Two-step pathways

Reaction mechanisms that have more than one step are referred to as complex. For these mechanisms, the rate of the overall reaction depends on the rate of the slowest (rate-determining) step. Thus experimental rate law for the overall process also depends on the rate law for the slowest step.

Thus when a mechanism is postulated, it is also necessary to postulate which step is rate-determining.

If the slow step is the first step in the reaction mechanism, the experimental rate law (equation) predicted by that mechanism will be the same as the rate law (equation) predicted by the rate-determining step.

Consider the two postulated mechanisms for the reaction shown below:

Overall:
CH₃Br + OH^–

CH₃OH + Br^–

Mechanism 1:
step 1: CH₃Br

⁺CH₃ + Br^– slow
step 2: H₃C⁺ + OH^–

CH₃–OH   fast
Predicts rate = k[CH₃Br]

Mechanism 2:
CH₃Br + OH^–

CH₃OH + Br^–
Predicts rate = k[CH₃Br][OH^–]

Thus these two mechanisms can be distinguished by comparing the experimental and predicted rate laws (equations).

Note, that If the same rate law (equation) is predicted by two different mechanisms, either of those mechanisms (or any other mechanism that has the same predicted rate law) could be operating.ss