- المجال: 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 ...
Catalysis by a Lewis base, involving formation of a Lewis adduct as a reaction intermediate. For example, the hydrolysis of acetic anhydride in aqueous solution catalyzed by pyridine:
<center>C<sub>5</sub>H<sub>5</sub>N + (CH<sub>3</sub>CO)<sub>2</sub>O → (C<sub>5</sub>H<sub>5</sub>NCOCH<sub>3</sub>)<sup>+</sup> + CH<sub>3</sub>CO<sub>2</sub><sup>-</sup>
(C<sub>5</sub>H<sub>5</sub>NCOCH<sub>3</sub>)<sup>+</sup> + H<sub>2</sub>O → C<sub>5</sub>H<sub>5</sub>N + CH<sub>3</sub>CO<sub>2</sub>H + H<sup>+</sup><sub>aq</sub></center>
Industry:Chemistry
Delocalization of a free electron pair (n) into an antibonding σ-orbital (s*).
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(1) A cation (with its counterion) derived by addition of a hydron to a mononuclear parent hydride of the nitrogen, chalcogen and halogen family, e.g. H<sub>4</sub>N<sup>+ </sup>ammonium ion.
(2) Derivatives formed by substitution of the above parent ions by univalent groups, e.g. (CH<sub>3</sub>)<sub>2</sub>S<sup>+</sup>H dimethylsulfonium, (CH<sub>3</sub>CH<sub>2</sub>)<sub>4</sub>N<sup>+</sup> tetraethylammonium.
(3) Derivatives formed by substitution of the above parent ions by groups having two or three free valencies on the same atom. Such derivatives are, whenever possible, designated by a specific class name. E.g. R<sub>2</sub>C=NH<sub>2</sub><sup>+</sup> iminium ion.
Industry:Chemistry
In a chemical reaction involving chiral reactants and products, the ratio of the optical purity of the product to that of the precursor, reactant or catalyst. This should not be confused with "enantiomeric excess". The optical yield is in no way related to the chemical yield of the reaction.
Industry:Chemistry
A concept expressing that the stereochemistry of approach of two reacting species is governed by the most favorable overlap of their appropriate orbitals.
Industry:Chemistry
The behavior of an atomic or localized molecular orbital under molecular symmetry operations characterizes its orbital symmetry. For example, under a reflection in an appropriate symmetry plane, the phase of the orbital may be unchanged (symmetric), or it may change sign (antisymmetric), i.e. the positive and negative lobes are interchanged.
A principal context for the use of orbital symmetry is the discussion of chemical changes that involve "conservation of orbital symmetry". If a certain symmetry element (e.g. the reflection plane) is retained along a reaction pathway, that pathway is "allowed" by orbital symmetry conservation if each of the occupied orbitals of the reactant(s) is of the same symmetry type as a similarly (e.g. singly or doubly) occupied orbital of the product(s). This principle permits the qualitative construction of correlation diagrams to show how molecular orbitals transform (and how their energies change) during idealized chemical changes (e.g. cycloadditions).
An idealized single bond is a sigma bond, i.e., it has cylindrical symmetry, whereas a p-orbital or pi-bond orbital has pi symmetry, i.e. it is antisymmetric with respect to reflection in a plane passing through the atomic centers with which it is associated. In ethene, the pi-bonding orbital is symmetric with respect to reflection in a plane perpendicular to and bisecting the C-C bond, whereas the pi*-antibonding orbital is antisymmetric with respect to this operation.
Considerations of orbital symmetry are frequently grossly simplified in that, for example, the pi orbitals of a carbonyl group would be treated as having the same symmetry as those of ethene, and the fact that the carbonyl group in, for example, camphor, unlike that in formaldehyde, has no mirror planes would be ignored. These simplified considerations nevertheless afford the basis of one approach to the understanding of the rules which indicate whether pericyclic reactions are likely to occur under thermal or photochemical conditions.
Industry:Chemistry
The behavior of an atomic or localized molecular orbital under molecular symmetry operations characterizes its orbital symmetry. For example, under a reflection in an appropriate symmetry plane, the phase of the orbital may be unchanged (symmetric), or it may change sign (antisymmetric), i.e. the positive and negative lobes are interchanged.
A principal context for the use of orbital symmetry is the discussion of chemical changes that involve "conservation of orbital symmetry". If a certain symmetry element (e.g. the reflection plane) is retained along a reaction pathway, that pathway is "allowed" by orbital symmetry conservation if each of the occupied orbitals of the reactant(s) is of the same symmetry type as a similarly (e.g. singly or doubly) occupied orbital of the product(s). This principle permits the qualitative construction of correlation diagrams to show how molecular orbitals transform (and how their energies change) during idealized chemical changes (e.g. cycloadditions).
An idealized single bond is a sigma bond, i.e., it has cylindrical symmetry, whereas a p-orbital or pi-bond orbital has pi symmetry, i.e. it is antisymmetric with respect to reflection in a plane passing through the atomic centers with which it is associated. In ethene, the pi-bonding orbital is symmetric with respect to reflection in a plane perpendicular to and bisecting the C-C bond, whereas the pi*-antibonding orbital is antisymmetric with respect to this operation.
