Which of the following potential energy (P.E.) diagrams represents the S$$_N$$1 reaction?

An $$S_N1$$ substitution proceeds in TWO distinct steps:
  1. Slow ionisation of the alkyl halide to give a carbocation and a halide ion.
  2. Fast attack of the nucleophile on this carbocation.

Potential-energy consequences of these steps:

• Two elementary steps ⇒ two transition states ⇒ the energy profile must show TWO maxima.
• The first step (ionisation) is rate-determining, so its activation energy is larger; therefore the first maximum is higher than the second.
• Between the two maxima lies a local minimum that represents the carbocation intermediate.

Among the given diagrams, only Option A shows
  • two humps (two transition states),
  • the first hump higher than the second, and
  • an intermediate valley between them.
Hence it correctly depicts the $$S_N1$$ mechanism.

Option A which is: the diagram with two maxima (first higher) and a carbocation intermediate.