- 行业: 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 ...
Parameters of the Kamlet-Taft solvatochromic relationship which measure separately the hydrogen bond donor (α), hydrogen bond acceptor (β), and dipolarity/polarizability (π*) properties of solvents as contributing to overall solvent polarity.
Industry:Chemistry
The term characterizes conditions (including reaction times) that lead to reaction products in a proportion governed by the relative rates of the parallel (forward) reactions in which the products are formed, rather than by the respective overall equilibrium constants.
Industry:Chemistry
The general effect of an added electrolyte (i.e. an effect other than, or in addition to, that due to its possible involvement as a reactant or catalyst) on the observed rate constant of a reaction in solution. At low concentrations (when only long-range coulombic forces need to be considered) the effect on a given reaction is determined only by the ionic strength of the solution and not by the chemical identity of the ions. For practical purposes, this concentration range is roughly the same as the region of validity of the Debye-Huckel limiting law for activity coefficients. At higher concentrations, the effect of an added electrolyte depends also on the chemical identity of the ions. Such specific action can usually be interpreted as the incursion of a reaction path involving an ion of the electrolyte as reactant or catalyst, in which case the action is not properly to be regarded just as a kinetic electrolyte effect.
Kinetic electrolyte effects are usually (too restrictively and therefore incorrectly) referred to as "kinetic salt effects".
A kinetic electrolyte effect ascribable solely to the influence of the ionic strength on activity coefficients of ionic reactants and transition states is called a "primary kinetic electrolyte effect".
A kinetic electrolyte effect arising from the influence of the ionic strength of the solution upon the pre-equilibrium concentration of an ionic species that is involved in a subsequent rate-limiting step of a reaction is called a "secondary kinetic electrolyte effect". A common case encountered in practice is the effect on the concentration of hydrogen ion (acting as catalyst) produced from the ionization of a weak acid in a buffer solution.
Industry:Chemistry
Parameters to measure separately the ability of a solvent to enter into non-specific solvent-solute interactions (permittivity ε and refractive index n<sub>D</sub>) and specific solvent-solute interaction (solvent basicity or nucleophilicity B and solvent acidity or electrophilicity E) as contributing to overall solvent polarity.
Industry:Chemistry
The term has loosely been used to describe a relatively unstable and transient chemical species or (less commonly) a relatively stable but reactive species. It must therefore not be used without explanation of the intended meaning.
Industry:Chemistry
An atom or group (charged or uncharged) that becomes detached from an atom in what is considered to be the residual or main part of the substrate in a specified reaction.
For example, in the heterolytic solvolysis of benzyl bromide in acetic acid
<center>PhCH<sub>2</sub>Br + AcOH → PhCH<sub>2</sub>OAc + HBr</center>
the leaving group is Br<sup>-</sup> ; in the reaction
<center>MeS<sup>-</sup> + PhCH<sub>2</sub>N<sup>+</sup>Me<sub>3</sub> → MeSCH<sub>2</sub>Ph + NMe<sub>3</sub></center>
the leaving group is NMe<sub>3</sub>; in the electrophilic nitration of benzene, it is H<sup>+</sup>. The term has meaning only in relation to a specified reaction. The leaving group is not, in general, the same as the substituent group present in the substrate (e.g. bromo and trimethylammonio in the substrates of the first two examples above.)
A slightly different usage of the term prevails in the (non-mechanistic) naming of transformations, where the actual substituent group present in the substrate (and also in the product) is referred to as the leaving group.
Industry:Chemistry
Arrangement of the structural formulae of the reactants so that the bonds to be made or broken form a linear array in which the electrons move from left to right.
Industry:Chemistry
The tendency of a solvent to make all Brønsted acids whose acidity exceeds a certain value appear equally acidic. It is due to the complete transfer to a protophilic solvent of a hydron from a dissolved acid stronger than the conjugate acid of the solvent. The only acid present to any significant extent in all such solutions is the lyonium ion . For example, the solvent water has a leveling effect on the acidities of HClO<sub>4</sub>, HCl, and HI: aqueous solutions of these acids at the same (moderately low) concentrations have the same acidities. A corresponding leveling effect applies to strong bases in protogenic solvents.
Industry:Chemistry
A molecular entity (and the corresponding chemical species ) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct , by sharing the electron pair furnished by the Lewis base. For example:
<center>Me<sub>3</sub>B (Lewis acid) + :NH<sub>3</sub> (Lewis base) → Me<sub>3</sub>B<sup>-</sup>-N<sup>+</sup>H<sub>3</sub> (Lewis adduct)</center>
Industry:Chemistry
The thermodynamic tendency of a substrate to act as a Lewis acid . Comparative measures of this property are provided by the equilibrium constants for Lewis adduct formation of a series of Lewis acids with a common reference Lewis base .
Industry:Chemistry
