Study guides › C/P › Organic Chemistry
Elimination Reactions
Elimination removes H and a leaving group from neighboring carbons to make a C=C bond. You will learn E2 and E1, predict Zaitsev versus Hofmann products, use anti-periplanar geometry, and decide when elimination beats substitution.
Study this chapter in PrepMed (free)
What this chapter covers
- The E2 Mechanism & Anti-Periplanar Geometry. E2 is the elimination twin of SN2: one step, a strong base, and a strict geometric requirement.
- Zaitsev vs Hofmann Products. When two alkenes can form, the base and the leaving group decide which one is major.
- The E1 Mechanism & Alcohol Dehydration. E1 shares its carbocation with SN1, so carbocation rearrangements are fair game here.
- Substitution vs Elimination. The MCAT loves to hand you a substrate and a reagent and ask which of four mechanisms wins. Use a checklist.
Key points to know
- E2: one step, rate = k[R–X][base]
- Needs a strong base; reactivity 3° > 2° > 1°
- Zaitsev: the more substituted alkene is major (small base, E1)
- Hofmann: the less substituted alkene is major (bulky base, poor leaving group)
- E1: two steps, carbocation, rate = k[R–X]
- Competes with SN1; heat favors E1 (entropy)
- 3° never does SN2; strong base gives E2, weak base gives SN1/E1
- 1° gives SN2 unless the base is bulky (E2)
Try a practice question
What is the rate law for an E2 reaction?
- rate = k[R–X]
- rate = k[R–X][base]
- rate = k[base]
- rate = k[R–X]²
Show the answer
B. E2 is bimolecular: the base and the substrate both take part in the single rate-determining step, so the rate depends on both concentrations.
Common questions
What does this chapter cover?
Elimination Reactions covers The E2 Mechanism & Anti-Periplanar Geometry, Zaitsev vs Hofmann Products, The E1 Mechanism & Alcohol Dehydration, Substitution vs Elimination.
What should I know first?
E2: one step, rate = k[R–X][base] Needs a strong base; reactivity 3° > 2° > 1° Zaitsev: the more substituted alkene is major (small base, E1)
21 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.