- Settore: 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 interception of a reactive molecule or reaction intermediate so that it is removed from the system or converted into a more stable form for study or identification.
Industry:Chemistry
The process by which a particle or a set of particles crosses a barrier on its potential energy surface without having the energy required to surmount this barrier. Since the rate of tunneling decreases with increasing reduced mass, it is significant in the context of isotope effects of hydrogen isotopes.
Industry:Chemistry
Any process by which the normal alternating donor and acceptor reactivity pattern of a chain, which is due to the presence of O or N heteroatoms, is interchanged. Reactivity umpolung is most often achieved by temporary exchange of heteroatoms (N, O) by others, such as P, S, and Se.
The original meaning of the term has been extended to the reversal of any commonly accepted reactivity pattern. For example, reaction of R-C≡CX (X = halide) as a synthon for "R-C≡C<sup>+</sup> (i.e. electrophilic acetylene) is an umpolung of the normal more common acetylide, R-C≡C<sup>-</sup> (i.e. nucleophilic) reactivity.
Industry:Chemistry
Failing to react with a specified chemical species under specified conditions. The term should not be used in place of stable, since a relatively more stable species may nevertheless be more reactive than some reference species towards a given reaction partner.
Industry:Chemistry
The opposite of stable, i.e. the chemical species concerned has a higher molar Gibbs energy than some assumed standard. The term should not be used in place of reactive or transient, although more reactive or transient species are frequently also more unstable.
(Very unstable chemical species tend to undergo exothermic unimolecular decompositions. Variations in the structure of the related chemical species of this kind generally affect the energy of the transition states for these decompositions less than they affect the stability of the decomposing chemical species. Low stability may therefore parallel a relatively high rate of unimolecular decomposition.)
Industry:Chemistry
The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.
Industry:Chemistry
A constitutional isomer interrelated with another by pericyclic reactions. For example, Dewar benzene, prismane and benzvalene are valence isomers of benzene.
Industry:Chemistry
The attractive or repulsive forces between molecular entities (or between groups within the same molecular entity) other than those due to bond formation or to the electrostatic interaction of ions or of ionic groups with one another or with neutral molecules. The term includes: dipole-dipole, dipole-induced dipole and London (instantaneous induced dipole-induced dipole) forces.
The term is sometimes used loosely for the totality of nonspecific attractive or repulsive intermolecular forces.
Industry:Chemistry
A quantity derived from the pressure dependence of the rate constant of a reaction (mainly used for reactions in solution), defined by the equation
<center>Δ<sup>†</sup>V = -RT(∂ln k/∂p)T</center>
providing that the rate constants of all reactions (except first-order reactions) are expressed in pressure-independent concentration units, such as mol dm<sup>- 3</sup> at a fixed temperature and pressure.
The volume of activation is interpreted, according to transition state theory as the difference between the partial molar volumes of the transition state (<sup>†</sup>V) and the sums of the partial volumes of the reactants at the same temperature and pressure, i.e.,
<center>Δ<sup>†</sup>V = <sup>†</sup>V - Σ(rV<sub>R</sub>)</center>
where r is the order in the reactant R and V<sub>R</sub> its partial molar volume.
Industry:Chemistry
A multiparameter extension of the Hammett equation to quantify the role of enhanced resonance effects on the reactivity of meta- and para-substituted benzene derivatives, e.g.
<center>lg k = lg k<sub>o</sub> + ρσ + r (σ<sup>+</sup> - σ))</center>
The parameter r gives the enhanced resonance effect on the scale (σ<sup>+</sup>-σ), or (σ<sup>-</sup>-σ), respectively.
Industry:Chemistry
