A small base (NaOEt, NaOH) gives the more-substituted Zaitsev alkene; a bulky base (KOtBu, LDA, DBU) can't reach the crowded internal hydrogen, so it takes the terminal one and gives the less-substituted Hofmann alkene.
2-Bromobutane + KOtBu gives the terminal alkene 1-butene, not the more-substituted 2-butene.
A bulky base takes the most accessible β-hydrogen and gives the Hofmann (less-substituted) alkene. Structures drawn live.
1. Zaitsev Is the Default: a Small Base Gives the More-Substituted Alkene
A small strong base like NaOEt reaches the internal β-hydrogen freely and builds the more-substituted, more-stable Zaitsev alkene.
A small strong base follows Zaitsev: 2-bromobutane → 2-butene (more substituted).
2. A Base Is "Bulky" When Branching Crowds the Basic Atom
Bulk is about shape, not strength: ethoxide's oxygen is exposed, but tert-butoxide's is buried under three methyl groups — equally basic, but sterically shielded.
LDA and DBU are bulky for the same reason: branching or rings crowd their basic nitrogen.
3. The Same Substrate Gives Zaitsev with NaOEt but Hofmann with KOtBu
Same substrate, same E2 mechanism, but swapping the base flips the major product from Zaitsev (2-butene) to Hofmann (1-butene) — compare with section 1.
Bulky base, same substrate: 2-bromobutane → 1-butene (Hofmann, less substituted).
4. Bulky Bases Take the Most Accessible β-Hydrogen — a Kinetic, Steric Choice
The internal β-hydrogen is congested and the terminal one is exposed, so a bulky base can only reach the terminal H — a kinetic, steric choice, not a thermodynamic one.
5. Bulky Bases Also Push E2 over SN2 Because They Can't Reach Carbon
SN2 needs backside attack on the crowded α-carbon, which a bulky base can't manage, so bulk also favors E2 over SN2.
6. A Second Example: 2-Bromo-2-methylbutane Makes the Rule Obvious
2-Bromo-2-methylbutane gives the trisubstituted 2-methyl-2-butene (Zaitsev) with a small base, but the terminal 2-methyl-1-butene (Hofmann) with KOtBu.
7. Summary
Small base → Zaitsev (more substituted) · bulky base (KOtBu, LDA, DBU) → Hofmann (less substituted) · bulk = crowded basic atom · kinetic/steric, not thermodynamic · same substrate: 2-butene with NaOEt, 1-butene with KOtBu · bulk also favors E2 over SN2 · see Zaitsev's rule.
Quiz yourself
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1-Butene, the less-substituted terminal alkene — the Hofmann product. The bulky tert-butoxide can't reach the crowded internal β-hydrogen, so it removes a terminal one. A small base like NaOEt would instead give the Zaitsev product, 2-butene.
No — it is about shape. Bulk means branching or rings crowd the atom carrying the lone pair. tert-Butoxide has three methyls wrapped around its oxygen, so even though it is a strong base its lone pairs are hard to bring up to a hindered hydrogen. Ethoxide is just as basic but slim, so it reaches internal hydrogens easily.
SN2 requires a direct backside attack on the crowded α-carbon, which a fat base cannot manage. E2 only needs the base to reach an outward-pointing β-hydrogen, which it still can. So bulk suppresses substitution and promotes elimination.
Kinetic and steric. The bulky base takes the fastest-accessible (least hindered) β-hydrogen rather than the one that leads to the most stable alkene. The Zaitsev product is still the more stable alkene — the bulky base simply can't get to the proton that would form it.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.