- 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 term applies to either of the equations
<center>k<sub>HA</sub>/p = G(K<sub>HA</sub>q/p)<sup>α</sup>
k<sub>A</sub>/q = G(K<sub>HA</sub>q/p)<sup>-β</sup></center>
(or their logarithmic forms) where α, β and G are constants for a given reaction series (α and β are called "Brønsted exponents"), k<sub>HA</sub> and k<sub>A</sub> are catalytic coefficients (or rate coefficients) of reactions whose rates depend on the concentrations of HA and/or of A<sup>-</sup>. K<sub>HA</sub> is the acid dissociation constant of the acid HA, p is the number of equivalent acidic protons in the acid HA, and q is the number of equivalent basic sites in its conjugate base A<sup>-</sup>. The chosen values of p and q should always be specified. (The charge designations of HA and A<sup>- </sup> are only illustrative.)
The Brønsted relation is often termed the "Brønsted catalysis law" (or the "Catalysis Law"). Although justifiable on historical grounds, this name is not recommended, since Brønsted relations are known to apply to many uncatalyzed and pseudo-catalyzed reactions (such as simple proton (hydron) transfer reactions). The term "pseudo-Brønsted relation" is sometimes used for reactions which involve nucleophilic catalysis instead of acid-base catalysis. Various types of Brønsted parameters have been proposed such as β<sub>lg</sub>, β<sub>nuc</sub>, β<sub>eq</sub> for leaving group, nucleophile and equilibrium constants, respectively.
Industry:Chemistry
The equations for the relation between lg((SH<sup>+</sup>)/(S)) + H<sub>o</sub> and H<sub>o</sub> + lg(H<sup>+</sup>) for base S in aqueous mineral acid solution, where H<sub>o</sub> is Hammett's acidity function and H<sub>o</sub> + lg(H<sup>+</sup>) represents the activity function lg(γ<sub>S</sub>γ<sub>H</sub>+)/γS<sub>H</sub>+ for the nitroaniline reference bases to build H<sub>o</sub>.
<center>lg((SH<sup>+</sup>)/(S)) - lg(H<sup>+</sup>) = (Φ - 1)(H<sub>o</sub> + lg(H<sup>+</sup>)) + pK<sub>SH</sub>+
lg((SH<sup>+</sup>)/(S)) + H<sub>o</sub> = Φ(H<sub>o</sub> + lg(H<sup>+</sup>)) + pK<sub>SH</sub>+ </center>
Industry:Chemistry
An aggregate of molecules, generally in the condensed phase, that surrounds the fragments formed, for example, by thermal or photochemical dissociation. Because the cage hinders the separation of the fragments by diffusion, they may preferentially react with one another ("cage effect") but not necessarily to re-form the precursor species. For example,
<center>R-N=N-R, heat → (R<sup>.</sup> + N<sub>2</sub> + R<sup>.</sup>)cage → R-R + N<sub>2</sub></center>
Industry:Chemistry
A polycyclic compound having the shape of a cage. The term is also used for inclusion compounds.
Industry:Chemistry
Effect on the stability of a carbon centered radical determined by the combined action of a captor (electron withdrawing) and a dative (electron releasing) substituent, both attached to the radical center. The term is also used for certain unsaturated compounds.
Industry:Chemistry
Generic name for anions containing an even number of electrons and having an unshared pair of electrons on a tervalent carbon atom (e.g. Cl<sub>3</sub>C<sup>-</sup> or HC≡C<sup>-</sup>) or - if the ion is mesomeric - having at least one significant contributing structure with an unshared pair of electrons on a tervalent carbon atom, for example,
<center>CH<sub>3</sub>C(-O<sup>-</sup>)=CH-C(=O)CH<sub>3</sub> ↔ CH<sub>3</sub>C(=O)-CH<sup>-</sup>-C(=O)CH<sub>3</sub></center>
Industry:Chemistry
Generic name for the species H<sub>2</sub>C: and substitution derivatives thereof, containing an electrically neutral bivalent carbon atom with two nonbonding electrons. The nonbonding electrons may have antiparallel spins (singlet state) or parallel spins (triplet state). Use of the alternative name "methylene" as a generic term is not recommended.
Industry:Chemistry
The three-coordinate carbon atom in a carbenium ion to which the excess positive charge of the ion (other than that located on heteroatoms) may be formally considered to be largely attributed, i.e., which has one vacant p-orbital. (N.B. It is not always possible to uniquely identify such an atom.) This formal attribution of charge often does not express the real charge distribution.
Industry:Chemistry
A generic name for carbocations, real or hypothetical, that have at least one important contributing structure containing a tervalent carbon atom with a vacant p-orbital. (The name implies a protonated carbene or a substitution derivative thereof.)
The term was proposed (and rejected) as a replacement for the traditional usage of the name carbonium ion.
To avoid ambiguity, the name should not be used as the root for the systematic nomenclature of carbocations. The corresponding difficulty confused carbonium ion nomenclature for many years. For example, the term "ethylcarbonium ion" has at times been used to refer either to CH<sub>3</sub>CH<sub>2</sub><sup>+</sup> (ethyl cation) or (correctly) to CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub><sup>+</sup> (propyl cation).
Industry:Chemistry
A carbene like chemical species but with properties and reactivity differing from the free carbene itself, e.g.
<center>R<sup>1</sup>R<sup>2</sup>C(Cl)M (M = metal)</center>
Industry:Chemistry
