Identifying SN1, SN2, E1, E2

Primary (1°) - Good nucleophile, weak base (I⁻, RS⁻)

SN2. The carbon is accessible, and the nucleophile prefers substitution.

Primary (1°) - Very weak nucleophile/base (H₂O, ROH)

Usually no reaction. An ordinary primary carbocation is too unstable for SN1/E1, and the nucleophile is too weak for SN2.

Primary (1°) - Strong base (OH⁻, RO⁻)

SN2 often favored. An unhindered primary carbon is accessible, although bulky bases or branching can favor E2.

Secondary (2°) - Good nucleophile, weak base (I⁻, RS⁻)

SN2 often favored. A good nucleophile can attack a secondary carbon, though conditions can affect competition.

Secondary (2°) - Very weak nucleophile/base (H₂O, ROH)

SN1 / E1 possible. These share a carbocation intermediate. Solvent and temperature matter.

Secondary (2°) - Strong base (OH⁻, RO⁻)

E2 often favored. Strong bases readily remove beta hydrogens; SN2 may compete.

Tertiary (3°) - Good nucleophile, weak base (I⁻, RS⁻)

SN1 possible. SN2 is blocked by crowding. A stable carbocation can allow SN1 under suitable conditions.

Tertiary (3°) - Very weak nucleophile/base (H₂O, ROH)

SN1 + E1. A tertiary carbocation is relatively stable; substitution and elimination compete.

Tertiary (3°) - Strong base (OH⁻, RO⁻)

E2. SN2 is blocked by crowding, and a strong base promotes elimination.