Brønsted catalysis equation
Encyclopedia
The Brønsted catalysis equation or law or correlation, after Johannes Nicolaus Brønsted
, gives the relationship between acid strength and catalytic activity in general acid catalysis.
A plot of the common logarithm
of the reaction rate constant k versus the logarithm of the ionization constant Ka for a series of acids (for example a group of substituted phenol
s or carboxylic acid
s) gives a straight line with slope α and intercept C. The Brønsted equation is a free-energy relationship
. The relationship implies that the Gibbs free energy
for proton dissociation is proportional to the activation energy
for the catalytic step. When the relationship is not linear, the chosen group of catalysts do not operate through the same reaction mechanism
.
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
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 acidity 1.
Johannes Nicolaus Brønsted
Johannes Nicolaus Brønsted born in Varde was a Danish physical chemist.He received a degree in chemical engineering in 1899 and his Ph. D. in 1908 from the University of Copenhagen. He was immediately appointed professor of inorganic and physical chemistry at Copenhagen.In 1906 he published his...
, gives the relationship between acid strength and catalytic activity in general acid catalysis.
A plot of the common logarithm
Common logarithm
The common logarithm is the logarithm with base 10. It is also known as the decadic logarithm, named after its base. It is indicated by log10, or sometimes Log with a capital L...
of the reaction rate constant k versus the logarithm of the ionization constant Ka for a series of acids (for example a group of substituted phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
s or carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s) gives a straight line with slope α and intercept C. The Brønsted equation is a free-energy relationship
Free-energy relationship
In physical organic chemistry, a free-energy relationship or linear Gibbs energy relation relates the logarithm of a reaction rate constant or equilibrium constant for one series of reactions with the logarithm of the rate or equilibrium constant for a related series of reactions...
. The relationship implies that the Gibbs free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
for proton dissociation is proportional to the activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
for the catalytic step. When the relationship is not linear, the chosen group of catalysts do not operate through the same reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
.
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
Transition state
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 energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
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
Phenalene
Phenalene is a polycyclic aromatic hydrocarbon . Like many PAHs, it is an atmospheric pollutant formed during the combustion of fossil fuels....
compounds it was concluded from Brønsted analysis that phenalene acidity is very different from either indene
Indene
Indene is a flammable polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring. This aromatic liquid is colorless although samples often are pale yellow...
acidity or phenylene acidity 1.