A small strong base (NaOEt, NaOH) removes the most accessible internal β-hydrogen and gives the more-substituted Zaitsev alkene. A bulky base — potassium tert-butoxide (KOtBu), LDA, DBU — is too big to fit next to the crowded internal hydrogen, so it grabs the least hindered (usually terminal) β-hydrogen and delivers the less-substituted Hofmann alkene. Same substrate, opposite regiochemistry, decided by the size of the base.
Here is the whole idea in one scheme: 2-bromobutane loses HBr, but with a bulky base like potassium tert-butoxide the product is the terminal alkene, 1-butene, not the more-substituted 2-butene you might expect.
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
When a β-elimination has a choice of β-hydrogens, the usual winner is the Zaitsev product — the more substituted, more stable alkene. A more-substituted C=C is lower in energy because the alkyl groups donate electron density into the π system (hyperconjugation and induction), so the transition state that leads to it is also lower. With a small, ordinary strong base such as sodium ethoxide (NaOEt) or hydroxide, nothing stops the base from reaching the internal β-hydrogen, and 2-bromobutane eliminates cleanly to 2-butene.
A small strong base follows Zaitsev: 2-bromobutane → 2-butene (more substituted).
2. A Base Is "Bulky" When Branching Crowds the Basic Atom
"Bulky" is a statement about the shape around the reactive site, not about how strong the base is. Compare the two oxygen bases below. In ethoxide, the negatively charged oxygen sits at the end of a slim two-carbon chain — it can slide right up to a hydrogen. In tert-butoxide, that same oxygen is flanked by three methyl groups on the neighboring carbon, wrapping it in a fat, cone-shaped shell. The lone pairs are just as basic, but they are buried. That steric bulk is the entire reason KOtBu behaves differently from NaOEt.
The same logic makes LDA and DBU bulky bases: their basic nitrogen is hemmed in by big isopropyl groups or a fused ring. Whenever branching or rings crowd the atom carrying the lone pair, the base can no longer squeeze into tight spaces.
3. The Same Substrate Gives Zaitsev with NaOEt but Hofmann with KOtBu
This is the payoff, and it is a classic exam trap: identical starting material, identical mechanism (E2), yet the major product flips just by swapping the base. Give 2-bromobutane a small base and you get mostly the internal Zaitsev alkene; give it the bulky tert-butoxide and the major product becomes the terminal Hofmann alkene, 1-butene. The scheme below is the KOtBu case — hold it next to the NaOEt scheme in section 1 to see the switch.
Bulky base, same substrate: 2-bromobutane → 1-butene (Hofmann, less substituted).
4. Bulky Bases Take the Most Accessible β-Hydrogen — a Kinetic, Steric Choice
Why does bulk change the outcome? Look at where the two β-hydrogens live on 2-bromobutane. The hydrogen that would give the Zaitsev alkene sits on an internal carbon that already carries a methyl group and the rest of the chain — a congested spot. The hydrogen that gives the Hofmann alkene sits on the exposed terminal CH₃ group. A slim base can reach either, so it takes the internal one and follows thermodynamics (Zaitsev). A bulky base physically cannot dock next to the crowded internal hydrogen; the only β-H it can comfortably reach is the terminal one, so it takes that. The bulky base makes a kinetic, steric decision — fastest accessible proton — rather than a thermodynamic one, and the less-substituted alkene results.
5. Bulky Bases Also Push E2 over SN2 Because They Can't Reach Carbon
The same fatness that steers regiochemistry also steers the substitution-versus-elimination contest. An SN2 reaction needs the nucleophile to make a direct, backside hit on the crowded α-carbon; an E2 only needs the base to reach an outward-pointing β-hydrogen. A bulky base is terrible at the first job and perfectly fine at the second, so bulky bases strongly favor elimination (E2) over substitution (SN2). This is exactly why tert-butoxide is a go-to reagent when you want a clean E2 and want to suppress the competing ether that SN2 would give. Bulk therefore buys you two things at once: elimination over substitution, and the Hofmann alkene over the Zaitsev one.
6. A Second Example: 2-Bromo-2-methylbutane Makes the Rule Obvious
The effect is even sharper on a substrate that offers a very congested Zaitsev hydrogen and a very exposed Hofmann one. 2-Bromo-2-methylbutane can eliminate toward the trisubstituted alkene 2-methyl-2-butene (Zaitsev) or toward the terminal, disubstituted 2-methyl-1-butene (Hofmann). A small base overwhelmingly gives the crowded Zaitsev product. Switch to KOtBu and the major product becomes 2-methyl-1-butene — the base simply cannot get to the internal hydrogens buried between all those methyl groups, so it strips a proton from the accessible terminal CH₃ instead.
7. Summary
Elimination normally follows Zaitsev — a small strong base (NaOEt, NaOH) reaches the crowded internal β-hydrogen and builds the more-substituted, more-stable alkene. A bulky base — potassium tert-butoxide, LDA, or DBU — is defined by branching or rings that crowd its basic atom, and that bulk keeps it from docking next to the hindered internal hydrogen. It instead removes the most accessible (usually terminal) β-hydrogen, giving the less-substituted Hofmann alkene: a kinetic, steric preference rather than a thermodynamic one. The same substrate can therefore give 2-butene with NaOEt but 1-butene with KOtBu. As a bonus, bulky bases can't reach the α-carbon for backside attack, so they also favor E2 over SN2. When you see a bulky base in a problem, expect elimination and expect the less-substituted product. For the flip side — the more-substituted default — review Zaitsev's rule.
Quiz yourself
Tap a question to reveal the answer — free, no login.
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.