Transition state

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.

S_N2 reaction mechanism showing 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.

Transition state in the base hydrolysis of bromoethane.

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.

Illustration of the Structure Correlation Principle.

Related pages

• Reaction mechanism
• Activation energy
• Transition state theory
• [[S_N2 reaction]]

References

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

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  Bromoethane SN2 reaction
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  Base hydrolysis of bromoethane, transition state
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  Structure Correlation Principle