- Branche: Chemistry
- Number of terms: 1965
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The step of a stepwise reaction, in which the product distribution is determined. The product-determining step may be identical to, or occur later than, the rate-controlling step on the reaction coordinate.
Industry:Chemistry
Capable of acting as a proton (hydron) donor strongly or weakly acidic (as a Brønsted acid). The term is preferred to the synonym "protic" or the more ambiguous expression "acidic" by itself. Also called hydrogen bond donor (HBD) solvent.
Industry:Chemistry
The negative of the enthalpy change in the gas phase reaction (real or hypothetical) between a proton (more appropriately hydron) and the chemical species concerned, usually an electrically neutral species to give the conjugate acid of that species. Proton affinity is often, but unofficially, abbreviated as PA.
Industry:Chemistry
A chemical reaction, the main feature of which is the intermolecular or intramolecular transfer of a proton (hydron) from one binding site to another. For example,
<center>CH<sub>3</sub>CO<sub>2</sub>H + (CH<sub>3</sub>)<sub>2</sub>C=O → CH<sub>3</sub>CO<sub>2</sub><sup>-</sup> + (CH<sub>3</sub>)<sub>2</sub>C=O<sup>+</sup>H</center>
In the detailed description of proton transfer reactions, especially of rapid proton transfers between electronegative atoms, it should always be specified whether the term is used to refer to the overall process (including the more-or-less encounter-controlled formation of a hydrogen bonded complex and the separation of the products; see microscopic diffusion control) or just to the proton transfer event (including solvent rearrangement) by itself.
Industry:Chemistry
Capable of acting as proton acceptor, strongly or weakly basic (as a Brønsted base). Also called hydrogen bond acceptor (HBA) solvent.
Industry:Chemistry
If an acid or base is present in nearly constant concentration throughout a reaction in solution (owing to buffering or the use of a large excess), it may be found to increase the rate of that reaction and also to be consumed during the process. The acid or base is then not a catalyst and the phenomenon cannot be called catalysis according to the well-established meaning of these terms in chemical kinetics, although the mechanism of such a process is often intimately related to that of a catalyzed reaction. It is recommended that the term pseudo-catalysis be used in these and analogous cases (not necessarily involving acids or bases). For example, if a Brønsted acid accelerates the hydrolysis of an ester to a carboxylic acid and an alcohol, this is properly called acid catalysis, whereas the acceleration, by the same acid, of hydrolysis of an amide should be described as pseudo-catalysis by the acid: the "acid pseudo-catalyst" is consumed during the reaction through formation of an ammonium ion. The terms "general acid pseudo-catalysis" and "general base pseudo-catalysis" may be used as the analogues of general acid catalysis and general base catalysis.
The term "base-promoted", "base-accelerated, or "base-induced" is sometimes used for reactions that are pseudo-catalyzed by bases. However, the term "promotion" also has a different meaning in other chemical contexts.
Industry:Chemistry
The building of structure-biological activity models by using regression analysis with physicochemical constants, indicator variables or theoretical calculations. The term has been extended by some authors to include chemical reactivity, i.e. activity is regarded as synonymous with reactivity. This extension is, however, discouraged.
Industry:Chemistry
A molecular entity such as <sup>.</sup>CH<sub>3</sub>, <sup>.</sup>SnH<sub>3</sub>, Cl<sup>.</sup> possessing an unpaired electron. (In these formulae the dot, symbolizing the unpaired electron, should be placed so as to indicate the atom of highest spin density, if this is possible.) Paramagnetic metal ions are not normally regarded as radicals. However, in the "isolobal analogy", the similarity between certain paramagnetic metal ions and radicals becomes apparent.
At least in the context of physical organic chemistry, it seems desirable to cease using the adjective "free" in the general name of this type of chemical species and molecular entity, so that the term "free radical" may in future be restricted to those radicals which do not form parts of radical pairs.
Depending upon the core atom that possesses the unpaired electron, the radicals can be described as carbon-, oxygen-, nitrogen-, metal-centered radicals. If the unpaired electron occupies an orbital having considerable s or more or less pure p character, the respective radicals are termed sigma or pi radicals.
In the past, the term "radical" was used to designate a substituent group bound to a molecular entity, as opposed to "free radical", which nowadays is simply called radical. The bound entities may be called groups or substituents, but should no longer be called radicals.
Industry:Chemistry
A molecular entity such as <sup>.</sup>CH<sub>3</sub>, <sup>.</sup>SnH<sub>3</sub>, Cl<sup>.</sup> possessing an unpaired electron. (In these formulae the dot, symbolizing the unpaired electron, should be placed so as to indicate the atom of highest spin density, if this is possible.) Paramagnetic metal ions are not normally regarded as radicals. However, in the "isolobal analogy", the similarity between certain paramagnetic metal ions and radicals becomes apparent.
At least in the context of physical organic chemistry, it seems desirable to cease using the adjective "free" in the general name of this type of chemical species and molecular entity, so that the term "free radical" may in future be restricted to those radicals which do not form parts of radical pairs.
Depending upon the core atom that possesses the unpaired electron, the radicals can be described as carbon-, oxygen-, nitrogen-, metal-centered radicals. If the unpaired electron occupies an orbital having considerable s or more or less pure p character, the respective radicals are termed sigma or pi radicals.
In the past, the term "radical" was used to designate a substituent group bound to a molecular entity, as opposed to "free radical", which nowadays is simply called radical. The bound entities may be called groups or substituents, but should no longer be called radicals.
Industry:Chemistry
The atom (or group of atoms) in a polyatomic radical on which an unpaired electron is largely localized. Attachment of a monovalent atom to a radical center gives a molecule for which it is possible to write a Lewis formula in which the normal stable valencies are assigned to all atoms.
Industry:Chemistry
