Transition State: Higher Energy Level In Reaction Energy Diagram

As we know that when the reaction takes place, the reactants are converted into products, but what really happens, is that the reactants are directly converted into products. So the answer is no, it does not happen. 

Then, what happens when the reaction occurs???

Generally, whenever a chemical reaction occurs, all the reactants are not directly converted into products. In an elementary reaction, first the reactant breaks their bond and forms an activated complex in the transition state and after that this activated complex is converted into product. Thus;

........"A Transition state is define as a position of a molecule (may be reactant molecule but not always) in the reaction path at which it's potential energy is maximum as shown in below figure"........

OR 

......."We can say that transition state is an intermediate state which lies between the state where molecules are reactant and the state where molecules are products"........

We can say that the total number of elementary steps involved in the reaction is equal to the total number of transition states present in the reaction. Thus, the higher the number of elementary steps ( or complex recations), the higher the number of transition states. 

Reaction Energy Diagram 

A transition state has extremely short lifetime due to that it is highly unstable and therefore reactant molecules don't stay there long time. Transition state structure has partial bonds because making and breaking of bonds are taking place simultaneously. And it can't be isolated from reaction mixture. 

            

Hence, we can say that a transition state is just a state before the formation of new molecules (or products) as shown in below curve;

Since, the transition state is highly unstable in nature so it readily breakdown into products. In the above figure, we see a peak point which is known as transition state that exhibits maximum energy of reacting species. 

Transition state is a molecular entity which is neither a reactant nor a product. 

For Example; 

         X─Y       ➝        Xᵟ⁻--- Yᵟ⁺         ➝      X⁽⁻⁾Y⁽⁺⁾

   (Reactants)     (Transition state)      (Products)

"Lifetime of transition state is equal to only one bond vibrational cycle''. 

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