|
Sterically
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel. Steric hindrance Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested in ''intermolecular reactions'', whereas discussion of steric effects often focus on ''intramolecular interactions''. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions. Steric hindrance between adjacent groups can also affect torsional ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Biphenyl
Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one hydrogen (the site at which it is attached) may use the prefixes xenyl or diphenylyl. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula (C6H5)2. It is notable as a starting material for the production of polychlorinated biphenyls (PCBs), which were once widely used as dielectric fluids and heat transfer agents. Biphenyl is also an intermediate for the production of a host of other organic compounds such as emulsifiers, optical brighteners, crop protection products, and plastics. Biphenyl is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule consists of two connected phenyl rings. Properties and occurrence Biphenyl occurs naturally in coal tar, crude oi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
A Value
400px, The A-value for a methyl group is 1.74 as derived from the chemical equilibrium above. This means it costs of energy to have a methyl group in the axial position compared to the equatorial position. A-values are numerical values used in the determination of the most stable orientation of atoms in a molecule (Conformational isomerism, conformational analysis), as well as a general representation of steric bulk. A-values are derived from energy measurements of the different cyclohexane conformations of a monosubstituted cyclohexane chemical. Substituents on a cyclohexane ring prefer to reside in the equatorial position to the axial. The difference in Gibbs free energy (ΔG) between the higher energy conformation (axial substitution) and the lower energy conformation (equatorial substitution) is the A-value for that particular substituent. Utility A-values help predict the conformation of cyclohexane rings. The most stable conformation will be the one which has the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Depolymerization
Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy. Ceiling temperature The tendency of polymers to depolymerize is indicated by their ceiling temperature. At this temperature, the enthalpy of polymerization matches the entropy gained by converting a large molecule into monomers. Above the ceiling temperature, the rate of depolymerization is greater than the rate of polymerization, which inhibits the formation of the given polymer. Applications Depolymerization is a very common process. Digestion of food involves depolymerization of macromolecules, such as proteins. It is relevant to polymer recycling. Sometimes the depolymerization is well behaved, and clean monomers can be reclaimed and reused for making new plastic. In other cases, such as polyethylene, depolymerization gives a mixture of products. These products are, for polyethylene, ethylene, propylene ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting fish, due to the presence of substituted phosphine and diphosphane (). With traces of present, is spontaneously flammable in air ( pyrophoric), burning with a luminous flame. Phosphine is a highly toxic respiratory poison, and is immediately dangerous to life or health at 50 ppm. Phosphine has a trigonal pyramidal structure. Phosphines are compounds that include and the organophosphines, which are derived from by substituting one or more hydrogen atoms with organic groups. They have the general formula . Phosphanes are saturated phosphorus hydrides of the form , such as triphosphane. Phosphine, PH3, is the smallest of the phosphines and the smallest of the phosphanes. History Philippe Gengembre (1764–1838), a student of Lavois ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Solid Angle
In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The point from which the object is viewed is called the ''apex'' of the solid angle, and the object is said to ''subtend'' its solid angle at that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unit called a '' steradian'' (symbol: sr). One steradian corresponds to one unit of area on the unit sphere surrounding the apex, so an object that blocks all rays from the apex would cover a number of steradians equal to the total surface area of the unit sphere, 4\pi. Solid angles can also be measured in squares of angular measures such as degrees, minutes, and seconds. A small object nearby may subtend the same solid angle as a larger object farther away. For example, although the Moon is much smaller ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Coordination Chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many metal-containing compounds, especially those that include transition metals (elements like titanium that belong to the Periodic Table's d-block), are coordination complexes. Nomenclature and terminology Coordination complexes are so pervasive that their structures and reactions are described in many ways, sometimes confusingly. The atom within a ligand that is bonded to the central metal atom or ion is called the donor atom. In a typical complex, a metal ion is bonded to several donor atoms, which can be the same or different. A polydentate (multiple bonded) ligand is a molecule or ion that bonds to the central atom through several of the ligand's atoms; ligands with 2, 3, 4 or even 6 bonds to the central atom are common. These com ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs, often through Lewis bases. The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands". Metals and metalloids are bound to ligands in almost all circumstances, although gaseous "naked" metal ions can be generated in a high vacuum. Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Ligand selection requires critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environment ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ligand Cone Angle
In coordination chemistry, the ligand cone angle (a common example being the Tolman cone angle or ''θ'') is a measure of the steric bulk of a ligand in a transition metal coordination complex. It is defined as the solid angle formed with the metal at the vertex and the outermost edge of the van der Waals spheres of the ligand atoms at the perimeter of the cone (see figure). Tertiary phosphine ligands are commonly classified using this parameter, but the method can be applied to any ligand. The term ''cone angle'' was first introduced by Chadwick A. Tolman, a research chemist at DuPont. Tolman originally developed the method for phosphine ligands in nickel complexes, determining them from measurements of accurate physical models. Asymmetric cases The concept of cone angle is most easily visualized with symmetrical ligands, e.g. PR3. But the approach has been refined to include less symmetrical ligands of the type PRR′R″ as well as diphosphines. In such asymmetric cases, the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Styrene
Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concentrations have a less pleasant odor. Styrene is the precursor to polystyrene and several copolymers. Approximately 25 million tonnes of styrene were produced in 2010, increasing to around 35 million tonnes by 2018. Natural occurrence Styrene is named after storax balsam (often commercially sold as ''styrax''), the resin of Liquidambar trees of the Altingiaceae plant family. Styrene occurs naturally in small quantities in some plants and foods (cinnamon, coffee beans, balsam trees and peanuts) and is also found in coal tar. History In 1839, the German apothecary Eduard Simon isolated a volatile liquid from the resin (called ''storax'' or ''styrax'' (Latin)) of the American sweetgum tree (''Liquidambar styraciflua''). He called th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
4-3D-balls.png "Click for more on -> Ligand")
