The table below summarises the relationship between the structure of the substrate and the relative amounts of substitution and elimination pathways.
| RCH2X(primary) | R2CHX(secondary) | R3CX (tertiary) |
Mainly substitution (SN2)
| Mainly substitution (SN2)
IF nucleophile
is strong and NOT basic (I–, CN–).
| Mainly substitution (SN1)
IF temperature is low and
nucleophile has no charge
|
May observe elimination (E2)
IF nucleophile basic and bulky (CH3)3COK) | Mainly elimination (E2)
IF nucleophile is basic | Mainly elimination (E2)
IF nucleophile is basic |
Summarising the summary!Substitution is the major pathway if the carbon bearing the leaving group is primary.
Substitution is more significant for other types of substrates if the nucleophile is NOT basic.
Elimination is the major pathway if the nucleophile is basic, particularly if the carbon bearing the leaving group is tertiary.
Elimination is more significant for reactions done at higher temperatures.