- 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 ...
This hypothesis states that, if in an acid catalyzed reaction, lg k<sub>1</sub> (first-order rate constant of the reaction) is linear in H<sub>o</sub> (Hammett acidity function), water is not involved in the transition state of the rate-controlling step. However, if lg k<sub>1</sub> is linear in lg(H<sup>+</sup>), then water is involved. This has been shown to be incorrect by Hammett himself.
Industry:Chemistry
An index of the ionizing power of a solvent based on the frequency of the longest wavelength electronic absorption maximum of 1-ethyl-4-methoxycarbonylpyridinium iodide in the solvent. The Z-value is defined by
<center>Z = 2.859 x 10<sup>4</sup>/λ</center>
where Z is in kcal mol<sup>-1</sup> and λ is in nm.
Industry:Chemistry
A neutral compound having electrical charges of opposite sign, delocalized or not on adjacent or nonadjacent atoms. Zwitterionic compounds have no uncharged canonical representations. Sometimes referred to as inner salts, ampholytes, dipolar ions (a misnomer).
For example: H<sub>3</sub>N<sup>+</sup>CH<sub>2</sub>C(=O)O<sup>-</sup>, glycine.
Industry:Chemistry
A chemical reaction resulting in a single reaction product from two or three reacting chemical species, with formation of two new chemical bonds to the same atom in one of the reactant molecular entities. The synonymous term 1/1/addition is also used. For example:
<center>Cl<sub>2</sub>C: + CH<sub>3</sub>OH → Cl<sub>2</sub>CHOCH<sub>3</sub></center>
(This particular example can also be viewed as an insertion reaction.) In inorganic chemistry such α-addition reactions, generally to a metallic central atom, are known as "oxidative additions".
α-Addition is the reverse of α-elimination or 1/1/elimination.
Industry:Chemistry
A chemical reaction resulting in a single reaction product from two or three reacting chemical species, with formation of two new chemical bonds to the same atom in one of the reactant molecular entities. The synonymous term 1/1/addition is also used. For example:
<center>Cl<sub>2</sub>C: + CH<sub>3</sub>OH → Cl<sub>2</sub>CHOCH<sub>3</sub></center>
(This particular example can also be viewed as an insertion reaction.) In inorganic chemistry such α-addition reactions, generally to a metallic central atom, are known as "oxidative additions".
α-Addition is the reverse of α-elimination or 1/1/elimination.
Industry:Chemistry
A positive deviation of an α-nucleophile (a nucleophile bearing an unshared pair of electrons on an atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg k<sub>nuc</sub> vs. pKa constructed for a series of related normal nucleophiles. More generally, it is the influence of the atom bearing a lone pair of electrons on the reactivity at the adjacent site. HOZ and BUNCEL (1985).
The use of the term has been extended to include the effect of any substituent on an adjacent reactive center, for example in the case of the "α-silicon effect".
Industry:Chemistry
A transformation of the general type
<center>RR'ZXY → RR'Z + XY (or X + Y, or X<sup>+</sup> + Y<sup>-</sup>)</center>
where the central atom Z is commonly carbon. The reverse reaction is called α-addition.
Industry:Chemistry
A measure of the susceptibility to the influence of substituent groups on the rate constant or equilibrium constant of a particular organic reaction involving a family of related substrates. Defined by Hammett for the effect of ring substituents in meta- and para-positions of aromatic side-chain reactions by the empirical "ρσ-equation" of the general form
<center>lg(k<sub></sub>x/k<sub>H</sub>) = ρσX</center>
in which σ<sub></sub>x is a constant characteristic of the substituent X and of its position in the reactant molecule.
More generally (and not only for aromatic series), ρ-values (modified with appropriate subscripts and superscripts) are used to designate the susceptibility of reaction series for families of various organic compounds to any substituent effects, as given by the modified set of σ-constants in an empirical ρσ-correlation.
Reactions with a positive ρ-value are accelerated (or the equilibrium constants of analogous equilibria are increased) by substituents with positive σ-constants. Since the sign of σ was defined so that substituents with a positive σ increase the acidity of benzoic acid, such substituents are generally described as attracting electrons away from the aromatic ring. It follows that reactions with a positive ρ-value are considered to involve a transition state (or reaction product) so that the difference in energy between this state and the reactants is decreased by a reduction in electron density at the reactive site of the substrate.
Industry:Chemistry
A measure of the susceptibility to the influence of substituent groups on the rate constant or equilibrium constant of a particular organic reaction involving a family of related substrates. Defined by Hammett for the effect of ring substituents in meta- and para-positions of aromatic side-chain reactions by the empirical "ρσ-equation" of the general form
<center>lg(k<sub></sub>x/k<sub>H</sub>) = ρσX</center>
in which σ<sub></sub>x is a constant characteristic of the substituent X and of its position in the reactant molecule.
More generally (and not only for aromatic series), ρ-values (modified with appropriate subscripts and superscripts) are used to designate the susceptibility of reaction series for families of various organic compounds to any substituent effects, as given by the modified set of σ-constants in an empirical ρσ-correlation.
Reactions with a positive ρ-value are accelerated (or the equilibrium constants of analogous equilibria are increased) by substituents with positive σ-constants. Since the sign of σ was defined so that substituents with a positive σ increase the acidity of benzoic acid, such substituents are generally described as attracting electrons away from the aromatic ring. It follows that reactions with a positive ρ-value are considered to involve a transition state (or reaction product) so that the difference in energy between this state and the reactants is decreased by a reduction in electron density at the reactive site of the substrate.
Industry:Chemistry
Specifically the substituent constant for meta- and for para-substituents in benzene derivatives as defined by Hammett on the basis of the ionization constant of a substituted benzoic acid in water at 25 <sup>o</sup>C, i.e. lg(K<sub>a</sub>/K<sub>a</sub><sup>o</sup>), where K<sub>a</sub> is the ionization constant of a m- or p-substituted benzoic acid and K<sub>a</sub><sup>o</sup> that of benzoic acid itself.
The term is also used as a collective description for related electronic substituent constants based on other standard reaction series, of which, σ<sup>+</sup>, σ<sup>-</sup> and σ<sup>o</sup> are typical; also constants which represent dissected electronic effects such as σ<sub>I</sub> and σ<sub>R</sub>. For this purpose it might be better always to spell out the term in full, i.e. as "Hammett sigma constant", and restrict σ-constants to the scale of substituent constants which is based on benzoic acid.
A large positive σ-value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative σ-value implies high electron-releasing power relative to H.
Industry:Chemistry
