- 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 ...
This is the term most commonly used to describe the influence (on reactivity, spectra, etc.) of a substituent through electron delocalization into or from the substituent. The use of the term obviates the need to attempt to distinguish between the operation of the mesomeric effect and the electromeric effect. (An alternative term with essentially the same meaning is "conjugative effect". At one time "tautomeric effect" was also used, but was abandoned because tautomerism implies reorganization of the atomic nuclei.) The effect is symbolized by R.
Industry:Chemistry
The difference in potential energy between the actual molecular entity and the contributing structure of lowest potential energy. The resonance energy cannot be measured, but only estimated, since contributing structures are not observable molecular entities.
Industry:Chemistry
The linear free energy relation
<center>log k<sub>N</sub> = log k<sub>0</sub> + N<sub>+</sub></center>
applied to the reactions between nucleophiles and certain large and relatively stable organic cations, e.g. arenediazonium, triarylmethyl, and aryltropylium cations in various solvents. k<sub>N</sub> is the rate constant for reaction of a given cation with a given nucleophilic system (i.e. given nucleophile in a given solvent). k<sub>0</sub> is the rate constant for the same cation with water in water, and N<sub>+</sub> is a parameter which is characteristic of the nucleophilic system and independent of the cation. A surprising feature of the equation is the absence of a coefficient of N<sub>+</sub>, characteristic of the substrate (cf. the s in the Swain-Scott equation), even though values of N<sub>+</sub> vary over 13 log units. The equation thus involves a gigantic breakdown of the reactivity-selectivity principle. The equation has been extended both in form and in range of application.
Industry:Chemistry
A term used in nuclear magnetic resonance. When a nucleus is strongly irradiated, its spin population may partly be transferred to another nucleus by an exchange process.
Industry:Chemistry
Dehydrohalogenation of secondary- and tertiary-alkyl halides proceeds by the preferential removal of the β-hydrogen from the carbon that has the smallest number of hydrogens. Originally formulated by A. Saytzeff (Zaitsev) to generalize the orientation in β-elimination reactions of alkyl halides, this rule has been extended and modified, as follows: When two or more olefins can be produced in an elimination reaction, the thermodynamically most stable alkene will predominate. Exceptions to the Saytzeff rule are exemplified by the Hofmann rule.
Industry:Chemistry
A substance that reacts with (or otherwise removes) a trace component (as in the scavenging of trace metal ions) or traps a reactive reaction intermediate.
Industry:Chemistry
A kinetic isotope effect that is attributable to isotopic substitution of an atom to which bonds are neither made nor broken in the rate-controlling step or in a pre-equilibrium step of a specified reaction, and is therefore not a primary isotope effect, is termed a secondary isotope effect. One speaks of α, (etc.) secondary isotope effects, where α, β (etc.) denote the position of isotopic substitution relative to the reaction center. The corresponding isotope effect on the equilibrium constant of such a reaction is called a "secondary equilibrium isotope effect".
Secondary isotope effects have been discussed in terms of the conventional electronic effects of physical organic chemistry, e.g. induction, hyperconjugation, hybridization, etc., since these properties are determined by the electron distribution, that depends on vibrationally averaged bond lengths and angles which vary slightly with isotopic substitution. While this usage is legitimate, the term "electronic isotope effect" should be avoided, because of the misleading implication that such an effect is electronic rather than vibrational in origin.
Industry:Chemistry
The discrimination shown by a reagent in competitive attack on two or more substrates or on two or more positions in the same substrate. It is quantitatively expressed by ratios of rate constants of the competing reactions, or by the decadic logarithms of such ratios.
Industry:Chemistry
A quantitative representation of selectivity in aromatic substitution reactions (usually electrophilic, for monosubstituted benzene derivatives). If the partial rate factor, f, expresses the reactivity of a specified position in the aromatic compound PhX relative to that of a single position in benzene, then the selectivity factor S<sub>f</sub> (expressing discrimination between p- and m-positions in PhX) is defined as
<center>S<sub>f</sub> = lg (f<sub>p</sub><sup>X</sup>/f<sub>m</sub><sup>X</sup>).</center>
Industry:Chemistry
In the context of NMR spectroscopy shielding is the effect of the electron shells of the observed and the neighboring nuclei on the external magnetic field. The external field induces circulations in the electron cloud. The resulting magnetic moment is oriented in the opposite direction to the external field, so that the local field at the central nucleus is weakened, although it may be strengthened at other nuclei (deshielding).
The phenomenon is the origin of the structural dependence of the resonance frequencies of the nuclei.
Industry:Chemistry
