- المجال: 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 ...
A chemical reaction or transformation, the main feature of which is the bimolecular removal of an atom (neutral or charged) from a molecular entity. For example:
<center>CH<sub>3</sub>COCH<sub>3</sub> + (i-C<sub>3</sub>H<sub>7</sub>)<sub>2</sub>N<sup>-</sup> → (CH<sub>3</sub>COCH<sub>2</sub>)<sup>-</sup> + (i-C<sub>3</sub>H<sub>7</sub>)<sub>2</sub>NH</center>
(proton abstraction from acetone)
<center>CH<sub>4</sub> + Cl<sup>. </sup> → H<sub>3</sub>C<sup>.</sup> + HCl</center>
(hydrogen abstraction from methane)
Industry:Chemistry
A quantitative measure, devised by Gutmann (1976), of Lewis acidity.
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A molecular entity or chemical species capable of donating a hydron (proton) or capable of forming a covalent bond with an electron pair.
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(1) Of a compound:
For Brønsted acids it means the tendency of a compound to act as a hydron donor. It can be quantitatively expressed by the acid dissociation constant of the compound in water or some other specified medium. For Lewis acids it relates to the association constants of Lewis adducts and π-adducts.
(2) Of a medium:
The use of the term is mainly restricted to a medium containing Brønsted acids, where it means the tendency of the medium to hydronate a specific reference base. It is quantitatively expressed by the appropriate acidity function.
Industry:Chemistry
Any function that measures the thermodynamic hydron-donating or -accepting ability of a solvent system, or a closely related thermodynamic property, such as the tendency of the lyate ion of the solvent system to form Lewis adducts. (The term "basicity function" is not in common use in connection with basic solutions.) Acidity functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured.
Commonly used acidity functions refer to concentrated acidic or basic solutions. Acidity functions are usually established over a range of composition of such a system by UV/VIS spectrophotometric or NMR measurements of the degree of hydronation (protonation or Lewis adduct formation) for the members of a series of structurally similar indicator bases (or acids) of different strength: the best known of these functions is the Hammett acidity function H<sub>o</sub> (for uncharged indicator bases that are primary aromatic amines).
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An activated complex, often characterized by the superscript <sup>†</sup>, is defined as that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential energy surface.
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(1) Refers to addition reaction or addition transformation.
(2) Loosely, the formation of an adduct. (For an example, see Lewis acid.)
(3) Loosely, any association or attachment.
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A chemical reaction of two or more reacting molecular entities, resulting in a single reaction product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants. The reverse process is called an elimination reaction. The addition may occur at only one site (α-addition, 1/1/addition), at two adjacent sites (1/2/addition) or at two non-adjacent sites (1/3/- or 1/4/addition, etc.). For example
<blockquote>(a) H<sup>+</sup> + Br<sup>-</sup> + (CH<sub>3</sub>)<sub>2</sub>C=CH<sub>2</sub> → (CH<sub>3</sub>)<sub>2</sub>CBr-CH<sub>3</sub> (1/2/addition)
(b) Br<sub>2</sub> + CH<sub>2</sub>=CH-CH=CH<sub>2</sub> → BrCH<sub>2</sub>-CH=CH-CH<sub>2</sub>Br (1/4/addition) and BrCH<sub>2</sub>-CH(Br)-CH=CH<sub>2</sub> (1/2/addition)</blockquote>
If the reagent or the source of the addends of an addition are not specified, then it is called an addition transformation.
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The hypothesis that each of several structural features of a molecular entity makes a separate and additive contribution to a property of the substance concerned. More specifically, it is the hypothesis that each of the several substituent groups in a parent molecule makes a separate and additive contribution to the standard Gibbs energy change (or Gibbs energy of activation) corresponding to a particular equilibrium (or rate of reaction).
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A new chemical species AB, each molecular entity of which is formed by direct combination of two separate molecular entities A and B in such a way that there is change in connectivity, but no loss, of atoms within the moieties A and B. Stoichiometries other than 1:1 are also possible, e.g. a bis-adduct (2:1). An "intramolecular adduct" can be formed when A and B are groups contained within the same molecular entity.
This is a general term which, whenever appropriate, should be used in preference to the less explicit term complex. It is also used specifically for products of an addition reaction. For examples see Lewis adduct, Meisenheimer adduct, π-adduct.
Industry:Chemistry
