- المجال: Chemistry
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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 ...
An organic or inorganic ion or molecule that occupies a cavity, cleft or pocket within the molecular structure of a host molecular entity and forms a complex with it or that is trapped in a cavity within the crystal structure of a host.
Industry:Chemistry
In a kinetic experiment, the time required for the concentration of a particular reacting species to fall to one-half of its initial value. (Its dependence on initial concentration depends upon the order of reaction. It is independent of initial concentration only for a first-order process.)
Industry:Chemistry
Halochromism means the color change which occurs on addition of acid (or base, or a salt) to a solution of a compound. A chemical reaction (e.g. ion formation) transforms a colorless compound into a colored one.
Industry:Chemistry
The equation in the form
<center>lg(k/k<sub>o</sub>) = ρσ</center>
or
<center>lg(K/K<sub>o</sub>) = ρσ</center>
applied to the influence of meta- or para-substituents X on the reactivity of the functional group Y in the benzene derivative m- or p-XC<sub>6</sub>H<sub>4</sub>Y. k or K is the rate or equilibrium constant, respectively, for the given reaction of m- or p-XC<sub>6</sub>H<sub>4</sub>Y; k<sub>o</sub> or K<sub>o</sub> refers to the reaction of C<sub>6</sub>H<sub>5</sub>Y, i.e. X = H; σ is the substituent constant characteristic of m- or p-X; ρ is the reaction constant characteristic of the given reaction of Y. The equation is often encountered in a form with lg k<sub>o</sub> or lg K<sub>o</sub> written as a separate term on the right hand side, e.g.
<center>lg k = ρσ + lg k<sub>o</sub></center>
or
<center>lg K = ρσ + lg K<sub>o</sub></center>
It then signifies the intercept corresponding to X = H in a regression of lg k or lg K on σ.
Industry:Chemistry
The hypothesis that, when a transition state leading to an unstable reaction intermediate (or product) has nearly the same energy as that intermediate, the two are interconverted with only a small reorganization of molecular structure. Essentially the same idea is sometimes referred to as "Leffler's assumption", namely, that the transition state bears the greater resemblance to the less stable species (reactant or reaction intermediate/product). Many text books and physical organic chemists, however, express the idea in Leffler's form, but attribute it to Hammond.
As a corollary, it follows that a factor stabilizing a reaction intermediate will also stabilize the transition state leading to that intermediate.
The acronym "Bemahapothle" (Bell, Marcus, Hammond, Polanyi, Thornton, Leffler) is sometimes used in recognition of the principal contributors towards expansion of the original idea of the Hammond postulate.
Industry:Chemistry
The hypothesis that, when a transition state leading to an unstable reaction intermediate (or product) has nearly the same energy as that intermediate, the two are interconverted with only a small reorganization of molecular structure. Essentially the same idea is sometimes referred to as "Leffler's assumption", namely, that the transition state bears the greater resemblance to the less stable species (reactant or reaction intermediate/product). Many text books and physical organic chemists, however, express the idea in Leffler's form, but attribute it to Hammond.
As a corollary, it follows that a factor stabilizing a reaction intermediate will also stabilize the transition state leading to that intermediate.
The acronym "Bemahapothle" (Bell, Marcus, Hammond, Polanyi, Thornton, Leffler) is sometimes used in recognition of the principal contributors towards expansion of the original idea of the Hammond postulate.
Industry:Chemistry
A measure of the capability of a solute for hydrophobic (lipophilic) interaction based on the partition coefficient P for distribution of the solute between octan-1-ol and water. The most general way of applying P in correlation analysis, QSAR, etc. is as log P, but the behavior of substituted benzene derivatives may be quantified by a substituent constant scale, π, which is defined in a way analogous to the Hammett σ scale. There are various π scales, depending on the substrate series used as reference.
Industry:Chemistry
The hapto symbol, η with numerical superscript, provides a topological description for the bonding of hydrocarbons and other pi-electron systems to metals, by indicating the connectivity between the ligand and the central atom. For example, η<sup>3</sup> indicates that three atoms of the ligand are bonded to the central atom.
Industry:Chemistry
A Lewis acid with an acceptor center of low polarizability. Other things being approximately equal, complexes of hard acids and bases or soft acids and bases have an added stabilization (sometimes called "HSAB" rule). For example the hard O- (or N-) bases are preferred to their S- (or P-) analogues by hard acids. Conversely a "soft acid" possesses an acceptor center of high polarizability and exhibits the reverse preference for coordination of a soft base. These preferences are not defined in a quantitative sense.
Industry:Chemistry
A Lewis base with a donor center (e.g. an oxygen atom) of low polarizability; the converse applies to "soft bases".
Industry:Chemistry
