the liquid, T 0 is the boiling temperature, ln is the natural logarithm.
Saturation pressure is the pressure for a corresponding saturation temperature at which a liquid boils into its vapor phase. Saturation pressure and saturation temperature have a direct relationship: as saturation pressure is increased, so is saturation temperature. If the temperature in a system remains constant, vapor at saturation pressure and temperature will begin to condense into its liquid phase as the system pressure is increased. A liquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is decreased. There are two conventions regarding the standard boiling point of water: The normal boiling point is 99.97 °C at a pressure of 1 atm. The IUPAC recommended standard boiling point of water at a standard pressure of 100 kPa is 99.61 °C. For comparison, on top of Mount Everest, at 8,848 m elevation, the pressure is about 34 kPa and the boiling point of water is 71 °C; the Celsius temperature scale was defined until 1954 by two points: 0 °C being defined by the wate
The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium; the melting point of a substance depends on pressure and is specified at a standard pressure such as 1 atmosphere or 100 kPa. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point; because of the ability of some substances to supercool, the freezing point is not considered as a characteristic property of a substance. When the "characteristic freezing point" of a substance is determined, in fact the actual methodology is always "the principle of observing the disappearance rather than the formation of ice", that is, the melting point. For most substances and freezing points are equal. For example, the melting point and freezing point of mercury is 234.32 kelvins. However, certain substances possess differing solid-liquid transition temperatures.
For example, agar melts at 85 °C and solidifies from 31 °C. The melting point of ice at 1 atmosphere of pressure is close to 0 °C. In the presence of nucleating substances, the freezing point of water is not always the same as the melting point. In the absence of nucleators water can exist as a supercooled liquid down to −48.3 °C before freezing. The chemical element with the highest melting point is tungsten, at 3,414 °C; the often-cited carbon does not melt at ambient pressure but sublimes at about 3,726.85 °C. Tantalum hafnium carbide is a refractory compound with a high melting point of 4215 K. At the other end of the scale, helium does not freeze at all at normal pressure at temperatures arbitrarily close to absolute zero. Many laboratory techniques exist for the determination of melting points. A Kofler bench is a metal strip with a temperature gradient. Any substance can be placed on a section of the strip, revealing its thermal behaviour at the temperature at that point. Differential scanning calorimetry gives information on melting point together with its enthalpy of fusion.
A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window and a simple magnifier. The several grains of a solid are placed in a thin glass tube and immersed in the oil bath; the oil bath is heated and with the aid of the magnifier melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, optical detection is automated; the measurement can be made continuously with an operating process. For instance, oil refineries measure the freeze point of diesel fuel online, meaning that the sample is taken from the process and measured automatically; this allows for more frequent measurements as the sample does not have to be manually collected and taken to a remote laboratory. For refractory materials the high melting point may be determined by heating the material in a black body furnace and measuring the black-body temperature with an optical pyrometer. For the highest melting materials, this may require extrapolation by several hundred degrees.
The spectral radiance from an incandescent body is known to be a function of its temperature. An optical pyrometer matches the radiance of a body under study to the radiance of a source, calibrated as a function of temperature. In this way, the measurement of the absolute magnitude of the intensity of radiation is unnecessary. However, known temperatures must be used to determine the calibration of the pyrometer. For temperatures above the calibration range of the source, an extrapolation technique must be employed; this extrapolation is accomplished by using Planck's law of radiation. The constants in this equation are not known with sufficient accuracy, causing errors in the extrapolation to become larger at higher temperatures. However, standard techniques have been developed to perform this extrapolation. Consider the case of using gold as the source. In this technique, the current through the filament of the pyrometer is adjusted until the light intensity of the filament matches that of a black-body at the melting point of gold.
This establishes the primary calibration temperature and can be expressed in terms of current through the pyrometer lamp. With the same current setting, the pyrometer is sighted on another black-body at a higher temperature. An absorbing medium of known transmission is inserted between this black-body; the temperature of the black-body is adjusted until a match exists between its intensity and that of the pyrometer filament. The true higher temperature of the black-body is determined from Planck's Law; the absorbing medium is removed and the current through the filament is adjusted to match the filament intensity to that of the black-body. This establishes a second calibration point for the pyrometer; this step is repeated to carry the calibration to hi