Hofmann's rule
/ HOFF-mahn /
Sometimes an elimination defies the usual wisdom and makes the less substituted alkene as its major product — the one with fewer alkyl groups on the double bond. This counter-current outcome is Hofmann's rule (named after August Wilhelm von Hofmann): under certain conditions, elimination favours the least substituted, terminal alkene, the opposite of Zaitsev orientation.
Two situations flip the preference. The first is a bulky base. A large base such as potassium tert-butoxide or LDA cannot easily reach the more crowded, internal beta hydrogens (which are flanked by other alkyl groups); it more readily plucks the exposed hydrogens on a less hindered terminal carbon, so the less substituted alkene wins on accessibility, not stability. The second is a bulky or charged leaving group — classically the trimethylammonium group of a quaternary ammonium salt in the Hofmann elimination — where steric strain and the electronics of the transition state again steer the base to the less hindered proton.
So Zaitsev and Hofmann are not contradictory laws but two outcomes of one balance: thermodynamic preference for the more stable alkene (Zaitsev) versus steric accessibility of the proton being removed (Hofmann). Recognizing the cue — a bulky base or a bulky leaving group — lets you predict which way an elimination will lean, which is exactly why these rules are taught as a contrasting pair rather than memorized in isolation.
2-bromo-2-methylbutane gives mostly the Zaitsev (more substituted) alkene with ethoxide, but switching to the bulky base potassium tert-butoxide flips the major product to the less substituted Hofmann alkene, 2-methylbut-1-ene.
A bulky base steers elimination to the less substituted (Hofmann) alkene.
Hofmann orientation is driven by sterics in the transition state, not by the product being more stable; the Hofmann alkene is the less stable one, formed because its beta proton is easier to reach.