SN1, SN2, E1, E2 Reactions

The Two Substitutions

SN2 is a single-step backside attack with rate law $\text{rate} = k[\text{RX}][\text{Nu}^-]$ It demands an unhindered (methyl or primary) substrate, a strong nucleophile, and a polar aprotic solvent (DMSO, acetone), and it inverts configuration (Walden inversion). SN1 is a two-step mechanism through a carbocation with $\text{rate} = k[\text{RX}]$ It favors tertiary substrates, weak nucleophiles, and polar protic solvents, gives racemization, and can rearrange via hydride or methyl shifts.

The Two Eliminations

E2 is one concerted step requiring a strong base and an anti-periplanar beta-hydrogen; Zaitsev's more-substituted alkene usually dominates, but bulky bases (t-BuOK) give the Hofmann product. E1 shares the SN1 carbocation, so tertiary substrates in protic solvents with heat give SN1/E1 competition.

Test-Day Decision Rules

Methyl or primary substrate plus strong nucleophile in aprotic solvent: SN2. Tertiary substrate in protic solvent: SN1/E1 mixture, elimination favored by heat. Strong bulky base with any beta-hydrogen: E2. Secondary substrates are the battleground where nucleophile strength and basicity decide.

Key Takeaways

• Bimolecular rate laws (SN2/E2) include the nucleophile or base; unimolecular ones (SN1/E1) do not.
• SN2 inverts; SN1 racemizes; E2 needs anti-periplanar geometry.
• Carbocation stability (3 > 2 > 1 degree) and rearrangements belong to SN1/E1 only.
• Heat and bulky bases push toward elimination.