Read peaks and valleys differently
A reaction-energy profile plots energy against progress along a reaction coordinate. It is not a plot of concentration against time.
A transition state lies at the top of an energy barrier for an elementary step. An intermediate occupies a local minimum between barriers: it has formed from the reactants but still must react to become the final products. A short lifetime does not make an intermediate a transition state.
Example: nucleophilic substitution
In the simple SN2 substitution of methyl iodide by hydroxide, C–O bond formation and C–I bond cleavage pass through one transition state. There is no intermediate between those bond changes.
A stepwise SN1 solvolysis can instead form a carbocation before solvent capture. That carbocation is an intermediate, with an energy barrier on either side. Subsequent proton transfer may add another step and another transition state.
Use the profile to ask better questions
- A peak identifies a barrier, not an isolable molecule.
- A valley between peaks identifies an intermediate in the proposed mechanism.
- Count barriers to count the elementary steps shown.
A drawn profile summarizes a mechanistic model. Its shape alone is not experimental proof, and a high-energy intermediate can be too short-lived to isolate.
Related question
Apply this knowledge
Use the concept guide to understand the reasoning, then return to the complete question and worked answer.
The Diels–Alder Reaction Is Concerted: Define ConcertedSources
These references support the core concepts and interpretation boundaries explained above.