- 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 ...
E<sub>a</sub> (SI unit: kJ mol<sup>-1</sup>) An operationally defined quantity expressing the dependence of a rate constant on temperature according to
<center>E<sub>a</sub> = RT<sup>2</sup>(∂ ln k /∂T)<sub>p</sub></center>
as derived from the "Arrhenius equation", k = A exp(-E<sub>a</sub>/RT), where A (SI unit: as for the corresponding rate constant) is termed the "pre-exponential factor".
Industry:Chemistry
E<sub>a</sub> (SI unit: kJ mol<sup>-1</sup>) An operationally defined quantity expressing the dependence of a rate constant on temperature according to
<center>E<sub>a</sub> = RT<sup>2</sup>(∂ ln k /∂T)<sub>p</sub></center>
as derived from the "Arrhenius equation", k = A exp(-E<sub>a</sub>/RT), where A (SI unit: as for the corresponding rate constant) is termed the "pre-exponential factor".
Industry:Chemistry
Variation of reaction parameters in a series of reactions proceeding in non-concerted steps may lead to a situation, where the putative intermediate will possess a lifetime shorter than a bond vibration, so that the steps become concerted. The transition state structure will lie on the coordinate of the More O'Ferrall-Jencks diagram leading to that of the putative intermediate.
Industry:Chemistry
An atom or group that forms a bond to what is considered to be the main part of the substrate during a reaction. For example: the attacking nucleophile in a bimolecular nucleophilic substitution reaction.
Industry:Chemistry
The standard enthalpy difference between the transition state and the ground state of the reactants at the same temperature and pressure. It is related to the temperature coefficient of the rate constant according to the equation:
<center>Δ<sup>†</sup>H = RT<sup>2</sup>(∂ ln k/∂T)<sub>p</sub> - RT = E<sub>a</sub> - RT
= -R(∂ln(k/T)/∂(1/T))<sub>p</sub></center>
where E<sub>a</sub> is the energy of activation, providing that the rate constants for reactions other than first-order reactions are expressed in temperature-independent concentration units (e.g., mol dm<sup>-3</sup>, measured at a fixed temperature and pressure). If lnk is expressed as
<center>ln k = (a/T) + b + c ln T + dT, </center>
then
<center>Δ<sup>†</sup>H = -aR + (c - 1)RT + dRT<sup>2</sup>.</center>
If enthalpy of activation and entropy of activation are assumed to be temperature independent, then
<center>Δ<sup>†</sup>H = -aR</center>
If the concentration units are mol dm<sup>-3</sup>, the true and apparent enthalpies of activation differ by (n - 1)/(αRT<sup>2</sup>), where n is the order of reaction and α the thermal expansivity.
Industry:Chemistry
The standard enthalpy difference between the transition state and the ground state of the reactants at the same temperature and pressure. It is related to the temperature coefficient of the rate constant according to the equation:
<center>Δ<sup>†</sup>H = RT<sup>2</sup>(∂ ln k/∂T)<sub>p</sub> - RT = E<sub>a</sub> - RT
= -R(∂ln(k/T)/∂(1/T))<sub>p</sub></center>
where E<sub>a</sub> is the energy of activation, providing that the rate constants for reactions other than first-order reactions are expressed in temperature-independent concentration units (e.g., mol dm<sup>-3</sup>, measured at a fixed temperature and pressure). If lnk is expressed as
<center>ln k = (a/T) + b + c ln T + dT, </center>
then
<center>Δ<sup>†</sup>H = -aR + (c - 1)RT + dRT<sup>2</sup>.</center>
If enthalpy of activation and entropy of activation are assumed to be temperature independent, then
<center>Δ<sup>†</sup>H = -aR</center>
If the concentration units are mol dm<sup>-3</sup>, the true and apparent enthalpies of activation differ by (n - 1)/(αRT<sup>2</sup>), where n is the order of reaction and α the thermal expansivity.
Industry:Chemistry
The standard entropy difference between the transition state and the ground state of the reactants, at the same temperature and pressure.
It is related to the Gibbs energy of activation and enthalpy of activation by the equations
<center>Δ<sup>†</sup>S = (Δ<sup>†</sup>H -Δ<sup>†</sup>G)/T
= Δ<sup>†</sup>H/T - R ln (k<sub>B</sub>/h) + R ln (k/T)</center>
or, if ln k is expressed as ln k = a/T + b + c ln T + dT,
<center>Δ<sup>†</sup>S = R (b - ln (k<sub>B</sub>/h) + (c - 1)(1 + ln T) + 2 dT)</center>
provided that rate constants for reactions other than first-order reactions are expressed in temperature-independent concentration units (e.g., mol dm<sup>-3</sup>, measured at a fixed temperature and pressure). The numerical value of Δ<sup>†</sup>S depends on the standard state (and therefore on the concentration units selected). If entropy of activation and enthalpy of activation are assumed to be temperature-independent,
<center>Δ<sup>†</sup>S = R(b - ln(k<sub>B</sub>/h))</center>
Strictly speaking, the quantity defined is the entropy of activation at constant pressure from which the entropy of activation at constant volume can be deduced.
The information represented by the entropy of activation may alternatively be conveyed by the pre-exponential factor A.
Industry:Chemistry
The standard entropy difference between the transition state and the ground state of the reactants, at the same temperature and pressure.
It is related to the Gibbs energy of activation and enthalpy of activation by the equations
<center>Δ<sup>†</sup>S = (Δ<sup>†</sup>H -Δ<sup>†</sup>G)/T
= Δ<sup>†</sup>H/T - R ln (k<sub>B</sub>/h) + R ln (k/T)</center>
or, if ln k is expressed as ln k = a/T + b + c ln T + dT,
<center>Δ<sup>†</sup>S = R (b - ln (k<sub>B</sub>/h) + (c - 1)(1 + ln T) + 2 dT)</center>
provided that rate constants for reactions other than first-order reactions are expressed in temperature-independent concentration units (e.g., mol dm<sup>-3</sup>, measured at a fixed temperature and pressure). The numerical value of Δ<sup>†</sup>S depends on the standard state (and therefore on the concentration units selected). If entropy of activation and enthalpy of activation are assumed to be temperature-independent,
<center>Δ<sup>†</sup>S = R(b - ln(k<sub>B</sub>/h))</center>
Strictly speaking, the quantity defined is the entropy of activation at constant pressure from which the entropy of activation at constant volume can be deduced.
The information represented by the entropy of activation may alternatively be conveyed by the pre-exponential factor A.
Industry:Chemistry
A diastereoisomer that has the opposite configuration at only one of two or more tetrahedral "stereogenic" centers present in the respective molecular entity.
Industry:Chemistry
Interconversion of epimers by reversal of the configuration at one of the "stereogenic" centers.
Industry:Chemistry
