- Branche: 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 ...
Quantitative measures of the capability of solvents for interaction with solutes. Such parameters have been based on numerous different physico-chemical quantities, e.g. rate constants, solvatochromic shifts in ultraviolet/visible spectra, solvent-induced shifts in infrared frequencies, etc. Some solvent parameters are purely empirical in nature, i.e. they are based directly on some experimental measurement. It may be possible to interpret such a parameter as measuring some particular aspect of solvent-solute interaction or it may be regarded simply as a measure of solvent polarity. Other solvent parameters are based on analysing experimental results. Such a parameter is considered to quantify some particular aspect of solvent capability for interaction with solutes.
Industry:Chemistry
Generally, reaction with a solvent, or with a lyonium ion or lyate ion, involving the rupture of one or more bonds in the reacting solute. More specifically the term is used for substitution, elimination and fragmentation reactions in which a solvent species is the nucleophile ("alcoholysis" if the solvent is an alcohol, etc.).
Industry:Chemistry
A solvent parameter defined by
<center>Sp = 1 - M/M(hexadecane)</center>
derived from the Gibbs energy of transfer (Δ<sub>t</sub>G<sup>o</sup>) of a series of solutes from water to numerous aqueous-organic mixtures and to pure solvents:
<center>Δ<sub>t</sub>G<sup>o</sup>(to solvent) = MR<sub>T</sub> + D</center>
where R<sub>T</sub> is a solute parameter, and M and D characterize the solvent. The M values are used to define a solvent solvophobic effect so that S<sub>p</sub> values are scaled from unity (water) to zero (hexadecane).
Industry:Chemistry
Irradiation with (often ultra)sound waves, e.g. to increase the rate of a reaction or to prepare vesicles in mixtures of surfactants and water.
Industry:Chemistry
The initial steep rate increase observed in the kinetic electrolyte effect on certain solvolysis reactions, upon addition of some non-common ion salts, especially LiClO<sub>4</sub>.
Industry:Chemistry
The acceleration of a reaction by a unique catalyst, rather than by a family of related substances. The term is most commonly used in connection with specific hydrogen-ion or hydroxide-ion (lyonium ion or lyate ion) catalysis.
Industry:Chemistry
The unpaired electron density at a position of interest, usually at carbon, in a radical. It is often measured experimentally by electron paramagnetic resonance (EPR, ESR (electron spin resonance)) spectroscopy through hyperfine coupling constants of the atom or an attached hydrogen.
Industry:Chemistry
A stable paramagnetic group (typically a nitryl radical) that is attached to a part of a molecular entity whose microscopic environment is of interest and may be revealed by the electron spin resonance (ESR) spectrum of the spin label.
When a simple paramagnetic molecular entity is used in this way without covalent attachment to the molecular entity of interest it is frequently referred to as a "spin probe".
Industry:Chemistry
In certain reactions in solution a transient radical will interact with a diamagnetic reagent to form a more persistent radical. The product radical accumulates to a concentration where detection and, frequently, identification are possible by EPR/ESR spectroscopy. The key reaction is usually one of attachment; the diamagnetic reagent is said to be a "spin trap", and the persistent product radical is then the "spin adduct". The procedure is referred to as spin trapping, and is used for monitoring reactions involving the intermediacy of reactive radicals at concentrations too low for direct observation. Typical spin traps are C-nitroso compounds and nitrones, to which reactive radicals will rapidly add to form nitryl radicals. A quantitative development, in which essentially all reactive radicals generated in a particular system are intercepted, has been referred to as "spin counting". Spin trapping has also been adapted to the interception of radicals generated in both gaseous and solid phases. In these cases the spin adduct is in practice transferred to a liquid solution for observation in order to facilitate interpretation of the EPR/ESR spectra of the radicals obtained.
Industry:Chemistry
As applied to chemical species, the term expresses a thermodynamic property, which is quantitatively measured by relative molar standard Gibbs energies. A chemical species A is more stable than its isomer B if Δ<sub>r</sub>G<sup>o</sup> > 0 for the (real or hypothetical) reaction A → B, under standard conditions. If for the two reactions
<center>P → X + Y (ΔrG1<sup>o</sup>)
Q → X + Z (ΔrG2<sup>o</sup>)</center>
Δ<sub>r</sub>G<sub>1</sub><sup>o</sup> > Δ<sub>r</sub>G<sub>2</sub><sup>o</sup>, P is more stable relative to the product Y than is Q relative to Z. Both in qualitative and quantitative usage the term stable is therefore always used in reference to some explicitly stated or implicitly assumed standard.
The term should not be used as a synonym for unreactive or "less reactive" since this confuses thermodynamics and kinetics. A relatively more stable chemical species may be more reactive than some reference species towards a given reaction partner.
Industry:Chemistry
