- 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 ...
A dual parameter approach to the correlation analysis of substituent effects, which involves a field constant (F) and a resonance constant (R). The original treatment was modified later.
The procedure has been considerably applied, but also much criticized.
Industry:Chemistry
The linear free-energy relation of the form
<center>lg(k/k<sub>0</sub>) = sn</center>
applied to the variation of reactivity of a given electrophilic substrate towards a series of nucleophilic reagents. n is characteristic of the reagent (i.e. a measure of its nucleophilicity) and s is characteristic of the substrate (i.e. a measure of its sensitivity to the nucleophilicity of the reagent). A scale of n values is based on the rate coefficients k for the reaction of methyl bromide with nucleophiles in water at 25 <sup>o</sup>C, s being defined as 1.00 for these reactions and n being defined as 0.00 for the hydrolysis of methyl bromide. (Other scales have been devised.)
Industry:Chemistry
The term was originally applied to describe the maximum flocking of either hard or soft ligands in the same complexes. For hydrocarbon molecules, symbiosis implies that those containing a maximum number of C-H bonds (e.g. CH<sub>4</sub>) or C-C bonds (e.g. Me<sub>4</sub>C) are the most stable. HO (1977).
Industry:Chemistry
A concerted process in which the primitive changes concerned (generally bond rupture and bond formation) have progressed to the same extent at the transition state is said to be synchronous. The term figuratively implies a more or less synchronized progress of the changes. However, the progress of the bonding change (or other primitive change) has not been defined quantitatively in terms of a single parameter applicable to different bonds or different bonding changes. The concept is therefore in general only qualitatively descriptive and does not admit an exact definition except in the case of concerted processes involving changes in two identical bonds.
Industry:Chemistry
Various equations are associated with R.W. Taft, but the term is most often used to designate the family of equations that emerged from Taft's analysis of the reactivities of aliphatic esters, and which involved the polar substituent constant σ* and the steric substituent constant E<sub>s</sub>
<center>lg k = lg k<sub>o</sub> + ρ<sup>*</sup>σ<sup>* </sup>+ δE<sub>s</sub></center>
or the one-parameter forms applicable when the role of either the polar term or the steric term may be neglected. Nowadays σ<sup>*</sup> is usually replaced by the related constant σ<sub>I</sub>.
Industry:Chemistry
Synonymous with kinetic equivalence, two reaction schemes are kinetically ambiguous if they imply the same rate law .
Industry:Chemistry
The isomerization by which tautomers are interconverted. It is a heterolytic molecular re-arrangement and is frequently very rapid.
Industry:Chemistry
The steps in a chain reaction in which reactive intermediates are destroyed or rendered inactive, thus ending the chain.
Industry:Chemistry
A reaction intermediate in which the bond arrangement around an initially double-bonded carbon atom (typically a carbonyl carbon atom) has been transformed from trigonal to tetrahedral. For example, aldol in the condensation reaction of acetaldehyde (but most tetrahedral intermediates have a more fleeting existence).
Industry:Chemistry
