The Brønsted catalysis equation or law of correlation, after
Johannes Nicolaus Brønsted, gives the relationship between
acid strength
Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a hydron (chemistry), proton, , and an anion, . The Dissociation (chemistry), dissociation or ionization of a strong acid in solution is effectivel ...
and catalytic activity in
general acid catalysis.
:
A plot of the
common logarithm
In mathematics, the common logarithm (aka "standard logarithm") is the logarithm with base 10. It is also known as the decadic logarithm, the decimal logarithm and the Briggsian logarithm. The name "Briggsian logarithm" is in honor of the British ...
of the
reaction rate constant
In chemical kinetics, a reaction rate constant or reaction rate coefficient () is a proportionality constant which quantifies the rate and direction of a chemical reaction by relating it with the concentration of reactants.
For a reaction between ...
k versus the logarithm of the
ionization constant K
a for a series of acids (for example a group of substituted
phenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
s or
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
s) gives a straight line with slope α and intercept C. The Brønsted equation is a
free-energy relationship
In physical organic chemistry, a free-energy relationship or Gibbs energy relation relates the logarithm of a reaction rate constant or equilibrium constant for one series of chemical reactions with the logarithm of the rate or equilibrium consta ...
. The relationship implies that the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...
for proton dissociation is proportional to the
activation energy
In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...
for the catalytic step. When the relationship is not linear, the chosen group of catalysts do not operate through the same
reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...
.
Specific and general catalysis is also found in base catalysed reactions and base Brønsted equation also exists with constant β.
The Brønsted equation gives information about a reaction mechanism. Reactions that have low values for proportionality constants α or β are considered to have a
transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
closely resembling the reactant with little proton transfer. With a high value, proton transfer in the transition state is almost complete. In a study of a group of
phenalene compounds it was concluded from Brønsted analysis that phenalene acidity is very different from either
indene acidity or
phenylene
In organic chemistry, the phenylene group () is based on a di-substituted benzene ring ( arylene). For example, poly(''p''-phenylene) is a polymer built up from ''para''-phenylene repeating units.p. C-9, Section 11.6, Handbook of Chemistry and ...
acidity.
See also
*
Free-energy relationship
In physical organic chemistry, a free-energy relationship or Gibbs energy relation relates the logarithm of a reaction rate constant or equilibrium constant for one series of chemical reactions with the logarithm of the rate or equilibrium consta ...
*
Bell–Evans–Polanyi principle In physical chemistry, the Evans–Polanyi principle (also referred to as the Bell–Evans–Polanyi principle, Brønsted–Evans–Polanyi principle, or Evans–Polanyi–Semenov principle) observes that the difference in activation energy between ...
References
Chemical kinetics
Catalysis
{{Catalysis-stub