Substitution & Elimination Exam Review
Evaluate substrate class, nucleophile, solvent, and temperature before selecting a mechanism.
1. Understand the Concept
One clue never determines a mechanism. Use a weighted decision: primary carbon plus an unhindered nucleophile in polar aprotic solvent suggests SN2; a hindered strong base favors E2; tertiary ionization in protic solvent can give competing SN1/E1 products.
Your goal: Evaluate substrate class, nucleophile, solvent, and temperature before selecting a mechanism.
2. Step-by-Step Exam Strategy
Methyl and primary substrates resist SN1; tertiary centers block normal SN2. Secondary substrates are particularly condition-dependent.
A strong bulky base typically favors E2. A good, weakly basic nucleophile can favor SN2 when steric access allows.
Polar aprotic solvents often help SN2. Polar protic solvents can stabilize ions; heat can increase elimination competition.
For E2 look for anti-periplanar beta H. For SN2 expect inversion of geometry. Rearrangements may occur only via carbocations.
3. Visual Reference
4. Worked Example
2-bromobutane + NaOEt, ethanol, heat
Expect E2 as a common major pathway with substitution competition; removing a β-hydrogen forms butenes. Zaitsev alkene usually predominates among allowed alkenes, with trans-2-butene generally favored over cis-2-butene.
5. Common Mistakes
- Treating every secondary substrate as SN2 regardless of base and heat.
- Assuming that all SN1 reactions produce a perfectly 50:50 racemate.
- Drawing E2 products without identifying an accessible anti β-H.
6. Practice & Reinforcement
Try the interactive questions below. In Practice mode, each answer includes an explanation. In Mock Exam mode, explanations appear after submitting the full set.
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