A chemical reaction always has to do with: 'redistribution of valency electrons, at the moment that certain particles with enough energy have contact with one another.
In such a process, chemical bonds disappear and new are made. We know this from module 7.
The reaction is the result of a complex process, with various steps.
There always will be a first step (initiation), the next steps (continuation) and something like a termination, the final steps that stop the reaction.
All these steps toghether are the reaction mechanism.
In such a mechanism, there is always something like an 'attack' of certain particles at other particles.
The attacking particles normally have sufficient energy for that (otherwise they would fail). They must be energy rich.
Besides that, catalysts have an important influence on the mechanism.
They participate, yes, but at the end they remain unchanged.
In biochemistry we talk about biocatalysts or enzyms.
Catalysts can stimulate a process, or better: certain steps in a process.
In stead of starting a chemical process with a catalyst, it can also be done with light radiation, that makes a bonding unstable and thus a particle more reactive.
Active particles are made (with sufficient energy) that can continue the reaction and eventually also can stop the reaction.
In Carbon Chemistry we know two types of reaction mechanisms:
- An attack is possible of a positive at a negative particle (electrophylic attack) or the opposite: an attack of a negative particle at a positive particle (nucleophylic attack).
- The other, non polar, mechanism is that of 'radicals'. Uncharged particles can be very reactive because of unpaired electrons; charges do not play any role here.