- 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 indicates a sequence of reactions such as shown in equations (1)-(3), leading from A to B:
<center>A + e<sup>-</sup> → A<sup>. -</sup> (1)
A<sup>. -</sup> → B<sup>. -</sup> (2)
B<sup>. -</sup> + A → B + A<sup>. -</sup> (3)</center>
An analogous sequence involving radical cations (A<sup>.+</sup>, B<sup>.+</sup>) is also observed.
The most notable example of electron-transfer catalysis is the S<sub>RN</sub>1 (or T+D<sub>N</sub>+A<sub>N</sub>) reaction of aromatic halides.
The term has its origin in a suggested analogy to acid-base catalysis, with the electron instead of the proton. However, there is a difference between the two catalytic mechanisms, since the electron is not a true catalyst, but rather behaves as the initiator of a chain reaction. "Electron-transfer induced chain reaction" is a more appropriate term for the situation described by equations (1)-(3).
Industry:Chemistry
(1) The property of being electrophilic (see electrophile).
(2) The relative reactivity of an electrophilic reagent. (It is also sometimes referred to as "electrophilic power".) Qualitatively, the concept is related to Lewis acidity. However, whereas Lewis acidity is measured by relative equilibrium constants, electrophilicity is measured by relative rate constants for reactions of different electrophilic reagents towards a common substrate (usually involving attack at a carbon atom).
Industry:Chemistry
The ratio of the rate constants of two reactions that differ only in the identity of the element of the atom in the leaving group, e.g., k<sub>Br</sub>/k<sub>Cl</sub>. As for isotope effects, a ratio of unity is regarded as a "null effect".
Industry:Chemistry
A reaction for which no reaction intermediates have been detected or need to be postulated in order to describe the chemical reaction on a molecular scale. An elementary reaction is assumed to occur in a single step and to pass through a single transition state.
Industry:Chemistry
The reverse of an addition reaction or transformation.
In an elimination two groups (called eliminands) are lost most often from two different centers (1/2/elimination or 1/3/elimination, etc.) with concomitant formation of an unsaturation in the molecule (double bond, triple bond) or formation of a new ring.
If the groups are lost from a single center (α-elimination, 1/1/elimination) the resulting product is a carbene or a "carbene analogue".
Industry:Chemistry
One of a pair of molecular entities which are mirror images of each other and non-superimposable.
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A complex of molecular entities produced at an encounter-controlled rate, and which occurs as an intermediate in a reaction mechanism. When the complex is formed from two molecular entities it is called an "encounter pair". A distinction between encounter pairs and (larger) encounter complexes may be relevant in some cases, e.g. for mechanisms involving pre-association.
Industry:Chemistry
A rate of reaction corresponding to the rate of encounter of the reacting molecular entities. This is also known as "diffusion-controlled rate" since rates of encounter are themselves controlled by diffusion rates (which in turn depend on the viscosity of the medium and the dimensions of the reactant molecular entities).
For a bimolecular reaction between solutes in water at 25 <sup>o</sup>C an encounter-controlled rate is calculated to have a second-order rate constant of about 10<sup>10</sup> dm<sup>3</sup> mol<sup>-1</sup> s<sup>-1</sup>.
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E<sub>a</sub> (SI unit: kJ mol<sup>-1</sup>) An operationally defined quantity expressing the dependence of a rate constant on temperature according to
<center>E<sub>a</sub> = RT<sup>2</sup>(∂ ln k /∂T)<sub>p</sub></center>
as derived from the "Arrhenius equation", k = A exp(-E<sub>a</sub>/RT), where A (SI unit: as for the corresponding rate constant) is termed the "pre-exponential factor".
Industry:Chemistry
