Transition state or Intermediate

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???

Let's have a look
What happens when there is a reaction???
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.
     A–B + C   ➝         A----B----C         ➝   A+B–C
   Reactants     Activated Complex      Products 
While in case of non-elementary reactions, inbetween two transition state one intermediate molecules may exists. These intermediate molecules can be free radicals, Ions (cations or anions) and Polar substances, Molecules, Transition complexes, etc.

'Transition State' will always be formed whenever a reaction occurs (or whenever a reactant molecule is converted into product molecule) while 'Intermediate' may or may not be form in between them. 
A transition state occurs at the top of the activation energy barrier. There is one transition state for each elementary Step. 
While intermediate will be formed between the two transition states. An intermediate is neither a reactant nor a product. Intermediates are short lived compounds formed in a chemical reaction.

Can the transition state be more than one?
Yes, the transition state can be more than one but only and only in case of non-elementry reactions. Whereas in case of elementary reactions, only one transition state exists. When more than one transition state exists then the 'intermediate' must be formed. And between the two transition states, an intermediate is present. Since, an intermediate can be present between two transition states so intermediate is not present in case of elementary reactions. 

Notes
We all know what are the Elementary reactions? Now we are discussing about what are the Transition state or Intermediates?
➣ For Elementary Reactions
   Reactants ➝ Transition state ➝ products
➣ For Non-elementary Reactions
   Reactants ➝ Transition state ➝
            Intermediates ➝ Transition state 
                      ➝ products
Fig:- Energy Flow Diagram
As shown in the above figure, potential energy increases in y-direction while reaction proceeding in x-direction. 


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