What is the difference between intermediate and transition state

You may or may not have one depending on the nature and the mechanism of the reaction. There are plenty of single-step reactions that have no intermediates at all. You will always have a transition state though!

Every reaction, no matter how simple it might be, has a transition state. They first had to stretch the bond to the brink of breaking explaining it very simplistically here and then break apart:. So, the transition state in this reaction is an elongated bond which is almost, but not quite, broken. So, you can always check the number of transition states by counting the steps in the mechanism and vice-versa. Reaction above was an example of a reaction with no intermediate.

This reaction is an example of an SN2 reaction where no intermediate is formed. You do have a transition state though in which the O-C bond is not quite formed yet and the C-Br bond is not quite broken.

The energy diagram for this reaction will look like this:. How does a reaction with an intermediate look like then? In the reaction above, an intermediate is formed. That intermediate is produced before the reaction proceeds into the next step. And it is the intermediate that reacts further giving you the final product. The first transition state is the process of the C-Br bond elongation that leads to the eventual bond dissociation and the formation of the carbocationic intermediate.

The second step in this reaction has its own transition state:. A quick rule-of-thumb for identifying the transition states and intermediates in the reaction is to look for the hilltops and valles on the diagram. Figure 2: Transition State. There is a high probability of the transition state to go forward to form products rather than falling back to form reactants again. In order to make a chemical reaction successful, reactant molecule should colloid with each other in proper orientation.

The transition state or the intermediate with highest potential energy is highly unstable. Therefore, it does not exist for a long period of time. This makes it difficult to capture the transition state of a chemical reaction. Activation energy of a chemical reaction is the energy barrier that has to be overcome in order to obtain products from the reaction.

It is the minimum energy required for a reactant to convert into a product. Therefore, activation energy equals the potential energy of the transition state of a chemical reaction.

Activated Complex: Activated complex refers to a collection of intermediate molecules formed during the progression of a chemical reaction. Transition State: Transition state is the intermediate of a chemical reaction that comprises the highest potential energy.

Activated Complex: Activated complex has a high potential energy than reactants. Transition State: Transition state has the highest potential energy among other intermediate structures.

Activated Complex: Activated complex may form the end product of the reaction or can go backward forming reactants without giving products. Transition State: Transition state has a high probability of forming the product rather than forming the reactants again. For this ideal case, it has been worked out in detail and forms the basis for more advanced theories.

This picture of transition state theory and activated complexes says nothing about the mechanism of the reaction other than that there is a single bond being broken - formed.

Your question comes down to kinetics. The fictitious transition state cannot be isolated. When people measure rate constants, they are typically not measuring an elementary process like that described by an activated complex transition state theory. Instead, they measure a combination of processes such as chemical reactions and physical processes, i. That experimental measurement of the rate constant gives the "apparent" activation energy. And is something not directly related to any single specific transition state.

Chemists try to explain their measured activation energy and pre-factors with a mechanism involving a collection of specific elementary steps for chemical conversion these should be elementary processes like those in transition state theory. This mechanistic "explanation" of a reaction can never be proven, but only supported through experimental rates and measurements of the chemical intermediates along their way to products where slick ways of measuring elementary processes becomes important.

That is why people are always always coming up with new methods to measure chemical intermediates. An illustrative example is that of a non-Arrhenius "negative" activation energy. That is, the reaction rate increases with lower temperatures. This is only the case if one has at least one reversible step involved in the reaction.