Reaction coordinate for the bimolecular nucleophilic substitution (SN2) reaction between bromomethane and the hydroxide anion, showing a new “forming bond” and old “leaving bond” in the reactants, two stretched leaving-forming bonds in transition state (hill region), and a completed forming bond and detached leaving bond in the products.

In chemistry, the transition state is an association of atoms, in the form of a short-lived intermediate, of highest energy formed during a chemical reaction, during the transition from reactants to products. [1] A transition stateis the highest point on the reaction coordinatecorresponding to the time period in which the activated complexforms, whereby dynamic molecular reconfigurations proceed. The concept of mechanism is a key component to the logic if steps in the transition state.

The theory of transition statewas conceived in 1935 by chemists Henry Eyring, Meredith Evans, and Michael Polanyiwho, using thermodynamics, kinetics, and statistical mechanics logic, began constructing potential energy surfaces for chemical reactions. [4]

Potential
The application of the thermodynamic concept of potential to the kinetics of chemical reactions (transition state theory) provides a criterion for selecting the optimal pathway for a transition, usually the pathway with the transition state of the lowest free energy. In this perspective, by providing a direction for systems to evolve and an optimal pathway, thermodynamics offers a way for answering why things happen the way they do. [2]

Classic model
The classic model of reaction mechanism between two reactants A and B is as follows

A+ B	↔	A ∙∙∙ B	↔	A – B	↔	AB
close proximity		encounter complex		transition state		bound state

The first step in the reaction, A + B, brings the two atoms or molecules into close proximity, resulting in the first ‘macrocollision’ and yielding the encounter complex: A ∙∙∙ B. In the second step, short range electrostatic effects result in a well-oriented conformation called the ‘transition state’ or A – B. At this point, orbital overlap effects are not yet significant factors. The last step of the binding process is the latter stage transition from the favorable intermediates to the bound state unit AB. This is a process of induced fit, limited by a rate constant, which requires structural rearrangements overcoming mostly enthalpic energy barriers.

Human chemistry
In the study of reactions between people, the transition state is defined as a short-lived heightened-energy configuration in a relationship, in which human chemical bonds are said to be stretched to their maximum, prior to stable product formation. [3] The term new relationship energy is a verbal way of describing the energy of transition state.

References
1. Daintith, John. (2004). Oxford Dictionary of Chemistry. Oxford University Press.
2. Di-Cera, Enrico. (2000). Thermodynamics in Biology, (pgs. 3-4). Oxford: Oxford University Press.
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One) (transition state, pgs. 62, 144, 241, 245, 252, 267-68, 282-84; transition state theory, pg. 267) (preview), (Google books). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two) (transition state, pgs. 377, 413, 414, 540-41, 599, 614-15; transition state theory, pg. 414, transition state complex, 413) (preview) (Google books). Morrisville, NC: LuLu.

External links
● Transition state – Wikipedia.
● Transition state theory – Wikipedia.