Substitution vs. Elimination
Substitution and elimination are always competing. Which one wins depends on the reactants and the reaction conditions, which determine the relative energies of the transition states (TSs) involved.
From the reactants (red plateau), the three processes - bimolecular substitution (blue pathway), bimolecular elimination (green pathway) and carbocation formation (gray pathway) - take place simultaneously. The carbocation (yellow plateau) can evolve by two different routes, unimolecular substitution (brown pathway) and unimolecular elimination (navy-blue pathway).
The fastest pathway, and therefore the predominant one, will be the one that goes through the lowest-energy TS.
The relative energies of the different TSs depend on:
- The structure of the starting material
- The nucleophilicity/basicity of the nucleophile/base
- The polarity of the solvent
Although it is always difficult to predict what will happen in a reaction, the following table gives some rules of thumb, depending on the structure of the compound bearing the leaving group and on the nucleophile/base.
In the case of the nucleophiles HO−, CH3O−, EtO−, H2N−,(CH3)3CO−, the solvent is usually the corresponding conjugate acid. In the remaining cases the solvent used is a polar aprotic one such as dimethylformamide, dimethylsulfoxide or acetone.
