Activated Complex: Definition & Theory

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

The activated complex is the step in a reaction when the compound is not stable. It is due to the activated complex that exothermic reactions do not occur spontaneously.

Slowing down Exothermic Reactions

If you left an iron nail outside for a long time, it would eventually rust. The reason is because the oxidized (rusted) iron is at a lower energy state than the normal iron. In fact it is a much lower energy state, and the entire reaction releases about 150 kcal of energy/mol!

If the energy is so much lower, why doesn't the iron nail immediately rust? What prevents reactions such as this from spontaneously occurring all of the time?

Point A is the starting material, point C is the product, and point B is the activated complex
Energy diagram

Exothermic reactions (reactions that release energy) often have a high activation energy. It is this high activation energy that prevents the reactions from occurring spontaneously. The activation energy refers the energy needed to start the reaction.

Additional energy is needed to start a reaction because typically, in the course of the reaction, there is a step where the compound is very unstable and thus requires a lot of energy to create this compound. This in-between compound is called the activated complex, and occurs at the highest point of the energy diagram.

Reaction Steps

When we look at a reaction step-by-step we can see where this activated complex occurs. Let's look at the reaction to make chlorobenzene from benzene. Benzene is a very stable compound, because it is aromatic (ring-like and flat in structure).

The reaction to make chlorobenzene is exothermic, but the step breaking the aromaticity requires a lot of energy because the activated complex is very unstable.

In the basic reaction we see that benzene becomes chlorobenzene by using chlorine and we get hydrochloric acid as a side product.

Circled in red we see the activated complex which is the unstable portion of the reaction
Chlorobenzene with activated complex

Yet when we look at the reaction step by step we see that there is a step where the benzene has a positive charge. During this step the benzene is no longer aromatic and requires a lot of energy to create this product. So this is the activated complex and the reason that this reaction does not occur spontaneously.

Rust Formation

Sometimes reactions occur all in one step, but even in this case there is still an activated complex. This activated complex occurs when the electrons are moving between two molecules. During this period the electron isn't fully associated with either molecule, making both molecules very unstable. This occurs when iron is oxidized, forming rust.

Part of rust formation includes this reaction where iron is oxidized
Iron oxidation reaction

There isn't an actual intermediate compound, but during the reaction the electrons are being shared between two molecules. Both of these molecules are very unstable, so it requires a lot of energy to form this compound. But once it has formed it is really easy to create the next product.

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