Rate-determining step
Encyclopedia
The rate-determining step (RDS) is a chemistry
term for the slowest step
in a chemical reaction
. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed
at which water is poured in. In similar manner, the rate of reaction depends on the rate of the slowest step. However it is not clear what 'slowest step' means. For example does it refer to the step with the smallest rate constant; if the reaction is reversible, does it refer to the ratio of the forward and reverse rate constants or does it simply refer to the step that has the smallest flux? If the latter, then at steady state all steps carry the same flux and therefore there is no slowest step. In metabolic pathways the rate limiting step is better defined and a measure, the flux control coefficient
is given to a step to signify how rate-limiting the step is. Theory and experiment also suggests that there is no single rate limiting step but a range of rate limitingness across the entire reaction network.
For example, the reaction + → + can be thought of as occurring in two elementary steps:
As the second step consumes the produced in the slow first step, it is limited by the rate of the first step. For this reason, the rate-determining step is reflected in the rate equation
of a reaction.
Another example of a rate-determining step is the formation of a carbocation
from a haloalkane
during the SN1 reaction
of tertiary haloalkanes with sodium hydroxide.
In the previous examples, the rate determining step was one of the sequential chemical reactions leading to a product. The rate-determining step can also be the transport of reactants to interact and form the product. This case is referred to as diffusion control
and, in general, occurs where the formation of product from the activated complex is very rapid and thus the supply of reactants and their interaction is rate determining.
The concept of the rate-determining step is very important to the optimization and understanding of many chemical processes such as catalysis
and combustion
.
In a reaction coordinate
, the transition state
with the highest energy is the rate-determining step of a given reaction.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
term for the slowest step
Reaction step
A reaction step of a chemical reaction is defined as: "An elementary reaction, constituting one of the stages of a stepwise reaction in which a reaction intermediate is converted into the next reaction intermediate in the sequence of intermediates between reactants and products"....
in a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
at which water is poured in. In similar manner, the rate of reaction depends on the rate of the slowest step. However it is not clear what 'slowest step' means. For example does it refer to the step with the smallest rate constant; if the reaction is reversible, does it refer to the ratio of the forward and reverse rate constants or does it simply refer to the step that has the smallest flux? If the latter, then at steady state all steps carry the same flux and therefore there is no slowest step. In metabolic pathways the rate limiting step is better defined and a measure, the flux control coefficient
Metabolic control analysis
Metabolic control analysis is a mathematical framework for describingmetabolic, signaling and genetic pathways. MCA quantifies how variables,such as fluxes and species concentrations, depend on network parameters....
is given to a step to signify how rate-limiting the step is. Theory and experiment also suggests that there is no single rate limiting step but a range of rate limitingness across the entire reaction network.
For example, the reaction + → + can be thought of as occurring in two elementary steps:
- + → NO + (slow step)
- + CO → + (fast step)
As the second step consumes the produced in the slow first step, it is limited by the rate of the first step. For this reason, the rate-determining step is reflected in the rate equation
Rate equation
The rate law or rate equation for a chemical reaction is an equation that links the reaction rate with concentrations or pressures of reactants and constant parameters . To determine the rate equation for a particular system one combines the reaction rate with a mass balance for the system...
of a reaction.
Another example of a rate-determining step is the formation of a carbocation
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
from a haloalkane
Haloalkane
The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and...
during the SN1 reaction
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
of tertiary haloalkanes with sodium hydroxide.
In the previous examples, the rate determining step was one of the sequential chemical reactions leading to a product. The rate-determining step can also be the transport of reactants to interact and form the product. This case is referred to as diffusion control
Diffusion control
Diffusion-controlled reactions are reactions that occur so quickly that the reaction rate is the rate of transport of the reactants through the reaction medium . As quickly as the reactants encounter each other, they react...
and, in general, occurs where the formation of product from the activated complex is very rapid and thus the supply of reactants and their interaction is rate determining.
The concept of the rate-determining step is very important to the optimization and understanding of many chemical processes such as catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
and combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
.
In a reaction coordinate
Reaction coordinate
In chemistry, a reaction coordinate is an abstract one-dimensional coordinate which represents progress along a reaction pathway. It is usually a geometric parameter that changes during the conversion of one or more molecular entities....
, the 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...
with the highest energy is the rate-determining step of a given reaction.