The nucleophile has a high concentration if it is a protic solvent (H2O, ROH, RCO2H), if it is in the same molecule as the electrophile, or if it is noted as being in high concentration.
| low [nuc] | high [nuc] | |
|---|---|---|
| 1° elec | No reaction | SN2* |
| 2° elec | E1 (maybe rearrangement)** | SN1 (maybe rearrangement)** |
| 3° elec | E1 | SN1 |
Notes:
If elimination is to occur, there must be at least one C adjacent to the electrophilic C that bears a H atom.
*The SN2 reaction occurs only under very strongly acidic conditions and very high concentrations of a nonbasic nucleophile, usually Cl–, Br–, or I–. Typical conditions include 37% aq. HCl and 48% aq. HBr. The electrophile is usually an alcohol.
**Whether rearrangements are expected to occur is difficult to predict. Normally you would not be expected to predict that a rearranged product is the major one.