Substitution and elimination reactions are among the most common in organic chemistry. It is key to understand how and when these reactions happen.

This section focuses on three concepts necessary for an understanding of substitution and elimination: the rate law, the leaving group, and the nucleophile.

The Rate Law

The rate law is a mathematical equation that relates the speed of a reaction and the concentration(s) of its reactants. For example, take this hypothetical reaction:

X + Y → Z    

The rate law of the above reaction is:

rate = k [X]a [Y]b    

k is a constant determined by the reactions conditions. For the purposes of this SparkNote, a and b will be zero or one. The rate law is useful because it describes what reactants are present in the transition state of the rate-limiting step.

The Leaving Group

The leaving group is a common feature to all substitution and elimination reactions. It is the part of a reactant molecule that leaves during the course of the reaction. In the process of leaving, it generally picks up an extra electron from its bond to the main molecule. The leaving group's stability after it has left the main molecule determines its efficacy.

The Nucleophile

All of the substitution reactions discussed in the SparkNote involve a nucleophile. In these reactions, the nucleophile substitutes for the leaving group. As the name suggests, nucleophiles generally like to pick up protons and other bare nuclei just like bases. However, nucleophilicity and basicity are fundamentally different concepts. A clear relationship does not always exist between the two. Nucleophilicity jumps dramatically in polar, aprotic solvents.