Aninduction period inchemical kinetics is an initial slow stage of achemical reaction; after the induction period, the reaction accelerates.[1] Ignoring induction periods can lead torunaway reactions.
In some catalytic reactions, a pre-catalyst needs to undergo a transformation to form the active catalyst, before the catalyst can take effect. Time is required for this transformation, hence the induction period. For example, withWilkinson's catalyst, one triphenylphosphine ligand must dissociate to give thecoordinatively unsaturated 14-electron species which can participate in the catalytic cycle:
Similarly, for anautocatalytic reaction, where one of the reaction products catalyzes the reaction itself, the rate of reaction is low initially until sufficient products have formed to catalyze the reaction.
Reactions generally accelerate when heat is applied. Where a reaction isexothermic, the rate of the reaction may initially be low. As the reaction proceeds, heat is generated, and the rate of reaction increases. This type of reaction often exhibits an induction period as well.
The reactions to formGrignard reagents are notorious for having induction periods. This is usually due to two reasons: Firstly, the thin film of oxide on the magnesium reagent must be removed before the bulk magnesium can react. Secondly, Grignard reactions, while exothermic, are typically conducted at low temperature for better selectivity. For these two reasons, Grignard reactions often can have a long induction period, followed by athermal runaway, even causing the reaction solvent to boil-off.