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An induction period in 
Similarly, for an autocatalytic 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 is exothermic, 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 form Grignard 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 a
An induction period in chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...
is an initial slow stage of a chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions.
In some catalytic reactions, a pre-catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
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, with Wilkinson's catalyst, one triphenylphosphine ligand must dissociate to give the coordinatively unsaturated 14-electron species which can participate in the catalytic cycle:
:
Similarly, for an autocatalytic 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 is exothermic, 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 form Grignard 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 a thermal runaway
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
, even causing the reaction solvent to boil-off.
References
{{reflist Catalysis Chemical kinetics