Substitution & Elimination Summary - Putting it all Together

A leaving group must be able to leave, and the intermediate must be a stable carbocation (such as from a 3º alkyl halide)
SN1 and E1 compete
Both proceed through a carbocation intermediate once the leaving group leaves.
- Reactions that produce carbocations will produce both SN1 and E1 products
- SN1 predominates at lower temperatures
- E1 predominates at higher temperatures
  - If “heat” or “Δ” is denoted in the question, it’s typically hinting at an elimination reaction instead of a substitution
If the leaving group is OH, E1 can be favoured by using a strong acid whose conjugate base is not nucleophilic (such as H2SO4).

Species which are both strong nucleophiles and strong bases will produce both SN2 and E2 products. E2 products will be favored at higher temperatures, although a mixture will still be produced.
Species which are strong nucleophiles but weak bases will produce SN2 products almost exclusively.
Species which are weak nucleophiles but strong bases (or just simply very sterically-hindered bases) will produce E2 products exclusively, as bulky bases cannot act as nucleophiles, regardless of how strong their charge is.

Answer the following questions first:

1. What is the substitution level of the carbon bonded to the leaving group?

2. What is the nature of the solvent?

3. Describe the species that has the lone pair (nucleophile vs base, weak vs strong)

4. Describe the leaving group.

5. Use either chart on the following two pages to determine the type of reaction.

SN1 vs E1 | SN2 vs E2 | substitution and elimination summary