Considerations of orbital symmetry are frequently grossly simplified in that, for example, the pi orbitals of a carbonyl group would be treated as having the same symmetry as those of ethene, and the fact that the carbonyl group in, for example, camphor, unlike that in formaldehyde, has no mirror planes would be ignored. These simplified considerations nevertheless afford the basis of one approach to the understanding of the rules which indicate whether pericyclic reactions are likely to occur under thermal or photochemical conditions.
Industry:Chemistry
(SI unit: 1) If the macroscopic (observed, empirical or phenomenological) rate of reaction (v) for any reaction can be expressed by an empirical differential rate equation (or rate law) which contains a factor of the form k (A)<sup>α</sup>(B)<sup>β</sup>... (expressing in full the dependence of the rate of reaction on the concentrations (A), (B)...) where α, β are constant exponents (independent of concentration and time) and k is independent of (A) and (B) etc. (rate constant, rate coefficient), then the reaction is said to be of order α with respect to A, of order β with respect to B,..., and of (total or overall) order ν = α+β+.... The exponents α, β,... can be positive or negative integral or rational nonintegral numbers. They are the reaction orders with respect to A, B,... and are sometimes called "partial orders of reaction". Orders of reaction deduced from the dependence of initial rates of reaction on concentration are called "orders of reaction with respect to concentration"; orders of reaction deduced from the dependence of the rate of reaction on time of reaction are called "orders of reaction with respect to time".
The concept of order of reaction is also applicable to chemical rate processes occurring in systems for which concentration changes (and hence the rate of reaction) are not themselves measurable, provided it is possible to measure a chemical flux. For example, if there is a dynamic equilibrium according to the equation
<center>aA ⇌ pP</center>
and if a chemical flux is experimentally found, (e.g. by NMR line shape analysis) to be related to concentrations by the equation
<center>φ-A/a = k(A)<sup>α</sup>(L)<sup>λ</sup>,</center>
then the corresponding reaction is of order α with respect to A... and of total (or overall) order ν (= α +λ +...).
The proportionality factor k above is called the (nth order) "rate coefficient".
Rate coefficients referring to (or believed to refer to) elementary reactions are called "rate constants" or, more appropriately "microscopic" (hypothetical, mechanistic) rate constants.
The (overall) order of a reaction cannot be deduced from measurements of a "rate of appearance" or "rate of disappearance" at a single value of the concentration of a species whose concentration is constant (or effectively constant) during the course of the reaction. If the overall rate of reaction is, for example, given by
<center>v = k(A)<sup>α</sup>(B)<sup>β</sup></center>
but (B) stays constant, then the order of the reaction (with respect to time), as observed from the concentration change of A with time, will be α, and the rate of disappearance of A can be expressed in the form
<center>vA = k obs(A)<sup>α</sup></center>
The proportionality factor kobs deduced from such an experiment is called the "observed rate coefficient" and it is related to the (α + β)th order rate coefficient k by the equation
<center>kobs = k(B)<sup>β</sup></center>
For the common case when α = 1, k<sub>obs</sub> is often referred to as a "pseudo-first order rate coefficient" (k<sub>ψ</sub>).
For a simple (elementary) reactions a partial order of reaction is the same as the stoichiometric number of the reactant concerned and must therefore be a positive integer (see rate of reaction). The overall order is then the same as the molecularity. For stepwise reactions there is no general connection between stoichiometric numbers and partial orders. Such reactions may have more complex rate laws, so that an apparent order of reaction may vary with the concentrations of the chemical species involved and with the progress of the reaction: in such cases it is not useful to speak of orders of reaction, although apparent orders of reaction may be deducible from initial rates.
In a stepwise reaction, orders of reaction may in principle always be assigned to the elementary steps.
Industry:Chemistry
An outer-sphere electron transfer is a reaction in which the electron transfer takes place with no or very weak (4 -16 kJ mol<sup>-1</sup>) electronic interaction between the reactants in the transition state. If instead the donor and the acceptor exhibit a strong electronic coupling, the reaction is described as inner-sphere electron transfer. The two terms derive from studies concerning metal complexes and it has been suggested that for organic reactions the term "nonbonded" and "bonded" electron transfer should be used.
Industry:Chemistry
(1) The complete, net removal of one or more electrons from a molecular entity (also called "de-electronation").
(2) an increase in the oxidation number of any atom within any substrate.
(3) Gain of oxygen and/or loss of hydrogen of an organic substrate.
All oxidations meet criteria (1) and (2), and many meet criterion (3), but this is not always easy to demonstrate. Alternatively, an oxidation can be described as a transformation of an organic substrate that can be rationally dissected into steps or primitive changes. The latter consist in removal of one or several electrons from the substrate followed or preceded by gain or loss of water and/or hydrons or hydroxide ions, or by nucleophilic substitution by water or its reverse and/or by an intramolecular molecular rearrangement.
This formal definition allows the original idea of oxidation (combination with oxygen), together with its extension to removal of hydrogen, as well as processes closely akin to this type of transformation (and generally regarded in current usage of the term in organic chemistry to be oxidations and to be effected by "oxidizing agents") to be descriptively related to definition (1). For example the oxidation of methane to chloromethane may be considered as follows:
<center>CH<sub>4</sub> - 2e<sup>-</sup> - H<sup>+</sup> + OH<sup>-</sup> = CH<sub>3</sub>OH (oxidation) → CH<sub>3</sub>Cl (reversal of hydrolysis)</center>
Industry:Chemistry
