Elimination of a haloalkane with strong base such as hydroxide ion (the E2 reaction) is an important and common method to prepare alkenes. That the elimination proceeds to give the most highly substituted alkene is called Zaitsev's rule (Section 7.7B).
If the haloalkane can yield a stable carbocation then elimination to form an alkene might proceed via the E1 mechanism. Zaitsev's rule is followed, and carbocation rearrangements may occur (Section 7.7B).
Alcohol dehydration to an alkene with a strong acid such as sulfuric acid (H2SO4) or phosphoric acid (H3PO4) usually proceeds via a carbocation, and follows Zaitsev's rule. Tertiary alcohols are dehydrated most easily, whereas primary alcohols are harder to convert to alkenes. This reactivity is a function of the stability of the carbocations that are formed after protonation of the HO group and departure of a molecule of water. Carbocation rearrangements are commonly observed. Elimination of a haloalkane and an alcohol with the same carbon skeleton leads to the same products because both reactions involve the same carbocation (Section 8.4E).
