- المجال: Chemistry
- Number of terms: 1965
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The International Union of Pure and Applied Chemistry (IUPAC) serves to advance the worldwide aspects of the chemical sciences and to contribute to the application of chemistry in the service of people and the environment. As a scientific, international, non-governmental and objective body, IUPAC ...
The coupling of two molecular entities through an oxidative process, usually catalyzed by a transition metal compound and involving dioxygen as the oxidant; e.g.,
<center>2 CO + 2 MeOH + 1/2 O<sub>2</sub> (+ catalyst) → MeOOCCOOMe + H<sub>2</sub>O </center>
Industry:Chemistry
Substances having a magnetic susceptibility greater than 0 are paramagnetic. They are drawn into a magnetic field.
Industry:Chemistry
A chemical reaction in which concerted reorganization of bonding takes place throughout a cyclic array of continuously bonded atoms. It may be viewed as a reaction proceeding through a fully conjugated cyclic transition state. The number of atoms in the cyclic array is usually six, but other numbers are also possible. The term embraces a variety of processes, including cycloadditions, cheletropic reactions, electrocyclic reactions and sigmatropic rearrangements, etc. (provided they are concerted).
Industry:Chemistry
The differentiation between two or more symmetry-allowed processes, for example the (2+4) vs. (4+6) cycloaddition of cyclopentadiene to tropone.
Industry:Chemistry
The term persistent is used to characterize radicals which have lifetimes of several minutes or greater in dilute solution in inert solvents. Persistence is a kinetic or reactivity property. In contrast, radical stability, which is a thermodynamic property, is expressed in terms of the C-H bond strength of the appropriate hydrocarbon. The lifetime of a radical is profoundly influenced by steric shielding of the radical center by bulky substituents.
Industry:Chemistry
The phenomenon of rate enhancement of a reaction between chemical species located in different phases (immiscible liquids or solid and liquid) by addition of a small quantity of an agent (called the "phase-transfer catalyst") that extracts one of the reactants, most commonly an anion, across the interface into the other phase so that reaction can proceed. These catalysts are salts of "onium ions" (e.g. tetraalkylammonium salts) or agents that complex inorganic cations (e.g. crown ethers). The catalyst cation is not consumed in the reaction although an anion exchange does occur.
Industry:Chemistry
The cleavage of one or more covalent bonds in a molecular entity resulting from absorption of light, or a photochemical process in which such cleavage is an essential part. For example:
<center>Cl<sub>2</sub> → 2 Cl<sup>.</sup></center>
The term is used incorrectly to describe irradiation of a sample, although in the combination flash photolysis this usage is accepted.
Industry:Chemistry
A plot of observed rate coefficient, or more usually its decadic logarithm, against pH of solution, other variables being kept constant.
Industry:Chemistry
For a reactant molecule RY, the polar effect of the group R comprises all the processes whereby a substituent may modify the electrostatic forces operating at the reaction center Y, relative to the standard R<sup>o</sup>Y. These forces may be governed by charge separations arising from differences in the electronegativity of atoms (leading to the presence of dipoles), the presence of unipoles, or electron delocalization. It is synonymous with electronic effect or "electrical effect" of a substituent as distinguished from other substituent effects, e.g. steric effects.
Sometimes, however, the term "polar effect" is taken to refer to the influence, other than steric, that non-conjugated substituents exert on reaction rates, i.e. effects connected with electron delocalization between a substituent and the molecular framework to which it is attached are excluded. Polar effect is then not synonymous with electronic effect.
Industry:Chemistry
When applied to solvents, this rather ill-defined term covers their overall solvation capability (solvation power) for solutes (i.e. in chemical equilibria: reactants and products; in reaction rates: reactants and activated complex; in light absorptions: ions or molecules in the ground and excited state), which in turn depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e. to van der Waals forces).
Industry:Chemistry
