- 行业: 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 number of electron-pair bonds between two nuclei in any given Lewis formula. For example in ethene the bond number between the carbon atoms is two, and between the carbon and hydrogen atoms is one.
Industry:Chemistry
A theoretical index of the degree of bonding between two atoms relative to that of a normal single bond, i.e. the bond provided by one localized electron pair. In the valence-bond theory it is a weighted average of the bond numbers between the respective atoms in the contributing structures. In molecular-orbital theory it is calculated from the weights of the atomic orbitals in each of the occupied molecular orbitals. For example, in valence-bond theory (neglecting other than Kekulé structures) the bond order between adjacent carbon atoms in benzene is 1.5; in Huckel molecular orbital theory it is 1.67. Other variants of molecular orbital theory provide other values for bond orders.
Industry:Chemistry
The enthalpy (per mole) required to break a given bond of some specific molecular entity by homolysis, e.g. for<br> CH<sub>4</sub> → H<sub>3</sub>C<sup>.</sup> + H<sup>.</sup><br> symbolized as D(CH<sub>3</sub>-H) (cf. heterolytic bond-dissociation energy).
Industry:Chemistry
A mechanism intermediate between two extremes, for example a nucleophilic substitution intermediate between S<sub>N</sub>1 and S<sub>N</sub>2, or intermediate between electron transfer and S<sub>N</sub>2.
Industry:Chemistry
A ligand attached to two or more, usually metallic, central atoms.
Industry:Chemistry
A molecular entity capable of donating a hydron (proton) to a base, (i.e. a "hydron donor") or the corresponding chemical species. For example: H<sub>2</sub>O, H<sub>3</sub>O<sup>+</sup>, CH<sub>3</sub>CO<sub>2</sub>H, H<sub>2</sub>SO<sub>4</sub>, HSO<sub>4</sub><sup>-</sup>, HCl, CH<sub>3</sub>OH, NH<sub>3</sub>.
Industry:Chemistry
A molecular entity capable of donating a hydron (proton) to a base, (i.e. a "hydron donor") or the corresponding chemical species. For example: H<sub>2</sub>O, H<sub>3</sub>O<sup>+</sup>, CH<sub>3</sub>CO<sub>2</sub>H, H<sub>2</sub>SO<sub>4</sub>, HSO<sub>4</sub><sup>-</sup>, HCl, CH<sub>3</sub>OH, NH<sub>3</sub>.
Industry:Chemistry
A molecular entity capable of accepting a hydron (proton) from an acid (i.e. a "hydron acceptor") or the corresponding chemical species. For example: OH<sup>-</sup>, H<sub>2</sub>O, CH<sub>3</sub>CO<sub>2</sub><sup>-</sup>, HSO<sub>4</sub><sup>-</sup>, SO<sub>4</sub><sup>2-</sup>, Cl<sup>-</sup>.
Industry:Chemistry
A molecular entity capable of accepting a hydron (proton) from an acid (i.e. a "hydron acceptor") or the corresponding chemical species. For example: OH<sup>-</sup>, H<sub>2</sub>O, CH<sub>3</sub>CO<sub>2</sub><sup>-</sup>, HSO<sub>4</sub><sup>-</sup>, SO<sub>4</sub><sup>2-</sup>, Cl<sup>-</sup>.
Industry:Chemistry
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
