E1 Reaction: Stereochemistry and Regiochemistry

Just like E2 reactions, E1 eliminations are regioselective and form more than one regioisomers.

In this example, water functions as a weak, non-bulky base, and the reaction is heated to favor elimination over substitution, forming two alkenes.

Recall that the stability of alkenes increases with the number of alkyl groups across the double bond. E1 reactions favor the Zaitsev product since it is more substituted and more stable than the Hofmann product.

Additionally, the transition state leading to the Zaitsev product has a trisubstituted partial double bond, which is lower in energy than the disubstituted counterpart. Therefore, not only is the Zaitsev product thermodynamically stable but it is also formed faster.

Unlike E2 reactions, E1 mechanisms are independent of the nature of the base. Consequently, the regioselectivity of E1 eliminations cannot be controlled using sterically hindered bases.

For example, the same reaction with a weak, bulky base like isopropyl alcohol still favors the Zaitsev over the Hofmann product.

In some E1 reactions, the expected alkene is not the major product because E1 reactions proceed via a carbocation intermediate.

In this example, the secondary ca