Transition state: Difference between revisions

Revision as of 23:56, 9 February 2025

Concept in chemistry

Transition state is a key concept in chemical kinetics and reaction mechanisms. It refers to a particular configuration along the reaction coordinate. It is the state corresponding to the highest energy along this path. The transition state is often marked by the formation of a high-energy, unstable arrangement of atoms that exists momentarily as reactants are converted into products.

Theory

The concept of the transition state is central to the transition state theory (TST), which provides a framework for understanding how chemical reactions occur and how reaction rates are determined. According to TST, the transition state is a transient configuration of atoms at the peak of the potential energy barrier separating reactants and products. The energy required to reach this state is known as the activation energy.

Characteristics

The transition state is characterized by a partial bond formation and bond breaking. It is not a stable state and cannot be isolated. Instead, it is a fleeting moment in the reaction pathway. The structure of the transition state can be inferred using various experimental techniques and computational methods, such as X-ray crystallography and quantum chemistry calculations.

Examples

One classic example of a transition state is found in the [[S_N2 reaction]] mechanism. In the reaction of bromoethane with a nucleophile, the transition state involves a pentavalent carbon atom where the nucleophile and the leaving group are both partially bonded to the carbon.

Another example is the base hydrolysis of bromoethane, where the transition state involves the simultaneous breaking of the C-Br bond and the formation of the C-OH bond.

Structure Correlation Principle

The Structure Correlation Principle is a concept that relates the structure of the transition state to the structure of the reactants and products. It suggests that the transition state resembles the structure of the species to which it is more similar in energy.

Related pages

References

Laidler, K. J. (1987). "Chemical Kinetics". Harper & Row.
Anslyn, E. V., & Dougherty, D. A. (2006). "Modern Physical Organic Chemistry". University Science Books.

@@ Line 1: / Line 1: @@
-'''Transition state''' is a term used in [[chemistry]] and [[biochemistry]] to refer to the highest-energy state of a molecule during a [[chemical reaction]]. It is a state through which a molecule must pass as it is being transformed from reactants to products. The transition state is often symbolized by the double dagger ‡ in chemical equations.
+{{Short description|Concept in chemistry}}
+{{Use dmy dates|date=October 2023}}
-== Overview ==
+'''Transition state''' is a key concept in [[chemical kinetics]] and [[reaction mechanism]]s. It refers to a particular configuration along the reaction coordinate. It is the state corresponding to the highest energy along this path. The transition state is often marked by the formation of a high-energy, unstable arrangement of atoms that exists momentarily as reactants are converted into products.
-The concept of the transition state is fundamental to the understanding of [[chemical kinetics]] and the rates of chemical reactions. It was first proposed by [[Henry Eyring (chemist)|Henry Eyring]], [[Michael Polanyi]] and [[Meredith Gwynne Evans]] in the 1930s, and is based on the principles of [[quantum mechanics]].
+==Theory==
+The concept of the transition state is central to the [[transition state theory]] (TST), which provides a framework for understanding how chemical reactions occur and how reaction rates are determined. According to TST, the transition state is a transient configuration of atoms at the peak of the potential energy barrier separating reactants and products. The energy required to reach this state is known as the [[activation energy]].
-In a chemical reaction, the transition state is the point at which the original bonds have stretched to their limit and are about to break, while the new bonds are forming. This state is very unstable and exists for only a very short time – typically on the order of femtoseconds (10^-15 seconds).
+==Characteristics==
+The transition state is characterized by a partial bond formation and bond breaking. It is not a stable state and cannot be isolated. Instead, it is a fleeting moment in the reaction pathway. The structure of the transition state can be inferred using various experimental techniques and computational methods, such as [[X-ray crystallography]] and [[quantum chemistry]] calculations.
-The energy required to reach the transition state from the reactants is known as the [[activation energy]]. The lower the activation energy, the faster the reaction will proceed.
+==Examples==
+One classic example of a transition state is found in the [[S<sub>N</sub>2 reaction]] mechanism. In the reaction of bromoethane with a nucleophile, the transition state involves a pentavalent carbon atom where the nucleophile and the leaving group are both partially bonded to the carbon.
-== Transition state theory ==
+[[File:Bromoethane_SN2_reaction.svg|thumb|center|500px|S<sub>N</sub>2 reaction mechanism showing the transition state.]]
-[[Transition state theory]] (TST), also known as activated complex theory, is a theory that explains the rates of chemical reactions. According to TST, the rate of a reaction is directly proportional to the number of molecules that have enough energy to reach the transition state.
+Another example is the base hydrolysis of bromoethane, where the transition state involves the simultaneous breaking of the C-Br bond and the formation of the C-OH bond.
-TST provides a way to calculate the [[rate constant]] for a reaction, based on the energies of the reactants and the transition state. It also allows for the prediction of reaction rates under different conditions, such as changes in temperature or pressure.
+[[File:Base_hydrolysis_of_bromoethane,_TS.png|thumb|center|500px|Transition state in the base hydrolysis of bromoethane.]]
-== See also ==
+==Structure Correlation Principle==
+The [[Structure Correlation Principle]] is a concept that relates the structure of the transition state to the structure of the reactants and products. It suggests that the transition state resembles the structure of the species to which it is more similar in energy.
-* [[Reaction rate]]
+[[File:Structure_Correlation_Principle.png|thumb|center|500px|Illustration of the Structure Correlation Principle.]]
-* [[Chemical kinetics]]
-* [[Potential energy surface]]
-* [[Arrhenius equation]]
-== References ==
+==Related pages==
+* [[Reaction mechanism]]
+* [[Activation energy]]
+* [[Transition state theory]]
+* [[S<sub>N</sub>2 reaction]]
-<references />
+==References==
+* Laidler, K. J. (1987). "Chemical Kinetics". Harper & Row.
+* Anslyn, E. V., & Dougherty, D. A. (2006). "Modern Physical Organic Chemistry". University Science Books.
 [[Category:Chemical kinetics]]
-[[Category:Physical chemistry]]
+[[Category:Reaction mechanisms]]
-[[Category:Quantum mechanics]]
-{{stub}}