The mechanism for a reaction is a set of elementary reactions that detail the bond–making and bond–breaking steps leading from reactants and products.
There is often more than one plausible mechanism (pathway) for a reaction.
Multistep mechanisms involve intermediates like +CH3. These are formed in one step of the mechanism and consumed in a subsequent step.
Note that the multistep mechanism not only shows the bond-breaking and bond-making, it also predicts the relative rates of the steps. The slower step is referred to as the rate-determining step because this step limits the rate of the overall reaction.
Elementary reactions occur on a single collision between the reacting species.
There is often more than one plausible mechanism (pathway) for a reaction.
For example, consider the reaction:
CH3Br + OH–
CH3Br + OH–
CH3OH + :Br–
| single step mechanism CH3Br + OH– CH3OH + Br–(simultaneous C–Br bond breaking and C–O bond formation) |
| two step mechanism step 1: CH3Br +CH3 + Br – (C–Br bond breaks - slow) step 2: H3C+ + OH– CH3–OH(C–O bond forms -fast) |
Multistep mechanisms involve intermediates like +CH3. These are formed in one step of the mechanism and consumed in a subsequent step.
Intermediates, like catalysts, do not appear in the overall equation for the reaction. Catalysts are different to intermediates in that they are a reactant in one step and a product in a subsequent step.
Note that the multistep mechanism not only shows the bond-breaking and bond-making, it also predicts the relative rates of the steps. The slower step is referred to as the rate-determining step because this step limits the rate of the overall reaction.