- Settore: 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 non-metallic material whose properties resemble those of metals, usually following oxidative doping; e.g. polyacetylene following oxidative doping with iodine.
Industry:Chemistry
A one-electron wavefunction describing an electron moving in the effective field provided by the nuclei and all other electrons of a molecular entity of more than one atom. Such molecular orbitals can be transformed in prescribed ways into component functions to give "localized molecular orbitals". Molecular orbitals can also be described, in terms of the number of nuclei (or "centers") encompassed, as two-center, multi-center, etc. molecular orbitals, and are often expressed as a linear combination of atomic orbitals.
An orbital is usually depicted by sketching contours on which the wavefunction has a constant value (contour map) or by indicating schematically the envelope of the region of space in which there is an arbitrarily fixed high (say 96%) probability of finding the electron occupying the orbital, giving also the algebraic sign (+ or -) of the wavefunction in each part of that region.
Industry:Chemistry
The number of reactant molecular entities that are involved in the "microscopic chemical event" constituting an elementary reaction. (For reactions in solution this number is always taken to exclude molecular entities that form part of the medium and which are involved solely by virtue of their solvation of solutes.) A reaction with a molecularity of one is called "unimolecular", one with a molecularity of two "bimolecular" and of three "termolecular".
Industry:Chemistry
An electrically neutral entity consisting of more than one atom (n > 1). Rigorously, a molecule, in which n > 1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.
Industry:Chemistry
Representation of some molecular entities solely by localized two-electron two-center bonds appears to be unsatisfactory. Instead, multi-center bonds have to be considered in which electron pairs occupy orbitals encompassing three or more atomic centers. Examples include the three-center bonds in diborane, the delocalized pi bonding of benzene, and bridged carbocations.
Industry:Chemistry
The direct interaction of the reaction center (usually, but not necessarily, an incipient carbenium center) with a lone pair of electrons of an atom or with the electrons of a sigma or pi bond contained within the parent molecule but not conjugated with the reaction center. A distinction is sometimes made between n, sigma, and pi participation.
A rate increase due to neighboring group participation is known as "anchimeric assistance". "Synartetic acceleration" is the special case of anchimeric assistance ascribed to participation by electrons binding a substituent to a carbon atom in a β-position relative to the leaving group attached to the α-carbon atom. According to the underlying model, these electrons then provide a three-center bond (or "bridge") "fastening together" (as the word "synartetic" is intended to suggest) the α- and β-carbon atoms between which the charge is divided in the intermediate bridged ion formed (and in the transition state preceding its formation). The term synartetic acceleration is not widely used.
Industry:Chemistry
Generic name for HN: and substitution derivatives thereof, containing an electrically neutral univalent nitrogen atom with four non-bonding electrons. Two of these are paired; the other two may have parallel spins (triplet state) or antiparallel spins (singlet state). The name is the strict analogue of carbene and, as a generic name, it is preferred to a number of alternatives proposed ("imene", "imine radical", "azene", "azylene", "azacarbene", "imin", "imidogen").
Industry:Chemistry
A carbocation the ground state of which has delocalized (bridged) bonding pi- or sigma-electrons. (N.B.: Allylic and benzylic carbocations are not considered nonclassical.)
Industry:Chemistry
A leaving group that carries away the bonding electron pair. For example, in the hydrolysis of an alkyl chloride, Cl<sup>-</sup> is the nucleofuge. The tendency of atoms or groups to depart with the bonding electron pair is called nucleofugality.
The adjective is nucleofugal.
Industry:Chemistry
A reagent that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons.
A "nucleophilic substitution reaction" is a heterolytic reaction in which the reagent supplying the entering group acts as a nucleophile. For example
<center>MeO<sup>-</sup> (nucleophile) + Et-Cl → MeOEt + Cl<sup>-</sup> (nucleofuge)</center>
The term "nucleophilic" is also used to designate the apparent polar character of certain radicals, as inferred from their higher relative reactivity with reaction sites of lower electron density.
Nucleophilic reagents are Lewis bases.
Industry:Chemistry
