"The principal olefin formed in the @D01547@ of quaternary ammonium hydroxides that contain different primary @A00228@ is always ethylene, if an ethyl group is present." Originally given in this limited form by A. W. Hofmann, the rule has since been extended and modified as follows: "When two or more @A00224@ can be produced in a β-@E02038@ reaction, the alkene having the smallest number of @A00228@ attached to the double bond carbon atoms will be the predominant product." This orientation described by the Hofmann rule is observed in @E02038@ reactions of quaternary ammonium salts and tertiary sulfonium salts, and in certain other cases.
See also:
Zaitsev rule
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1121 [Terms] [Paper]