Why Are Third Order Reactions Uncommon?
For simple reactions the order of the reaction is the number of particles that react in the slowest step of the reaction – the rate-determining step. So essentially in a second order reaction two particles come together in this slow step; in a first order reaction the slow step involves the breakdown of one particle (e.g. the ionisiation of a tertiary alkyl halide in an SN1 mechanism) So for a reaction to be third order three particles would have to come together in this slow step, and statistically it’s fairly unlikely that three particles would happen to collide with the correct orientation at a particular moment in time.
