In general, an elimination that gives the less-substituted product in excess is said to follow the "Hofmann rule" (as opposed to the Zaitsev rule). What's important is why this happens. There are several reasons that will drive a system to give the Hofmann product:
An excess of iodomethane does an SN2 reaction repeatedly on the nitrogen of an amine, with base neutralizing the HI byproduct. Then silver oxide precipitates AgI leaving hydroxide as the counterion. This basic anion associates with the positively charged nitrogen, leading to syn elimination (predominantly). |
We can see the propensity for syn elimination and the trans product by looking at the structure of the trimethylammonium cation: Look at the H-C-C-N Dihedral Examine the ESP map Translucent ESP map |
And the structure of the trans-cyclooctene product:
Black background White background Spacefilling model Wireframe Ball & Stick |
Note that NO anti-periplanar hydrogens are oriented correctly with respect to the trimethylammonium leaving group, and that there is one syn hydrogen oriented correctly--this will lead to creation of the E double bond. As a further note, E-cyclohexene is the smallest cycloalkene that can be isolated with a trans double bond. |