- Settore: 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 ...
A cation containing an even number of electrons with a significant portion of the excess positive charge located on one or more carbon atoms. This is a general term embracing carbenium ions, all types of carbonium ions, vinyl cations, etc. Carbocations may be named by adding the word "cation" to the name of the corresponding radical. Such names do not imply structure (e.g. whether three-coordinated or five-coordinated carbon atoms are present).
Industry:Chemistry
The term should be used with great care since several incompatible meanings are currently in use. It is not acceptable as the root for systematic nomenclature for carbocations.
(1) In most of the existing literature the term is used in its traditional sense for what is here defined as carbenium ion.
(2) A carbocation, real or hypothetical, that contains at least one five-coordinate carbon atom.
(3) A carbocation, real or hypothetical, whose structure cannot adequately be described by two-electron two-center bonds only. (The structure may involve carbon atoms with a coordination number greater than five.)
Industry:Chemistry
A substance that participates in a particular chemical reaction and thereby increases its rate but without a net change in the amount of that substance in the system. At the molecular level, the catalyst is used and regenerated during each set of microscopic chemical events leading from a molecular entity of reactant to a molecular entity of product.
Industry:Chemistry
If the rate of reaction (v) is expressible in the form
<center>v = (k<sub>0</sub> + Σk<sub>i</sub>(C<sub>i</sub>)<sup>n<sub>i</sub></sup>) (A)<sup>α</sup> (B)<sup>β</sup>...</center>
where A, B, ... are reactants and Ci represents one of a set of catalysts, then the proportionality factor k<sub>i</sub> is the catalytic coefficient of the particular catalyst Ci. Normally the partial order of reaction (n<sub>i</sub>) with respect to a catalyst will be unity, so that k<sub>i</sub> is an (α+β+...+1)th order rate coefficient. The proportionality factor k<sub>0</sub> is the (α+β+...)th order rate coefficient of the uncatalyzed component of the total reaction.
Industry:Chemistry
A reaction in which one or more reactive reaction intermediates (frequently radicals) are continuously regenerated, usually through a repetitive cycle of elementary steps (the "propagation step"). For example, in the chlorination of methane by a radical mechanism, Cl<sup>.</sup> is continuously regenerated in the chain propagation steps:
<center>Cl<sup>.</sup> + CH<sub>4</sub> → HCl + H<sub>3</sub>C<sup>.</sup>
H<sub>3</sub>C<sup>.</sup> + Cl<sub>2</sub> → CH<sub>3</sub>Cl + Cl<sup>.</sup></center>
In chain polymerization reactions, reactive intermediates of the same types, generated in successive steps or cycles of steps, differ in relative molecular mass, as in
<center>RCH<sub>2</sub>C<sup>.</sup>HPh + H<sub>2</sub>C=CHPh → RCH<sub>2</sub>CHPhCH<sub>2</sub>C<sup>.</sup>HPh</center>
Industry:Chemistry
The abstraction, by the radical end of a growing chain-polymer, of an atom from another molecule. The growth of the polymer chain is thereby terminated but a new radical, capable of chain propagation and polymerization, is simultaneously created. For the example of alkene polymerization cited for a chain reaction, the reaction
<center>RCH<sub>2</sub>C<sup>.</sup>HPh + CCl<sub>4</sub> → RCH<sub>2</sub>CHClPh + Cl<sub>3</sub>C<sup>.</sup></center>
represents a chain transfer, the radical Cl<sub>3</sub>C<sup>.</sup> inducing further polymerization:
<center>H<sub>2</sub>C=CHPh + Cl<sub>3</sub>C<sup>.</sup> → Cl<sub>3</sub>CCH<sub>2</sub>C<sup>.</sup>HPh
Cl<sub>3</sub>CCH<sub>2</sub>C<sup>.</sup>HPh + H<sub>2</sub>C=CHPh → Cl<sub>3</sub>CCH<sub>2</sub>CHPhCH<sub>2</sub>C<sup>.</sup>HPh</center>
The phenomenon occurs also in other chain reactions such as cationic polymerization.
Industry:Chemistry
The net electric charge on a specified atom in a molecular entity, as determined by some prescribed definition such as that by Mulliken.
Industry:Chemistry
A ground state adduct which exhibits an observable charge transfer absorption band.
Industry:Chemistry
The formation or presence of bonds (or other attractive interactions) between two or more separate binding sites within the same ligand and a single central atom. A molecular entity in which there is chelation (and the corresponding chemical species) is called a "chelate". The terms bidentate (or didentate), tridentate, tetradentate... multidentate are used to indicate the number of potential binding sites of the ligand, at least two of which must be used by the ligand in forming a "chelate". For example, the bidentate ethylenediamine forms a chelate with CuI in which both nitrogen atoms of ethylenediamine are bonded to copper. (The use of the term is often restricted to metallic central atoms.)
The phrase "separate binding sites" is intended to exclude cases such as (PtCl<sub>3</sub>(CH<sub>2</sub>=CH<sub>2</sub>))<sup>-</sup>, ferrocene, and (benzene)tricarbonylchromium in which ethene, the cyclopentadienyl group, and benzene, respectively, are considered to present single binding sites to the respective metal atom, and which are not normally thought of as chelates.
Industry:Chemistry
